Differential Responses of the Cowpea Aphid, Aphis craccivora, to Host Plant and Pesticidal Plant Odours

The black cowpea aphid, Aphis craccivora, is a key pest of Vigna unguiculata in tropical and sub-tropical countries. Olfactometry experiments were conducted in the laboratory to study the effects of host plant odours on apterae and alatae forms of A. craccivora. The host plants were fresh leaves of cowpea, V. unguiculata and groundnut, Arachis hypogaea, while the non-host plants were seeds of neem, Azadirachta indica and Pepper fruit, