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Bloodworm andEarthwormControl in Rice
A report for the Rural Industries Researchand Development Corporation
by M. M. Stevens
December 2000
RIRDC Publication No 00/183RIRDC Project No DAN-146A
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© 2000 Rural Industries Research and Development Corporation.All rights reserved.
ISBN 0 642 58214 9ISSN 1440-6845
Bloodworm and earthworm control in ricePublication No. 00/183Project No. DAN-146A
The views expressed and the conclusions reached in this publication are those of the author and notnecessarily those of persons consulted. RIRDC shall not be responsible in any way whatsoever to any personwho relies in whole or in part on the contents of this report.
This publication is copyright. However, RIRDC encourages wide dissemination of its research, providing theCorporation is clearly acknowledged. For any other enquiries concerning reproduction, contact thePublications Manager on phone 02 6272 3186.
Researcher Contact DetailsMark M StevensYanco Agricultural InstituteNSW AgriculturePMB Yanco NSW 2703
(02) 6951-2611(02) [email protected]
RIRDC Contact DetailsRural Industries Research and Development CorporationLevel 1, AMA House42 Macquarie StreetBARTON ACT 2600PO Box 4776KINGSTON ACT 2604
Phone: 02 6272 4539Fax: 02 6272 5877Email: [email protected]: http://www.rirdc.gov.au
Published in December 2000Printed on environmentally friendly paper by Canprint
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ForewordThe New South Wales rice industry produces over a million tonnes of paddy each year, all of which ismilled and packaged locally. The industry contributes over one billion dollars to the NSW economyannually, and is heavily export-oriented. The rice growers of NSW, in partnership with RIRDC, arecommitted to the ethic of sustainable agricultural production. Minimising the use and off-targeteffects of pesticides is a key component of this philosophy.
This project focuses on two of the most significant invertebrate pests that impact on riceestablishment and subsequent crop yields. Bloodworms, the larvae of chironomid midges, attack thenewly-sown rice seed, whilst aquatic earthworms interact with the rice crop in a number of ways,modifying soils structure and water quality in a way that prejudices successful plant establishment.
Research presented in this report has contributed to a 77% reduction in the amount of insecticideneeded to control bloodworms, relative to 1994 levels. Evaluation of the pyrethroid insecticidealphacypermethrin as a potential tool for bloodworm control suggests that this reduction may beincreased to over 94% within the next few years. Reductions in insecticide inputs have also beenaccompanied by reductions in aerial spraying and reduced risks of drainage contamination. Studieson the colonisation of rice fields by different bloodworm species are a first step towards unravellingthe complex ecology of rice/bloodworm interactions, a prerequisite for the development of controlpractices that may eliminate synthetic pesticides from bloodworm control.
Whilst chemical control of aquatic earthworms has largely been unsuccessful, ecological studies haveshown how this pest impairs crop establishment, and how dryland cereal cultivation reducesearthworm populations. Several important recommendations for farmers have arisen from thisresearch, and these are actively being promoted.
This project was funded from industry revenue that is matched by funds provided by the FederalGovernment.
This report, a new addition to RIRDC’s diverse range of over 600 research publications, forms part ofour Rice R&D program, which aims to improve the profitability and sustainability of the Australianrice industry.
Most of our publications are available for viewing, downloading or purchasing online through ourwebsite:
• downloads at www.rirdc.gov.au/reports/Index.htm• purchases at www.rirdc.gov.au/eshop
Peter CoreManaging DirectorRural Industries Research and Development Corporation
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AcknowledgmentsI am indebted to Dr Stuart Helliwell of Charles Sturt University, who collaborated with me instudies on the environmental persistence of bloodworm control chemicals. Neil Coombes,Biometrician at Wagga Wagga Agricultural Research Institute is thanked for his advice onstatistical analyses. I would also like to thank Glen Warren, Liesl Schiller, Kathryn Fox andRichard Faulder for their unfailing technical assistance in both the field and laboratory.
Steve Plant, Alan Walsh and Mick Whelan are thanked for allowing my team to conductseveral of the earthworm experiments on their properties.
Mark StevensJanuary, 2000
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ContentsForeword .................................................................................................................... iii
Acknowledgments ...................................................................................................... iv
Executive Summary....................................................................................................vi
1. Introduction ........................................................................................................ 1
2. Objectives .......................................................................................................... 3
3. Methodology, results, and discussion ................................................................ 43.1 Field evaluation of bloodworm treatments ................................................. 43.2 Small plot evaluation of EC fipronil (Regent®) ........................................ 103.3 Chemical control of aquatic earthworms.................................................. 293.4 Interactions between aquatic earthworms and the rice ........................... 363.5 Ecology of aquatic earthworms................................................................ 44
4. Discussion of results compared with objectives .............................................. 47
5. Implications ...................................................................................................... 48
6. Recommendations ........................................................................................... 49
7. Appendices ...................................................................................................... 507.1 Appendix 1 – Publications arising............................................................ 50
8. References....................................................................................................... 52
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Executive SummaryChironomid midge larvae (bloodworms) are the most widespread and serious pests of rice inNew South Wales. Commercial trials of the fipronil seed treatment Cosmos® demonstratedthat this material is at least as effective as the industry standard spray treatment (chlorpyrifos),and Cosmos® is now registered and available to growers, allowing substantial reductions inchemical inputs for bloodworm management and a reduction in aerial spraying.
Field trials of alphacypermethrin conducted across 2 seasons demonstrated that this material(Dominex®) provides bloodworm control equivalent to chlorpyrifos, but at a much lower rateand with a reduced risk of contamination to downstream environments. A commercial trialpermit for alphacypermethrin is being sought.
Studies on the colonisation of rice fields by bloodworms have identified 18 different speciesthat occur in the first 40 days after flooding. These studies are the first step towardsidentifying which species, other than Chironomus tepperi, occur in sufficient numbers to posea potential threat to crop establishment. Once identified and confirmed as pests managementstudies on these species can be undertaken.
Extensive field trials were undertaken on the chemical control of the aquatic earthwormEukerria saltensis, however no chemicals were identified that have the potential to controlthis pest once the rice fields are inundated. Carbofuran appeared to suppress E.saltensis ifwatered into dry soil or incorporated into the soil as a granule prior to flooding, howeverapplication rates of over 1 kg active.ha-1 were required to achieve an impact, and it is doubtfulwhether such treatments are environmentally sustainable.
Studies on the influence of E.saltensis on water quality and rice growth, conducted in thelaboratory, have demonstrated that this pest retards crop establishment indirectly throughreleasing nutrients into the water column (thereby promoting algal growth), increasingturbidity, and transporting seeds deeper into the soil. Understanding these damagemechanisms has allowed recommendations to be developed for minimising crop damage, andthese are being widely promoted. Ecological studies on the influence of crop rotations onE.saltensis suggests that 2 or more years of dryland cropping drastically reduces bothearthworm and cocoon populations.
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1. IntroductionRelatively few invertebrate pests impact on the New South Wales (NSW) rice industry. Climatic factors(low winter temperatures and low summer humidities) mitigate against the survival of many rice pests thatrequire more tropical conditions, and an extensive quarantine area has contributed to the ongoing exclusionof other exotic pests. Of the serious pests that do affect the crop, all except the armyworm Leucaniaconvecta (Walker) are largely confined to the crop establishment period. The leafminer Hydrellia michelaeBock is a sporadic pest during late establishment, whilst the tadpole shrimp Triops australiensisaustraliensis (Spencer and Hall) also occurs sporadically during early establishment, however the areaaffected is usually quite small.
Water snails (Isidorella and Glyptophysa) attack rice plants from early establishment through to tillering,whilst bloodworms and aquatic earthworms damage plants only during the early and middle phases of cropestablishment. These three pests – snails, bloodworms, and aquatic earthworms - are the most significantpests affecting the NSW rice crop. This project deals with aspects of the biology and control of bothbloodworms and aquatic earthworms.
Bloodworms are the larvae of chironomid midges. Many different species are found in rice bays during thefirst month after sowing and, whilst some species are apparently harmless and others beneficial predators,some species, particularly Chironomus tepperi Skuse, cause extensive damage to young rice plants.Bloodworms often enter the newly-sown seed and consume the embryo and endosperm, however the mostcommon form of crop damage involves the attack of developing root systems. This form of damage slowscrop establishment, since the ability of the plants to absorb nutrients from the soil is diminished. Rootdamage also prevents plants from anchoring securely in the soil, making them particularly vulnerable touprooting through wave action.
A large amount of information is now available on the biology, ecology, and toxicology of Chironomustepperi, largely as a result of other RIRDC-funded projects (DAN73A, DAN97A). It is known thatC.tepperi has only a single generation in rice fields, and is generally absent by 18 to 20 days post-flooding.Although seed treatments eliminate C.tepperi, growers who do not use chlorpyrifos post-sowing oftenexperience severe crop losses. Species other than C.tepperi are responsible for this damage, however littleinformation is available on what species, other than C.tepperi, are associated with rice crop establishment.This has largely been a consequence of the difficulties previously associated with identifying bloodwormlarvae. New keys written by Dr P S Cranston, CSIRO Division of Entomology, have made faunistic studieson the bloodworm fauna of rice fields a viable proposition, and, once the species have been identified andtheir colonisation patterns determined, it will be possible to test different species to determine their potentialimpact on rice crops. Control measures will be able to be more effectively targeted once the damagingspecies have been identified and their chemical susceptibility determined.
The importance of this work is highlighted by bloodworm problems experienced in the 1995/6 season,where some growers reported that chlorpyrifos (normally applied at 75 g ai.ha-1) failed to give effectivebloodworm control at rates of up to 200 g ai.ha-1. Several of these crops were investigated, and C.tepperiwas not located in any of them; the numerically dominant species were Polypedilum nubifer Skuse andCryptochironomus griseidorsum Kieffer. It is presently unclear which of these species was responsible forthe crop damage, and how control could be effected using currently registered compounds.
Current bloodworm control practices are based on the use of broad-spectrum organophosphorus insecticides,particularly chlorpyrifos. A major problem associated with an increased reliance on chlorpyrifos forbloodworm control involves drainage water residues. The Environmental Protection Limit established bythe EPA for chlorpyrifos in drainage systems is 0.001 parts per billion (i.e. 1 part per trillion). Studiesconducted by CSIRO Division of Water Resources have shown that the required lock-up time to achieve thislevel following a standard chlorpyrifos application of 75 grams active.ha-1 is approximately 34 days.Retention periods this long would be difficult to achieve in commercial situations, and would lead to poorestablishment, algal growth, and possibly the establishment of mosquito populations in stagnant water.Although EPA ‘action’ levels are currently well above the environmental protection limit, it should be
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anticipated that actionable residue levels will fall in the future. The National Registration Authority forAgricultural and Veterinary Chemicals is currently reviewing the use of chlorpyrifos in Australia, placing adegree of uncertainty over its future availability.
There is an extensive body of literature available on the control of bloodworms, however much of itinvolves the control of adults that represent a nuisance in urban areas. The majority of research publishedon rice field chironomid control in the past decade has been published by the Principal Investigator.Faunistic studies on rice field colonisation by chironomids have been published for most countries wherechironomid larvae are recognised as rice pests, including Italy (Ferrarese, 1992) and the USA (Darby, 1962;Clement et al., 1977), and in several other countries where chironomid problems are either minor or non-existent, such as Japan and India. The only papers available that refer to bloodworm colonisation of NSWrice fields are those by Stevens (1994), who examined emergence phenology for 2 of the dominant species,and Pettigrove et al. (1995), who examined only treated bays and listed only the numerically dominantspecies. Pettigrove’s work was, however, directed towards looking at the induction of mouthpartabnormalities induced by pesticide use. The vast majority of recently published work on larval chironomidcontrol involves organophosphorus compounds; there is only very limited data available on the fieldperformance of other chemical groups.
The aquatic worm Eukerria saltensis (Beddard) has been known from Australia for almost 100 years, andhas colonised the NSW ricegrowing area during the last 2 decades. Although Eukerria can be regarded as abeneficial species in many crops, it is implicated in causing severe damage to aerially sown rice crops grownunder permanent irrigation. The problem is particularly severe on dispersable clay soils in the MurrayValley.
Eukerria is incapable of feeding directly on rice plants, since it lacks chewing mouthparts. Damage appearsto be caused indirectly through their effects on the rice bay environment, and investigating earthworm/cropinteractions forms a significant part of this project. When Eukerria tunnel through the soil they producesmall pellets of excreted soil (castings) on the soil surface. Under flooded conditions these castings collapseto form a layer of loose, unstructured sediment at the soil/water interface. Seed that sinks into this layer isdeprived of light and warmth, and fails to establish. The sediment itself is composed of very fine material,and is believed to be high in soluble nutrients as a consequence of the worms digestive processes.Convection currents and wind action result in this sediment being suspended in the water column, resultingin high turbidity and apparently triggering algal blooms, which retard or in some cases prevent plantestablishment. At present there are no simple and effective control measures available, although somefarmers that have used continuous crop flushing or water-run gypsum to reduce turbidity have achievedimprovements in crop establishment. These processes involve considerable costs, and have only partiallyalleviated, rather than solved, the Eukerria problem.
There has been comparatively little research conducted on oligochaete worms (such as Eukerria) in ricefields. In Japan, closely related tubificid worms are considered to play a partly beneficial role, as theircontinual tunnelling activity reduces the germination of several weed species (Kurihara and Kikuchi, 1988;Kurihara, 1989). They are however, known to reduce soil nitrogen levels by moving nitrogen andphosphorus from the soil into the water column (Gardner et al., 1981; Kurihara and Kikuchi, 1988), whichwould be anticipated to promote algal growth. They are also known to increase the water content of surfacesediments (Fukuhara, 1987). Similar studies have also been conducted in India (Kale et al., 1989). Incountries such as Japan, where rice is transplanted, any detrimental effects of oligochaetes on cropestablishment would be minimised. The factors most closely associated with Eukerria damage in NSWappear to be aerial sowing, dispersable soils, deep water, and, to some extent, large numbers of ibis. Thereis a considerable body of information available on the toxicity of pesticides to earthworms, mainly as aconsequence of work on maintaining worm populations rather than controlling them. The general consensusis that carbamate insecticides are the most toxic to earthworms, and they therefore represent a potentiallyuseful tool for Eukerria control. The only recent published research work on Eukerria itself is that ofBlackwell and Blackwell (1989), who deliberately introduced Eukerria into the Whitton area fromDarlington Point in an effort to improve soil structure. Eukerria is now well established around Whitton,but causes only limited problems due to the nature of soils in the area..
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2. ObjectivesThe objectives of this project, as stated in the project application, are;
To develop sustainable strategies for the control of aquatic earthworms and bloodworms in NSW rice cropsby:
(a) Identifying chemical treatments that will control the aquatic worm Eukerria saltensis, and quantifyingthe relationship between worm populations and factors associated with crop establishment failure.
(b) Evaluating low drainage residue alternatives to chlorpyrifos for bloodworm control in aerially sown ricecrops, and investigating the colonisation of rice fields by species other than Chironomus tepperi.
