EFFICACY OF Steinernema karii AND Heterohabditis. Indica ... · 1 EFFICACY OF Steinernema karii AND Heterohabditis. Indica NEMATODES IN THE MANAGEMENT OF THE SWEET POTATO WEEVIL (

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EFFICACY OF Steinernema karii AND Heterohabditis. Indica NEMATODES IN THE MANAGEMENT OF THE SWEET POTATO

WEEVIL ( Cylas puncticollis) IN KIBWEZI

A THESIS SUBMITED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR

THE AWARD OF A MASTER OF SCIENCE DEGREE IN CROP PROTECTION IN THE

FACULTY OF AGRICULTURE, UNIVERSITY OF NAIROBI

By

MARTHA M. SILA

B.SC (Hons) Agriculture and Home Economics EGERTON UNIVERSITY

2004

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DECLARATION I declare that the work presented in this thesis is my own, and apart from the acknowledged assistance, is a record of my own research. The material has never been submitted for a degree in any other university or any other establishment for an academic award. Signed………………………………………….Date……………………………… M.M. SILA UNIVERSITY OF NAIROBI, DEPARTMENT OF CROP PROTECTION,

Supervisors This thesis has been submitted for examination with our approval as supervisors PROFESOR J.H. NDERITU, UNIVERSITY OF NAIROBI, DEPARTMENT OF CROP PROTECTION, Signed………………………………………….Date………………………………. DR. G.H.N. NYAMASYO, UNIVERSITY OF NAIROBI, DEPARTMENT OF ZOOLOGY, Signed………………………………………….Date……………………………….

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DEDICATION

To my son James, parents, brothers and sisters for their prayers, understanding and support.

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ACKNOWLEDGMENT

I want to thank God for the strength and ability He has given me to undertake this study.

My profound gratitude to my supervisors, Dr. G.H.N.. Nyamasyo and Dr. J.H.Nderitu,

for their guidance, comments and constructive criticisms, which enabled the successful

completion of the research study.

Heartfelt thanks go to the Manager and staff of Kibwezi Irrigation Project where the

study was undertaken for their moral and logistic support. I am much indebted to

Rockefeller Foundation (FORUM) through Dr. Nyamasyo for the finances which

supported this research study, and to my employer Ministry of Agriculture for granting

me study leave. The guidance and encouragement given by the entire crop protection

department staff is also highly appreciated.

Finally, I want to express sincere gratitude to my family, relatives and friends who

through their prayers, encouragement and moral support has seen me through this study.

May God bless you all.

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TABLE OF CONTENTS PAGE TITLE……………………………………………………………………………..i DECLARATION…………………………………………………………………ii DEDICATION……………………………………………………………………iii ACKNOWLEDGEMENT………………………………………………………...iv TABLE OF CONTENTS………………………………………………………….v LIST OF TABLES………………………………………………………………...viii LIST OF FIGURES………………………………………………………………..ix ABSTRACT……………………………………………………………………….x CHAPTER 1. 1.0 Introduction And Literature Review………………………………………..1

1.1 Sweet Potato Production……………………………………………………1

1.2 Constraints to sweet potato production in Kenya………………………….2

1.2.1 Arthropod Pests of Sweet Potato……………………………………..3

1.3 Sweet potato weevil (Cylas puncticolli Boheman)………………………….4

1.3.1 Taxonomy, Nomenclature and Geographic distribution……………………4

1.3.2 Morphology…………………………………………………………………4

1.3.3 Biology and Ecology………………………………………………………..5

1.3.4 Host Range………………………………………………………….……….6

1.3.5 Economic Impact…………………………………………………………….6

1.4 Current management strategies for sweet potato weevil on sweet potato…..8

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1.4.1 Biological Control………………………………………………………………8

1.4.1.1 Life cycle and Development of entomopathogenic nematodes…………………9

1.4.2 Chemical Control………………………………………………………………12

1.4.3 Cultural Control………………………………………………………………..12

1.4.4 Host Plant Resistance………………………………………………………….13

1.4.5 Use of Sex Pheromones………………………………………………………..14

1.5 Justification of the study………………………………………………………14

1.6 Objectives ………………………………………………………………………..16

1.6.1 0verall 0bjectivce………………………………………………………………...16

1.6.2 Specific 0bjectives……………………………………………………………….16

1.7 References………………………………………………………………………...17

CHAPTER 2

2.0 GENERAL MATERIALS AND METHODS ………………………………27

2.1 Experimental Site……………………………………………………….27

2.2 Establishment of Experiment……………………………………………27

2.3 Statistical Analysis……………………………………………………....28

2.4 Reference………………………………………………………………..28

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CHAPTER 3

3.0 INSECT SPECIES ASSOCIATED WITH SWEETPOTATO IN KIBEZI,

MAKUENI DISTRICT, EASTERN PROVINCE OF KENYA

3.1 Abstract…………………………………………………………………29

3.2 Introduction……………………………………………………………..30

3.3 Materials and methods…………………………………………………..31

3.4 Results…………………………………………………………………..32

3.5 Discussion……………………………………………………………....37

3.6 References………………………………………………………………40

CHAPTER 4

4.0 EFFECTS OF Steinernema karii AND Hetrohabditis indica ON SWEET

POTATO WEEVIL (Cylas pucticollis)……………………………………….43

4.1 Abstract…………………………………………………………………43

4.2 Introduction……………………………………………………………..44


4.3.1 Persistence of the nematodes in the soil………………………………….47

4.4 Results…………………………………………………………………...48

4.5 Discussion……………………………………………………………….57

4.6 References……………………………………………………………….60

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CHAPTER 5

5.0 EFFICACY OF Steinernema karii AND Heterohabditis indica

AGAINST THE SWEET POTATO WEEVIL (Cylas puncticollis)…61

5.1 Abstract…………………………………………………………………..61

5.2 Introduction……………………………………………………………62


5.3.1 Presence of entomopathogenic nematodes in soils samples

collected from Kibwezi farm……………………………………………63 5.3.2 Application of nematode cultures……………………………………..…64

5.4 Results…………………………………………………………………...65

5.5 Discussion……………………………………………………………….78

5.6 Reference………………………………………………………………...81

CHAPTER 6

GENERAL DISCUSSION AND CONCLUSIONS…………………………………….84

LIST OF TABLES

Table 3.1 Mean count of insect pests collected from sweet potato in

Kibwezi Farm…………………………………………………………..34

Table 4.1 Mean number of nematodes recovered per larvae

of Galleria. mellonella………………………………………………………..57

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Table 5.1 Presence or absence of entomopathogenic nematodes

(EPNs) from soil samples collected at Kibwezi Farm………………….67

Table 5.2 Mean count of sweet potato adults, larvae, pupae weevils

after treatment application………………………………………………74

Table 5.3 Mean nematode count after dissecting cadavers of sweet potato

adults, larvae, pupae weevils (n=5)……………………………………...74

Table 5.4 % Root damage caused by sweet potato weevils and total

% marketable tubers…………………………………………………......76

LIST OF FIGURES

Figure 4.1 Mean weevil mortality on and around potted sweet potato plant

after treatment with H. indica and S. karii………………………………49

Figure 4.2 Mean live weevils on and around the treated sweet potato potted

plants…………………………………………………………………….50

Figure 4.3 Mean count of larvae in treated sweet potato vines……………………..51

Figure 4.4 Mean count of larvae in sweet potato tubers ………………………........52

Figure 4.5 Mean count of pupae in sweet potato vines……………………………...53

Figure 4.6 Mean count of pupae in sweet potato tubers……………………………54

Figure 4.7 Mean weights of tuber parts damaged by the sweet potato weevil……....55

Figure 4.8 Mean Percent mortality of G. mellonella larvae

after exposure to treated soils……………………………………………56

Figure 5.1 Mean counts of dead weevils recorded on sweet potato plants

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21 days after treatment application……………………………………..69

Figure 5.2 Mean number of live weevils counted within the sweet potato plant

21 days after treatment application…………………………………….70

Figure 5.3 Mean number of dead weevils counted in infected sweet potato

tubers 21 days after treatment application………………………………71

Figure 5.4 Mean number of dead larvae counted in infected

tubers 21 days after treatment application……………………………….72

Figure 5.5 Mean number of dead pupae after dissecting infected sweet potato

tubers days after treatment application…………………………………..73

Figure 5.6 Percent mortality of G. mellonella exposed to soil collected

from the field at four intervals post treatment.………………………….77

Figure 5.7 Mean number of nematodes recovered per larvae of G. mellonela..

Exposed to soil from the field at four intervals post treatment…………..78

Plate 1 Damage caused by Cylas puncticollis…………………………………….7

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ABSTRACT

Studies were carried out in Kibwezi Irrigation scheme of the University of Nairobi with

the main objective of evaluating the efficacy of newly identified indigenous

entomopathogens Steinernema karii and Heterohabditis indica nematodes in the

management of Cylas puncticollis in sweetpotato, and to review the insects associated

with sweetpotato.

Sweet potato was found to be attacked by a wide spectrum of insects. Eight of the insect

spp. were major pests inflicting damage on the leaves, vines or tubers, twenty one were

minor pests whose damaging effects were not easily noticeable on the plants while seven

of them were beneficial insects. The most important pest species were sweet potato

weevil (C. puncticollis) and the clearwing moth (Synathedon dascyceles). Defoliating

pests also dominated the spectrum of the pests observed, Aspidormopha spp. and Systates

spp, being the most abundant.

A scereenhouse experiment whereby potted sweet potato plants were infested with ten

male adult weevils and ten adult females showed that both S. karii and H. indica

significantly suppressed weevil population. Larvae and pupae were more susceptible to

both species of nematodes fourteen days after treatment application (P<0.001), with the

larvae being more susceptible compared to the pupae. Both nematode species did not,

however, have any significant effect on either male or female weevils but greater

mortality was achieved in males compared to females (2.3% mortality in males compared

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to 1.1% in females). It was also noted that more male weevils were observed on the vines

and leaves of the plants compared to females, which were more abundant in the tubers.

Field experiments whereby nematodes were applied at a rate of 1.0x1010 per Ha also

revealed that S. karii and H.indica infected C. puncticollis immatures in the tuber.

Applications of S. karii and H. indica were found to be more effective than Dimethoate

insecticides, Bacillus thuringensis insecticide and untreated control at reducing weevil

densities on plants. H. indica was found to be more effective compared to S. karii,

however, S.karii persisted in the soil for a longer period (more than 21 days) compared to

H. indica which persisted for 14 days.

The findings have shown that C. puncticolis and S. dascyceles were the most destructive

pests and control measure should target the two pests. H. indica was found to be more

effective than S. karii and would be a better control option that can be incorporated in

Integrated Pest Management programmes. More than one application, however, would

give better results

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CHAPTER ONE INTRODUCTION AND LITERATURE REVIEW

1.1 Sweet Potato Production

Sweet potato (Ipomea batatas L.) is an important crop in the developing

world, ranking fourth in importance after rice, wheat and maize (Karyeija

et al., 1998). Although the bulk of production is in China, it is an

important staple food for smallholder farmers in much of the sub Saharan

Africa. In the African continent, production is concentrated along Lake

Victoria (Gibson et al., 1997). About 90% of Kenya’s sweet potato is

produced in Nyanza, Western, Central and Eastern Provinces (Lenne,

1991). Sixty percent of the farmers produce sweet potato primarily for

home consumption but surplus is sold in the local markets (Mutuura,

1990).

