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
1
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……………………………….
2
DEDICATION
To my son James, parents, brothers and sisters for their prayers, understanding and support.
3 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.
4 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
5 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
6 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 Materials and methods…………………………………………………..45
4.3.1 Persistence of the nematodes in the soil………………………………….47
4.4 Results…………………………………………………………………...48
4.5 Discussion……………………………………………………………….57
4.6 References……………………………………………………………….60
7 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 Materials and methods…………………………………………………..63
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
8 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
9 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
10 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
11 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
12 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
13 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)
14
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.
15 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
16 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.
17 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
18 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
19 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
20 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
21 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).
22 Infective Dauer Juvenile (DJ)
Penetration into the host via Emigration from Natural openings or Insect Cadaver Through the Cuticle In the soil
In the Insect Formation of DJ Bacteria stored in intestine Penetration into the Haemocoel interaction with Host defence
J2D Stage J2 Stage
Recovery from DJ stage Induced by food signal,
release of symbiotic Propagative bactaria, insect dies J3 J1 Stage
J4 Stage J4 Stage
Amphimictic Adults Copulation, Egg Production
Figure 1.1 Life cycle of steinernematids (after Ehlers et al., 1996)
23 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
24 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
25 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.
26 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.
27 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.
28 1.6 References
Allard G.B (1990), Integrated Control of Arthropod pests of root crops
November 1988. December 1989. Mid-Term Report. CAB
International Institute of Biological Control, Nairobi, Kenya.
Allard G.B., Cock M.J.W., Rangi D.K., (1991). Integrated control of
arthropod pests of root crops, final report. Nairobi, Kenya:
CAB International.
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
Allard G. B., Cock M. J. W., Rangi D. K.(1991). Integrated control of
arthropod pests of root crops, final report. Nairobi, Kenya:
CAB International
Anota, T. and J.A. Odebiyi. (1984) Resistance in sweet potato to Cylas
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38 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
39 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
40 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
41 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
42 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.
43 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
44 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.
45 Table 3.1 Mean Count of insect pests collected from sweet potato in Kibwezi farm.
Order/ family Scientific name Part Pest Season 1 Season 2 damaged 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 L N tomentosicolis 158 132 Orthoptera Orthopterans Gryllidae cicindeloide L N Rambur 172 463 Acrididae Chrohongonus L N senegalensis bol 184 118 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 L N lse 41 146 Cassididae Laccoptera afrata 15 12 L N Cassididae Aspidomorpha L N matalensis 165 194 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
46 Curculionidae Sternochetus mangiferae L 0 (F) 12 7 Curculionidae Cosmopolites sordidus L 0 (Germ) 7 8 Galeradae Auracophora foreicallis 7 83 L N Meloidae Mylarbris 14 96 F N Meloidae Epicuata albovit lata LF N Gestro 45 87 Meloidae Coryna apiciconis 137 206 F N Curculiunidae Blosyrus abliquatus Duv 197 401 LT M Tenebrrionidae Raytinota acuticollis 88 112 LT U Curculiunidae Prezotrachellus L N gestackerii Faust 145 462 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
47 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
48 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.
49
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
50 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
51 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
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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
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Geddes, A.M.W. (1990). The relative importance of crop pests in sub-
saharan Africa. Natural Resources Institute, Bullettin No. 36pp
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R.O.M., Seal S. E., and Vetten, H. J., (1998). Symptoms,
52 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
Sathula, R.A.L. (1993). Studies on the biology and control of Cylas
puncticollis Boheman( Coleoptera: Curculionidae ), a pest of
Ipomea Batatas (L) Larmark, the sweetpotato. M.Sc. Thesis,
University of Malawi.
53 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
54 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.
55 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.
4.3 Materials and methods
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
56 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.
57 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
58 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.
5 4.5 4 3.5 3 2.5 2 H.indica 1.5 S.karii 1 0.5 0 Mean mortality of weevils Mean mortality 2 7 14 21 2 7 14 21
Dead Females Dead Males around Plant around Plant Days after treatment
59 Fig 4.1 Mean weevil mortality on and around the sweet potato plant after treatment with H. indica and S. karii
12 10 8 Control 6 H.indica S.karii 4 2 Meancountweevil 0 2 7 14 21 2 7 14 21
Live Females Live Males around Plant around Plant Days after treatment
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
60 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.
20
18
16
14
12 Control H.indica 10 S.karii 8
6 Meancount of larvae 4
2
0 2 7 14 21 2 7 14 21 Dead Larvae in Vines Live Larvae in Vines Days after treatment
Figure 4.3 Mean count of larvae in treated sweet potato vines.