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3. Methodology, results, and discussion3.1 Field evaluation of bloodworm treatments
3.1.1 Commercial scale evaluations of fipronil (Cosmos®)
IntroductionIn 1995 aerial operators placed a ban on the use of malathion seed treatments for the control of bloodwormsin NSW rice crops. Their decision to do so was based on concerns about occupational health and safety, asthe emulsifiable concentrate formulations of malathion were claimed to give pilots headaches. For 2seasons growers had to sow untreated seed, and then apply chlorpyrifos as a direct to water spray forbloodworm control. This increased costs to growers (who had to pay for an extra pass by aircraft), andincreased the risk of spray drift to non-target areas. Seed treatments are an effective way of protecting thecrop whilst avoiding the drift hazards associated with spray application.
Locating an alternative seed treatment that would be both effective, and acceptable to aerial operators,became a major priority for the NSW rice industry. Small plot trials at Yanco Agricultural Institute during1995 showed that the phenylpyrazole insecticide fipronil (Cosmos®) applied to seed at rates as low as 12.5g ai.ha-1 provided far superior bloodworm control to malathion, and in fact had a similar level of residualactivity (14-18 days) to chlorpyrifos (see Final Report, RIRDC Project DAN 97A).
Prior to a registration claim by Rhône-Poulenc for the use of fipronil as a rice seed treatment, commercialscale trails of Cosmos® were undertaken in the Murrumbidgee Irrigation Area during the 1996/7 riceseason. The trials were established by Rhône-Poulenc staff, and monitored by NSW Agriculture staff aspart of this project.
Materials and Methods
Trial establishmentTrials were conducted on 4 commercial farms during spring 1996. At each trial site 3 treatments wereexamined, each in a single landformed field of variable size. The treatments were;
1. fipronil seed treatment (Rhône-Poulenc EXP 80415A, 12.5-15.75 g active.ha-1, see Table 1) appliedimmediately before sowing.
2. fipronil seed treatment (as for treatment 1) applied immediately before sowing, followed bychlorpyrifos (Rhône-Poulenc Chlorfos® 500 g.l-1 EC, 75 g active.ha-1) applied by air or amphibiousmotorbike 10-17 DAS (days after sowing).
3. untreated seed sown, chlorpyrifos EC (as for treatment 2) applied by air or amphibious motorbike 2-5 DAS, followed by an identical second chlorpyrifos treatment 10-17 DAS.
At the Yenda site an additional post-sowing chlorpyrifos application was added to treatments 2 and 3(treatment 2 chlorpyrifos: 10, 15 DAS; treatment 3 chlorpyrifos: 3, 10, 15 DAS) due to heavy recolonisationactivity being observed. Rice varieties, sowing rates, and the timing of chemical applications variedbetween the 4 sites, and are summarised in Table 1. Fipronil was applied to pregerminated seed using asimple spray nozzle mounted inside the shaft of a grain auger. The application rate was initially calibratedto 1 litre of spray per 100 kg seed (dry weight), delivering a final application rate of 12.5 grams active.ha-1,however in practice the rate varied between 12.5 and 15.75 g active.ha-1 (Table 1) because long grain ricevarieties travelled more slowly through the auger.
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Table 1Locations, sowing data, treatment rates and treatment scheduling for commercial scale evaluations of fipronil against chironomid larvae. DAS, days after sowing.
* chlorpyrifos at 75 g active.ha-1 at all sites.
Site Sowing date Variety Seeding rate (kg dry.ha-
1)Fipronil rate (treatments
1 and 2, g.active.ha-1)Chlorpyrifos 1 (treatment
3 only, DAS)*Chlorpyrifos 2 (treatments
2 and 3, DAS)*Coleambally 15.x.96 Langi 145 15.75 4 14
Yenda 30.x.96 Langi 135 13.5 3 10 and 15
Hanwood 15.x.96 Amaroo 125 12.5 2 12
Whitton 2.xi.96 Langi 125 13.5 5 17
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Trial monitoring and assessmentLarval samples were taken at 6 and 17-19 DAS from all treatments. At one site (Whitton) larvalpopulations were only examined for treatments 1 and 3, since the post-sowing chlorpyrifosapplications were not applied to treatments 2 and 3 until 17 DAS, and treatments 1 and 2 weretherefore identical up to that time.
Chironomid populations were assessed using soil core sampling. Samples were obtained by pushing a96 mm diameter plastic corer into the mud to a depth of 50 mm, sealing the top aperture, and theninserting a rigid perspex sheet under the corer. The sample, corer, and perspex sheet could then belifted out of the water (Stevens and Warren, 1992). Samples were transferred to plastic containersand frozen at -17oC until larval extraction. On each sampling day 8 core samples were taken fromrandom positions in each bay. Magnesium sulfate flotation (Stevens and Warren, 1992) was used toextract larvae from the thawed soil cores. Plastic cups (11 cm diameter, 6 cm deep) were half filledwith saturated aqueous MgSO4, and small quantities of each thawed sample were stirred into thesolution. After the heavier material had settled larvae were collected from the solution surface andtransferred to 70% ethanol for counting and classification. Data for Chironominae and 'otherchironomids' were transformed to y' = loge (y+1) and analysed across treatments within each site usingANOVA and Tukey's HSD test to separate means.
Plant establishment counts were taken from each treatment at each site at 27-30 DAS. A total of 45counts were made at random points in each bay using 35 cm diameter sampling rings. Once eachcount had been made, the most central plant within the sampling ring was carefully removed forassessment of shoot length and root weight. Shoot length was measured manually, whilst rootsystems were removed and dried individually to constant weight at 105oC. Untransformed data wereanalysed using ANOVA and Tukey's HSD test. Sites were analysed separately due to variations intreatment rates, treatment scheduling, and agronomic and climatic conditions.
Results
Larval populationsLarval populations at the 4 sites are shown in Fig. 1. Differences between treatments were onlysignificant on one occasion. At the Yenda site significant recolonisation by Chironominae hadoccurred in the fipronil only treatment at 19 DAS, however populations remained low in the fieldsreceiving chlorpyrifos applications at 10 and 15 DAS.
Populations of Chironominae were considerably lower at all other sites, and were not detected at all atthe Hanwood site. In the absence of untreated controls, it is not possible to determine whether thiswas a consequence of effective control being provided by all treatments, or simply a consequence oflow colonisation activity.
Plant establishment and vigourPlant establishment counts, shoot lengths and root dry weights for the 4 commercial scale trials areshown in Fig. 2. Significant differences were found between treatments at all sites and for allparameters, except shoot length at the Coleambally site. Since larval populations did not differsignificantly between treatments at 3 of the 4 sites, it is apparent that other factors contributed to theobserved differences in crop establishment and vigour. These may have included effects of thetreatments on plant growth, differential soil fertility across fields, herbicide interactions, and theinfluence of other pests, such as snails and ducks.
At the Yenda site, however, significant differences in populations of Chironominae were found at 19DAS. The establishment and vigour data supports what would be expected from significantcolonisation activity by Chironominae. Relative to the fipronil/double chlorpyrifos treatment in use at
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Figure 1. Larval chironomid populations in commercial scale field trials. nsd: no significantdifference (P > 0.05). Columns followed by different letters are significantly different (P < 0.05)(Yenda trial, Chironominae at 19 DAS only).
this site, plants from the fipronil-only field had significantly shorter shoots, smaller root systems, andlower establishment density. Poor establishment in the triple chlorpyrifos treatment at this site mayreflect seed damage during the 3 day delay between sowing and application of the first chlorpyrifostreatment.
Chironominae I other chironomids6 DAS 17 DAS 6 DAS 17 DAS
0.0
0.2
0.4
0.6
0.8(a) Coleambally
Chironominae I other chironomids6 DAS 19 DAS 6 DAS 19 DAS
larv
ae.s
ampl
e-1(±
SE
)
0
2
4
6
8
10 (b) Yenda
Chironominae I other chironomids6 DAS 17 DAS 6 DAS 17 DAS
0.0
0.2
0.4
0.6
Chironominae I other chironomids6 DAS 17 DAS 6 DAS 17 DAS
0
1
2
3
4
(c) Hanwood
(d) W hitton (fipronil/chlorpyrifos not assessed)
nsd
nsd
nsd
nsd
nsdnsd nsd
a
b b
nsdnsd
nsd
nsd
nsd
nsd
nsdnsd
fipronil f ipronil/chlorpyrifoschlorpyrifos/chlorpyrifos
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Figure 2. Plant establishment densities, shoot lengths and root system dry weights, commercialscale trials at 27-30 DAS. (a): plant establishment; (b): shoot lengths; (c): root system dry weights.On each graph columns within groups followed by different letters are significantly different (P <0.05).
plants.m-2 (+SE)
0 50 100 150 200 250 300 350 400 450
W hitton
Hanwood
Yenda
Coleambally
shoot length (mm + SE)
0 50 100 150 200 250 300 350 400 450
W hitton
Hanwood
Yenda
Coleambally
root system dry weight (mg + SE)
0 5 10 15 20 25 30
W hitton
Hanwood
Yenda
Coleambally
a bb
a
bc
a abb
aa
b
nsd
bb
a
aa
b
ab
c
aa
b
aa
b
ab
c
aa
b
fipronil fipronil/chlorpyrifos chlorpyrifos/chlorpyrifos
(a)
(b)
(c)
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At the other sites double chlorpyrifos regimes provided better establishment than fipronil alone at all3 sites, and better establishment than fipronil/chlorpyrifos at one site (Whitton), where thechlorpyrifos post-sowing treatment was delayed until 17 DAS. Stronger crop establishment in thedouble chlorpyrifos treatments, despite a 2-5 d delay between sowing and the first chlorpyrifosapplications, suggests that these sites were under considerably less colonisation pressure.
DiscussionInterpreting the results of commercial scale evaluations of bloodworm control chemicals is alwayscomplicated by the lack of untreated control sites. Without control sites, it is difficult to determinewhether treatments under evaluation are actually effective, or whether their similarity to standardtreatments reflects low levels of pest pressure. The most useful data obtained from this study camefrom the Yenda trial, where the use of fipronil seed treatments without a supplementary chlorpyrifosapplication led to significant recolonisation by Chironominae by 19 DAS. This recolonisation isreflected in significantly poorer plant establishment in comparison to the fipronil/double chlorpyrifostreated field at the same site. Whilst fipronil alone provided control statistically equivalent to thatprovided by either fipronil/chlorpyrifos or chlorpyrifos/chlorpyrifos treatments at 3 of the 4 sites, theresults from the Yenda site show that under conditions of strong colonisation pressure fipronil alonemay be in adequate, and a supplementary chlorpyrifos application will be needed to provide optimalcrop protection. The optimal timing for such a treatment is approximately 12 DAS, and it should onlybe applied in response to observed plant damage or visible bloodworm infestations.
ConclusionsFipronil as Cosmos® applied as a seed treatment to pregerminated rice at 12 g ai.ha-1 is a moreefficacious seed treatment than malathion, and provides a similar, or slightly better level of residualcontrol than chlorpyrifos sprayed to water at 75 g ai.ha-1. For many farmers, Cosmos® will providesingle-application bloodworm control, however in areas with heavy recolonisation pressure asupplementary application of chlorpyrifos approximately 12 DAS may still be required to achieveoptimum bloodworm control.
FootnotesCosmos® was registered for bloodworm control in rice in time for the 1997/8 season, and has provenhighly effective, although their has been substantial resistance to its use by aerial operators.
This work was published in Field Crops Research, 1998, 57, 195-207 (see Appendix 1).
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3.2 Small plot evaluation of EC fipronil (Regent®)
IntroductionSince the deregistration of organochlorine compounds chironomid control has been based on a rangeof organophosphorus treatments, particularly malathion and chlorpyrifos. An initial malathion seedtreatment or chlorpyrifos spray has been used to eliminate early C.tepperi infestations, whilst asubsequent chlorpyrifos spray 5 to 14 days after sowing has been used to control mixed chironomidcommunities during the later part of the establishment period. The phenyl pyrazole insecticidefipronil has substantially replaced malathion as a seed treatment since 1997, and provides far superiorcontrol at a much lower application rate. The residual control provided by fipronil may not always besufficient to protect the crop during the full period of vulnerability, and a supplementary spray ofchlorpyrifos may still be required (Stevens et al., 1998). Fipronil has not yet been registered as adirect spray treatment for chironomid control.
Although chlorpyrifos is the mainstay of chironomid midge control programs in NSW rice fields, itsfuture availability to rice growers is uncertain. Chlorpyrifos is highly toxic to a broad range ofaquatic organisms, and its use is currently under review by the National Registration Authority forAgricultural and Veterinary Chemicals, a federal regulatory agency. It is essential that a highlyeffective material for spray application remain available to NSW rice producers, and doubts about theongoing availability of chlorpyrifos have prompted a research program aimed at developingalternative compounds.
Materials and Methods
Trial location, establishment and treatments.An emulsifiable concentrate (EC) formulation of fipronil (Regent® 300) was evaluated in a singlereplicated small plot trial during the 1997/8 rice season at Yanco Agricultural Institute (34o37’S,146o26’E) in south-west NSW. The trial was conducted on a Birganbigil clay loam soil (van Dijk,1961).
Two rows of 9 rectangular bays with earthen banks (each approximately 30 m2) were used in the trial.Each bay was supplied with water from a central channel. Alternate bays in each row were used astreatment bays, with intervening bays being used as buffer zones. A water depth of approximately 14cm was maintained in all bays throughout the trial. Bays were measured individually prior tocalculating chemical dosages. In each trial 2 bays were designated as untreated controls, 2 weretreated with a standard chlorpyrifos treatment (Lorsban® 500 EC (DowElanco Australia Ltd), 500g.L-1 applied at 75 g active ingredient (ai).ha-1), and 6 bays (2 at each of 3 application rates) weretreated with fipronil (Regent® 300 EC, Rhône-Poulenc, 300 g.L-1 EC). The fipronil application rateswere 3, 4.5, and 6 g ai.ha-1. All chemical treatments were applied to the water surface inapproximately 5 L of water using a single nozzle hand sprayer. Treatments were applied 6 days afterflooding.
All treatment and control bays were sown with pregerminated rice (cv ‘Namaga’, 120 kg (dry).ha-1) byhand broadcasting within 2 hours of chemical treatments being applied.
Monitoring of environmental conditionsTemperature at the soil/water interface in one of the control bays was recorded at hourly intervalsusing a miniature data logger, with readings being taken from immediately after pesticide applicationuntil 25 days after application (DAA). Conductivity and pH were assessed twice on every second daythroughout the trial until 25 DAA. A single 250 mL water sample was taken from each of the controlbays twice each day (0745 - 1015 and 1545 - 1945 daylight saving time), and measurements weremade in the laboratory using regularly recalibrated electronic meters. Rainfall was monitored at anearby automated weather station.