Sweet potato is considered to be a warm season crop in spite of its wide

adaptation to varying ecological zones ( Onweme, 1978). Optimal

conditions for sweet potato growth are temperatures at or above 240C, and

when temperatures fall below 10oC growth is severely retarded (Onweme,

1978, Wolfe 1992). Optimal rainfall is 750–1000mm per annum with

approximately 500mm falling during the growing period (Wolfe, 1992). A

soil PH of 5.6 – 6.6 is preferred as the sweet potato plant is sensitive to

alkaline and saline conditions (Onweme, 1978). Soils, which are deeply

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worked, free draining and fairly light result in better tuberization (Wolfe,

1992). In tropical latitudes, it flowers readily but the plant usually sets few

viable seeds. However, many genotypes do not readily flower, others are

sterile and most are self-incompatible (Basset, 1986).

1.2 Constraints to sweet potato production in Kenya

In Kenya sweet potato is the most widely distributed root crop (Wambugu,

1991). The annual production has fluctuated over the years and the mean

yield stands at 9.2 tonnes/ha (FAO 2000). Farmers suffer significant yield

losses and the yield levels are 20% of the crop’s potential (50 tonnes/ha)

observed under experimental conditions (Ndolo et al., 1997, Qaim, 1999).

Thus there is ample opportunity to increase yields.

Constraints to increased production for sweet potato include, poor

agronomic practices, lack of improved cultivars and planting material,

poor soil fertility and lack of marketing prospects for the crop due to poor

transport systems. (Moyer and Kennedy, 1978; Horton, 1989; Wambugu

et al, 1991; Carey et al., 1996; Ateka et ,al 2001)

In addition biotic factors such as pests and diseases also cause yield loss.

Pests include weevils, monkeys, moles, rats and porcupines. The crop is

also attacked by a wide range of pathogens which include fungi, bacteria,

nematodes and viruses (Ames et al.,1996, Geddes,1990;Moyer and

Salazar, 1989)

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1.2.1 Arthropod Pests of Sweet Potato

Losses due to insect feeding, especially the sweet potato weevils, may

often reach 60 to 100% because most sweet potatoes are produced in low

input agricultural systems (Chalfant et al., 1990). West (1977) listed 100

insects and three mite species attacking sweet potato worldwide. The

majority of these were leaf feeders (58) followed by stem and vine feeders

(32) root feeders (9) and flower feeders (4). Talekar (1982) listed 280

insects and 18 mite species attacking sweet potato in the field and in

storage around the world of which weevils C. formicarius (Fabricius) and

Cylas punticollis (Boheman) are most damaging.

Although the plant is attacked by a large number of pest species, few

cause significant crop loss. For example, many foliar feeders do not cause

yield reductions because of a compensatory ability of the plant to tolerate

high levels of defoliation (Chalfant et al., 1990). Amongst all the pests of

sweet potato, the sweet potato weevil of the genus Cylas are the most

devastating pests world wide (Jansson, 1991, Chalfant et al., 1990). In

addition, to finding out the insects associated with sweet potato in

Kibwezi, the study focused on the management of Cylas puncticollis

which was the only Cylas species found in the study area.

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1.3 Sweet potato weevil (Cylas puncticollis Boheman)

1.3.1 Taxonomy, Nomenclatature and Geographic

Distribution

Boheman first described Cylas Punticollis (Coleoptera;curculionidae) in

1833 from Senegal. According to Allard (1990), C. puntiocollis is the

most dominant and prevalent Cylas species in East Africa. In Africa the

Cylas species has been detected from 22oN South-to-South Africa and

Madagascar. There are notable gaps in distribution in central – South-

Western Africa, especially in Angola, Zambia and Zimbabwe and in

northern Africa especially in Chad, Niger and Egypt. (Jasson et al.,1991).

Cylas punticollis is uniformly black with the eyes dorsally narrowly

separated in males.

1.3.2 Morphology

Eggs of Cylas Punticollis are oval, cream coloured and are laid singly in

cavities excavated by females in vines or tubers (Jayaramaiah, 1975;

Sutherland, 1986). There are 3 to 5 larval instars (Gonzales, 1975;

Sheman and Tamashiro, 1954, Jayaramaiah, 1975a) and according to

Allard (1990) the head width of the various larval instars ranges from 0.25

to 1.00mm. Larvae are whitish, legless, slightly curved with

approximately 5-10mm in length and a maximum width of 1.5mm. Pupae

are white and approximately 5-6 mm in length. Adults are entirely black

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with faint metallic blue luster (Jansson, 1991). The body length is about

6.8mm. Cylas puncticollis is distinguished from the other two pest species

in the genus Cylas by its characteristics uniformly black colour and by the

fact that it’s larger than the other two species (Wolfe, 1991). C.

Formicarius (Fabr) has a red thorax and bluish black abdomen while C.

Brunneus is often smaller in size with a reddish brown thorax.

1.3.3 Biology and Ecology

The female lays its eggs in small cavities, which are hidden in the base of

the stem or in the tubers, when the latter can be reached. Larvae hatch

after approximately 1 week and feed in the tubers and veins, causing

mining symptoms. Larval period lasts for 2-3 weeks depending on

temperature and there are four larval instars. Pupation takes place either in

the tuber or in the soil nearby and lasts for approximately one week. The

adult weevils remain within the pupal chamber for some days before

leaving the plant. To reach the soil surface, they tunnel through the stems

or make their way through the soil. The development period from egg to

adult is 20-28days at 27±1oC and 45±5%RH. Newly emerged adult

weevils can survive for up to 8 days in the absence of any food source and

for up to 90 days if fed on sweet potato foliage. Adults are long lived and

activities of up to 45 days have been observed, even in storage (Allard et

al., 1991). Mean adult longevity is 80.5±14.4 for females and 54.8 ± 6.7

days for males. The sex ratio is 1:1.

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1.3.4 Host Range

Cylas punticollis feeds on herbaceous convolvulaceae, especially Ipomea

Spp. It has also been reported on sesame in Uganda, Cassia acutifolia in

Sudan (Booth et al., 1990) and cowpea in Nigeria (Nonveille, 1984). The

primary host however is Ipomea batatas (Sweet Potato). The secondary

hosts include Coffea (coffee) Zea mays (Maize), Vigna unguiculata

(Cowpea), Sesamum (Sesame) and Ipomoea (morning glory).

1.3.5 Economic Impact

Cylas Punticollis is one of the most important biotic factors limiting sweet

potato production in Africa notably, Uganda, Rwanda, Kenya and

Cameroon (Chalfant et al., 1990; Pfeiffer, 1982; Smit and Matengo,

1995). The feeding action of both adult and larval stages of sweet potato

weevils causes damages to sweet potato. The adult weevils feed on vines,

petioles, leaves and fleshy roots but they prefer the latter. On leaves the

adults prefer to feed on the periderm than on the inner core (Nottigham et

al., 1987). The larvae feed and develop within leaf petioles, vines and

roots and the tunneling of root causes the most serious damage (Sathula,

1993). Damage to the vascular system caused by larval tunneling and

secondary rots reduces the size and number of roots. Damage symptoms

are characterized by malformation, thickening and cracking of vines and

tunneling of the root tubers (Sherman and Tamashiro, 1954), reduced plant

vigor and paling leaves (Trehan and Bagal, 1957). Besides the physical

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damage to the storage roots (Plate 1), feeding by the sweet potato weevils

stimulate production of terpene phytoalexins which renders tubers unfit

for consumption (Proshold, 1983; Sutherland, 1986b). The tubers produce

these toxic sesquiterpenes in response to weevil feeding (Uritani et al.,

1975), weevil densities may therefore cause devastating crop losses of up

to 60-100% as both quality and marketable yields are reduced (Chalfant et

al., 1990; Geisthardt and Van Harten, 1992)

Plate 1: Damage caused by Cylas puncticollis in sweet potato tuber

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1.4 Current management strategies for sweet potato

weevil on sweet potato.

1.4.1 Biological Control

Worldwide the known natural enemies of sweet potato weevils include

parasitoids, predators and pathogens (Jansson, 1991; Allard and Rangi,

1995; Smit, 1997). Allard and Rangi (1995) reported on several

Hymenopteran and Dipteran natural enemies of Cylas Spp in Eastern

Africa, however, none of these parasitoid on Cylas spp. was known to

cause high enough mortality to be effective for use in biological control

programmes. This is because several life stages of sweet potato are

apparently difficult for parasitoids to locate them. Ground-dwelling insect

predators, entomopathogenic fungi, nematodes and bacteria are better

suited to the underground conditions and have greater potential as

biological control agents.

Among the predators recorded attacking the sweet potato weevils are

Pheiodole megacephala, Tetramonium guineense and Drapetis exilis

(Castineira et al., 1982; Morales, 1988; Jansson, 1991; Rajamma, 1980).

The other pathogens known to infect and kill the sweet potato weevils are

fungi and bacteria. Entomopathogenic fungi recorded infecting Cylas spp

include Beauveria bassiana (Bals) and Fusarium spp (Jansson,1991),

while the known bacterial pathogen is Bacillus thuringiensis. Field

experiments in East Africa with the most pathogenic strains of B. bassiana

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in soil applications showed that it is difficult to create the right condition

for the fungus to have a controlling effect (Smit, 1997).

Entomopathoenic nematodes, Steinernematid and Hetrorhabditid have

received more attention than any other nematodes as biological control

agents for insect pests. The efficacy of entomopathogenic nematodes is

optimal in soil (Klein, 1990) and Cryptic habits (Begley, 1990) because

they are sheltered from environmental extremes. Two unidentified strains

of entomopathogenic nematodes recovered from soils in Kenya killed

Cylas punticollis larvae within four days under laboratory conditions

(Allard and Rangi, 1995). Field research on entomopathogenic nematodes

of Cylas formicarius conducted in the U.S.A, showed inconsistent results

(Jansson, 1991; Jansson et al., 1993).

1.4.1.1 Life cycle and development of entomopathogenic nematodes

The life cycle of Steinernematids and Heterohabditids is illustrated in figure 1.

(Ehlers-1996). It includes a non-feeding third stage juvenile otherwise known

as Dauer Juvenile (DJ). The DJ is free living in soil, resistant to

environmental conditions and can survive for long periods in favorable

conditions including humidity, temperature and oxygen (Woodring and Kaya

1988). Differences in the life cycle of Steinernematids and Heterohabditids

occur after the ineffective DJs enter the insect haemocoel. Heterorhabditid

ineffective DJs develop into protandrous hermaphroditic first generation

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females and subsequently into ampimictic second-generation females and

subsequently into ampimictic second-generation males and females (Dix et

al., 1992). In contrast to the Steinernematid ineffective DJs develop into

amphimictic first and second-generation males and females consecutively.