61
30
25
20 Control 15 H.indica S.karii 10 Mean count of larvae
5
0 2 7 14 21 2 7 14 21
Dead Larvae in Live Larvae in Tubers Tubers Days after treatment
Figure 4.4 Mean count of Larvae in sweet potato tubers
62 9 8 7 6 Control 5 H.indica 4 3 S.karii 2 1
Mean count of pupae of count Mean 0 2 7 14 21 2 7 14 21 Dead Pupal in Vines Live pupal in Vines Days after treatment
Figure 4.5 Mean count of pupae in sweet potato vines
63 18 16 14 12 Control 10 H.indica 8 S.karii 6 4
Mean count of pupal of count Mean 2 0 2 7 14 21 2 7 14 21 Dead Pupae in Live pupae in Tubers Tubers Days after treatment
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.
64 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).
0.45
0.4
0.35
0.3
0.25 Control H.indica 0.2 S.karii 0.15
0.1
Mean damged tuber weight (Kg) weight tuber damged Mean 0.05
0 2 7 14 21 Days post treatment
Figure 4.7 Mean weights of tuber parts damaged by the sweet potato weevil
65 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
100 90 80 70 60
G. G. mellonella H. indica 50 S. karii 40 30 20
% of% Mortality 10 0 2 7 14 21 Days after treatment
4.8 Mean percent mortality of G.Mellonella larvae after exposure to treated soils.
66 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 14666.0 5107667 868.3 95.0
H. indica 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
67 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.
68 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
69 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.
70 Mullen, M. A. (1984). Influence of sweet potato weevil infestation on
yields of twelve sweet potato lines. Journal of Agriculural
Entomology. 122 : 309-319
Sathula, R.A.L. (1993). Studies on the biology and control of Cylas
puncticollis Boheman( Coleoptera: Curculionidae ), a pest of
Ipomea Batatas (L) Larmark, the sweetpotato. M.Sc. Thesis,
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
71 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
72 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.
5.3 Materials and methods
73 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
74 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
75 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 Presence or absence of nematodes Nematode three days counts
76 Soil Sweet Sweet potato Galleria Sweet potato l Sweet Galleria Sweet sample potato adult weevils larvae weevil larvae potato larvae potato
larvae weevil larvae
weevils adults 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
77 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.
78 25
20 S.karii H.indica 15 Bt Dimethoate 40EC 10 Control 5
0 Mean counts of dead weevils weevils dead of counts Mean 2 7 14 21 days after treatment
Figure 5.1 Mean count of dead weevils recorded on the sampled sweet potato plants 21 days after treatment application.
79 40 35 30 S.karii 25 H.indica 20 Bt Dimethoate 40EC 15 Control 10 5
Mean count of live weeevils 0 2 7 14 21 Days after treatment
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
80 were more susceptible to the entomopathogenic nematodes compared to both pupae and adult weevils. ( Fig 5.3).
9 8 7 S.karii H.indica 6 Bt 5 Dimethoate 40EC 4 Control 3 2
Meanno of dead weevils in tuber 1 0 2 7 14 21 Days after treatment
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
81 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)
16
14
12 S. karii H. indica
10 Bt 8 Dimethoate 40EC Control 6
4
Mean no. of dead larva in tuber larva dead no. of Mean 2
0 2 days 7 days 14 days 21 days Days after treatment
Figure 5.4: Mean number of dead larvae counted in infected tubers 21 days after treatment application
82 14
12
10 S. karii
8 H. indica
Bt tuber 6 Dimethoate 4 40EC Control 2 Mean no. of dead pupae in in pupae dead of no. Mean
0 2 7 14 21 Days after treatment
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.
83 Table 5.2 Mean total count of sweet potato adults, larvae, and pupae
after treatment application
Treatment Adult Larvae Pupae
weevil
22.14 55 17.3 H. indica
19.08 32.5 7.2 S.karii
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)
84 Table 5.3 Mean nematode counts after dissecting cadavers of sweet
potato adult, larvae and pupae (n=5)
Treatment Adult Larvae Pupae
S. karii 6.2±2.2 19±0.1 17.3±2.0
H.indica 3.6±0.3 5.8±1.3 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).
85 Table 5.4 % Root damage caused by the sweet potato weevil and total
% marketable tubers
Treatment % % Marketable
Damage roots
free
32.0±3.8 32.1±2.8 S. karii
42.3±2.3 43.4±4.6
H.indica
26.3±2.9 21.2±0.0
Bt
29.2±2.5 28.6±1.2
Dimethoate 40EC
19.2±2.0 8.1±2.1 Control
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)
86 120 100 80 60 40 S.karii H.indica
% mortality of Galleria larvae of Galleria % mortality 20 0 1 2 3 4 Days post treatment
Fig 5.6 Percent mortality of G. Mellonella exposed to soil collected from the field at four intervals post treatment
87 6000 5000 4000
3000 S.karii H.indica 2000 1000 0 mean nematodes per larva per nematodes mean 2 7 14 21 Days after treatment
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
88 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
89 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
90 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
91 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),
92 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
93 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
94 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
95 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
96 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
97 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.
98