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Chironomid population assessmentsSoil core sampling combined with magnesium sulfate flotation (Stevens and Warren, 1992) was usedto quantify larval chironomid populations. Samples were obtained by pushing a 96 mm diameterplastic cylinder 50 mm into the sediment, sealing the top, and then digging around the outside of thecylinder and sliding a thin perspex sheet underneath. The complete assembly was then lifted out, andthe water and sediment within the cylinder was transferred to 1 L capacity plastic containers andfrozen at -17oC until sample analysis. Magnesium sulfate flotation of larvae from thawed samplesfollowed the procedure of Stevens and Warren (1992). Three samples were taken from each bay at 4,9, 14, 19, 24 and 29 DAA. Extracted larvae were divided into 2 groups, Chironominae and ‘otherchironomids’ (predominantly Tanypodinae). Data were transformed to y’ = loge (y+1) and analysedusing ANOVA and Tukey’s HSD test to separate means.
Plant establishmentPlant establishment counts were made using a 35 cm internal diameter sampling ring. Thirty plantcounts were made at 30 DAA, however severe duck damage was evident across the trial.
Plants damaged by bloodworms and ducks are easily separated; bloodworm-damaged plants havereduced root systems and the seed, although sometimes damaged, remains attached, whilst in duck-damaged plants the root system is generally intact and the entire seed is missing. An attempt wasmade to adjust establishment counts for duck damage by collecting these plants from each bay,counting them, and adjusting the measured values by the mean plant loss to ducks per sample area.Both the raw and ‘duck-adjusted’ data sets were analysed using ANOVA, with Tukey’s HSD testbeing used to separate means.
Results
Environmental conditionsTable 2 summarises the environmental conditions in the control bays during the trial. Watertemperatures were consistent with those experienced at the trial site in previous seasons, conductivityremained low throughout the trial while the pH varied from approximately neutral to moderatelyalkaline.
Table 2Environmental parameters in experimental rice bays, fipronil EC bloodworm trial, 1997/8 season, first25 DAA.
Variable Mean RangepH 9.1 7.2 - 10.0Conductivity (µS.cm-1) 90.2 71.3 – 115.2water temperature (oC) 23.8 12.7 - 38.5rainfall (mm) 0.9 0 - 14.8
Chironomid population assessmentResults from the larval chironomid population assessments are shown in figure 3, and summarised intable 3.
12
Figure 3. Influence of EC fipronil (Regent®) and chlorpyrifos standard on larval chironomid population in small plot rice bays, 1997/8 rice season. Error barsrepresent standard errors. *, significantly (P < 0.05) lower than equivalent control population. b, significantly (P < 0.05) higher population than chlorpyrifos standard(Regent ® treatments only).
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13
Table 3Cumulative percentage reductions in target Chironominae (relative to untreated controls) resultingfrom applications of EC fipronil (Regent®) and chlorpyrifos, 1997/8 rice season.
Treatment first 14 DAA first 19 DAAchlorpyrifos 75 g ai.ha-1 82 73fipronil 3 g ai.ha-1 52 22fipronil 4.5 g ai.ha-1 58 20fipronil 6 g ai.ha-1 94 49
Pest pressure by target Chironominae was low during the trial period, as shown by the lowpopulations of Chironominae in the control bays (figure 3). None of the treatments (including thechlorpyrifos standard) significantly suppressed the target group on any individual sampling occasion,however this result is largely an artifact of the low control populations, rather than an indication ofcontrol failure. All treatments significantly suppressed non-target groups for at least 14 DAA. Thesummary data in table 3 shows that only the highest rate of fipronil (6 g ai.ha-1) provided suppressionof target Chironominae equivalent to, or better than the chlorpyrifos standard over the first 14 DAA,however over the first 19 DAA none of the fipronil treatments provided equivalent control tochlorpyrifos. Results for plant establishment assessments are shown in figure 4.
controlfipronil 3
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Figure 4. Plant establishment in small plot rice bays at 30 DAA. Modified data has been adjustedfor losses due to ducks. On each graph numerical values next to treatment labels indicate applicationrates in g ai.ha-1 Columns with different letters are significantly different (P < 0.05). Error barsrepresent standard errors.
14
Although there were significant differences found between treatments, there was no dose-dependentimprovement in establishment for the fipronil treatments after adjustment for duck damage, and notreatments showed superior crop establishment to the untreated controls, either before or after dataadjustment.
Discussion and conclusionsThis trial provided only limited information about the efficacy of EC fipronil for chironomid control.The overriding reason for this was the poor recruitment of target Chironominae during the 1997/8trial season, which was probably related to dry conditions during spring limiting the buildup of midgepopulations. Despite this problem, some useful information can be obtained from the trial results.
Examination of figure 3 and table 3 suggests that EC fipronil (Regent ®) has the potential to provideeffective chironomid control in rice fields, but that the application rates evaluated in this trial wereprobably too low. Only the 6 g ai.ha-1 rate provided any evidence of adequate suppression of thetarget larval group. Further trials of this material are warranted, and when they are conductedapplication rates between 5 and 10 g ai.ha-1 should be evaluated.
15
3.1.3 Small plot evaluations of EC alphacypermethrin (Dominex®)
IntroductionAs discussed in the previous section, the future availability of chlorpyrifos for bloodworm control inrice is currently in doubt, and locating and developing suitable alternative compounds is a majorpriority for the NSW rice industry.
Since the deregistration of organochlorine compounds, chironomid control in rice has been based ona range of organophosphorus treatments, such as chlorpyrifos and malathion. The only non-organophosphorus material that has been evaluated in the field for bloodworm control is the phenylpyrazole compound fipronil. No pyrethroid compounds have been evaluated in the field in Australia,despite their known high levels of toxicity to Chironomus tepperi in laboratory bioassays (Stevens,1993). This study was conducted to evaluate the potential of alphacypermethrin (Dominex®) for thecontrol of bloodworms in establishing rice crops. Alphacypermethrin is substantially more toxic tobloodworms than chlorpyrifos under laboratory conditions (Stevens, 1992, 1993) and already has anestablished maximum residue limit (MRL) in rice, which should make this material substantiallyeasier to register for use against bloodworms than other pyrethroid compounds. The only previousstudy on the efficacy of alphacypermethrin for the control of rice field chironomid larvae is thatconducted in Italy by Pasini et al. (1997), who found that an application of formulated product at 23.7g ai.ha-1 provided effective control of mixed chironomid populations for approximately 15 days.
Materials and Methods
Trial location, establishment and treatmentsAlphacypermethrin was evaluated in 2 trials conducted during the 1997/8 (trial 1) and 1998/9 (trial 2)rice seasons at Yanco Agricultural Institute (34o37’S, 146o26’E) in south-west NSW. The two trialswere conducted on adjacent sites on a Birganbigil clay loam soil (van Dijk, 1961).
Two rows of 9 rectangular bays with earthen banks (each approximately 30 m2) were used in eachtrial. Each bay was supplied with water from a central channel. Alternate bays in each row wereused as treatment bays, with intervening bays being used as buffer zones. A constant water depth ofapproximately 14 cm was maintained in all bays throughout the trials. Bays were measuredindividually prior to calculating chemical dosages. In each trial 2 bays were designated as untreatedcontrols, 2 were treated with a standard chlorpyrifos treatment (Lorsban® 500 EC (DowElancoAustralia Ltd), 500 g.L-1 applied at 75 g active ingredient (ai).ha-1), and 6 bays (2 at each of 3application rates) were treated with alphacypermethrin (Dominex® 100 EC, FMC International AG,100 g.L-1 EC).
In trial 1, alphacypermethrin was applied at 10, 20, and 30 g ai.ha-1, whilst in trial 2, application ratesof 6, 10, and 20 g ai.ha-1 were evaluated. All chemical treatments were applied to the water surfacein approximately 5 L of water using a single nozzle hand sprayer. All treatments were applied 6 daysafter flooding.
All treatment and control bays were sown with pregerminated rice (cv ‘Namaga’, 120 kg (dry).ha-1)by hand broadcasting within 2 hours of chemical treatments being applied. Following severe damageto trial 1 by ducks, cages (2.5 m x 1.5 m x 0.45 m (height)) covered with wire mesh were constructed,and randomly placed (one per bay) in all control and treatment bays immediately after trial 2 wassown, in order to protect at least part of each bay from duck damage.
16
Environmental conditionsTrial 1 was conducted simultaneously with the Regent® trial (previous section) at an adjacent site,and the environmental data collected for that trial is also applicable to 1997/8 Dominex® trial. Intrial 2 temperature at the soil/water interface in one of the control bays was recorded at hourlyintervals using a miniature data logger, with readings being taken from immediately after pesticideapplication until 18 DAA, the day on which the final water samples were taken for chemical analysis.Conductivity and pH were assessed twice on every second day throughout the trial until watersampling was completed. A single 250 mL water sample was taken from each of the control baystwice each day (0800 - 1015 and 1615 - 1730 daylight saving time), and measurements were made inthe laboratory using regularly recalibrated electronic meters. Rainfall was monitored at a nearbyautomated weather station
Chironomid population assessmentsSoil core sampling combined with magnesium sulfate flotation (Stevens and Warren, 1992) was usedto quantify larval chironomid populations. Samples were obtained by pushing a 96 mm diameterplastic cylinder 50 mm into the sediment, sealing the top, and then digging around the outside of thecylinder and sliding a thin perspex sheet underneath. The complete assembly was then lifted out, andthe water and sediment within the cylinder was transferred to 1 L capacity plastic containers andfrozen at -17oC until sample analysis. Magnesium sulfate flotation of larvae from thawed samplesfollowed the procedure of Stevens and Warren (1992). In trial 1, 3 samples were taken from each bay4, 9, 14, 19, 24 and 29 days after chemical application (DAA), whilst in trial 2, 4 samples were takenfrom each bay 4, 9, 14, 19 and 24 DAA.
Extracted larvae were divided into 2 groups, Chironominae and ‘other chironomids’ (predominantlyTanypodinae). Data were transformed to y’ = loge (y+1) and analysed using ANOVA and Tukey’sHSD test to separate means.
Plant establishmentPlant establishment counts were made in both trials using a 35 cm internal diameter sampling ring. Intrial 1, 30 plant counts were made at 30 DAA, however severe duck damage was evident across thetrial.
Since plants damaged by bloodworms and ducks are easily separated (see previous section), anattempt was made to adjust establishment counts by collecting the plants uprooted by ducks fromeach bay, counting them, and adjusting the measured values by the mean plant loss to ducks persample area. Both the raw and ‘duck-adjusted’ data sets were analysed.
In trial 2, severe duck damage also occurred, and plant establishment counts were restricted to theareas under the duck proof cages. Eight plant counts were made per bay at 30 DAA using 35 cminternal diameter sampling rings. Results from both trials were analysed using ANOVA, withTukey’s HSD test being used to separate means.
Sampling of water, sediments, and plants for chemical analysisSingle 1 L composite water samples were taken from the control and alphacypermethrin treated baysat set intervals. All glassware used in sample collection and storage was treated with Coatasil® glasstreatment (APS Ajax Finechem, 2% W/W dimethyldichlorosilane in 1,1-dichloro-1-fluoroethane) tominimise pesticide adsorption. Small (200 mL) amber glass bottles were fitted to an aluminiumhandle and used as dippers, with 5 subsamples taken from random points in each bay being used tomake up each composite sample. In trial 1, samples were collected at 1, 2, 5, 9, 14, 19 and 25 DAA.Each sample was acidified with 10 mL of 6 M HCl and then chilled to 5oC until extraction andanalysis. All extractions were conducted within 1 week of sample collection. In trial 2, sampleswere collected at 1, 2, 3, 4, 7, 10, 14 and 18 DAA, and chilled to 5oC. Samples were not acidified,however extractions were completed within 48 hours of sample collection.
17
Samples of sediment were collected for chemical analysis from control and alphacypermethrintreated bays in both trials. In trial 1, 2 samples were collected from each bay at 5, 14 and 25 DAA,whilst in trial 2, 2 samples were collected from each bay at 15 and 38 DAA. Samples were obtainedusing 25 cm lengths of square-section aluminium cylinder (internal wall width 44 mm). Cylinderswere hammered through the sediment at random points within the bays, and then twisted, causing thecore to break free from the underlying clay. The clay layer effectively sealed the bottom of thecylinder, which was then lifted free. The cylinder containing the core was maintained upright whileboth ends were covered with plastic film, and then frozen at -17oC in the upright position untilpreparation for extraction and analysis.
Plant samplesPlant samples were collected for residue analysis from trial 1 only. Twenty complete plants wereremoved at random from each control and alphacypermethrin treated bay at 42 DAA, whilst grainsamples (20 random panicles per bay) and above ground forage samples (6 random plants per bay)were collected at grain maturity (136 DAA). Plant and grain samples were stored at –17oC prior totransport and analysis.
Chemical analysisWater. Samples from trial 1 were analysed using solvent extraction (2 x 25 mL hexane, dried overNa2SO4, evaporated to dryness under nitrogen and taken up in 0.2 mL acetonitrile) while solid phasemicroextraction (SPME) (100 µm thick polydimethylsiloxane fibre, Supelco) exposed for between 10- 30 min. at 25oC) was used for trial 2. Quantification was carried out via gas chromatography withan electron capture detector on a DB-5 (J&W) capillary column (30 m x 0.25 mm i.d.) with a filmthickness of 0.25 µm.
Soil. Sediment cores were thawed and the top 2 cm from duplicate cores were combined and air-dried. 10 g subsamples were sonicated and analysed according to Clark et al. (1989) using solventextraction followed by gas chromatography.
Plant Material. Samples (40 g) of rice forage, stems or grain was chopped for 15 minutes using anOmnimix (Sorvall) and 80 g of anhydrous Na2SO4 was added to the sample. This was then extractedwith 150 mL of 1:1 acetone:petroleum ether. Following filtering through Na2SO4, the sample wasevaporated to dryness with a rotary evaporator set at 60°C and a pressure of 600 mm Hg. The residuewas taken up in 10 mL of petroleum ether. The extract was then loaded onto a column containing5.5g of florisil (6%) and eluted with 40 mL of 10% diethyl ether in petroleum ether. The eluate wasevaporated to dryness using a rotary evaporator with the extract being taken up in 10mL of petroleumether. Recovery rates for control samples spiked with 0.5 ppm of alphacypermethrin were 93.7 ±6.4% (n = 3). The level of reporting using a 3 µL injection was 0.02 ppm.
A Shimadzu 17A gas chromatograph equipped with electron capture detectors and a AOC 17autoinjector was used. A DB-17 (J&W) capillary column (30m x 0.25mm i.d.) with a film thicknessof 0.25 µm and a DB-5 (J&W) capillary column (30m x 0.25mm i.d.) with a film thickness of 0.25µm were fitted. The initial oven temperature was set at 240°C and was held for 1 minute beforeramping at 20°C min-1 to 270°C with a hold time of 8.5 minutes. The GC was calibrated using anexternal standard of alphacypermethrin (0.1 ppm) before and during the run. Injector and detectortemperatures were 240 and 300°C respectively. A 3µL injection was made in the splitless mode witha wait time of 36 seconds.