The second-generation males and females reproduce in the dead host cadaver

and progeny develop through two juvenile stages into third stage DJs. High

population density and lack of nutrition are known to induce formation of DJs

(Popiel et al., 1989) which escapes into the soil ready to infect other hosts.

In the absence of these triggers, propagative DJs develop and another

generation occurs in the insect. These nematodes release mutualistic bacteria,

Xenorhabdus spp., from their intestines into the insect host’s hemolymph.

Bacteria multiply rapidly, causing septicemia and death of the host.

Developing nematode progeny (infective juveniles) subsequently feed on

bacteria and host tissues. Stenernematid nematodes are associated with four

subspecies of X. nematophilus bacteria, whereas heterorhabditid nematodes

care associated with one bacterium, X.luminescens (Woodring & Kaya, 1998).

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Figure 1.1 Life cycle of steinernematids (after Ehlers et al., 1996)

In the soil

In the Insect

Penetration into the host via Natural openings or Through the Cuticle

Infective Dauer Juvenile (DJ)

Emigration from Insect Cadaver

Formation of DJ Bacteria stored in intestine

J2D Stage J2 Stage

Propagative J3

J1 Stage

Amphimictic Adults Copulation, Egg Production

Penetration into the Haemocoel interaction with Host defence

Recovery from DJ stage Induced by food signal, release of symbiotic bactaria, insect dies

J4 Stage J4 Stage

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1.4.2 Chemical Control

Sutherland (1986a) tested 59 different insecticides, most of which were

applied as post-planting foliar sprays, which resulted in varying levels of

control. Weevils control after planting is difficult with convectional

spraying as only above-soil adults are killed and repeated application

would be necessary to kill newly emerged adults. Systemic insecticides

have been used to control weevils in planting material. By dipping vine

cutting in these insecticides, weevil life stages within the vine are killed

and the plant is protected for at least one month after planting in the field.

(Talekar, 1991.). This type of treatment is usually more economical than

post-plant insecticides and applications, but the systemic insecticides are

highly toxic bringing about health risks and alternative tactics are needed

to improve weevil management on sweet potato. Additionally, the

majority of sweet potato farmers are also small scale and resource-poor

producers who find use of expensive agricultural inputs such as pesticides

out of reach.

1.4.3 Cultural Control

Principle sources of weevils infesting new sweet potato plantings are:

carry-over of weevils when cuttings are taken from old infested fields,

emergence of weevils from crop residues and immigration of weevils from

host plants and weevils infested neighboring crops. The recommended

cultural practices include crop rotation and sanitation which act by

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avoiding weevil emergence from infested crop residues, use of clean tip

cuttings as planting material, which reduce initial weevil population,

planting away from weevil infested fields to avoid immigrations, hilling

up to reduce soil cracking to deny females access to access roots for

reproduction, mulching, removal of alternate hosts and prompt harvesting

(Taleker, 1991, Rajamma, 1983).

Although cultural control practices are appropriate for the management of

sweet potato weevils in Africa, as they do not require costly inputs (Smit

1997), they also have drawbacks. Firstly, some of the cultural control

practices, such as extra hilling up of mounts and filling of soil cracks, are

very labour intensive. Farmers may not therefore be willing to provide

extra labour especially during periods when other crops also require care

( Qaim, 1999).

1.4.4 Host Plant Resistance

Numerous studies have been conducted in the last 50years to identify

sweet potato germplasm resistant to Cylas spp (Anota and Odebiyi, 1984;

Bong and Saad; 1987, Pole 1988) In this period, various plant traits have

been identified to be important in weevil resistance. These traits included

fleshy and root density (Martin, 1984), high dry matter and starch contents

(Pillai and Kamalan, 1977), crown hardness (Jayaramaiah, 1975) fleshy

root surface chemistry (Nottingham et al., 1989 and deep rooting (Singh et

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al., 1987). Breeding commercial sweet potato cultivars utilizing parents

with known source of resistance sweet potato weevils has not been

successful and this has led to the suggestion by researchers that sweet

potato weevil resistance does not exist in sweet potato germplasm

(Talekar, 1987). Development of transgenic sweet potato with proteinase

inhibitors for Cylas spp is going on.

1.4.5 Use of Sex Pheromones

The sex pheromone for the Cylas puncticollis has been identified as decyl

(E) -2- buteonate. The pheromone has been successfully tried in Uganda

for mass trapping of male Cylas puncticollis. A restriction of the use of

weevil pheromone as a management tool is that only male weevils are

attracted to the traps and also the availability and price of pheromone

lures.

1.5 Justification of the study

The Kenyan government is giving sweet potato a priority as part of a

national strategy to guarantee food security. Sweet potato is a fallback to

the majority of Kenyans especially during famine periods. In the hope of

improving food security, it’s imperative that all effort be made towards

improving production of this crop.

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Losses due to sweet potato weevil and plant diseases that often allow

weevil attack is estimated at 35-95%. Complete control using chemical is

rarely achieved due to the cryptic nature of the immatures developing

within vines and roots of sweet potato. Microbial pesticides have acquired

increasing importance in view of their target specific efficacies, lack of

potential for development of resistance and environmental safety.

A study carried out in the 1997 resulted in the first contribution to the

science from Africa describing a new Steinernema species, S.Karii

(Waturu et al., 1997) and confirmation of H. indica as indigenous to

Kenya. Laboratory studies carried out then revealed that the sweet potato

weevil (Cylas punticollis) was susceptible to both pathogens. There was

therefore a need to determine the potential of this indigenous

entomopathogens in the management of the sweet potato weevil under

field conditions. It was along this line of thinking that this research was

formulated.

In view of the economic importance of sweet potatoes as an important

food and income earner for the resource poor people in Kenya, it is critical

to understand other insects limiting or favoring increased production of

this crop.

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1.6.1 Overall objective

The overall objective of the research was to determine the potential of the

existing indigenous entomopathogenic nematodes as biological control

agents of sweet potato weevil as a component of an integrated sweet

potato weevil management package.

1.6.2 Specific objectives

1. To identify insects associated with sweet potato at Kibwezi

Research Farm, Makueni District, Eastern Province of Kenya

2. To determine field efficacy and persistence of S. karii and H.

indica for the control of C. puncticollis in Kibwezi Research Farm,

Makueni District, Eastern Province of Kenya.

29

1.6 References

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Allard G. B., Cock M. J. W., Rangi D. K.(1991). Integrated control of

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Anota, T. and J.A. Odebiyi. (1984) Resistance in sweet potato to Cylas

puncticollis ( Coleoptera : Curculionidae). Biol. Afr. 1: 21-

30.

Ateka, E.M., Njeru, R.W., Gibson. R.W., Vetten, H.J., Kimenju, J.

W.., Barg,E. and Kibaru, A.G. (2001). Identification and

distribution of viruses infecting sweet potato in Kenya. In

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Basset, M. J. (1986). Breeding vegetable crops. The AVI publishing

company, INC, Connecticut, USA pp. 1-35

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Begley, J. W (1990). Efficacy against insects in habitants other than soil.

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Rato, Florida.

Bong C. F. J. and M.S. Saad. (1987). Preliminary screening for

resistance to Cylas formicarius (Fab) in sweet potato in East

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Carey E.E Mwanga, R.O.M Fuentes, S. Kasule; Macharia C; Gichuki

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Tropical Root crops – Africa Branch (ISTRC – AB) October

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Efectividad tecnico-economico del empleode la hormiga

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boniato Cylas formicarius elegantulus Cienciay Tecnica la

Agricultura, Protection Plant, 5 (Suppl.): 103-109

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Chalfant, R.B Jansson r.K; Shalk J.M (1990). Ecology and

Management of sweet potato insects. Annual Review of

entomology 35: 157-190

Dix, I., Burnell, A.M., Griffin, C.T., Joyce, S.A.., Nugent, M. J. and

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219-223. In Improvement of Sweet potato (Ipomoea batatas)

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to sweet potato virus disease (SPVD) in the wild East Africa

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Florida.

33

Lenne J.M (1991). Disease and pests of sweet potatoes. South East Asia,

the pacific and East Africa. Natural Resources Institute

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Africa 23-28 1997 pp 94-96

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35

Pillai,K. S. and Kamalan P.(1977). Screening sweet potato germplasm

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Proshold, F.I (1983) Mating activity and management of Cylas

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(1997). Adult longevity and oviposition characteristics of

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Sathula, R.A.L. (1993). Studies on the biology and control of Cylas

puncticollis Boheman( Coleoptera: Curculionidae ), a pest of

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their effects on pest control in sweet potato in South Nyanza,

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21 ref.

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potato weevil Cylas formicarius (Fab.) Tropical. Pest

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37

Sutherland J.A (1986b) Damage by Cylas formicarius Fab.to sweet

potato vines and tubers, and the effect of infestations on total

yield in Papau New Guinea. Trop Pest Manage. 32: 316-323

Talekar, N.S. (1982). Effects of sweet potato weevil (Coleoptera:

Curculionidae) infestation on sweet potato root yields. J.

Econ. Entomol. 75: 1042-1044

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Insect Sc. Applic. 8. (4/5/6):819-823

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of resistance to sweet potato vine borer (Lepidoptera:

Pyralidae) in sweet potato, j. Econ. Entomol. 80: 788-791

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of sweet potato weevil (Cylas formicarius F.) with short

notes on some other pests of sweet potato in Bombay. Indian

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and Heterohabditidae) from Kenya Ph D. Thesis, University

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potato stem borer Megastes grandalis Guen. In Trinidad.

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management. A global perspective. Westview press, Boulder,

Colorado, USA. Pp. 13-43

39

CHAPTER 2

2.0 GENERAL MATERIALS AND METHODS

2.1 Experimental Site

The study was carried out at the University of Nairobi, Kibwezi Irrigation

Project (K.I.P), Makueni District, Eastern Province of Kenya. It lies in

latitude20 21.5’S and longitude380 025’E (Macmillan, 1995) at an

elevation of 750m above sea level. This area receives an erratic mean

annual rainfall of 700mm, which is bi-modally distributed with peak

periods occurring from the months of November to December and April,

followed by a long dry spell from May to November. The temperatures

range from 25-300C

2.2 Establishment of Experiment

Sweet potato plants were planted on 20th November 2002 and 4th June

2003 for the first and second season respectively. Kemp 10, a popular

local variety grown by Kenyan farmers was used during both seasons.

Experimental plots measuring 4x4 meters were arranged in Randomized

Complete Block Design (RCBD) with three replicates. Planting was done

on 4m long and 0.5 wide ridges made one meter apart. The vines were cut

into 30cm sections, each with three internodes. The vine cuttings were

planted singly 30cm apart along each ridge with each plot having four

ridges and 32 plants. During planting two internodes were buried in the

40

soil. The outer ridges in each experimental plot were used as guard rows.

