Abstract:
Vegetable production is increasing in order to feed the growing urban populations in subSaharan Africa. Net houses are one possible solution to increase the quality and yield of cowpea, an important leafy vegetable, and to reduce the use of pesticides. This study tested the hypothesis that net houses do not protect cowpea against the Black Legume Aphid,Aphis craccivora or Bean Flower Thrips, Megalurothrips sjostedti, two small but major cowpea pests. Thus, a possibility to supplement the physical barrier with repellent volatiles to prevent these pests from getting through the netting was examined. Two sources of repellent volatiles were investigated thus, host-and non-host -plant volatiles.This study focused on four specific objectives that included: (i) to identify cowpea cultivars (vegetative stage) and their bioactive odours which attract or repel alate A.craccivora; (ii) to identify cowpea cultivars at different phenelogical stages (vegetative and flowering stages) and their bioactive odours which attract or repel male and female M. sjostedti; (iii) to evaluate the repellent effect of Lemongrass, Cymbopogon citratus and Mexican marigold, Tagetes minuta plants and their major compounds against male and female M. sjostedti; and (iv) to evaluate the efficacy of repellent volatiles from Lemongrass and Mexican marigold combined or not with netting to reduce cowpea pests in a ‘push-pull’ system in the field. Behavioural assays were conducted to study host preference and gas chromatography-mass spectrometry (GC/MS) for chemical analysis of volatiles. To this end of both studies the host preference and volatiles emitted by four cowpea cultivars viz. Katumani 80, Ex-Luanda, Machakos 66 and Ken Kunde 1 that were tested on A. craccivora and M. sjostedti using a cage and Y- tube olfactometer respectively. Results of the olfactory tests showed that A. craccivora was attracted by the volatiles emitted by Ex-Luanda cultivar but repelled by the cultivar Katumani 80.Machakos 66 and Ken Kunde 1 elicited neutral response. The Bean Flower Thrips, M. sjostedti females were repelled by volatiles emitted during the vegetative stage of cowpea cultivars Katumani 80, Machakos 66, Ex-Luanda while Ken Kunde 1 elicited a neutral response. The males were repelled only by the vegetative stage of cultivar Ken Kunde 1.Females were attracted by the flowers of cultivar Ken Kunde 1. The volatiles emitted by the flowers of cultivar Katumani 80 were repellent to female M. sjostedti but not to males.Among the 23 compounds identified in the vegetative stage of cowpea cultivars, (E)-2-Hexenal tested alone or in combination with attractant cowpea flower (KK1) was repellent to female M. sjostedti at a 0.01% concentration but not at 1% concentration. The blend of hexanal and (E)-2-hexenal added to cowpea cultivar Ex-Luanda reduced the attractiveness of the cultivar to A. craccivora while the addition of 1-octen-3-ol and p-xylene to cowpea cultivar Katumani 80 did not reduce the repellence of the cultivar to A. craccivora. (E)-βOcimene and 1-octen-3-ol were only detected in the volatiles of flower of repellent cultivar Katumani 80 to the bean flower thrips. Tested at concentrations of both 0.01%and 1%, these compounds elicited a neutral response from female M. sjostedti. In addition,olfactory tests revealed that both male and female M. sjostedti were repelled by fresh cut leaves of C. citratus. A combination of fresh cut leaves of C. citratus and cowpea flower was less attractive to females than cowpea flowers alone. However, males were not repelled by the combination. Female thrips were more repelled by volatiles emitted by vegetative T. minuta and by the combination of either vegetative or flowering T. minuta with cowpea flower than by cowpea flowers alone. However, males were not repelled/attracted by the volatiles from T. minuta. Citral, a major compound of C. citratus and a blend of 4 compounds: dihydrotagetone, (Z)-3-hexenyl acetate, limonene and (Z)-β-ocimene with natural ratio were repellent to female M. sjostedti. However,dihydrotagetone alone was an attractant for females M. sjostedti. Alone, myrcene was not an attractant but enhanced the attraction of cowpea flowers for female M. sjostedti. In the field trials, the net house alone reduced the number of large pests (body length > 5 mm)such as the Brown Pod-Sucking Bug, Clavigralla tomentosicollis, Leafhoppers, Empoasca sp, Bean Pod Borer Maruca vitrata and small pests (body length < 5 mm) such as Bean Flower Thrips, M. sjostedti, Greenhouse Whitefly, Trialeurodes vaporariorum.Conversely, the population of Black Legume Aphid, A. craccivora was significantly higher inside the net house. The populations of T. vaporariorum and A. craccivora were lower in the push-pull treatment than in either control or net house treatments,respectively. The yield was significantly more abundant and the quality of the pods and grains of cowpea better in the net house than in the open field. This work showed that the volatiles from host or non-host plants can repel A. craccivora and M. sjostedti. The nethouse was effective in protecting cowpeas against most pests and improved the yield of pods. The repellent compounds identified in thisstudy could be used in the field through dispensers to improve the control of small insects in the net house.