Directed sprays and liquid baits to manage ants in vineyards and citrus groves

Directed Sprays and Liquid Baits to Manage Ants inVineyards and Citrus Groves1

John H. Klotz, Michael K. Rust, Daniel Gonzalez, Les Greenberg,Heather Costa, Phil Phillips,2 Carmen Gispert,3 Donald A. Reierson, and

Ken Kido

Department of Entomology, University of California at Riverside,Riverside, California 92521

J. Agric. Urban Entomol. 20(1): 31–40 (January 2003)ABSTRACT Chemical treatments were evaluated for efficacy against spe-cific ant pests in two perennial field crop systems. A directed spray of Lorsbanapplied in grape vineyards resulted in significant reductions of Formica per-pilosa Wheeler (Hymenoptera: Formicidae) for 9 wk. Significant reductions inthe number of foraging Argentine ants, Linepithema humile (Mayr) (Hy-menoptera: Formicidae), were obtained in citrus for the 4-mo duration of a testwith a liquid bait delivery system containing either 0.0001% fipronil or thia-methoxam in 25% sucrose water.

KEY WORDS Hymenoptera, Formicidae, Linepithema humile, Formicaperpilosa, Argentine ants, ant baits, residual spray

The Argentine ant, Linepithema humile (Mayr) (Hymenoptera: Formicidae) isa significant invasive pest species of urban, agricultural, and natural environ-ments (Vega & Rust 2001). It is common in California orange orchards and has foryears been especially numerous in Los Angeles and Riverside Counties (Horton1918). In the Ojai Valley (Ventura County, California), Argentine ants attaintremendous population levels in citrus. In San Diego County, California, Markin(1967) estimated that 50,000 to 600,000 foraging ants could ascend an individualcitrus tree in a single day.

Unlike the Argentine ant, the status of Formica perpilosa Wheeler (Hy-menoptera: Formicidae) as a pest is not documented. Indigenous to the UnitedStates, this ant ranges from central Texas, Oklahoma, and Kansas westward toCalifornia (Wheeler & Wheeler 1986). In the arid southwest, it nests around theroots of bushes or trees near streams (Wheeler & Wheeler 1986). It is thereforeideally suited to infest the irrigated soils of vineyards in the Coachella Valley(Riverside County, California), where it nests at the base of grapevines in closeproximity to its major honeydew source, the vine mealybug, Planococcus ficus(Signoret) (Homoptera: Pseudococcidae).

Ants reduce the effectiveness of biological control by their cultivation andprotection of homopteran pests from their predators and parasitoids (Prins et al.

1Accepted for publication 24 July 2003.2University of California Cooperative Extension, Ventura, California 93003.3University of California Cooperative Extension, Indio, California 92201.

31

1990). Homopteran populations grow larger and more stable, and their rate ofdispersal increases when ants are present (Hölldobler & Wilson 1990). A strategythat weakens or breaks the mutualistic association between ants and homopter-ans can thus have major consequences for pest management. For example, re-ducing the number of ants tending and protecting the homopterans will leave thelatter vulnerable to predation and parasitism, thereby reducing their populationas well (Moreno et al. 1987).

Chemical control of ants is primarily accomplished by the application of re-sidual insecticides or use of toxic baits (Hedges 1997). Unless properly applied,however, residual insecticides are ineffective (Rust et al. 1996) and kill a signifi-cant proportion of beneficial organisms (Smith et al. 1996). To overcome thisproblem in grapes, Phillips & Sherk (1991) used a hand-held compressed airsprayer to direct insecticide to the base of vines where the ants were concentratedwhile avoiding application to the foliage, where beneficial organisms are found.This approach was effective, but it was difficult for growers to treat large areasusing this method. Therefore, our first objective was to develop a practical methodto apply residual insecticides in a target-specific manner to control F. perpilosa invineyards.

Moreno et al. (1987) used residual insecticides to control Argentine ants incitrus. In combination with skirt-pruning, they applied chlorpyrifos or diazinon asa barrier on the trunk or on the ground around trees. Tree banding with repel-lents or sticky substances can also prevent Argentine ants from tending honey-dew-producing homopterans (James et al. 1995; Shorey et al. 1996). Becausethese strategies are labor-intensive and not practical or cost-effective, growershave avoided using them. Another drawback of residual insecticides is that theykill foraging ants but not reproductives nor the majority of workers in the nest(Knight & Rust 1990, 1991). The use of baits would be practical and more effectiveapproach to ant control in citrus.