18
Results
Environmental conditionsTable 4 summarises the environmental conditions in control bays during the 2 trials. There weresubstantial differences in pH and conductivity between the trials, with conditions in trial 1 being farmore alkaline than in trial 2. Conductivity was also much lower in trial 1. A comparison of themeans from the first 18 DAA of trial 1 with data from trial 2 shows that these are genuinedifferences, rather than artefacts of the longer sampling period in trial 1.
Table 4Environmental parameters in experimental rice bays, trials 1 and 2.
Trial 1 (first 25 DAA, first 18 DAA means in brackets)Variable Mean Range
pH 9.1 (9.1) 7.2 - 10.0Conductivity (µS.cm-1) 90.2 (89.6) 71.3 - 115.2Water temperature (oC) 23.8 (22.8) 12.7 - 38.5Rainfall (mm) 0.9 (0.3) 0 - 14.8
Trial 2 (first 18 DAA)Variable Mean Range
pH 7.7 6.7 - 8.5Conductivity (µS.cm-1) 172.3 155.4 - 189.9Water temperature (oC) 19.0 8.9 - 28.3Rainfall (mm) 1.0 0 - 12.2
Chironomid population assessmentsResults from the larval chironomid population assessments are given in figures 5 and 6. Percentagereductions in target Chironominae relative to equivalent control populations are summarised in table5.
In trial 1 populations of target Chironominae remained low in the control bays throughout the trial,reaching a density of approximately 485 larvae.m-2 at 14 DAA. Control was relatively poor duringthe first 14 DAA, with the standard chlorpyrifos treatment providing only 81.8% control of the maintarget group. Whilst the 10 g ai.ha-1 alphacypermethrin treatment provided 97% control over thisperiod, its performance was not matched by higher application rates. Analysis of larval populationsat set intervals after treatment (figure 5) shows that chlorpyrifos only had significant (P<0.05)impacts on the non-target group of larvae; whilst alphacypermethrin affected both the non-targetgroup and provided significant (P<0.05) reductions in target Chironominae at 9 DAA (all rates) andat 14 DAA (10 g ai. ha-1 rate only). At no time, however, did alphacypermethrin provide a level ofcontrol of either chironomid group statistically (P<0.05) better or worse than the chlorpyrifosstandard.
Target Chironominae populations were far higher in trial 2 (estimated density in control bays at 14DAA approximately 13,100 larvae.m-2), and much higher levels of control were achieved. Both thechlorpyrifos standard and all rates of alphacypermethrin provided > 99% control of the target groupfor 14 DAA, and only the chlorpyrifos standard had dropped below 99% control (to 97.1%) at 19DAA. All treatments significantly (P<0.05) reduced populations of Chironominae at each samplingperiod up to and including 19 DAA (figure 6). The only occasion on which the level of controldiffered significantly between chlorpyrifos and an alphacypermethrin treatment was at 24 DAA,where the non-target larval group was more heavily suppressed by chlorpyrifos than by the 6 g ai.ha-1
alphacypermethrin treatment.
19
Table 5Percentage reductions in target Chironominae larvae (relative to untreated controls) resulting fromapplications of chlorpyrifos and alphacypermethrin, cumulative data. DAA, days after sowing.
Trial 1Treatment a first 14 DAA first 19 DAA
chlorpyrifos 75 81.8 73.2alphacypermethrin 10 97.0 73.2alphacypermethrin 20 70.0 53.7alphacypermethrin 30 84.8 65.6
Trial 2Treatment a first 14 DAA first 19 DAA
chlorpyrifos 75 99.6 97.1alphacypermethrin 6 99.8 99.1alphacypermethrin 10 99.9 99.3alphacypermethrin 20 100 99.1
a values represent application rates in g ai.ha-1
Plant establishmentPlant densities at 30 DAA in trial 1 are given in table 6. No significant differences were recordedbetween treatments, either before or after correction for differential levels of duck damage across the10 bays.
Table 6Plant densities in control and treatment bays at 30 DAA in trial 1. nsd, no significant difference.Treatment a raw data
plants.m-2 (mean (SE))adjusted for losses to ducks
plants.m-2 (mean (SE))control 107.0 (12.0) 167.4 (10.0)chlorpyrifos 75 98.6 (6.8) 168.1 (6.4)alphacypermethrin 10 117.1 (8.9) 160.0 (9.2)alphacypermethrin 20 131.3 (12.2) 167.7 (10.9)alphacypermethrin 30 122.3 (6.8) 157.2 (9.2)significance (P = 0.05) nsd nsd
a values represent application rate in g ai.ha-1
In contrast, substantial differences in plant establishment were recorded in trial 2 (figure 7). Allchemical treatments significantly (P<0.05) improved plant establishment from the control level (23plants.m-2) to between 169 and 204 plants.m-2. There were no significant differences between thechemical treatments themselves.
20
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Figure 5. Effect of alphacypermethrin (Dominex®) and chlorpyrifos standard on chironomid larvae in trial 1, Yanco Agricultural Institute, 1997/8 season.Error bars represent standard errors. Solid bars, target Chironominae, hatched bars, other chironomids. *, significantly lower than equivalent control population(P < 0.05). No Dominex® treated populations significantly (P < 0.05) different to equivalent chlorpyrifos treated populations. No treated populationssignificantly (P < 0.05) higher than equivalent control populations.
21
Figure 6. Effect of alphacypermethrin (Dominex®) and chlorpyrifos standard on chironomid larvae in trial 2, Yanco Agricultural Institute, 1998/9 season.Error bars represent standard errors. Solid bars, target Chironominae, hatched bars, other chironomids. *, significantly lower than equivalent control population(P < 0.05). α, significantly higher than equivalent chlorpyrifos treated population (P < 0.05). No other differences significant.
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22
Chemical residuesWater. Alphacypermethrin concentrations in both trials dropped in an exponential-like manner as thealphacypermethrin became incorporated into the sediment. In trial 1 levels for the 10 and 20 g ai.ha-1
treatments were below 0.01 µg.L-1 at 5 DAA. During trial 2 water column alphacypermethrinconcentrations for the highest application rate (20 g ai.ha-1) had fallen to 0.008 µg.L-1 by 18 DAA, andwere below detection limits (0.001 µg.L-1) for the lowest (6 g ai.ha-1) rate (figure 8).
alphacypermethrin 20
alphacypermethrin 10
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0 50 100 150 200 250
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Figure 7. Plant establishment in trial 2, small plot rice bays, 1998/9 season at 30 DAA. Bars withdifferent letters are significantly different (P < 0.05). Error bars represent standard errors.
No alphacypermethrin was detected in any of the plant samples taken at 42 DAA, or in grain or foragesamples collected at harvest (detection limit 20 µg.kg-1).
DiscussionThe two trials provided substantially different results, primarily as a consequence of the differentlevels of colonisation by target Chironominae (particularly Chironomus tepperi) in the two seasons.Although the environmental data suggests conditions were extremely different during the 2 trials, thislargely reflects the differences in chironomid populations themselves. In trial 1 larval populationsremained very low and there was no obvious turbidity in the control bays where the measurementswere made. In trial 2, however, the control bays became extremely turbid, which kept the waterrelatively cool and probably also raised the conductivity. High turbidity and low water temperatureswould have minimised photosynthesis by macrophytes and algae, reducing the large diel variations inpH typically observed in establishing rice fields (eg. trial 1).
Over 80 larval Chironominae per sample were found in the trial 2 control bays at 14 DAA, anuncharacteristically high number in comparison to earlier trials (eg. Stevens and Warren, 1992;Stevens et al. 1998), where turbidity related to larval activity was much less noticeable. Theoccurrence of such dense infestations suggests that it may be more appropriate to take environmentalmeasurements from both control and chemically treated bays in future studies.
In percentage terms (table 2), the control provided by all chemical treatments in trial 1 appearsrelatively poor. This can be explained by the fact that the main target species, C.tepperi, was totallyabsent during trial 1, and those Chironominae that were present belonged to different taxa that may bemore tolerant to the pesticides being evaluated. The response of chironomid populations to increasingrates of alphacypermethrin in trial 1 was not clearly dose-related, suggesting only limited emphasiscan be placed on the result of this trial because of the very low pest pressure. Similarly, low pest
23
pressure led to no significant differences in plant establishment, even when losses to ducks werecompensated for (table 6).
Figure 8. Alphacypermethrin levels detected in water and soil during trials 1 and 2. Error barsrepresent standard errors.
In contrast, C.tepperi was the dominant component of the Chironominae found in trial 2 control bays,and its high susceptibility to both alphacypermethrin and chlorpyrifos resulted in > 97% control ofChironominae being achieved in the first 19 DAA (table 5). In both trials, alphacypermethrin at thelowest rates evaluated provided control of the main target group at least equivalent to that provided bychlorpyrifos at 75 g ai.ha-1. Our results indicate that alphacypermethrin at an application rate ofapproximately 8 g ai.ha-1 will be an effective alternative to chlorpyrifos for chironomid control in ricecrops. This result is in accordance with the findings of Pasini et al. (1997), who found thatalphacypermethrin applied at a higher rate (23.7 g ai.ha-1) provided effective control of chironomids inItalian rice fields.
Alphacypermethrin rapidly partitions from surface water and bonds to sediments, indicating that therisk of losing alphacypermethrin from rice fields in clean drainage water is very low. In trial 1,however, fluctuating levels of alphacypermethrin were found in the water column 14 and 25 days afterapplication at the highest (30 g ai.ha-1) rate evaluated. These apparently anomalous results can beaccounted for by chemical analysis methodology used in trial 1, where the samples were not filteredprior to solvent extraction. Higher turbidity levels in the bays, caused either by ducks or thecollection of sediment samples, would have resulted in greater levels of sediment-boundalphacypermethrin occurring in the water samples. The SPME technique used in trial 2 would nothave been affected by sample turbidity in the same way.
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24
Alphacypermethrin has the potential to be an effective, low-cost alternative to chlorpyrifos forbloodworm control. The estimated optimum field rate is 8 g ai.ha-1, which will pose minimal risk todrainage quality provided turbidity levels remain low. Alphacypermethrin is more persistent insediments, and the possibility exists that turbid drainage water may pose a greater risk to downstreamenvironments. This risk is considered minimal, however, due to the low proposed commercial rate,and the reduced bioavailability of sediment-bound pyrethroids. Commercial scale trials ofalphacypermethrin as an alternative to chlorpyrifos for bloodworm control are strongly recommended.
25
3.2 Colonisation of rice fields by bloodworms
IntroductionThe majority of invertebrate pest problems in NSW rice crops are characterised by the involvement ofonly one or two species. For example, armyworm damage is caused by a single species, Leucaniaconvecta, whilst snail problems are usually caused by Isidorella newcombi, although Glyptophysa sp.is also an occasional pest.
The situation with bloodworms is much more complex. It is well established that Chironomus tepperiis the principal pest species in early establishment, has a single generation in rice fields, and issubsequently replaced by a range of other species about which little is known. At least one of theseadditional species, and possibly several, are pests of rice. This is demonstrated by the fact that manyrice crops experience significant damage if they are not treated with insecticides at least twice duringthe crop establishment period. Only the first chemical application is needed to eliminate C.tepperi -the second application is targeted at other species of unknown identity.
Faunistic studies, such as the on reported here, are fundamental to developing an understanding of theecology of bloodworm communities in rice crops. Basic faunistic studies on chironomids have beenconducted in many rice growing areas, and particularly where they have been recognised as croppests. Darby (1962) and Clement et al. (1977) studied chironomid colonisation of Californian ricefields, whilst studies on Italian rice field communities have been published by Cocchi (1966), andmore recently by Ferrarese (1992). Until recently similar studies on the chironomid faunistics ofAustralian rice fields have not been possible due to an absence of taxonomic studies on larvalchironomids in this country. Recent work by Cranston (1996) has changed this situation, and it isnow possible to identify most NSW rice field species to at least generic level. Determining whichspecies are present in rice fields and quantifying their abundance and temporal distribution is the firststep towards elucidating which species other than C.tepperi may be contributing to plant damage, andnecessitating the second spray application required by at least some growers each season. The factthat some species, such as Polypedilum nubifer and Cryptochironomus griseidorsum, are apparentlymuch more tolerant to insecticides than C.tepperi means that determining whether these species arecausing crop damage is of particular significance to NSW rice producers.
Materials and Methods
Collection of specimensSpecimens used in this study was sourced from small untreated rice bays used as controls in pesticideevaluation studies during the 1995/6 and 1997/8 rice seasons. All material was collected at YancoAgricultural Institute using soil coring. Samples were obtained by pushing a 96 mm diameter plasticcylinder 50 mm into the sediment, sealing the top, and then digging around the outside of the cylinderand sliding a thin perspex sheet underneath. The complete assembly was then lifted out, and thewater and sediment within the cylinder was transferred to 1 L capacity plastic containers and frozen at-17oC until sample analysis. Magnesium sulfate flotation of larvae from thawed samples followed theprocedure of Stevens and Warren (1992). Three samples were taken from each bay 10, 15, 20, 25, 30,35 and 40 days after bay flooding.
Specimen storage and preparationChironomid larvae extracted from the soil cores were initially preserved in 70% ethanol. Prior toslide mounting, they were cleared overnight in 10% w/v potassium hydroxide, acidified in glacialacetic acid, and then transferred through 2 changes of n-propanol. Larvae were then mountedindividually under cover slips in euparal. Larvae were examined using a Leica DMLB compoundmicroscope equipped with interference contrast optics, and were identified using the keys of Cranston(1996).
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ResultsLarval recoveries from core samples taken in the 1995/6 and 1997/8 seasons are summarised in tables7 and 8. A total of 1,512 larvae were recovered from the 2 trials. Eighteen different species wererecorded across the 2 seasons.
There were considerable differences across the two trial seasons. In the 1995 season Chironomustepperi dominated the fauna during the first 15 days after flooding and was then replaced byProcladius paludicola, a species that is largely predatory and not known to attack rice. Polypedilumnubifer was the third most common species, occurring from day 20 onwards, whilst no other speciesconstituted more than 2% of the total fauna. In 1997 chironomid densities were substantially lower,C.tepperi was totally absent from the fields, and P.paludicola was again numerically dominant overthe full 40 day period. Other species remained at low densities until 40 days after flooding, whenthere was a substantial increase in the densities of both P.nubifer and Cladopelma ‘Y1’.
DiscussionThe results of this study serve to highlight the variability in chironomid populations at a single siteacross different seasons. C.tepperi, acknowledged as the principal pest species in rice, showedmoderate densities in 1995, but was totally absent in 1997. A preliminary examination of specimenscollected during the 1998 sowing season has shown that C.tepperi densities were far higher than in1995, and collection of data from at least 2 more seasons is necessary before meaningful analysis ofthe data (ie. through rank-abundance curves) will be possible.