Remoulding of the ridges was done at 6 and 12 weeks after planting and

weeding was done as need be until harvest. Weevil infestation was

natural.

2.3 Statistical Analysis

Excel computer package was used to key in all the raw data collected from

the experiments and the package was also used to plot graphs presented.

Each experimental data was subjected to several analysis models but

general Analysis of Variance (ANOVA) was used in all the experiments

with several adjustments as indicated at each experiment. For the

treatments that showed significant F-statistic means were separated by

standard error of difference of means.

2.4 Reference

Macmillan Kenya publishers (1995). Macmillan Atlas, pp14-25

41

CHAPTER 3

3.0 INSECT SPECIES ASSOCIATED WITH

SWEETPOTATO IN KIBWEZI, MAKUENI

DISTRICT, EASTERN PROVINCE OF KENYA.

3.1 Abstract

Studies were carried out in Kibwezi Irrigation Scheme of the University of

Nairobi to review the insects associated with sweet potato. Over fifty

insects representing several orders in different stages of development were

found associated in variable numbers within crop eight of the insect

species were major pests inflicting damage on the leaves, vines and tubers,

twenty-one were minor pests whose damaging effects were not easily

noticeable on the plant and seven of them were beneficial insects three

were refuge insects and did not affect the crop in any way. The most

important economic pest species were the sweet potato weevil (C.

puncticollis) and the clear wing moth (Synathedon dascyeles). Defoliating

pests also dominated the spectrum of pests observed, Aspidomorpha sp

and Systates sp. being the most abundant. The studies revealed that the

minor pests were prevalent during the first season and the major pests

during the second season. The findings show that

C. Puncticollis and S. dascyceles were the most destructive pests and

should dominate any control programme in Kibwezi

42

3.2 Introduction

The sweet potato ecosystem is inhabited by many pest species of insects

and mites. Talekar, (1982) reported at least two hundred and seventy

species of insects and seventeen species of mites feeding on sweet potato

in the field and in storage worldwide. All plant parts namely; roots, foliage

and even seeds are damaged by these pests.

Insects have the potential to increase and adjust their numbers in response

to the dynamic environment in which they occur (Rideway and Vinson,

1977). Hence, population fluctuations are influenced by the important

biotic and abiotic factors. These include weather and other physical

factors, food, interspecific and intraspecfic competition, natural enemies

and spatial or territorial requirements (De back, 1974).

In order to have an effective pest management programme, information on

population dynamics and seasonal distribution of a pest or pest species is

of great importance. This information can be used to determine when and

whether an insect pest management programme is necessary or not. Crop

monitoring or scouting is important in any successful pest and disease

management programme and this is unfortunately a missed out element by

many smallholder growers (Mills, 1993).

This study was carried out to determine insect pests associated with sweet

potato at specific plant development stage. This information would

43

contribute to the understanding of the pest species changes in sweet potato

and recommend appropriate time for controlling them. Integrated pest

management approach requires that all major pests be considered

individually.

3.3 Materials and methods

One and half months after the sweet potato crop had established,

fortnightly sampling for insects was done. Sampling was done using

random square metal frame quadrants (0.5x0.5m). Four quadrants samples

made one sampling unit representing a total area of 1m2. Insects found

inside the quadrant were identified and counted (Sutherland et al, 1996).

Those that could not be identified immediately were transported to the

National Agricultural Research Laboratories for the same. For whiteflies

and scales, five leaf samples picked randomly from each quadrant was

picked and the number of nymphs counted to give an estimate of the

population.

Sampling was done fortnightly for a period of five months giving a total of

nine samplings during the growth period.

44

A damage scale on leaves. tubers and stems of one to five was used to

classify the pests as major or minor. This was done as follows:

1= 0-10% damage

2= 11-25% damage

3= 26-50% damage

4= 51-75% damage

5= 76-100% damage.

Insects categorized between scale 1 and 2 were classified as minor pests..

Insects categorized between scale 3 and 5 were regarded as major pests.

3.4 Results

Over fifty insects representing several orders in different stages of

development were found associated in variable numbers within the crop.

Identified insect species associated with sweet potatoes in Kibwezi are

summarized in Table 3.1. Eight species belonging to seven families were

major pest species. Twenty-one species were recorded as minor pests and

eight species belonging to six families were observed as beneficial insect

species.

There was no time when the vines were completely free from infestation.

The crop attracted a wide spectrum of pests and was a refuge of several

others. Of these coleopteran pests were the most abundant and widely

distributed representing over 40% of the total insect count. Systates

45

pollinosus, followed by Blosyrus obliquatus Dev were the most abundant

coleopterans pest during the 1st season while Cylas punticollis and

Blosyrus obliquatus dominated during the second season. Cylas

puncticollis caused serious damage on the tuber greatly affecting yield and

quality of tubers. Systates pollinosus was the predominant Coleopteran

during the first season, its defoliating effects were, however, minimal on

the crop. The damaging effects of Blosyrus obliquatus could be seen on

the tubers.

46

Table 3.1 Mean Count of insect pests collected from sweet potato in

Kibwezi farm.

Order/ family Scientific name Season 1 Season 2

Part damaged

Pest status

Hemiptera Aleyrodidae Bemisia tabaci 1792 6 L M Margarodidae Icerya purchasi Mask 1632 0.0 LV N Pentatomidae Nezara viridula L 79 55 L N Coreidae Cletus Fuscescens Wlk 160 391 L N Coreidae Harpactor segmentarius 82 231 L N Coreitae Harpactor tibialis 21 83 L N Coreidae Dysdercus nigrofasciatus 123 213 L N Pentatomidae Aspavia armigera 93 285 L N Pentatomidae Eysarcoria incospicus 485 171 L N Alphidiphae Myzus persice 642 8 L M Coccidae Pulvinaria spp 6442 11 LV N Acanthomia Acanthomia

tomentosicolis 158 132 L N

Orthoptera Orthopterans Gryllidae cicindeloide

Rambur 172 463 L N

Acrididae Chrohongonus senegalensis bol 184 118

L N

Acrrididae Morpharic Fasciata 168 90 L N Acrididae Zonocerous variegates 203 148 L N Tetttigoniidae Eugaster Loricatus Gerst 139 0 L N

Lepidoptera Gelechiidae Branchmia Convolvuuli 226 548 L M Sphingidae Agrius Convolvuuli 54 28 VL M Sessidae Synathedon dascyleles 102 571 V M Nymphlidae Acrea ccerate 132 12 V N

Coleptera Cassididae Aspidomorpha Concinna

lse 41 146 L N

Cassididae Laccoptera afrata 15 12 L N Cassididae Aspidomorpha

matalensis 165 194 L N

Curculiunidae Cylas puncticollis 273 577 LVT M Tenebrionidae Gonocephalum spp 252 182 T N Carabidae Cocarabomorphus Spp 12 174 T N Carabidae Harpalus gregoryi 7 159 T N Curculionidae Alcidodes dentipes 12 157 VT M Curculionidae Systates pollinous 323 361 L N

47

Curculionidae Sternochetus mangiferae (F) 12 7

L 0

Curculionidae Cosmopolites sordidus (Germ) 7 8

L 0

Galeradae Auracophora foreicallis 7 83 L N Meloidae Mylarbris 14 96 F N Meloidae Epicuata albovit lata

Gestro 45 87 LF N

Meloidae Coryna apiciconis 137 206 F N Curculiunidae Blosyrus abliquatus Duv 197 401 LT M Tenebrrionidae Raytinota acuticollis 88 112 LT U Curculiunidae Prezotrachellus

gestackerii Faust 145 462 L N

Staphylanidae Paederus sabaeus 123 77 L B Curculiunidae Alcidodes erroneous 8 101 VT M Coccinalidae Cheilomensis lunataF 96 81 B

Curculionidae Systates sauberlichi (FST) 89 297 L N

Epiladininae Chnootriba chisomelinae 51 82 L B

Hymenoptera Formicidae Polyrharchis gagates Sn 896 301 B Formicidae Schistalla gerhadae 1859 106 B Formicidae Pheidole spp 1982 215 B Braconidae IphIaulux vanpalpus 86 464 B Apidae Apis adansoni Lartr 11 15 B

Diopsidae Diopsdae Diopsis tenuipes 485 151 L N Tachniidae Tachnid 80 171 B

Pest status: M= major, O= occasional, and N= minor and B= beneficial Parts damaged: L= leaves, V= vines, T= tubers

The major Coleopteran defoliators were the Cassid beetles the most

frequent being Aspidomorpha cocinna Use. Others were Laccoptera

afrata and Aspidomorpha matalensis.

Hemipteran insects were more abundant between the third and sixth

sampling periods, which represented the vegetative stage of the crop.

These insects were the most frequently observed during the first season.

The most frequent were the scales (Pullvinaria spp), which was also the

48

most abundant pest during the first season. Their abundance was closely

followed by attendant ants mainly Polyrharchis gagates, Schistacea

gerstaecler and Phoeidole spp. Low frequency of occurrence of

hemipterans was observed during the second season.

The lepidopterans were less distributed compared to the other orders. As

far as damage to the crop is concerned the clearwing moth (Synathedon

dascyceless) was the most important pest in this group. The pest also

recorded the highest frequency during the second season and was the

second most economical pest recorded after C. puncticollis. The

infestation was heavy resulting in vines breaking off easily at the base, and

damage to the storage roots. The leaf-rolling caterpillar (Brachmia

convolvuli) was also frequent during the second season compared to the

first.

Apart from the giant cricket (Eugaster loricatus), which was only

observed during the first season all the other insects were observed during

both seasons but with varying degree of occurrence. The abundance of the

species was, however, higher in the first season compared to the second

season. Pests, which caused damaging effects on the roots included sweet

potato weevil, Dust brown beetles and the Rough sweet potato beetle. The

clear wing moth was occasionally observed on the tip of the storage roots.

Pests that affected the vines included the clearwing moth, Alcidodes

49

dentipes, Alcidodes erroneous, sweet potato butterfly (Acrea acerata),

and Agrius convolvule.

Beneficial insects observed included lady bird beetles, parasitic flies

(Tachnidae), Hemipteran bugs and Hymenoptera insects (Table 3.1)

Pests classified as minor pests had low population levels and the damage

they caused was minimal. The major pests had high populations causing

major losses on the crop.

3.5 Discussion

Kibata (1973) reported most of the major and minor insects identified with

the exception of Systates spp. In a survey conducted in Zimbabwe, of the

thirteen pests recorded from sweet potato tubers, stems, crowns and

leaves, its only Cylas formicarius elegantulus and termites that were not

observed in this study. Bohlen (1973) recorded the most important sweet

potato pests in Tanzania as the sweet potato butterfly (Acrea acerata),

coleopterans mainly the sweet potato weevil (Cylas puncticollis), the

stripped sweet potato weevil (Alcidodes dentipes) and occasionally the

Cassid beetles mainly Aspidormorpha spp and Lagna vissola which

defoliate the plants. Ames et al., (1997), reported most of the insect pest

identified on sweet potato in this study. Hill (1975) recorded Bermisia

tabaci, Synathedon dasysceles, Agrius convolvuli, Acraea acerata, Cylas

puncticollis, Alcidodes dentipes and Aspidormorpha spp. as the major pest

of sweet potato in Africa.

50

Although many insect species were recorded, only a few were important

pests of sweet potato. The coleopterans were the key pests. This included

Cylas puncticollis, Blosyrus obliqutus, Systates polinosus, Cassid beetles,

the most frequent being Aspidormorpha concina. Coleopterans have also

been recorded as major pests of sweet potatoes in Tanzania (Bohlen,

1973). Prezotrachelus gestackerii (Coleoptera: Curculionidae) was the

pest observed that had not been recorded in many places. It was the sixth

most important pest observed during the second season. It’s however a

very minor pest and its effects do not cause any economical loss to the

sweet potato. It has been recorded as a major pest in legumes (Panizzi and

Slansky, 1985b).

Among the homopterans pests, the white fly (Bemisia tabaci) and the

Aphids (Myzus persicae) are vectors of diseases. Whitefly-borne virus

infecting sweet potatoes include Sweet potato Chlorotic Stunt Virus

crinivirus (SPCSV), a closterovirus (Cohen et al., 1992; Hoyer et al.,

1996; Schaefer & Terry, 1976; Gibson et al., 1998) and Sweet potato Mild

Mottle Virus (SPMMV) (Hollings et al., 1976a). The only known aphid-

borne viral disease in Africa is the Sweet potato Feathery Mottle Virus

(SPFMV) (Cali Moyer, 1981; Brunt et al., 1996). Sweet potato Virus

Disease (SPVD), the most important disease of sweet potato in sub-

Saharan Africa (Geddes, 1990) is caused by dual infection of SPFMV and

51

SPCSV (Sheffield, 1957b). These diseases have been reported in most

parts of the country. This points out to the need for proper management of

Sweet potato pests including the disease vectors.

Most of the minor pests observed are cosmopolitan, polyphagous and are

pests of other crops. These includes Nezara viridula, which is also a host

of french beans, cowpea, grams, castor, sorghum, sunflower, sesame, Soya

and cotton, Myzus persicae on turnips, cucurbits, potato, tobacco and

sesame; Acanthomia tomentosicollis on French beans, cow pea, grams,

soya, castor, sorghum, sesame, sunflower and cotton. (Hill, 1975).

Some insects present at the time of sampling were transients (tourists),

which had no direct effect on the crop. These included banana weevil

(Cosmopolites sordidus (Germ.), Mango weevil (Sternochetus mangiferae

(F) ), Dysdercus nigrofasciatus, which is a major pest of okra and cotton.

Alternate host of this insect is the baobab tree which is a common tree in

the region. The study revealed that Synathedon dascyceles was a serious

pest of sweet potato in the region, and there is need to study the biology

and management of this moth. Though the study did not assess yield loss,

continued infestation of the crop by these insects evidently showed that

there were losses which were incurred overtime

52

3.6 References Ames, T., Smit, N., Braun, A. R., OSullivan, J. N. and Skoglund, L. G.

(1996). Sweet potato: Major pests, diseases and nutritional

disorders. Lima, Peru: International Potato Center (CIP). Pp 4-

60

Bohelen E. (1973). Crop pests in Tanzania and their control. Berlin,

Germany: Verlag Paul Parey.

Brunt, A .,Grabtree K. , P allwitz, M., Gibbs,A. and Watson L

.(1996). Virus of ; Description and lists from the VIDE

Database. CAB international Wallingford, United kingdom

Cali ,B .B R Moyer J.W (1981) Purification , serology and particle

morphology of two russet cract strains of sweet potato

feathery mottle virus phytopathology 71 ,302-305

Cohen, J .Frank, A. Vetten, H .J Lesemann D .E and Loebenstejn G,

(1992). Purification and properties of clostero - like particles

associated with a whitefly transmitted of diseases of sweet

potato. Annals of applied Biology 121, 251-268.

Deback, P. (1974). Biological control by Natural Enemies. Cambridge

University press, London. 272 pp.

Geddes, A.M.W. (1990). The relative importance of crop pests in sub-

saharan Africa. Natural Resources Institute, Bullettin No. 36pp

69.

Gibson, R .W., Mpembe I., Alicai ,T., Carey, E .E., Mwanga,

R.O.M., Seal S. E., and Vetten, H. J., (1998). Symptoms,

53

etiology and serological analysis of sweet potato virus disease

in Uganda. Plant pathology. 47, 95-102

Hill, D. (1975). Agricultural Insect Pests of the Tropics and their control.

Cambridge University Press, London.

Hollings, M., Stone, O.M. and Bock, K. R. (1976a). Purification and

properties of sweet potato mild mottle, a whitefly borne virus,

from sweet potato. (Ipomea batatas) in East Africa of Annals

of Applied Biology, 82:511-528..

Kibata, G.N. (1973) Studies on varietal susceptibilitry and pest control of

sweet potato (Ipomea batatas ) in Central Kenya. Advances in

Medical Vetinary and Agricutural entomology in eastern

Africa. Pp 93

Mills, P. (1993).Scouting in peas. Unpublished paper

Panizzi, A.R., and Slansky, Jr.,F. (1985b). Legume host impact on

performance of adult Piezodorus guilinii (Westwood)

(Hemiptera: Pentatomidae). Environmental Entomology, 14,

237-242

Ridgeway, R.C and Vinson, E. (1977). Biological control by

Augumentation of Natural Enemies. Plenum New York




University of Malawi.

54

Schaefers, G .A and Terry, E .R (1976). Insect transmission of sweet

potatoes diseases agents in Nigeria. Phytopathology, 66(5):

642-645.

Sheffield, F.M., (1957b). Virus diseases of sweet potato in East Africa.

Identification of the viruses and their insect vectors.

Phytopathology, 47: 582-590.

Sutherland J. A., Kibata G. N., and Farrell G. (1996). Field sampling

methods for crop pests and diseases in Kenya. pp 6-7

KARI/ODA

Talekar, N.S. (1982). Effects of sweet potato weevil (Coleoptera:

Curculionidae) infestation on sweet potato root yields. J. Econ. Entomol.

75: 1042-1044

55

4.2 Introduction

Among the arthropod pests of sweet potato, the sweet potato weevil (

Cylas pucticollis and Cylas brunneus ) are ranked as major pests in Kenya

and have been known to contribute between 60- 70 % yield loss ( Smit,

1997; Jansson et al.,; 1987, Mullen, 1984 ). It has been recognized that the

subterranean habitat of Cylas spp makes the weevils less accessible to

chemicals, predators and parasitoids but would increase the impact of

pathogens and entomophilc nematodes which require protected, cool and

humid environments for survival and reproduction (Allard et al; 1993).

Steinernematids, and heterohabditids have received more attention than

other nematodes as biological control agents for insect pests (Kaya, 1986).

Identification of a new Steinernema species, S. Karii and confirmation of

H. Indica as indigenous to Kenya (Waturu et al., 1998) initiated these

studies. Results obtained from laboratory tests confirmed that the sweet

potato weevil larval was susceptible to both S. Karii and H. Indica.

Laboratory results, for the susceptibility of important insect pests to newly

isolated indigenous nematode species may however not be good indication

of the ability of the test nematodes to control pests in the field, where the

environment is different.

56

The aim of this work was to find the effects of S. karii and H. indica on

the sweet potato weevil Cylas pucticollis under controlled semi-field

conditions.


A mixture of sterilized soil, ballast and sand in the ratio 6:2:2 was put in

25 liter capacity plastic buckets. The buckets were perforated at the base

with holes measuring 1 cm to avoid water logging of the soil. Two sweet

potato vines each measuring 30 cm were planted in each pot. The cuttings

were dipped in a solution of Carbofuran before planting to disinfect the

cuttings. After the vines had established one of the vines was thinned out.

The experiment was laid out in a Complete Randomized design (CRD).

Two months after establishment, the pots were caged using netting cloth

(35 cm square), which was held into a square frame around the pots using

1-meter sticks. The netting material was placed in such a way to ensure

that a height of 30 cm above the sweet potato crown was maintained.

Artificial infestation of ten pairs of weevils aged between four and seven

days was done on the caged plants and netting material properly held in

place at the base using soil, to ensure no weevil escaped, and to prevent

other insects from investing the plants.

Treatments were replicated three times each represented by single plants

and these were applied two months after the weevil infestation. The

57

indigenous nematodes S. karii and H. indica used in the tests were

cultured and mass produced in larvae of the G. mellonella at the National

Fiber Research Center-Mwea Tabere. The treatments were as shown

below:

Treatment Rate of application

1. S.karii 500,000 Dauer juveniles /plant

2. H.Indica 500,000 Dauer juveniles /plant

3. Untreated control (Weevils infested but no nematodes applied)

4. Untreated control with no weevils

The nematodes were applied by drenching the nematode suspensions over

the crown part of the plant using a watering can. The nematodes

eventually drained onto the soil surface. Watering of all the sweet potato

plots was done before and after treatment application to provide moist

conditions.

Plants were uprooted and tubers dug out, the following evaluation

parameters were then measured at 2, 7, 14 and 21 days after treatment

application with three plants on each day;

1. Weevil density on the vines and crown region.

2. Weevil population and life stages inside tubers.

3. Weevil population and life stages in 13 cm vine section

immediately above the tubers.

58

4. Weight of the damaged tuber parts. This was done by chopping off

the damaged parts using a knife and measuring the weights using a

weighing balance.

4.3.1 Persistence of the nematodes in the soil.

Two 200ml soil samples were collected around three plants in each pot at

day 2,7, 14, and 21 post treatment. Four late instar larvae of G. mellonella

were buried in each soil sample placed in 250 ml containers. The

containers were then incubated in room temperature and larval mortality

recording and dissection of cadavers was carried out. The cadavers were

washed in distilled water to remove nematodes on the body surface. To

determine nematodes penetration, the cadavers were carefully dissected in

Taylor and bakers (1978) Ringer solution ( Nad 6.75g, Kcl 0.09g, Cacl2

0.115g. and NaHco3 (2H2O) 0.215g in 1 liter of distilled water) under a

binocular dissecting microscope. The dissected insect cadavers were

allowed to stand on the bench for at least 30 minutes for the nematodes to

escape from the dissected tissues into Ringers solution. Counting of

nematodes was carried out in a 9cm diameter Petri dish with a grid

engraved on the bottom. The number of nematodes counted was recorded

with the help of a tally counter.

The data was transformed into square root (X+0.5) and subjected to

ANOVA.

4.4 Results

59

There was a significant reduction in the adult weevil count around the

crown and vines for the plants treated with the two species of nematodes

21 days post-treatment compared to the untreated plants (Fig 4.2). The

reduction was greater in plants treated with H. indica compared to plants

treated with S. karii. Adult mortality was high 7 days post- treatment and

after this weevil mortality attributed to nematodes was not observed (Fig

4.1). Male weevil mortality was higher compared to the female weevil

mortality and this was 18.99% and 15.86% respectively for plants treated

with H.indica and 8.37% and 9.52% for plants treated with S. karii.

00.5

11.5

22.5

33.5

44.5

5

2 7 14 21 2 7 14 21

Dead Femalesaround Plant

Dead Malesaround Plant

Days after treatment

Mea

n m

orta

lity

of w

eevi

ls

H.indicaS.karii

60

Fig 4.1 Mean weevil mortality on and around the sweet potato plant after treatment with H. indica and S. karii

0

2

4

6

8

10

12

2 7 14 21 2 7 14 21

Live Femalesaround Plant

Live Malesaround Plant


Mea

n w

eevi

l cou

nt

ControlH.indicaS.karii

Fig 4.2 Mean live weevils on and around the treated sweet potato plants

The treatments effect on larval mortality both in tubers and vines was

highly significant (P<0.001). H.indica was more effective causing 23.33%

mortality in veins and 51.09% in tubers. Mortality was greatest seven days

post- treatment application both in the vine and tuber (fig4.3 and 4.4).

Larvae mortality caused by S. karii continued up to 21 days post treatment

61

in the tubers unlike in plants treated with H.indica where mortality ceased

14 days after treatment (Fig 4.4). Effects of S. karii and H. indica, on the

pupae in the vine ceased 14 days after treatment application (fig 4.5),

however, Pupae mortality in the tuber was recorded up to 21 days post

treatment application. (fig4.6). It was noted that larvae were more

susceptible to both strains of nematodes compared to pupae.

0

2

4

6

8

10

12

14

16

18

20

2 7 14 21 2 7 14 21

Dead Larvae in Vines Live Larvae in Vines


Mea

n co

unt o

f lar

vae


Figure 4.3 Mean count of larvae in treated sweet potato vines.

62

0

5

10

15

20

25

30

2 7 14 21 2 7 14 21

Dead Larvae inTubers

Live Larvae inTubers


Mea

n co

unt o

f lar

vae


Figure 4.4 Mean count of Larvae in sweet potato tubers

63

0123456789

2 7 14 21 2 7 14 21

Dead Pupal in Vines Live pupal inVines


Mea

n co

unt o

f pup

ae


Figure 4.5 Mean count of pupae in sweet potato vines

64

02468

1012141618

2 7 14 21 2 7 14 21

Dead Pupae inTubers

Live pupae inTubers


Mea

n co

unt o

f pup

alControlH.indicaS.karii

Figure 4.6 Mean count of pupae after dissecting tubers

The larva stage was more susceptible compared to the pupa both in the

plant veins and tubers. Pupae mortality was maximum 7 days post-

treatment and significant mortality was achieved in plants treated with H.

indica compared to S. karii. As in the case of larvae, pupae mortality in

the tuber was only observed on plants treated with S. karii 21 days after-

treatment.

65

No significant differences were observed between treatments for the

weight of tubers. Differences in damage level were however significant

between treatments 21 days post treatment, with the untreated plants

having significantly higher levels of damage (fig 4.7).

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

2 7 14 21

Days post treatment

Mea

n da

mge

d tu

ber w

eigh

t (K

g)


Figure 4.7 Mean weights of tuber parts damaged by the sweet potato weevil

66

The effect of nematodes on percent mortality of G. mellonella larvae

exposed to the soil collected from around the plants at four intervals post

treatment are shown in fig 4.8

0102030405060708090100

2 7 14 21Days after treatment

% M

orta

lity

of G

. mel

lone

lla

H. indicaS. karii

4.8 Mean percent mortality of G.Mellonella larvae after exposure to treated soils.

67

These results indicate that persistence of S. karii (21days) was longest

compared to H. indica (14 days). Results for the mean number of

nematodes recovered from Galleria larvae at each interval (Table 4.1)

followed the same pattern with mortality, and further confirmed longer

persistence of S. karii. The number of nematodes recovered from cadavers

varied and declined over time for the two nematode species with S. karii

infecting G. mellonella larvae up to 21 days post treatment.

Table 4.1 Mean number of nematodes recovered per larva of

Galleria mellonella exposed to soil at four intervals post

treatment

Nematodes used in the treatments Mean nematode per larva

2 days 7days 14 days 21 days

S. karii

H. indica

14666.0 5107667 868.3 95.0

3769.3 1133.3 31.6 0

`4.5 Discussion

Ability of any pest control approach to cause adequate mortality of target

pest within a short period is the best measure for its success in controlling

the pest and hence reducing damage on crops .The results obtained

showed that the use of S. karii and H. indica significantly (p<0.05)

suppressed adult weevil emergence from the tubers, compared to the

untreated plants (control). The fact that various stages of sweet potato

68

weevil are found within roots at the same time favored the

entomopathogens in reducing weevil abundance, by virtue of their

attraction and mobility towards the insect host. The larval period was

found to be the most susceptible probably due to the fact that the larva is

soft and the larval period lasts for 2-3 weeks, pupation on the other hand

lasts for 1 week. The hard outer elytra and limited in tersegmental soft

areas probably served as barriers to nematode penetration leading to a

lower mortality of the adult weevils. These results which indicate that

sweet potato weevil larvae are more suitable for targeting with nematode

was in agreement with observations made under laboratory conditions

using S. karii against the sweet potato larvae (Waturu et al., 1997).

. Males were frequent on the crown and leaves than females and these

results are in agreement with Sathula et al., (1993). The males move to the

upper leaves and wait for females seeking mates. It was noted that

maximum mortality was achieved 7 days post- treatment and these

declined 21days after. S. karii persisted up to the 21st day after treatment,

indicating that S karii was more persistent than H. indica though; H.

indica caused greater mortality compared to S. karii.

No significant difference in the weight of tubers was obtained between the

nematode treated pots and the control confirming the fact that weevil

feeding does not reduce the total yield, but the quality of the infected

tubers.

69

The results obtained in this study support the fact that heterohabditis are

more effective than steinernematids in controlling root weevils (Beddings

et al., 1983). The favorable results obtained with hetrorhabditid nematodes

were attributed to their tendancy to move downward from the point of

application and their strong host- finding ability in soil. (Georgis & Poinr,

1989).

4.6 References

70

Allard, G. B.( 1993). Integrated control of arthropod pests of root crops:

Annual report. CAB International Institute of Biological

Control, Kehya station, Nairobi, Kenya.

Bedding, R. A. and Molyneux, A. S. (1983). Heterorhabditis spp.,

Neoaplectana spp., and Stinernema kraussei: Interspecific and

intraspecific differences in infectivity for insects. Experimental

Parasitology 55: 249-257

Georgis, R. and Poinar, G. O. Jr. (1989). Field effectiveness of

entomophilic nematodes Neoaplectana and Heterohabditis. In

Integrated pest management for turf grass and ornamentals, pp.

213. ( Eds A. R. Leslie and R. L. Metcalf). U.S. Environmental

protction Agency, Washington D. C.

Jansson, R.K., Bryan H.H. and Sorensen, KA. (1987). Within –vine

distribution and damage of sweet potato weevil, Cylas

formicarius elegantulus ( Coleoptera: Curculionidae ), on four

cultivars of sweet potato in southern Florida. Fla. Entomology

70: 523-526.

Kaya, H. K. (1986). Constraints associated with commercialization of

entomogenous nematodes. In Fundamental and applied aspects

of invertebrate pathology, pp 661. (Eds R. A. Samson, J.M.

Vlak and d. Peters, ). Wagenigen, Ponsen and Looijen.

71

Mullen, M. A. (1984). Influence of sweet potato weevil infestation on

yields of twelve sweet potato lines. Journal of Agriculural

Entomology. 122 : 309-319




University of Malawi. pp

Smit, NE.J.M. (1997). Integrated Pest management for sweet potato in

Eastern Africa. Ph. D. Thesis,Agricultural University

Wagenigen Pp. 151.

Sutherland J.A (1986b) Damage by Cylas formicarius Fab.to sweet

potato vines and tubers, and the effect of infestations on total

yield in Papua New Guinea. Trop Pest Manage. 32: 316-323

Taylor A. E. R. and Baker, J. R. (ed) ( 1978). Methods of collaborating

parasites in vitro. Academic press, London, 301pp

Waturu, C. N. ( 1998) Entomopathogenic Neamatodes (

Steinernematidae and Heterorhabditidae) from Kenya Ph. D.

Thesis, the University of Reading

CHAPTER 5

72

EFFICACY OF Steinernema karii AND

Heterohabditis indica AGAINST THE SWEET

POTATO WEEVIL (Cylas puncticollis) 5.1 Abstract

The potential of the Entomopathogenic nematodes (EPNs) Steinernema

Karii and Heterohabditis indica, for the control of sweetpotato weevil

(Cylas puncticollis) was tested under field trials in Kibwezi. Nematodes

were applied to the soil directly at a rate of 1.0x1010 per Ha. Field

applications of S. karii and H. indica were more effective than chemical

insecticides (Dimethoate 40EC) and Bacillus thuringiensis at reducing

weevil densities on sweet potato plants. H. Indica was superior to S. karii,

but both entomopathogens significantly killed larva and pupal of C.

puncticollis inecting tubers compared to chemical insecticide and Bacillus

thuringensis (p<0.001). Development of adults in tubers was suppressed

hence leading to low numbers of adults emerging from tubers. Protection

was provided for a period of 2-3 weeks. The study indicates that the two

pathogens are effective in managing sweet potato weevil and can be

incorporated in an Integrated Pest Management programme.

5.2 Introduction

73

Microbial pesticides have acquired increasing importance in view of their

target specific efficacies, lack of potential for development of resistance

and environmental safety. Entomopathogenic nematodes from the families

Steinernematidae and Herterohabditidae have been shown to have great

potential for use as biological control agents for soil insect pests.

Entomopathogenic nematodes such as S. karii and H. indica possess many

of the attributes associated with ideal biological organisms.

Entomopathogens are safe, are capable of actively seeking out even well

concealed hosts, possess high virulence and reproductive rates, kill most

hosts in less than 48 hours and thereby reduce pest feeding (Gaugler

1981). These nematodes may be easily applied by low-input methods such

as by pouring nematode suspensions over plants, or by high input methods

such as with high-pressure sprays (up to 70.3Kg/cm3) (Kaya 1985,

Woodring and Kaya 1988).

The study was initiated to determine field efficacy and persistence of S.

karii and H. indica for control of C. puncticollis in Kibwezi. Prior to this,

studies were done to find out if entomopathogenic nematodes were present

in soil samples collected from Kibwezi research farm.


74

The study was carried out at the Nairobi University Kibwezi irrigation farm

located in Eastern province, Makueni District. The altitude is 750m above sea

level Establishment of the experiment is outlined in Chapter 2.

5.3.2 Presence of entomopathogenic nematodes in soils samples collected from Kibwezi farm.

Nine soil samples were randomly collected from the sweet potato plot. A

garden spade was used to collect approximately 1kg of soil to a maximum

depth of 15cm. 500grams of soil from each sample was placed in 500ml

container and each soil sample was replicated three times. Five late instar

larvae of Galleria mellonella was placed in the first replicate, five larvae of

the sweet potato weevil in the second and five adult sweet potato weevil in

the third.. To ensure that the sweet potato larvae and adult would be

recovered, due to their small size, a circular 15cm diameter white poplin cloth

was used. The cloth was inserted at the top of the plastic container containing

the soil to prevent the insect from penetrating to the soil. The containers were

then inverted to ensure that the soil is in contact with the cloth. This would

enable the nematodes if present to move from the soil to the cloth and infect

the baiting insect. The containers were then incubated at room temperatures.

(25±2oC). After three days the insects were removed from the soils, surface

sterilized by dipping in 70% Ethanol and rinsed in distilled water. Insects

found alive were then dried in tissue paper and placed in Petri dishes lined

with wet 90mm Wattman filter paper. The Petri dishes were sealed and placed

under room temperatures until the larvae died. Cadavers were then transferred

into modified white traps, (White, 1927), assembled using a 250ml plastic

75

container, an inverted Pyrex Petri dish covered by 90mm Whattman filter

paper and distilled water. Ten days later, harvesting of EPNs present in the

distilled water was done and counts recorded.

5.3.3 Application of nematode cultures

Indigenous nematodes S. karii and H indica used in the study were cultured

and mass-produced on the larvae of Galleria mellonella at the National Fiber

Research Center-Mwea Tabere. Nematode suspensions in water adjusted to

concentrations of 1000 DJs per liter were transported to the site on the day of

treatment in containers tightly sealed to prevent spillage. Doses of 500,000

DJs per plant contained in 2500ml water suspensions were applied around

each plant with a watering can at 1800 hours. Watering of all the sweet potato

plots was done before and after treatment application to provide moist

conditions.

These treatments were compared with sweet potato plants treated with

Bacillus thuringensis commercially available as Thuricide and

organophosphate commercially available as Dimethoate 40EC. The

insecticides were applied with a knap-sack sprayer at the recommended dose.

All the plots were irrigated before nematode application and after.

The following evaluation parameters were measured at 2, 7,14 and 21 days

post treatment, with three plants randomly selected per treatment plot. Plants

were dug, vines and roots were dissected and the numbers of live and dead

weevil, larvae, pupae and adults recorded. Weight of the tubers per plant was

76

then recorded. The damaged tuber parts were then cut off using a knife and

these damaged parts were also weighed. Persistence of the entomopathogens

was determined as outlined in chapter 3

The data was analyzed using analysis of variance procedures with an F- test at

5% level of probability. The means were separated by the least significant

difference (LSD)=0.05

5.4 Results

Analysis of the soil samples collected from the experimental plot, prior to

treatment application gave negative results for the presence of

Entomopathogenic nematodes as shown in table 5.1.

Table 5.1 Presence or absence of entomopathogenic nematodes (EPN’s)

from soil samples collected at Kibwezi Farm.

Baiting specimen (Number of deaths) after

three days

Presence or absence of nematodes Nematode

counts

77

Soil

sample

Sweet

potato

larvae

weevils

Sweet potato

adult weevils

Galleria

larvae

Sweet potato l

weevil larvae

Sweet

potato

weevil

adults

Galleria

larvae

Sweet

potato

larvae

weevils

1 1 0 0 -ve -ve -ve 31

2 2 2 0 +ve -ve -ve 42

3 2 0 0 -ve -ve -ve 0

4 4 0 0 +ve -ve -ve 12

5 2 0 0 +ve -ve -ve 77

6 4 0 0 +ve -ve -ve 14

7 2 0 0 +ve -ve -ve 0

8 4 0 1 +ve -ve -ve 13

9 4 0 0 +ve -ve -ve 20

Totals 25 2 1 209

% 55% 4.4% 2.2%

Entomopathogenic nematodes were only found infecting the larvae of the

weevils and not adult sweet potato weevils and Galleria larvae. Galleria

larvae, for a long time has been used to bait entomopathogenic nematodes

from the soil. The sweet potato larvae, however, were more sensitive to the

entomopathogens compared to the Galleria moth larvae. This indicated that

78

the soils had very low counts of entomopathogens (209), which could only be

recovered using the sweet potato weevil larva. This showed that

entomopathogens could be used as bio-controls for the sweet potato weevil

larvae even at low doses compared with the weevil adult. The results also

showed that timing of application of entomopathogens is very crucial to be

able to capture the larvae stage, which is the most susceptible stage.

Greatest numbers of dead weevils were found on plants treated with

Dimethoate 40EC two days after treatment application (Fig 5.1). More live

weevils were found per root and per plant on non treated plants compared to

plants treated with nematodes 21 days after treatment application (Fig 5.2).

There was no significant difference in the number of live weevils between

plants treated with Bt, Dimethoate 40EC and the untreated plants (control), 21

days after treatment application (Fig 5.2). Adult mortality inside the tubers

was not significantly affected by the treatments. (P > 0 .05).

(Fig 5.3). Only S. karii and H. indica were able to cause mortality inside the

tubers. Highest mortality inside the tubers for the larvae and pupae were

recorded 7 days after treatment application and the mortality rate dropped 14

days after treatment application (Fig 5.4 and Fig5.5).

21days after treatment application, less live weevils were recorded on plants

treated with S.karii and H. indica. (Fig 5.2). The total number of dead weevils

ranged from 3 to 19. The total numbers of the nematode infected weevils

ranged from 0 to 9.

79

0

5

10

15

20

25

2 7 14 21

days after treatment

Mea

n co

unts

of d

ead

wee

vils

S.kariiH.indicaBtDimethoate 40ECControl

Figure 5.1 Mean count of dead weevils recorded on the sampled sweet potato plants 21 days after treatment application.

80

05

10152025303540

2 7 14 21


Mea

n co

unt o

f liv

e w

eeev

ilsS.kariiH.indicaBtDimethoate 40ECControl

Figure 5.2 Mean number of live weevils counted within the sweet potato plant 21 days after treatment application.

Weevils inside the tubers were not significantly affected by the treatments.

The mortality ranged from 0 to 8. However for larvae and pupae in the tubers,

there was a highly significant difference in the treatments (P<0.001), with no

larvae and pupae mortality recorded on plants treated with Bt and Dimethoate

40C (Fig 5.4, Fig 5.5). High mortality of larvae and pupae was observed on

plants treated with H. indica and S. karii. Highest mortality of the weevil

immatures was recorded seven days after treatment application. Weevil larvae

81

were more susceptible to the entomopathogenic nematodes compared to both

pupae and adult weevils. ( Fig 5.3).

0

1

2

3

4

5

6

7

8

9


Mea

n no

of d

ead

wee

vils

in tu

ber

S.karii

H.indica

Bt

Dimethoate 40EC

Control

Fig 5.3 Mean number of dead weevils counted in infected sweet potato tubers 21 days after treatment application

Larvae were more susceptible to the Entomopathogenic nematodes compared

to the pupae and adult weevil (Table 5.2). Larvae mortality was not

82

recorded on plants treated with Dimethoate, Bt and control. The treatments

did not have any effect on the larvae and the pupae both in the tubers and

vines ( Fig 5.4, 5.5)

0

2

4

6

8

10

12

14

16

2 days 7 days 14 days 21 daysDays after treatment

Mea

n no

. of d

ead

larv

a in

tube

r

S. karii

H. indica

Bt

Dimethoate40ECControl

Figure 5.4: Mean number of dead larvae counted in infected tubers 21 days after treatment application

83

0

2

4

6

8

10

12

14

2 7 14 21


Mea

n no

. of d

ead

pupa

e in

tu

ber

S. karii

H. indica

Bt

Dimethoate40ECControl

Figure 5.5: Mean number of dead pupae counted in infected tubers, 21 days after treatment application.

H. indica accounted for 45.22% of the total dead progeny, S.indica

28.14%, Bt, 6.84, and Dimethoate 40EC 18.99 and control 0.8. This

indicates H. indica was more efficacious compared to the other treatments

as shown in Table 5.2.

84

Table 5.2 Mean total count of sweet potato adults, larvae, and pupae

after treatment application

Treatment Adult

weevil

Larvae Pupae

H. indica 22.14 55 17.3

S.karii 19.08 32.5 7.2

Bt 14.28 0 0

Dimethoate 40 EC 39.66 0 0

Control 1.7 0 0

Mean number of nematodes counted on dissecting five cadavers of each

insect were significantly different (F=3.25; P=0.05). S. karii had the highest

mean number of nematodes (Table 5.2)

85

Table 5.3 Mean nematode counts after dissecting cadavers of sweet

potato adult, larvae and pupae (n=5)

Treatment Adult Larvae Pupae

S. karii

H.indica

6.2±2.2

3.6±0.3

19±0.1

5.8±1.3

17.3±2.0

27.2±1.2

Root damage caused by the sweet potato weevil and the yields

In all the plots damage ratings differed although this was not significant

(P.>0.05) among treatments. Damage ratings were lower on plants treated

with H. indica than on plants treated with S. karii, Dimethoate 40C, and Bt.

The percentage of total root weight free of weevil damage was higher on

plants treated with H. indica than on those treated with Bt, S. karii,

Dimethoate 40C and non-treated plants (Table5.4).

86

Table 5.4 % Root damage caused by the sweet potato weevil and total

% marketable tubers

Treatment %

Damage

free

% Marketable

roots

S. karii

H.indica

Bt

Dimethoate 40EC

Control

32.0±3.8

42.3±2.3

26.3±2.9

29.2±2.5

19.2±2.0

32.1±2.8

43.4±4.6

21.2±0.0

28.6±1.2

8.1±2.1

Persistence of the entomopathogens in the soil

In the first experiment to determine whether entompathogenic nematodes

were present in soils collected from the experimental plots, no nematode

infected larvae were found before application was made. After nematode

applications levels of recovery increased considerably and differed (P<0.001)

between the two nematodes strains. The highest level of recovery was found

in plots treated with S. karii. Persistence of S. karii remained stable for up to

21 days after treatment application. ( Fig 5.6)

87

0

20

40

60

80

100

120

1 2 3 4Days post treatment

% m

orta

lity

of G

alle

ria la

rvae

S.karii

H.indica

Fig 5.6 Percent mortality of G. Mellonella exposed to soil collected from the field at four intervals post treatment

88

0

1000

2000

3000

4000

5000

6000


mea

n ne

mat

odes

per

larv

a

S.kariiH.indica

Fig 5.7 Mean number of nematodes recovered per larva of G.mellonella exposed to soil from the field at four intervals post treatment

Nematode recovery from the larvae of G. mellonella was highest 2 days

post treatment but this declined tremendously to no recovery 21 days after.

This shows that the nematodes were not able to survive for long in the

soils 21 days after application

5.5 Discussion Analysis of the soil samples collected from the experimental plots prior to

treatment application gave negative results for the presence of entomopathogens

89

Field studies indicated that application of S.karii and H. indica

significantly suppressed the number of weevils and consequently tuber

damage leading to increased yields. The larvae and pupae of the sweet

potato weevil was previously found to be susceptible to the nematodes in

screenhouse experiment and these results are presented in chapter 4. The

two nematodes caused significantly higher mortality of C. puncticolis

larvae and pupae compared to using B. thuringensis and Dimethoate 40C.

Consequently the effect of weevil damage to tubers was significantly

reduced in nematode treated plots compared to plants treated with B.

thuringensis and insecticide 21 days post treatment. Chemical application

was able to kill only the adult weevils, 2 days post treatment, but their

numbers increased tremendously thereafter. Evidence during the study

indicated persistence of nematode juveniles up to two weeks post

treatment for H. indica and three weeks post treatment for S. karii.

Entomopathogenic nematodes in the family Steinernematidae and

Heterohabditidae have been tested and are infectious against several

weevils, including the sweet potato weevil (Jansson et al, 1990). The fact

that nematodes species are pathogenic to weevils in roots and vines was

established by Jansson et al., (1992) as well as Mannion et al (1992) but

with Cylas formicarius. These results suggest that H. indica could be a

more preferable microbial control agent for C. puncticollis compared to S.

karii. Their low toxicity to humans and non-target invertebrates compared

90

with chemical insecticides is an advantage, especially in developing

countries, where the risk of misuse of pesticides is very high. Persistence

of the nematode isolates was reasonably good with infection of G.

mellonella larvae occurring after three weeks in soil treated with S. karii

and two weeks for H. indica. The results indicate that more than one

application is needed to provide adequate protection and ensure that

weevil free tubers are obtained. It was worth noting that the nematodes did

not cause mortality to the many insects associated with sweet potato

reported in Chapter 3. Field results with the entomopathogenic nematodes

was not very encouraging considering the cost involved in the production

and that not more than 50% marketable roots were obtained.

Laboratory production of the indigenous nematodes which was done in the

larvae of the G. mellonella at the National Fiber Research Center-Mwea

Tabere proved to be expensive. The total cost of purchasing the

nematodes, used in the study amounted to Ksh 46,000. This was rather

prohibitive and was probably due to the fact that the institution has not yet

started producing nematodes on large scale and that production is done on

demand. The cost of hiring casuals increased the cost of production. To

rear Galleria, one needs the Galleria diet, which is normally put in

ventilated 3.4-liter plastic lunch boxes. One is able to rear more than 500

Galleria larvae using a lunch box with this capacity. The cost of the diet in

the 3.3 plastic lunch box is Ksh 300. One Galleria larvae can produce

91

20,000-50,000 infective juveniles. This translates to production of 1.0x108

nematodes (considering the minimum production of 20,000) at a cost of

Ksh. 300, which amounts to Ksh 7,500 per Ha (rate of application being

2.5x109 per Hectare. (Georgis and Grewwal, 1994).There is therefore an

urgent need to focus research towards a more friendly production method

that can be adapted by farmers and at a lower cost.

5.6 References

Akawzwa, T.,Uritani,I.and Kubota, H. (1960). Isolation of Ipomea

marone and two coumarine derivatives from sweetpotato roots

92

injured by the weevil. Cylas formicarius elegantulus. Arch

Biochem Biophys. 88:150-156

CAB International, (1993). Distribution maps of pestts. Series A: Map

No. 278. Cylas formicarius (Fabricius) Map No. 279. Cylas

puncticollis (Boheman ). Map No. 537. Cylas brunneus

(Fabricius).

FAO, (1996). FAO Production Year Book 1996. Rome FAO

FAO, (1998) FAO production Year Book 1998

Gaugler, R. (1981). Biological control potential of neoaplectanid nematodes.

Journal of Nematology 13: 241-249

Georgis, R. and Grewal, P. S. ( 1994) Commercial application of

entomopathogenic nematodes. Proceedings of the V’th

Colloquium on Invertebrate Pathology and Microbial Control

and 11’nd International Conference on Bacillus thuringiensis1:

157-163. Montpellier France.

Jansson, R.K., Bryan, H.H. and Sorenssen, K. A. (1987). Within-vine

distribution and damage of sweetpotato weevil, Cylas formicarius

elegantulus ( Coleoptera: curculionidae)on four cultivars of sweet

potato in southern Florida. Fla. Entomol. 70: 523 -526.

Jansson, R.K.F.R. and Misorley. (1990). Sampling plans for the sweet

potato weevil ( Coleoptera: curculioidae) on sweet potato in

southern Florida. J. Econ. Entomol 18: 1901-1906

Kaya,H.K. (1985). Entomogenous nematodes for insect control in IPM

systems, pp. 282-302. In M.A. Hoy & D.C. Herzog (eds),

93

Biological control in Agricultural IPM systems. Academic, New

York.

Mullen,M.A. (1984). Influence of sweet potato weevil infestation on the

yields twelve sweet potato lines. J. Agric. Entomol. 1: 227-230.

Sato, K., Uritani I. &Saito T. (1981). Characterization of the terpene-

inducing factor isolated from the larvae of the sweet potato

weevil, Cylas formicarius Fabricius (Coleoptera: Brenthidae).

Appl. Entomol. Zool. 16:103-112.

Smit, N.E.J.M. and Matengo L.O. (1995). Farmers’ cultural practices and

their effects on pests control in sweet potato in South Nyanza,

Kenya. Int. J. Pest manage. 41:2-7

Uritani, I., Saito, H., Honda, H. and Kim, W.K. (1975). Introduction of

Furano-terpenoids in sweet potato roots by the larval components

of the sweet potato weevils. Agric. Biol. Chem37:1852-1862.

Wambungu,F.M., Brunt, A.A.and Fermardez, E.M., (1991). Viruses and

virus diseases of sweetpotato (Ipomea batatas L.) in Kenya and

Uganda. In proceedings of the 2ndTriennial meeting and

influence of the African Potato Association, maunhus 22-27July

1990.Redient Mauritius, pp. 91-96.

White,G.F. (1927). A method for obtaining infective nematode larvae from

cultures. Science 66:302-303.

Woodring, J.L. and Kaya, H.K. (1998). Steinernematid and Heterohabditid

nematodes. Life cycle In a hand book of biology and techniques

94

pp3. A southern coorperative series. Bulleitin 331, Arkansas

Agricultural Experimental station, Fayettville Arkansas.

CHAPTER 6

GENERAL DISCUSSION, CONCLUSION AND

RECOMMENDATIONS

The need to determine the potential of S. karii and H. indica, in

management of the sweet potato weevil was the basis of the study, in

95

addition to determining the insects associated with sweet potato at specific

plant development stage.

Sweet potato was found to be highly attractive to many insects. This is

probably due to its attractive canopy. Though the study did not assess

yield loss, continued infestation of the leaves, vines and tubers evidently

showed that there was losses incurred overtime. Most of the insects

observed were defoliating pests. The coleopterans dominated the pest

complex and caused the most important damage. The sweet potato weevil

(Cylas puncticollis and the Clearwing moth (Synathedon dasyceles) were

found to be the most destructive pests on sweet potato in Kibwezi, and the

control of these two key pest should dominate any control programme. It

took almost three months before the first C. puncticollis weevil was noted

on the crop. Considering that the crop had not been grown in the farm

since its inception, it was concluded that the pest could have been

harbored in the many convolvulacea weeds alternate hosts observed

around the farm which also act as alternate host for C,.puncticollis.

Control of these weeds should also be included in any control programme.

There is need for more studies to be done on the biology and control of the

clearwing moth to shed more light on this important pest of sweet potato

in Kibwezi.

The success of a biological control programme depends on several factors

such as the ability of the agent to establish itself and remain in the

96

ecosystem for a long time`. The soils from Kibwezi, however, were found

to have no entomopathogenic nematodes. This probably accounts for the

high infestation rate of sweet potato by the sweet potato weevil. That is, in

the absence of a bio-control agent, sweet potato weevil multiplied

extremely fast. This might also explain the low hectarage of the crop in the

region. Sweet potato is a traditional food crop for the Akamba who

dominate the District,. The farmers might have been discouraged by the

damage caused by the weevil hence the low hectarage covered by the crop

in the region. The main objective of this study was to enhance food

security in the region by promoting sweet potato growing through

development of a farmer friendly Integrated Pest Management strategy in

which entomopathogens would be a component.

Field studies indicated that S. karii and H. indica reduced weevil damage

and consequently tuber damage leading to increased marketable tubers

compared to using Bt and Dimethoate. The fact that various stages of the

sweet potato weevil are found within roots at the same time favored the

entomopathogens in reducing weevil abundance, by virtue of their

attraction and mobility towards the insect host. The larval period was

found to be the most susceptible probably due to the fact that the larval is

soft and the larval period lasts for 2-3 weeks. Pupation on the other hand

lasts for 1 week. The entomopathogens easily penetrated through natural

openings and soft tissues. The hard outer elytra and limited intersegmental

97

soft areas probably served as barriers to nematode penetration leading to a

lower mortality of the adult weevils. Infection of the sweet potato weevil

within its natural habitat in the sweet potato tuber was a significant result

in that it provided evidence of possibility to control the pest in the field

using S. karii and H. indica. H. indica was found to be more efficacious

than S. karii and would be a better microbial control option that can be

cooperated in an IPM programme. The results indicate that more than one

application is needed to provide adequate protection and ensure weevil

free tubers are obtained and use of these nematodes should be integrated

with other sweet potato management strategies.

The findings of the study can be summarized as follows:

1. Synathedon dascyceles is a serious pest of sweet potato in Kibwezi

and there is need to study the biology and management of this

moth.

2. C. puncticollis and S. dascyceles are the most destructive pests and

should dominate any control programme in Kibwezi.

3. Entomopathogenic nematodes are more effective in managing C.

pucticollis compared to using organophosphates and Bacillus

thuringiensis.

4. H. indica was a more preferable microbial control agent for C.

puncticollis compared to S. karii, despite the fact that S. karii

98

persisted more in the soil compared to H. indica. The pathogen can

therefore be in cooperated in an IPM programme.

5. More than one application is needed to provide adequate protection

and ensure that weevil free tubers are obtained.

6. There is an urgent need to focus research towards a more friendly

production method that can be adapted by farmers and at a lower

cost.

Documents

EFFICACY OF Steinernema karii AND Heterohabditis. Indica ... · 1 EFFICACY OF Steinernema karii AND Heterohabditis. Indica NEMATODES IN THE MANAGEMENT OF THE SWEET POTATO WEEVIL (