An effective liquid bait reduces the need for broadcast sprays, thereby decreas-ing the amount of insecticide used to control ants (Hooper-Bui & Rust 2001). Aslow-acting toxin in a highly attractive bait may potentially be distributedthroughout the colony by exploiting the recruitment and food-sharing behavior ofants. Bait stations reduce the exposure of the crop and laborers to insecticide,eliminate the need for food tolerances, and do not interfere with biological control.We believe that low-toxicity baits delivered in stations would provide growerswith a powerful and safe method to control ants. Therefore, the second objectivewas to evaluate the efficacy of liquid baits in reducing the number of Argentineants in citrus groves.

Materials and Methods

Test sites. Four growers in the Coachella Valley (Riverside County, Califor-nia) and one in the Ojai Valley (Ventura County, California) provided us withacreage of table grape vineyards and Valencia orange trees, respectively. Bothsites are located inland in southern California, however, the hot, arid climate ofCoachella contrasts with the more moderate conditions in Ojai.

Experimental design. Each of the four growers in the Coachella Valleyprovided a 2.8-ha block of vineyards. Each block was divided into four 0.7-ha plotswhere the two treatments, insecticide spray, and untreated control were ran-

32 J. Agric. Urban Entomol. Vol. 20, No. 1 (2003)

domly assigned, providing two replicates of each per farm. Each plot consisted of24 rows with approximately 50 vines per row.

In the Ojai Valley, we were provided 15.8 ha of orange trees. Two different baitapplications and untreated controls were randomly assigned to 12 0.6-ha plotssuch that each treatment was replicated four times. Each plot consisted of a 9 ×12 block of trees. An additional seven rows of trees served as a buffer zone be-tween adjacent plots.

Monitoring. To estimate ant numbers in vineyards, we set 18 pitfall trapsthrough the center three rows of each plot with six traps per row. Outer rows ofeach side of the sampled area served as a buffer zone between treatments. Trapswere made from 250-ml plastic cups containing approximately 50 ml of antifreeze.Traps were set in the soil so that the top of the cup was flush with the groundsurface. A plywood cover was set above the top of the trap to prevent dirt anddebris from falling in but leaving enough space for ants to crawl under and fallinto the trap. Traps were left for 24 h, after which the number of ants caught wascounted.

In citrus, sucrose water monitoring tubes for Argentine ants were constructedaccording to specifications described by Klotz et al. (2000). Tubes were filled with25% sucrose water, weighed on an electronic top-loader scale (model GT4100,OHAUS Corp., Florham Park, New Jersey) and taped to the trunks of nine treeslocated in the center of each plot. To correct for evaporative water loss frommonitoring tubes, two additional tubes were filled with 25% sucrose water,weighed and each placed in a tree in the center of each plot. To prevent ants fromfeeding at control tubes, the tubes were suspended from a branch on a stringcoated with Stikem Special (Seabright, Emeryville, California). After 24 h, thetubes were collected and reweighed. Consumption of sucrose water from moni-toring tubes by the ants was obtained by correcting for evaporative water lossfrom the control tubes. Consumption is directly related to the number of ants thatvisit a monitoring tube (Reierson et al. 1998), thus providing an estimate of antnumbers. Estimates of ant numbers were made at all sites both before and aftertreatments on a biweekly basis.

Insecticide treatments. For ant control in vineyards, two applications of aninsecticide were made early in the growing season. Chlorpyrifos (Lorsban-4E,Dow AgroSciences, Indianapolis, Indiana) was applied with a sprayer mounted onan all terrain vehicle (Fig. 1) at the label rate of 2.35 l per hectare (2.0 pints/acre)on February 23, and March 15, 2000. Spray nozzles attached at ends of the boomwere directed below the leaf canopy at the base of the vines where ant nests werelocated. Because the cultivated region between vine rows was found to have littleor no ant activity, the spray-rig was designed to target nests and foraging antsconcentrated in the soil berm at the base of the vines.

Bait treatments in citrus consisted of liquid baits containing 0.0001% technicalthiamethoxam (Syngenta, Greensboro, North Carolina) or technical fipronil(Aventis, Montvale, New Jersey) in 25% sucrose water. Baits were deliveredin ≈320-ml capacity AdvanceTM ACE Stations (Whitmire Micro-Gen ResearchLaboratories, Inc., St. Louis, Missouri). The stations were placed on the groundalong the irrigation lines at every fourth tree in a row (27/plot). Stations wereremoved from treated plots before monitoring and replaced and filled with freshbait afterwards. Control plots were not baited.

33KLOTZ et al.: Targeted Ant Control in Groves and Vineyards

Statistical methods. Using Systat statistical software (Systat 1999) a re-peated measures analysis of variance (ANOVA) was performed on each data set(ant counts in pitfall traps in vineyards and consumption of sucrose water incitrus groves) before and after insecticide treatment. We were primarily inter-ested in the main treatment effect, which compares the height of the treatmentprofiles over time. Furthermore, the “simple effects” of treatments are the sepa-rate ANOVAS at each sample date (Keppel 1991). These simple effects can pin-point which dates had significant differences among the treatments.