In 1997 no significant plant damage was recorded in chemical control trials conducted adjacent to thesample site (see sections 1.2, 1.3). The only species present in significant numbers during the 1997season was P.paludicola (table 8), and the absence of plant damage serves to reinforce the theory thatP.paludicola is essentially predatory and does not attack rice.
Whilst it has been recognised that C.tepperi has a single generation in rice fields (Stevens, 1994), andthat infestation densities are highly variable, a more important finding from this study is the identityof species that are present in sufficient numbers to have the potential to cause plant damage in thelater part of the crop establishment period (from day 20 onwards). Polypedilum nubifer is a primecandidate for laboratory studies on pest potential because of its reasonably consistent abundance.Other species that are of potential concern include Cladopelma ‘Y1’, because of it’s rapid increase indensity at the end of the 1997 season, and Cryptochironomus griseidorsum. Although C.griseidorsumwas only present at low densities at the study site in 1995 and 1997, this species is known to reachhigh densities in resown crops where extensive bloodworm damage is occurring. The list of specieswith potential pest status will only be refined through additional sampling and analysis.
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Table 7Chironomid larval recoveries, summary, YAI 1995 sowing season, 888 larvae in total. *, identity requires confirmation; ** possibly pseudoconjunctus,however some characters obscured.
Taxon Days since bay flooding10 15 20 25 30 35 40 TOTAL (%)
Chironomus tepperi Skuse 66 77 1 - - - - 144 (16.2)Chironomus ‘Y1’ 2 - - - - - - 2 (<1)Procladius paludicola Skuse 1 6 93 73 128 157 116 574 (64.6)Polypedilum nubifer Skuse - - 16 29 26 21 8 100 (11.3)Cladopelma ‘Y1’* - 1 1 3 2 1 2 10 (1.1)Cladopelma curtivalva Kieffer - - - - - - - -Cryptochironomus griseidorsum Kieffer - - 3 3 5 3 3 17 (1.9)Parachironomus sp. - - - - - - - -Kiefferulus martini Freeman - - - - - 1 - 1 (<1)Kiefferulus ‘tinctus’ - - - 1 - 2 - 3 (<1)Kiefferulus ‘Y1’ - - - - - 1 - 1 (<1)Dicrotendipes pseudoconjunctus Epler - - - - - - - -Dicrotendipes sp.** - - - - - 1 - 1 (<1)Tanytarsus nr spinosus Freeman - 1 4 3 4 3 2 17 (1.9)Cladotanytarsus sp. - - 4 4 2 1 - 11 (1.2)Ablabesmyia notabilis Skuse - - 1 - - - - 1 (<1)Cricotopus sp. - - - - - - 1 1 (<1)Gymnometriocnemus sp. - - - - - - - -Orthocladiinae genus nr ‘SO2’ - - 1 - 2 - - 3 (<1)Undetermined - - - 1 - 1 - 2 (<1)
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Table 8Chironomid larval recoveries, summary, YAI 1997 sowing season, 624 larvae in total. *, identity requires confirmation.
Taxon Days since bay flooding10 15 20 25 30 35 40 TOTAL (%)
Chironomus tepperi Skuse - - - - - - - -Chironomus ‘Y1’ - - - - - - - -Procladius paludicola Skuse 10 25 77 49 66 58 114 399 (63.9)Polypedilum nubifer Skuse - 4 11 2 - 5 89 111 (17.8)Cladopelma ‘Y1’* - - 8 1 1 13 37 60 (9.6)Cladopelma curtivalva Kieffer - 1 - 1 1 1 1 5 (<1)Cryptochironomus griseidorsum Kieffer 1 - - - - 4 5 10 (1.6)Parachironomus sp. - 1 - - - - - 1 (<1)Kiefferulus martini Freeman - - - - - - - -Kiefferulus ‘tinctus’ - - - - - - - -Kiefferulus ‘Y1’ - - - - - - - -Dicrotendipes pseudoconjunctus Epler - - - - 1 - 1 2 (<1)Dicrotendipes sp. - - - - - - - -Tanytarsus nr spinosus Freeman 1 4 3 3 4 2 4 21 (3.4)Cladotanytarsus sp. - - - - 3 - - 3 (<1)Ablabesmyia notabilis Skuse - - 1 - 1 - - 2 (<1)Cricotopus sp. - - - - - - 6 6 (<1)Gymnometriocnemus sp. - - 1 - - 1 - 2 (<1)Orthocladiinae genus nr ‘SO2’ - - - - - 1 - 1 (<1)Undetermined - - - - 1 - - 1 (<1)
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3.3 Chemical control of aquatic earthworms
IntroductionIn southern New South Wales over 90% of the rice crop is sown by fixed-wing aircraft. Thispractice contrasts strongly with rice-growing practices in Asia, where manual transplantation ofseedlings from seedbeds is the most widely practiced method of crop establishment. Aerial sowingof pregerminated rice seed is fast, allows optimal use of irrigation water, and helps facilitateeffective weed management, however there are costs associated with this practice. Seed sown byaircraft is left at the soil/water interface, where it is vulnerable to damage from invertebrate pests,notably chironomid larvae, planorbid snails, and the oligochaete worm Eukerria saltensis(Beddard).
Eukerria saltensis (Beddard) is a widely distributed peregrine oligochaete (Jamieson, 1970) thathas become a major pest of rice in the last decade. Unlike most earthworms, E.saltensis cansurvive beneath a flooded rice crop for a complete growing season, and affects rice in a number ofways. Dense infestations attract large flocks of ibis which can trample young seedlings into thesoil, however even in the absence of ibis E.saltensis can seriously damage establishing rice cropsby interacting with the rice field environment in ways that limit, or in some cases prevent cropgrowth (see section 4). E.saltensis has its greatest impact on aerially sown crops growing onheavy, dispersive clay soils, and consequently the Murray Valley is the worst affected area. Cropsshowing Eukerria-related establishment failure are characterised by turbid water, loss of soilcompaction, high levels of algal growth, and reduced plant establishment, particularly in areas ofdeeper water.
Oligochaetes have been recognised as sporadic pests of rice in several countries, including Japan(Tamura, 1961), Nepal (Manandhar, 1985; Pradhan, 1986), the Philippines (Otanes and Sison,1947; Anon, 1985; Barrion and Litsinger, 1997) and India (Verma et al., 1975; Rao et al., 1992).Although earthworms do not feed on living rice plants, they have been recognised as indirectlydamaging rice in several ways. Newly hatched Limnodrilus sp. (Tubificidae) have been found inthe basal parts of plants, whilst mature worms cluster around the roots, causing the plants todiscolour and in some cases die (Manandhar, 1985; Pradhan, 1986). Similar damage has beencaused to rice in India by Malabaria sulcata Gates (Ocnerodrilidae) and Drawida pellucida(Bourne) (Moniligastridae) (Rao et al., 1992) and by unidentified Tubificidae and Naididae(Verma et al., 1975). Kale et al. (1989) found that if water levels were inadequate, Curgionanarayani (Michaelsen) could build cast mounds around plant bases, and this could interfere withtiller formation. Malabaria paludicola Stephenson has also been reported as forming cast moundsaround the bases of rice plants (Vasanthraj David et al., 1976). Barrion and Litsinger (1997)concluded that damage to rice plants in the Philippine Cordilleras by Dichogaster nr. curgensisMichaelsen was caused by setal abrasion of the root systems, which led to plant stunting andseedling death. Plants infested during the vegetative stage were more severely damaged than thoseinfested only during the reproductive stage. Burrowing activity may interfere with plantgermination by burying seeds to deeper levels within the seedbeds (Barrion and Litsinger, 1997),and may also lead to levees becoming unstable and developing leaks (Otanes and Sison, 1947;Anon, 1985; Barrion and Litsinger, 1997).
Developing an understanding of how E.saltensis interacts with the rice field environment (seesection 4) has allowed several guidelines to be developed in regard to cultural management ofinfestations (Clampett and Stevens, 1998, 1999). Whilst these management practices reduce thelevel of crop damage, they do not eliminate it, and consequently there is a need to developsupplementary control methods, including chemical control. This section reports the results of aseries of field trials aimed at evaluating selected pesticides for E.saltensis control, either in floodedcrops or prior to field inundation.
30
Materials and methods
Flooded field trialsFive trials were conducted to evaluate the effect of thiodicarb (1 trial), niclosamide, andbendiocarb (2 trials each) on E.saltensis populations in flooded rice fields. The basic methodologywas the same in each trial. Twelve enclosures were used per trial, each made from stainless steelsheet, 22 x 22 x 46 cm (height). A vertical series of 6 evenly spaced 2 mm diameter holes weredrilled along one side of each enclosure, and sealed with silicone sealant prior to deployment in thefield. Once a densely infested area of crop was located, the 12 enclosures were forcedapproximately 15 cm into the soil. A 4 x 3 pattern was used, with between 0.5 and 1.5 metresseparating adjoining enclosures. Four enclosures were designated at random as controls, whilst theremaining 8 were treated with the selected pesticide, 4 at 1 kg ai.ha-1 and 4 at 2 kg ai.ha-1. Alltreatments were prepared by serial dilution of formulated pesticide in distilled water, and wereapplied to each enclosure using a final volume of between 5 and 20 mL. After chemicals wereapplied, a large spatula was used to gently mix the contents of each enclosure. Two hours afterchemical application a fine piece of wire was used to remove the silicone sealant from one of theholes below the waterline of each enclosure so that water could enter the enclosure to compensatefor evaporation.
Trials were assessed at either 8 or 14 days post-treatment. The open hole in each enclosure wasblocked, and the water within the enclosure was bailed out. The soil was then removed to a depthof 20 cm, packed in large plastic bags, and returned to the laboratory. Locations, treatments, andassessment times for the 5 flooded field trials are summarised in table 9.
Preflood, fallow, and incorporation trialsThree trials were conducted to evaluate the efficacy of chemical treatments incorporated into thesoil prior to rice flooding or applied to fallow paddocks. Details of the trial sites and treatmentsevaluated are given in table 10.
In trials 6 and 7 a 2 x 2 metre galvanised steel frame with vertical walls was used to isolate 4square metre areas for treatment. The central square metre of each treatment area was marked with4 wooden pegs, and a pretreatment soil core sample was taken from each bay prior towater/chemical application. The samples were 21.5 x 21.5 x 26 cm (depth). Once pretreatmentsamples were removed a plastic bag was pushed into the hole, filled with sand, and sealed torestore a flat treatment surface. The control bays were treated by watering can with cleanirrigation water to determine the maximum soil uptake volume, which depended on the amount ofmoisture initially in the soil. Once the uptake volume had been determined, the required amountof pesticide for each treatment plot was diluted to 10 L in a watering can, and half was applied tothe plot initially. Clean water was used to water the treatment further into the soil profile until halfthe uptake volume was reached. The second half of the chemical was then applied, followed bythe remainder of the uptake volume. Post-treatment assessments were made by taking soil coresamples (same dimensions as pretreatment samples) from the centra1 area of each plot. In trial 6, astainless steel cylinder (as used in the flooded field trials) was pushed into the soil and the waterwas bailed out prior to collecting the soil core.
Trial 8 involved incorporating carbofuran granules into the surface layer of the soil prior toflooding. Plots one square metre in size were used. The surface of each plot was carefully levelledwith a trowel and 11 equidistant parallel grooves 3 cm deep were cut into the soil surface. Theappropriate weight of granules for each groove was spread uniformly along a 1 metre foldedaluminium strip and then tipped into the groove, which was then refilled with soil. Post-treatmentsampling was as for trial 6.
Four replicates of each treatment were conducted in each of trials 6, 7 and 8.
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Table 9Site and treatment details for flooded field pesticide trials against E.saltensis.
Trial # Farmer Location Compound Formulation* Period /WD*1 Steve Plant 25 km SW Deniliquin thiodicarb Larvin®375 (Rhône-
Poulenc)8 / 23.5
2 Steve Plant " " niclosamide Bayluscide®250(Bayer)
14 / 25.2
3 Steve Plant " " niclosamide Bayluscide®250(Bayer)
8 / 22.9
4 Steve Plant " " bendiocarb Ficam® 14 / 27.15 Mick Whelan 15 km N Darlington Point bendiocarb Ficam® 8 / < 5.0*** period between treatment and assessment in days / water depth at assessment in cm** greater than 10 cm at treatment
Table 10Site and treatment details for fallow and preflood pesticide trials against E.saltensis.
Trial # Farmer Location Trial type Treatments* Period6 Steve Plant 25 km SW
Deniliquinpreflood, liquidswatered in,infiltrationvolume 22.5 Lper square metre
carbofuran (FMC Furadan®360), thiodicarb (Rhône-Poulenc Larvin® 375),niclosamide (BayerBayluscide® 250), all at 2kgai/ha
Treated 7 dayspreflood, assessed22 days post-flood
7 Steve Plant " " fallow field,liquids wateredin, infiltrationvolume 9.5 Lper square metre
carbofuran (FMC Furadan®360) at 0.5 and 1.0 kg ai/ha
Assessed at 21and 63 days afterapplication
8 Alan Walsh 18 km NEDeniliquin
preflood,granules soilincorporated
carbofuran 10% W/Wgranules (FMC)
Treated 3 dayspreflood, assessed11 days post-flood
Worm extractionEach soil sample was divided into several 10 L plastic buckets. A small quantity of food-gradeCalgon® (amorphous sodium polyphosphate) was added to assist clay dispersal, and the bucketswere filled with water. The contents of each bucket were then stirred with a wooden paddle tobreak up the soil structure as much as possible, and the soil was then washed through a modifiedSalt-Hollick soil washing apparatus (Walker, 1983) using a garden hose. The residue (organicmatter, worms, seeds etc.) was collected from the screens and sorted by hand. Total worm biomasswas used as the assessment parameter. Worms and worm fragments were fixed in 8% formalin for48 to 72 hours and then transferred to 70% ethanol. After 1 to 2 weeks the worms were removed,air-dried for 24 hours and then oven dried to constant weight at 80oC prior to weighing on ananalytical balance. Data are expressed as milligrams (dry weight).sample-1.
Data analysisDifferences between treatments were assessed using one-way analysis of variance and Tukey’sHSD test.
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Results
Flooded field trialsResults from the flooded field trials (trials 1 to 5) are shown in figure 9. None of the flooded fieldtreatments resulted in statistically significant reductions in Eukerria biomass. Non-significantdose-related reductions in worm biomass were only apparent in trial 5 (bendiocarb), which wasconducted under unusual water management conditions and at a different site. In trial 5 waterdepth at treatment exceeded 10 cm, but by assessment this had fallen to less than 5 cm on average,and in some enclosures there was no water remaining at all. This inadvertent decline in waterlevels may have led to deeper infiltration of the chemical as the water dropped, and may representa potential management tool for enhancing the effectiveness of flooded-field treatments. TheDarlington Point site where this trial was conducted is on a much lighter soil than the MurrayValley soils typically supporting damaging Eukerria infestations, and the difference in soil typemay also have facilitated better chemical infiltration in this trial. Trial 4, conducted in the MurrayValley and also using bendiocarb, provides no evidence of this compounds efficacy and theapparent dose-related decline in worm biomass in trial 5 therefore need to be viewed with extremecaution.