Vineyards. There were two pretreatment and nine post-treatment counts forants caught in pitfall traps in vineyards. We did separate repeated measureANOVAS on the pre- and post-treatment data. We reasoned that pretreatmentdata should show no differences between treatment conditions, while post-

Fig. 1. Spray-rig for ant control: (1) 4’ spray boom mounted in front of ATV; (2)1.5� adjustable extension sleeve secured by two bolts; (3) 6” steel tubing(1” diameter) with v-shaped welding rod in front. This unit is attached to(2) by a hinge and spring, which allows it to fold back when it hits anobject; (4) two spray nozzles attached to each end of the boom. The nozzlesare controlled by independent ON-OFF valves; (5) pressure valvemounted between two control valves; (6) 25 gallon fiber-glass tankequipped with electric pump mounted in back of ATV.


treatment data should show differences in the heights of the profiles, and thatcombining them into one data set would confound the interpretation of the re-sults.

The data were plotted to check for deviations from normality. A logarithmictransformation gave the best visual fit to a normal distribution by removing muchof the skewness apparent in the data. In the repeated measures design we con-sidered the four vineyards and the two treatments as non-repeating fixed factors,and trials over time as the repeated factor. There was significant variabilityamong vineyards; in using vineyards as a fixed factor we are describing whathappened at these specific locations (Wilkinson et al. 1996). We would need tosample many more vineyards to generalize about them. In all cases replicateswithin vineyards were not a significant source of variability, so we pooled thatterm with the model’s error term (Sokal & Rohlf 1969), which was then used forthe tests of significance.

Citrus. There were four pre- and seven post-treatment measurements of su-crose water consumption by ants in citrus. These data were plotted, and we foundthat a square root transformation gave the best visual fit to a normal distribu-tion. As in the above, we did separate repeated measure ANOVAS on the pre- andpost-treatment data. In the repeated measures design we considered treat-ment as the nonrepeated factor and trials over time as the repeated factor.Furthermore, the four plots for each treatment were a random variable nestedwithin treatments. The mean squares of this variable were used as the model’serror team for the treatment effect. The first post-treatment data (July 13)were excluded from the analysis because the treatments did not have time to befully effective at this date. Subsequent to that date the treatment profiles did notcross.

Results

Vineyards. Figure 2 shows the pooled results for these trials. First we ana-lyzed the two pretreatment dates. For the main treatment effect the ANOVA wasnot significant (F � 0.4; df � 1, 280; P > 0.5). For the nine post-treatment dates,the ANOVA for the treatment factor was highly significant (F � 40.8; df � 1, 279;P < 0.00001). This result means that there was a difference in average height ofthe treatment profiles (the control and the Lorsban) where height is a measure ofthe number of ants in pitfall traps. The “simple” effects for the treatment variableare the ANOVAS for each date, which contrast the control and Lorsban. TheseANOVAS of the simple effects show that for each sample date from March 9through May 18 the number of ants in pitfall traps was significantly lower in theLorsban-treated areas. From June 1 through the end of the experiment on June29, there was no significant difference between the treated and untreated areas.

Citrus. Figure 3 shows the results for these trials. The decline in sucrosewater consumption on several of the monitoring dates was due to irrigation or lowtemperature, which reduced ant foraging. First we analyzed the four pre-treatment dates. For the main treatment effect the ANOVA was not significant (F� 0.61; df � 2, 9; P � 0.6). Therefore, there were no differences in the heightprofiles of the control or treatments.

For the last six post-treatment data samples the ANOVA for the treatmentfactor was significant (F � 11.9; df � 2, 9; P � 0.003). Multiple comparisons


among the treatment means (with Bonferroni correction) show that the controlprofile is significantly higher than that of the thiamethoxam (F � 11.9; df � 1, 9;P � 0.02) or the fipronil (F � 22.2; df � 1, 9; P � 0.003). Furthermore, thefipronil and thiamethoxam profiles do not differ significantly (F � 1.6; df � 1, 9;P � 0.75).

The “simple” effects for the treatment variable are the ANOVAS for each date,which contrast the control, fipronil, and thiamethoxam. From August 9 untilSeptember 20, the control and treatment profiles were significantly different. OnOctober 4, the treatment means are still different. However, an unusually largevariability among the replicates within plots that we are using for the ANOVA’serror term prevented us from obtaining a significant result for that date (P �0.09).

Discussion

The species composition of the ant–homopteran–plant association in the twocropping systems is different, but they share similar characteristics that allow forchemically focused ant treatments. In both systems, ants nest at the base ofplants and ascend into the canopy to forage. In the hot, arid climate of the Coach-ella Valley, F. perpilosa build their nests in the shade below the vine canopywhere drip irrigation provides them with water. The majority of ants trappedwere F. perpilosa, with a few Solenopsis spp.; however, this study only refers to

Fig. 2. Natural logarithm of the mean number of ants per pitfall trap in vine-yards.


the former species, which is the primary pest. In the citrus grove, each tree isassociated with its own nest of L. humile, usually located on the south side(Markin 1968). The ants ascend the trunk into the canopy on a single well-definedtrail to tend citrus mealybugs, Planococcus citri (Risso) and various species ofscale (Markin 1970a).

Our chemical treatments in the two cropping systems were designed to inter-cept the foraging traffic into the canopy. In vineyards, insecticide spray wasdirected below the foliage to create an insecticidal barrier on the vine trunks(Phillips & Sherk 1991). In the citrus grove, the bait stations were placed adjacentto irrigation lines, which the ants use as guidelines for travel. Use of baits anddirected sprays not only minimizes pesticide drift, but also reduces potentialresidual deposits of insecticides on the foliage (J. H. K., unpublished data), whichcould negatively impact biological control agents.

Residual insecticides like Lorsban are effective against foraging ants but donot eliminate the colony. Consequently, ants return when the residual insecticidedegrades, which can occur within weeks in the hot, dry conditions in the Coach-ella Valley. However, if properly timed, applications of Lorsban early in the sea-son can provide a significant reduction of ants for nine weeks. In spring, the antcolonies are undergoing rapid growth, and acquisition of an adequate food supplyis critical to their success. By eliminating many of the foraging ants we were ableto inhibit colony growth resulting in low numbers of ants later. With the regula-tory discontinuance of organophosphate and carbamate insecticides, however,

Fig. 3. Sucrose water consumption by Argentine ants in citrus. Numbers aremean values of sucrose water consumed during a 24-h monitoring period.


alternative treatments are necessary. Baiting is the preferred method to controlF. perpilosa, but unfortunately this species will not forage most granular fire antbaits or liquid Argentine ant baits (M. K. R., unpublished data), requiring theneed to use sprays for control and pitfall traps for monitoring. Additional researchon the feeding preferences of F. perpilosa is needed.

Similarly, for L. humile, the application of baits in citrus should commence inlate February to early March when the new seasonal life cycle of Argentine antsbegins (Markin 1970b). The unicoloniality of Argentine ants makes them particu-larly amenable to bait treatment. Markin (1968) estimated that in 5 d the ex-change of ants between neighboring nests is >50% of the worker population.Therefore, a toxic bait consumed by ants from one nest in a citrus grove will begradually spread into surrounding nests. A liquid bait formulation is ideal forArgentine ants because 99% of the food brought into the nest is in liquid form(Markin 1970c). Water is a good carrier because during the hot dry months ofAugust, September, and October, Argentine ants recruit to it (Markin 1970a), andwhen combined with sucrose it becomes a highly attractive bait to Argentine ants(Baker et al. 1985). Sucrose is a primary component of honeydew (Auclair 1963),which is the dietary staple of Argentine ants (Markin 1970a). Fipronil and thia-methoxam were chosen as bait toxicants due to their excellent performance inlaboratory tests. Hooper-Bui & Rust (2000) obtained 100% queen kill in Argentineants with 0.0001% fipronil in 25% sucrose water. In field tests in Chilean citrusgroves, Ripa et al. (1999) used a dye to measure the distance and speed of dis-persion of this bait. At 72 h, the bait was found 27 m away from the feeding site.

In conclusion, directed sprays and baits can be used effectively to reduce thenumber of ants in vineyards and citrus orchards, and are less likely to kill ben-eficial organisms than full canopy treatments. Our goal is to develop bait formu-lations and delivery systems that are more effective against ants and less dis-ruptive to the environment than directed insecticidal sprays. In addition, to befeasible as a cost-effective pest management strategy, we must also demonstratethat reductions in ant populations through bait technologies will ultimately re-duce the number of pest homopterans in these agricultural systems.

Acknowledgment

We thank growers in the Coachella and Ojai valleys for providing the experimental sitesto conduct our research and J. Hampton-Beesley for her assistance in the laboratory. Wethank Dr. Robert Beaver, Department of Statistics, UC Riverside, for his statistical advice.We appreciate the invaluable assistance in the field of Rosa Maria and Gloria Ibarra in theCoachella Valley and John Rodgers and Coralie Clement in the Ojai Valley. We also thankthe California Citrus Research Board, Department of Pesticide Regulation, Desert GrapeAdministrative Committee, and the Table Grape Commission for their financial support ofthis study.

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Directed sprays and liquid baits to manage ants in vineyards and citrus groves