Preflood, fallow, and incorporation trialsThe results of trial 6 are shown in figure 10. Thiodicarb and niclosamide at 2 kg ai.ha-1 had noeffect on worm populations, despite the high water infiltration rate achieved. Although carbofuranprovided a sizeable reduction in worm populations, its effect was not statistically significantrelative to the control populations due to their high error component. Carbofuran did, however,significantly reduce populations relative to those in the thiodicarb and niclosamide treatments.The lack of statistical significance associated with the difference between the control andcarbofuran treatments appears to be a consequence of the small number of replicates (4) of eachtreatment.
Results of trial 7 are shown in figure 11. Carbofuran applied at 0.5 and 1.0 kg ai.ha-1 did notsignificantly affect Eukerria populations when watered into a fallow rice paddock, however theinfiltration volume achieved was much lower than that in trial 6.
Results from trial 8 are shown in figure 12. Although there was a dose-related decrease inEukerria biomass at higher application rates, none of the evaluated treatments provided significant(P<0.05) reductions relative to the untreated control plots.
DiscussionOf the compounds examined as part of this project, only 2, bendiocarb and carbofuran, showed anyeffect of Eukerria populations. In the case of bendiocarb, a non-significant dose related responsewas observed in trial 5, but not in trial 4, which was conducted on a soil type more typical of thoseadversely affected by Eukerria.
Carbofuran watered into dry soil prior to flooding at 2 kg ai.ha-1 appears to suppress Eukerriapopulations quite well provided infiltration volumes are high enough, although the lack ofstatistical significance in the results of trial 6 are a cause for concern. Increasing the number ofreplicates should improve statistical resolution in subsequent trials. At lower infiltration volumescombined with application rates of 1 kg ai.ha-1 or less, however, carbofuran appears to beineffective (trial 7).
There is also evidence to suggest that soil incorporated carbofuran granules may suppress Eukerriapopulations, however the required application rate is likely to be in excess of 2 kg ai.ha-1, whichmay be unsustainable from both economic and environmental perspectives. Carbofuran is currentlyin the process of being deregistered for use in rice by the US Environmental Protection Agency,and it is doubtful whether this material could be registered for use in Australian rice crops,particularly at application rates of 2 kg ai.ha-1 or higher. Further research is required on alternativecompounds, and priority should be place on developing laboratory bioassay procedures that willallow faster screening of potential vermicides, since the current field trial assessment proceduresare unavoidably slow and labour-intensive.
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Figure 9. Influence of pesticides applied to enclosures in flooded rice fields on biomass ofE.saltensis 8 to 14 days after application. Error bars represent standard errors.
control 1000 2000
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nsd = no significant difference between treatments
34
Figure 10. Results of Eukerria trial 6. Influence of pesticides watered into dry soil prior (7D)to flooding and assessed at 22D post-flooding. Infiltration volume 22.5L per square metre.Treatments accompanied by different letters are significantly different (P<0.05). Error barsrepresent standard errors.
Figure 11. Results of Eukerria trial 7. Influence of carbofuran watered into a dry fallow ricefield and assessed at 3 and 9 weeks after treatment. Infiltration volume 9.5L per square metre. Nosignificant differences between treatments on any individual day. (P>0.05). Error bars representstandard errors
treatment
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35
Figure 12. Results of Eukerria trial 8. Influence of 10% w/w carbofuran granules incorporatedinto the soil prior to flooding on subsequent worm populations. Treatments accompanied bydifferent letters are significantly different (P<0.05). Error bars represent standard errors.
carbofuran treatment (kg ai.ha-1)
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36
3.4 Interactions between aquatic earthworms and the riceenvironment
IntroductionEukerria saltensis has become a major pest of rice, particularly in the Murray Valley, but one ofthe main questions associated with it’s pest status is how it interacts with plants and/or theirenvironment to cause crop damage. Like all earthworms, Eukerria lacks biting mouthparts,preventing it from feeding on living macrophytes. The impact of large flocks of ibis trampling onthe crop whilst feeding on E.saltensis is widely recognised, however crop establishment failuresare commonly reported even when ibis are absent. This, in combination with the reduced levels ofdamage seen in infested drill-sown crops (relative to aerially-sown ones) suggests that much of theproblem may be related to ways in which Eukerria modify the rice field environment.
Preliminary studies on the physical and chemical effects of Eukerria on the rice bay environmentwere conducted as part of RIRDC Project DAN137A, ‘Preliminary studies on biology and controlof the rice worm Eukerria saltensis’. During the course of this project, some of the experimentsconducted as part of DA137A were replicated to bring them up to publication standard, whilstseveral new experiments were also undertaken. Experiments that include partial data fromDAN137A are identified in the ‘Materials and Methods’ section.
Materials and methods
Preparation of experimental containers
All experiments were conducted using translucent round polypropylene containers (base diameter89 mm, top diameter 110 mm, height 115 mm). Containers were filled to a depth of 55 mm withmilled Riverina Clay topsoil (Johnston, 1953) taken from a Murray Valley rice farm (‘ToranaPark’, 35o39’S 144o39’ E, 30 km WSW of Deniliquin, NSW). Each container was then flooded to45 mm above soil level with 1x Martin’s solution (Martin et al., 1980), an artificial rearingsolution containing 75 ppm total dissolved solids. Soil samples containing E.saltensis werecollected from ‘Torana Park’ and transferred to the laboratory. Worms were extracted manuallyfrom the soil for use in all experiments, and the required number were placed on the soil/waterinterface of each container immediately after the containers were flooded. Any worms that did notburrow below the surface within 24 hours were removed and replaced.
Influence of E.saltensis on physical and chemical aspects of surface water quality(25% of data from DAN137A)Sixteen containers were prepared, four each with 0, 10, 20, and 40 mature E.saltensis. Thecontainers were placed in a constant temperature room set at 25±1oC, and maintained for 21 days,with distilled water being used to compensate for evaporation during the course of the experiment.
Water turbidity and pH were measured 7, 14, and 21 days after the establishment of each replicate,using a Lovibond DRT-15CE portable turbidimeter (HF Scientific Inc., Fort Myers) and a Hanna8417 benchtop pH meter (Hanna Instruments SpA, Padova), respectively. After 21 days, thesurface water was carefully pipetted from each container down to a depth of ca. 5 mm. Watersamples from the four containers of each treatment were combined, mixed, and analysed for N asNH4
+, N as nitrate/nitrate (NOx), and both total and soluble phosphorus. Nitrogen as NH4+ was
measured using APHA method APHA 4500 NH3 E (ammonium-selective electrode method usingknown addition), whilst phosphorus was determined using APHA methods 4500 PB and PE(ascorbic acid ± persulphate digestion) (APHA/AWWA/WEF, 1995). Nitrogen as NOx wasdetermined spectrophotometrically according to Australian Standard 2029-1997 (SAA, 1977).Three replicates of the experiment were conducted using a 15L:9D photoperiod (illumination withcool fluorescent tubes), whilst a further three replicates were conducted in total darkness in orderto determine whether algal growth may have contributed to changes in water chemistry observedunder cyclical illumination.
37
Influence of E.saltensis on rice plant developmentTo evaluate the influence of E.saltensis on rice plant growth, 12 containers were established, foureach with 0, 20, and 40 mature E.saltensis. Containers were placed in a constant temperature roomilluminated with cool fluorescent tubes (22/13oC, 12: 12 cycle with a 12L:12D photoperiod) for 7days prior to the addition of 25 pregerminated rice seeds (cv. ‘Millin’) to each container. Thecontainers were then maintained for a further 25 days under the same conditions, with distilledwater being used to compensate for evaporation. All plants were then removed and brushed cleanin water using a soft paintbrush. Root systems were examined microscopically for signs ofabrasion or other physical damage. Shoots and roots were then separated from the seeds, prior tomeasuring shoot lengths to the nearest mm. Average shoot and root weights were obtained bydrying the roots and shoots taken from each container to constant weight at 105oC, and thendividing by the number of plants present. The experiment was replicated three times.
Influence of burrowing activity on seed stratificationTwelve containers were prepared, four each with 0, 20, and 40 mature E.saltensis, and were usedto determine whether the burrowing activities of E.saltensis could lead to rice seeds beingtransported below the soil surface. Twenty-five pregerminated rice seeds (cv. ‘Millin’) wereplaced on the soil surface in each container, and the containers were then placed in a constanttemperature room set at 25±1oC and illuminated with cool fluorescent tubes (15L:9D photoperiod).The containers were maintained for 7 days, with distilled water being used to compensate forevaporation. At the end of this period the surface water was pipetted off, and the containers weredried down overnight at 55oC. This rapidly killed the worms, and subsequently allowed the soiland seedlings to be removed as a solid column. Rice seeds remaining on the soil surface wereremoved and counted, and then a sharp knife was used to gradually slice the soil away to reveal theposition of seeds below the soil surface. Seeds recovered from below the surface were dividedinto two categories, where the uppermost part of the seed was either less than or more than 2 mmbelow the surface of the dried soil column.
Influence of E.saltensis on chlorophyll a levelsTo determine if Eukerria-mediated changes in water chemistry could stimulate algal growth underlaboratory conditions, 12 containers were established (see section 2.1), three each with 0, 10, 20,and 40 mature E.saltensis. The containers were placed in a constant temperature roomilluminated with cool fluorescent tubes (22/13oC, 12: 12 cycle with constant illumination), andwater levels were maintained using distilled water. Three weeks after initial flooding all surfacewater was pipetted from the containers, and the container sides were swabbed to remove any algalgrowth. The surface layer of soil (2 mm) was also removed and added to the water and swabbedmaterial to form a composite sample containing all the recoverable algal material from eachcontainer. Chlorophyll a was extracted from the samples in acetone and analysedspectrophotometricaly using the technique of Vowles and Connell (1980).
Statistical analysisResults were analysed using ANOVA and Tukey’s HSD test. Where necessary, data were log-transformed to stabilise variances across the range of response prior to analysis. Data from theseed stratification experiment and seedling survival data from the plant development experimentwere converted to percentages and subjected to inverse-sine transformation prior to analysis.
38
Results
Effect of Eukerria on pH and turbidityResults of the turbidity and pH assessments are shown in figure 13. Under conditions of cyclicalillumination, turbidity increased significantly (P < 0.05) relative to controls at densities of 20 and40 worms.container-1 after 7 days. At 14 days after establishment, turbidity at densities of 20 and40 worms.container-1 had increased significantly (P < 0.05) relative to the levels at 7 days, andwhilst this trend continued 21 days after establishment, differences within treatments between days14 and 21 were not significant. In darkness a similar pattern occurred, however higher levels ofturbidity were achieved much more quickly, particularly at the maximum density of 40worms/container.
Figure 13. Influence of Eukerria density on turbidity and pH in experimental containers. ● , 7days after establishment; ∗∗∗∗ , 14 days after establishment; ▼, 21 days after establishment. On eachgraph points followed by different letters are significantly different (P < 0.05). Error barsrepresent standard error of least squares mean.
Under cyclical illumination pH dropped significantly even at the lowest Eukerria density (10worms.container-1) 7 days after establishment. At 14 and 21 days all treatments had risenprogressively in pH, often significantly (P < 0.05) relative to the day 7 levels. However,significant differences remained between at least some treatments at each time interval, withhigher worm densities continuing to reduce pH relative to the controls. In darkness thisrelationship was less well-defined. Whilst pH increased over time within treatments, changesacross treatments were only significant 21 days after establishment.
0 10 20 40
turb
idity
(N
TU
)
0
100
200
300
400
500
600
700
0 10 20 40
turb
idity
(N
TU
)
0
100
200
300
400
500
600
700
800
w orm s / conta iner
0 10 20 40
pH
6 .8
7 .0
7.2
7.4
7.6
7.8
8.0
8.2
w o rm s / conta iner
0 10 20 40
pH
6 .6
6 .8
7.0
7.2
7.4
7.6
(a ) turb id ity , il lumina ted (b ) tu rb id ity , da rkness
(c) pH , illum ina ted (d ) pH , da rkness
aa
aab
abab bc
d
de
cd
e
e
aabab
abcbc
cd
cd
de
effgfg
g
ba
a a
fg
ef
bcdbc
gh
h
decde
a a aab
c
c
bcc
bc bcc
d
39
Influence of Eukerria on nitrogen and phosphorus levels in the water columnThe effects of E.saltensis activity on water column levels of nitrogen as NH4
+ and total phosphorusare shown in figure 14. Concentrations of both N as NH4
+ and total P increased progressively withincreasing E.saltensis density. Significant (P < 0.05) differences in total P concentration werefound between the untreated controls and containers with the highest worm density (40worms.container-1) under both cyclical illumination and continuous darkness. A similar significantdifference was found in N as NH4
+, but only under the cyclical illumination regime. Data on theresponses of nitrogen as NOx and soluble P to E.saltensis are given in table 11. There was noapparent trend in NOx levels inresponse to increasing worm density. Soluble P increased with increasing worm density undercyclical illumination, and a similar trend was apparent in darkness, however the relatively highlevels of soluble P found in the controls under conditions of darkness are anomalous and cannot bereadily explained. None of the differences in NOx or soluble P were significant (P > 0.05).
Figure 14. Influence of Eukerria density on nitrogen as NH4+ and total phosphorus inexperimental containers. On each graph points followed by different letters of the same case aresignificantly different (P < 0.05). Error bars represent standard error of least squares mean. nsd,no significant differences (P > 0.05).
Influence of E.saltensis on rice plant developmentPlant growth parameters in the presence of different levels of Eukerria infestation are shown infigure 15. The mean number of plants surviving at assessment varied from 20 to 21plants.container-1 across the three treatments. Differences in plant survival between treatmentswere not significant (P > 0.05). Eukerria infestation did not significantly affect plant root systemweight (P > 0.05), however shoot weight was significantly (P < 0.05) increased by Eukerriadensities of 20 and 40 worms.container-1. Shoot length also increased, significantly so at thehighest worm density. Increased shoot growth at higher worm densities was also reflected insignificant increases in shoot/root weight ratio at Eukerria densities of 20 and 40 worms.container.
worms / container0 10 20 40
tota
l P (m
g.L
-1)
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7(b) total phosphorus
a
ab
ab
b
A
AB
ABB
worms / container0 10 20 40
N a
s N
H4+ (m
g.L-1
)
0
1
2
3
4 (a) nitrogen as NH4+
a
ab
ab
b
nsd
illuminateddarkness
illuminateddarkness
40
Table 11Effect of different E.saltensis densities on nitrogen as NOx (nitrate/nitrite) and soluble phosphorusin overlying water of experimental containers.
Eukerriadensity / container
N as NOx (ppm) Soluble P (ppm)
cyclicalillumination
darkness cyclicalillumination
Darkness
40 2.17 2.96 0.040 0.02920 4.61 3.97 0.018 0.01710 2.71 6.10 0.004 0.0170 5.66 4.75 0.005 0.053
SE of least square mean 1.463 2.123 0.0167 0.0126significance* nsd (P > 0.05) nsd (P > 0.05) Nsd (P > 0.05) nsd (P > 0.05)
* nsd = no significant differences between treatments
Eukerria-exposed plants had emerged from the water prior to harvest, at least part of the increasedshoot weight can be attributed to photosynthesis, rather than the faster mobilisation of seedreserves. Microscopic examination of plant root systems did not yield any evidence of abrasion orother structural damage.
Figure 15. Influence of Eukerria density on rice plant shoot and root weights, shoot length andshoot/root weight ratio in experimental containers. On each graph points followed by differentletters of the same case are significantly different (P < 0.05). Error bars represent standard error ofleast squares mean. nsd, no significant differences (P > 0.05).
Seed stratification arising from Eukerria activityStratification of rice seeds in the soil column arising from Eukerria activity is shown in figure 16.After seven days exposure to worm activity, between 18 and 20% of seeds were between 0 and 2mm below the surface of the soil columns after drying. This result was significant (P < 0.05)relative to the controls at both worm densities. A smaller proportion (3 to 10%) of seeds weremore than 2 mm below the surface of the dried soil columns.
worms.container -10 20 40
dry
wei
ght (
mg)
0
1
2
3
4
5
worms.container -1
0 20 40
shoo
t len
gth
(mm
)
30
40
50
60
70
80
shoo
t / ro
ot w
eigh
t rat
io
1
2
3
4
a
b b
nsd
a
abb
A
B
B
(a) shoot and root weights (b) shoot length and shoot / root weight ratio
shoot weight root weight shoot length shoot/root ratio
41
Figure 16. Effect of different Eukerria densities on stratification of rice seeds in experimentalcontainers after soil column drying. Within each category, bars with different letters aresignificantly different (P < 0.05). Error bars represent sample standard errors.
Influence of Eukerria activity on chlorophyll a levels.Results from the analysis of chlorophyll a levels are shown in table 12. All treatments includingthe control led to chlorophyll a concentrations of over 440 µg.mL-1. There were no significantdifferences between treatments (P > 0.05)
DiscussionThe majority of studies on the interactions between oligochaetes and the rice field environmenthave been conducted using tubificids, and have been reviewed by Kurihara (1989). The feedingand tunnelling activities of tubificids transport relatively smaller soil particles to the surface(Kikuchi and Kurihara, 1977), and may suppress plant growth by transporting seeds too large toingest to deeper levels in the soil, where germination is inhibited (Kurihara, 1989). Tubificidsmodify the bacterial fauna of rice fields (Fukuhara et al., 1980), accelerate the diffusion ofdissolved substances from the soil, increase ammonium concentrations in overlying water, and canlead to increased algal production (Kikuchi and Kurihara, 1982). The results of this study showthat E.saltensis alters the rice field environment in ways that are largely consistent with some ofthe changes caused by tubificids, and that E.saltensis infestations in a heavy clay soil canindirectly affect the growth of rice plants.
0.6230.7700.9171.0641.2111.3581.5051.6521.7991.9462.0932.2402.387
% o
f see
ds
0
5
10
15
20
25
30
0 w
orm
s
20 w
orm
s
40 w
orm
s
0 w
orm
s
20 w
orm
s
40 w
orm
s
seed 0 - 2 mm belowsoil surface
seed > 2 mm belowsoil surface
a a
b b
b
ab
42
Table 12.Effect of different E.saltensis densities on chlorophyll a concentration in overlying water ofexperimental containers.
Eukerriadensity / container
chlorophyll a concentrationµg.mL-1 (± sample SE)*
40 496 (88)
20 446 (61)
10 511 (84)
0 598 (38)
significance** nsd (P > 0.05)
* based on total chlorophyll a per container divided by containerwater volume.** nsd = no significant differences between treatments
Eukerria infestations dramatically increase turbidity, which in a field situation would lead toreduced photosynthesis in submerged seedlings, and also reductions in water temperature. Underlaboratory conditions pH is also reduced by higher Eukerria densities, however rice fields oftenexperience far broader diel pH ranges than those encountered in this study (eg. Stevens et al.,1998), and it is unlikely that Eukerria-mediated changes in water pH alone would detrimentallyaffect plant growth.
Rice plants grown in the presence of Eukerria showed markedly different growth parameters tocontrol plants. Although root dry weights were not affected by Eukerria, shoot length, weight, andshoot/root weight ratio all increased significantly in response to higher worm densities. This isprobably a response to water turbidity, as the plants partition more of their available resources intoshoot elongation in order to emerge from the dirty water and begin photosynthesis. Thisinteraction between Eukerria, water turbidity and rice plants is of major significance in aerially-sown rice crops, and provides an explanation as to why plant establishment in Eukerria-infestedfields is always worst in areas of deeper water. In turbid water the fully submerged shoots ofyoung seedlings cannot photosynthesise, and growth depends on seed reserves alone. The deeperthe water, the smaller the proportion of seeds that will have adequate reserves to support growththrough to surface emergence. This situation would be further complicated by lower watertemperatures slowing plant growth, and by the disproportionately long shoots making the plantsvulnerable to uprooting by wave action.
Although adequate to demonstrate the occurrence of seed stratification, our technique of dryingsoil columns prior to assessing seed position would have led to substantial underestimation of thedepth to which seeds had been transported, since the upper layers of the soil lost a large proportionof their volume on drying. Barrion and Litsinger (1997) found that Dichogaster nr curgensis couldbury seeds 6 to 10 cm below the soil surface, and that buried seeds either decayed or producedseedlings 4 to 6 cm shorter than normal. Under field conditions seed burial by E.saltensis maywell be more severe than suggested by this study, due to water movement continually resuspendingsediment and depositing it over the seeds. Seed burial would be expected to exacerbate the effectsof deep, turbid water, since it effectively increases the amount of shoot growth necessary beforephotosynthesis can begin. Whilst D. nr curgensis was found to physically damage rice rootsthrough setal abrasion (Barrion and Litsinger, 1997), no evidence was found to support a similarmechanism of plant damage by E.saltensis. The construction of cast mounds around the stems ofrice plants by Curgiona narayani, reported as potentially affecting tillering by Kale et al. (1989),
43
has also been observed in Eukerria infested fields in NSW, but usually occurs only after crops aredrained prior to harvest.
Under conditions of cyclical illumination, nitrogen as NH4+ and total phosphorus were found to
increase significantly with increasing Eukerria densities. Increased NH4+ levels may be primarily a
consequence of ammonia excretion by the worms themselves, whilst the increased totalphosphorus levels reflect the greater sediment load in the water column at higher worm densities.Our experiments failed to demonstrate significant density-dependent changes in either nitrogen asnitrate/nitrite or soluble phosphorus in the water column. Kikuchi and Kurihara (1977) andFukuhara et al. (1980) showed that tubificids increase NH4
+ levels in overlying water, howevernitrogen as nitrate/nitrite only increases above control levels after approximately 50 days,suggesting our water analyses at 21 days may have been too early to detect Eukerria-mediatedchanges in nitrate/nitrite levels. Phosphorus as phosphate is also known to be released into thewater column by tubificids (Kikuchi and Kurihara, 1982; Kurihara, 1989) and our failure to detectdifferences in soluble phosphorus may have been caused either by an inadequate period ofincubation, or the use of a soil with inherently low levels of soluble P. Kikuchi and Kurihara(1982) found that soluble phosphate concentrations in overlying water were generally greater inthe presence of tubificids during the course of a three month experiment, but briefly droppedbelow control levels 2 to 3 weeks after the experiment was established. Analysis of chlorophyll alevels failed to show significant differences in algal growth between control containers and thosecontaining Eukerria, reflecting the minimal differences in soluble P concentrations acrosstreatments.
Although Eukerria clearly have an effect on the chemistry of water overlaying heavy clay soils,their most pronounced effect is in creating conditions of high turbidity, which cause seedlings toproduce disproportionately long shoots relative to the size of their root systems. These seedlingsare vulnerable to uprooting through wind damage, and may not survive if they cannot break thoughthe water surface before their seed reserves are exhausted. Our results suggest that maintaining thelowest possible water levels in the period immediately after sowing should assist cropestablishment by minimising the impact of water turbidity. Additional studies on chemical controland on the effect of crop rotations on Eukerria populations are required before a completeintegrated management plan for this pest can be developed.
44
3.5 Ecology of aquatic earthworms
IntroductionSubstantial progress has been made in developing an understanding of how Eukerria saltensisinteracts with rice plants and their environment, and this had led to several importantrecommendations as to how earthworm infestations can be managed in establishing rice crops(Clampett and Stevens, 1998, 1999). Progress with developing suitable chemical control strategieshas been limited, however. No chemicals have been identified that provide effective control inflooded fields, and only carbofuran has been found effective when watered into soil duringflooding. For this treatment to be effective, the surface soil layers must be capable of taking uplarge volumes of water, and application rates of over 1 kg ai ha-1 are needed. It is doubtful whethercarbofuran could be commercialised for Eukerria control, given its high level of toxicity to non-target organisms.
An alternative approach to Eukerria management involves the use of crop rotations, and a longterm study has been initiated to determine how crop rotations affect Eukerria populations. It hasbeen noted by Murray Valley growers that Eukerria infestations are particularly dense if rice isplanted immediately following a clover pasture, but that less damage occurs in subsequent ricecrops. To evaluate the validity of these observations, and make clear recommendations on croprotation strategies for Eukerria management, extensive data on population fluctuations needs to becollected and correlated with land management practices.
This study was initiated in September 1996, and is ongoing. Several more years of monitoringdata from these and other sites is required before any form of statistical analysis can beundertaken, however several important trends are becoming apparent.
Materials and methodsThe study is being conducted at ‘Archdale’, approximately 15 km NE of Deniliquin, NSW, in twoseparate fields. Site 1 had three consecutive rice crops grown on it prior to commencement of thestudy, and had been reported as supporting dense E.saltensis populations during each of the crops.Site 2 had similarly been reported as supporting large E.saltensis populations in previous ricecrops, but had been left fallow and not irrigated for three years prior to the commencement of thestudy.
A 40 x 40 m block in each of the 2 paddocks was pegged out as a sampling site. On each samplingday, 4 soil cores were taken from random points in each block. Each core was 21.5 x 21.5 cm inarea, and was excavated down to a depth of 26 cm. Cores were bagged and returned to thelaboratory for worm and cocoon extraction. Small (100 g) soil samples were taken from 40 mmbelow the surface adjacent to the site of each soil core, and placed in airtight jars for soil moisturemeasurements.
Each soil sample was divided into several 10 L plastic buckets. A small quantity of food-gradeCalgon® (amorphous sodium polyphosphate) was added to assist clay dispersal, and the bucketswere filled with water. The contents of each bucket were then stirred with a wooden paddle tobreak up the soil structure as much as possible, and the soil was then washed through a modifiedSalt-Hollick soil washing apparatus (Walker, 1983) using a garden hose. The residue (organicmatter, worms, seeds, cocoons etc.) was collected from the screens and sorted by hand. The entireresidue from each sample was sorted for worms, however due to the time consuming nature ofmanually extracting cocoons from the residue, only 25% of the sand/gravel fraction and 12.5% ofthe gross organic fraction of each sample were sorted for cocoons. The cocoon recovery valueswere then multiplied appropriately and added together to give an estimate of the total number ofcocoons present in the sample. Soil samples for moisture assessment were wet-weighed inpreweighed glass tubes, dried to constant weight at 105oC, and then weighed again to allowpercentage moisture content to be calculated.
45
Preliminary ResultsEukerria populations, cocoon densities and sub-surface soil moistures at Site 1 are shown in figure 17.
Figure 17. Eukerria populations and sub-surface soil moisture at Site 1 (dryland cereal croppingfollowing 3 consecutive rice crops). Error bars represent standard errors.
At the onset of monitoring after three consecutive rice crops, relatively few earthworms werepresent, and earthworm populations have not exceeded 8 worms.sample-1 at any time during themonitoring period. Cocoon densities were extremely high after the final crop (63 cocoons.sample-
1), but progressively declined to around 1 cocoon.sample-1 over a 2 ½ year period of drylandcropping.
Figure 18 shows the corresponding data for Site 2, which had a 3 year fallow prior toreturning to rice. Despite the long fallow period, worm populations rose to 51worms.sample-1 shortly after flooding the first rice crop in 1996. By late in the seasonworm populations had fallen, but cocoon densities had increased. During the winter wormand cocoon densities fell, but increased in the spring in response to increasing soilmoisture levels. By the middle of the second rice crop cocoon densities had increased to148 cocoons.sample-1. Cocoon densities have fallen since the conclusion of the 1997/8rice season.
Sep
tem
ber
Oct
ober
Nov
embe
rD
ecem
ber
Janu
ary
Feb
ruar
yM
arch
Apr
ilM
ayJu
neJu
lyA
ugus
tS
epte
mbe
rO
ctob
erN
ovem
ber
Dec
embe
rJa
nuar
yF
ebru
ary
Mar
chA
pril
May
June
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ust
Sep
tem
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embe
rD
ecem
ber
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ary
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yM
arch
Apr
ilM
ay
soil
moi
stur
e (%
)
0
4
8
12
16
20
24
wor
ms
or c
ocoo
ns p
er s
ampl
e
0
40
80
120
160
1996 1997 1998 1999
barley stubblestubbleoats barley stubble
soil m oistureworm s.sam ple -1
cocoons.sam ple -1
46
Figure 18. Eukerria populations and sub-surface soil moisture at Site 2 (returning to rice after a3 year fallow). Error bars represent standard errors.
DiscussionThe results of this study to date show several important trends, but are also somewhatcontradictory. The data from site one shows that when land is taken away from rice andput into dryland cereal cropping Eukerria populations drop progressively over time. Wormdensities decline rapidly, however cocoon densities fall much more slowly, indicating thatEukerria survives periods of soil dryness primarily in the egg stage. After 2½ years of lowsoil moistures, cocoon populations are reduced to negligible levels. This would suggestthat long periods without irrigation can reduce Eukerria populations to the point where atleast a single rice crop should be able to establish without significant damage.
This theory is contradicted by the data from Site 2. After a three year fallow wormpopulations rose dramatically as soon as this site was inundated for rice production.Despite high worm populations during the first rice crop at this site, large increases incocoon densities were not recorded until midway through the second rice crop, a seasonin which worm populations remained relatively low. Although this study has demonstratedsome important aspects of Eukerria biology, notably that survival in dry soil relies primarilyon dormant eggs, further data from these and other sites are required to clarify theinfluence of crop rotations on Eukerria populations and resolve apparent contradictionsbetween the two sites. Significant data is anticipated when Site 1 either returns to rice oris used for irrigated pastures. The shortage of water in the Murray Valley in recentseasons has delayed this transition.
Sep
tem
ber
Oct
ober
Nov
embe
rD
ecem
ber
Janu
ary
Febr
uary
Mar
chA
pril
May
June
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ust
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ber
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ober
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embe
rD
ecem
ber
Janu
ary
Febr
uary
Mar
chA
pril
May
June
July
Aug
ust
Sep
tem
ber
Oct
ober
Nov
embe
rD
ecem
ber
Janu
ary
Febr
uary
Mar
chA
pril
May
soil
moi
stur
e (%
)
0
4
8
12
16
20
24
28
32
wor
ms
or c
ocoo
ns p
er s
ampl
e
0
40
80
120
160
200
240
280
320
1996 1997 1998 1999
stubbleRICEbare barley stubbleRICE
soil moistureworms.sample -1
cocoons.sample -1
47
4. Discussion of results compared with objectives
The results of this project have satisfied all the project objectives in terms of improving thesustainable control of bloodworms in rice. Commercial evaluation of fipronil seed treatments wasundertaken during the first year of the project, and its successful completion has led to theregistration of fipronil (Cosmos®) for bloodworm control in rice. Growers now have aenvironmentally acceptable seed treatment available for bloodworm control for the first time sinceaerial operators placed a ban on the use of malathion for this purpose. The introduction of fipronilhas meant that active chemical inputs required for bloodworm control have been reduced from 375grams.hectare-1 down to 87.5 grams.hectare-1, a reduction of almost 77%.
Although poor bloodworm activity in the 1997/8 season prevented any firm conclusions beingdrawn about the efficacy of direct-to-water fipronil treatments, the combined data onalphacypermethrin from the 1997/8 and 1998/9 trial seasons indicates that alphacypermethrin willeffectively control bloodworms at a rate of approximately 10 grams active.hectare-1. Thisdiscovery may allow even greater reductions in chemical inputs for bloodworm control, combinedwith improved sustainability arising from the tendency of this material to bind strongly to the soilsurface and not be lost into drainage systems.
The data obtained on the colonisation of rice fields by different bloodworm species represents thefirst steps towards understanding the community ecology of bloodworms within the riceenvironment. This understanding is long overdue, a situation brought about by difficultiesassociated with identifying the larval stages of different bloodworm species that have only recentlybeen overcome.
All the proposed work associated with the ecology and control of Eukerria saltensis has beencompleted, however a lesser degree of success has been achieved than with the bloodwormresearch. Studies on interactions between Eukerria and the ricefield environment, begun as part ofDAN137A, have been successfully completed, and the knowledge generated has allowed severalpractical recommendations to be made on how farmers may limit the damage caused by thisincreasingly important pest. Ecological studies have commenced that will ultimately allowrecommendations to be made on how crop rotations may be used to suppress Eukerria populationsprior to rice cultivation. In regard to chemical control, however, no substantial breakthroughs havebeen made. Carbofuran at rates of 1 kg active.hectare-1 and above watered into dry soil appears toreduce Eukerria activity, however complete control is not obtained, and the problem must beanticipated prior to flooding, rather than responded to afterwards. The high application raterequired and the high non-target toxicity of carbofuran make its future registration unlikely. Nochemical has been identified that will reliably control Eukerria in flooded fields.
48
5. ImplicationsThe results of this study have several important implications for rice production in NSW. Firstly,the registration of fipronil has provided an alternative for growers to paired sprays of chlorpyrifosfor bloodworm control. The use of fipronil as a seed treatment has the potential to remove anaerial chemical application from most NSW rice crops, allaying community concerns about aerialspraying and the risk of spray drift. The low application rate of fipronil means that rice crops cannow be grown with 77% less active bloodworm control chemical than 6 years ago – in an averageseason, this means over 30 tonnes of active insecticide won’t be entering the environment. Ifcommercial trials of alphacypermethrin are successful and registration for bloodworm control isobtained, the reduction in insecticide use could be as high as 94.7% over a 7 year period. This willprovide both economic benefits fro growers and environmental benefits for the wider community.
The bloodworm ecology study reported here will continue until substantially more data has beenobtained from both experimental and commercial crops. Ultimately, this data will form thefoundation for investigations on bloodworm species other than Chironomus tepperi, in an effort todetermine which are involved in crop damage. Whilst the ecology of C.tepperi and itssusceptibility to pesticides are well understood, several of the other common species present in ricefields are implicated in crop damage, however the identity of these species and their susceptibilityto control measures remains unknown.
Elucidating the ways in which Eukerria saltensis impacts on establishing rice plants allows severalrecommendations to be made on how Eukerria infestations can best be managed. Theserecommendations, that involve reducing the slope on lasered paddocks to obtain uniform shallowwater, and flooding and sowing smaller paddock layouts, are already being promulgated throughthe annual Rice crop Protection Guides, and will allow growers to minimise crop damage while thesearch for additional control measures continues.
49
6. RecommendationsBloodworm ecology and control1. The results of this study indicate that alphacypermethrin represents a viable and more
sustainable alternative to chlorpyrifos for bloodworm control in rice. Commercial scale trialsshould be conducted as soon as possible, subject to obtaining an appropriate permit from theNational Registration Authority for Agricultural and Veterinary Chemicals. Fipronil (as a seedtreatment only) has already achieved full commercial registration.
2. Additional trials of fipronil as a direct-to-water spray are justified due to the known efficacy ofthis material when used as a seed treatment, and because if its low environmental impact.
3. Support should be given for further studies on the colonisation of rice fields by bloodworms.Data from at least three more seasons needs to be collected to ensure that the full range ofvariability is documented in the final analyses.
Aquatic earthworm biology and control1. The labour-intensive nature of field experiments involving Eukerria saltensis has slowed
progress with studies on the ecology and control of this important pest. Sample processing andsorting to accurately quantify worm populations is extremely slow, and there is little that canbe done to alleviate this problem, at least in regard to the ecological studies. The developmentof a laboratory bioassay technique that reflects field conditions would, however, substantiallyexpedite the chemical control program, and should be supported.
50
7. Appendices7.1 Appendix 1 – Publications arising
BooksSTEVENS, M. M. 1997. Common Invertebrates of New South Wales Rice Fields. Biology, Pest
Status and Control. (NSW Agriculture / Rural Industries Research and DevelopmentCorporation), 55 pp
Refereed Journal PapersSTEVENS, M. M., HELLIWELL, S. and WARREN, G. N. 1998. Fipronil seed treatments for the
control of chironomid larvae (Diptera: Chironomidae) in aerially-sown rice crops.Field Crops Research, 57, 195-207.
HELLIWELL, S. and STEVENS, M. M. 20 . Efficacy and environmental fate ofalphacypermethrin applied to rice fields for the control of chironomid midge larvae(Diptera: Chironomidae). Field Crops Research, (submitted).
STEVENS, M. M. and WARREN, G. N. 20 . Laboratory studies on the influence of Eukerriasaltensis (Beddard) (Oligochaeta: Ocnerodrilidae) on overlying water quality andrice plant establishment. International Journal of Pest Management (submitted).
Conference PresentationsSTEVENS, M. M. 1996. Strategies to reduce the environmental impact of current chironomid
management practices in Australian rice fields. INVITED PAPER, XX InternationalCongress of Entomology, Florence, Italy (abstract 11-045, p. 349).
STEVENS, M. M. and HELLIWELL, S. 1998. Fipronil for the control of chironomid larvae inestablishing rice crops. IV International Congress of Dipterology, Oxford, UK pp.219-220.
STEVENS, M. M. 1999. Environmental aspects of rice production in Australia. KEYNOTEPAPER, 3rd International Environmental Meeting, Shinshu/Aspen CulturalExchange Committee on the Environment, Nagano, Japan.
HELLIWELL, S. and STEVENS, M. M. 20 . Efficacy and environmental fate ofalphacypermethrin applied to rice fields for the control of chironomid midge larvae.2nd Temperate Rice Conference, Sacramento, California, USA (in press).
CLAMPETT, W. S., LEWIN, L. G, WILLIAMS, R. L., BATTEN, G., BEECHER, H. G, LACY, J.M., FITZGERALD, M. and STEVENS, M. M. 1999. An overview of temperate riceproduction, technology and development in New South Wales, Australia. 2ndTemperate Rice Conference, Sacramento, California, USA (in press).
Extension PublicationsCLAMPETT, W. S. and STEVENS, M. M. 1996. Rice Crop Protection Guide 1996. NSW
Agriculture, 8pp.
CLAMPETT, W. S. and STEVENS, M. M. 1997. Rice Crop Protection Guide 1997. NSWAgriculture, 12 pp.
CLAMPETT, W. S. and STEVENS, M. M. 1998. Rice Crop Protection Guide 1997. NSWAgriculture, 12 pp.
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STEVENS, M. M. 1998. Sustainable control of bloodworms and aquatic earthworms in rice.IREC Farmers' Newsletter (Large Area), 150, 26-28.
STEVENS, M. M. 1998. Cosmos® seed treatment offers greater flexibility for bloodwormcontrol. Advances in Rice 1:9-10.
CLAMPETT, W. S. and STEVENS, M. M. 1998. Rice Crop Protection Guide 1998. NSWAgriculture, 12pp
STEVENS, M. M. 1998. Better techniques for managing bloodworms and aquatic earthworms.IREC Farmers' Newsletter (Large Area), 152, 22-23.
STEVENS, M. M. 20 . Alphacypermethrin - a future alternative for bloodworm control? IRECFarmers' Newsletter (Large Area) (in press).
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ANON, 1985. Black giant earthworms attackingBenguet rice terraces. Animal Husbandry andAgriculture Journal, 19, 25.
BARRION, A. T. and LITSINGER, J. A. 1997.Dichogaster nr. curgensis Michaelsen (Annelida:Octochaetidae): An earthworm pest of terracedrice in the Philippine Cordilleras. CropProtection, 16, 89-93.
BLACKWELL, P. S. and BLACKWELL, J. 1989.The introduction of earthworms to an ameliorated,irrigated duplex soil in south-eastern Australia andthe influence on macropores. Australian Journalof Soil Research, 27, 807-814.
CLAMPETT, W.S. and STEVENS, M.M. 1998.Rice Crop Protection Guide 1998. NSWAgriculture, 12pp.
CLAMPETT, W.S. and STEVENS, M.M. 1999.Rice Crop Protection Guide 1999. NSWAgriculture, 12 pp.
CLARK, J. R., GOODMAN, L. R., BORTHWICK,P. W., PATRICK Jr, J. M., CRIPE, G. M.,MOODY, P. M., MOORE, J. C. and LORES, E.M. 1989. Toxicity of pyrethroids to marineinvertebrates and fish: a literature review and testresults with sediment-sorbed chemicals.Environmental Toxicology and Chemistry, 8, 393-401.
CLEMENT, S. L., GRIGARICK, A. A. and WAY,M. O. 1977. The colonization of California ricepaddies by chironomid midges. Journal ofApplied Ecology, 14, 379-389.
COCCHI, G. F. 1966. Ricerche sui DitteriChironomidi dannosi al riso nella BassaBolognese. Bolletino Osservatorio malattiepinate Bologna, 1, 39-66.
CRANSTON, P. S. 1996. Identification guide to theChironomidae of New South Wales. AWTIdentification Guide Number 1. (Sydney:Australian Water Technologies), 376 pp.
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FUKUHARA, H. 1987. Effect of tubificids andchironomids on particle redistribution of lakesediment. Ecological Research, 2, 255-264.
FUKUHARA, H., KIKUCHI, E. and KURIHARA,Y. 1980. The effect of Branchiura sowerbyi(Tubificidae) on bacterial populations insubmerged ricefield soil. Oikos, 34, 88-93.
GARDNER, W. S., NALEPA, T. F., QUIGLEY, M.A. and MALCYZK, J. M. 1981. Release ofphosphorus by certain benthic invertebrates.Canadian Journal of Fisheries and AquaticSciences, 38, 978-981.
JAMIESON, B. G. M. 1970. A taxonomic revisionof the oligochaete genus Eukerria Michaelsen,1935. Bulletin of the British Museum (NaturalHistory), Zoology. 20, 134-172.
JOHNSTON, E. J. 1953. Pedology of theDeniboota Irrigation District, New South Wales.Soil Publication No. 1. (Melbourne: CSIRO), 55pp.
KALE, R. D., BANO, K., SECILIA, J. andBAGYARAJ, D. J. 1989. Do earthworms causedamage to paddy crop? Mysore Journal ofAgricultural Science, 23, 370-373.
KIKUCHI, E. and KURIHARA, Y. 1977. In vitrostudies on the effects of tubificids on thebiological, chemical and physical characteristicsof submerged ricefield soil and overlying water.Oikos, 29, 348-356.
KIKUCHI, E. and KURIHARA, Y. 1982. Theeffects of the oligochaete Branchiura sowerbyiBeddard (Tubificidae) on the biological andchemical characteristics of overlying water andsoil in a submerged ricefield soil system.Hydrobiologia, 97, 203-208.
KURIHARA, Y. 1989. Ecology of somericefields in Japan as exemplified by some benthicfauna, with notes on management. InternationaleRevue der gesamten Hydrobiologie, 74, 507-548.
KURIHARA, Y., and KIKUCHI, E. 1988. Use oftubificids for weeding and aquaculture in paddyfield in Japan. Journal of Tropical Ecology, 4,393-401.
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MARTIN, J., KUVANGKADILOK, C., PEART, D.H. and LEE, B. T. O. 1980. Multiple sexdetermining regions in a group of relatedChironomus species (Diptera: Chironomidae).Heredity, 44, 367-382.
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PETTIGROVE, V., KORTH, W., THOMAS, M.,and BOWMER, K. H. 1995. The impact ofpesticides used in rice agriculture on larvalchironomid morphology. pp. 81-88 in: Cranston,P. (Ed.), Chironomids: From genes to ecosystems(CSIRO: Melbourne).
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SAA, 1977. Determination of nitrate and nitrite inwaters. Australian Standard 2029-1977.(Standards Association of Australia: Sydney),15pp.
STEVENS, M.M. 1992. Toxicity oforganophosphorus insecticides to fourth-instarlarvae of Chironomus tepperi Skuse (Diptera:Chironomidae). Journal of the AustralianEntomological Society 31, 335-337.
STEVENS, M.M. 1993. Acute toxicity of syntheticpyrethroids to final instar larvae of the ricebloodworm, Chironomus tepperi Skuse (Diptera:Chironomidae). General and AppliedEntomology, 25, 61-64.
STEVENS, M. M. 1994. Emergence phenology ofChironomus tepperi Souse and Procladiuspaludicola Souse (Diptera: Chironomidae) duringrice crop establishment in southern New SouthWales. Australian Journal of ExperimentalAgriculture, 34, 1051-1056.
STEVENS, M.M., and WARREN, G.N. 1992.Insecticide treatments used against a ricebloodworm, Chironomus tepperi (Diptera:Chironomidae): suppression of larval populations.Journal of Economic Entomology, 85, 1606-1613.
STEVENS, M. M., HELLIWELL, S. andWARREN, G. N. 1998. Fipronil seed treatmentsfor the control of chironomid larvae (Diptera:Chironomidae) in aerially-sown rice crops. FieldCrops Research, 57, 195-207.
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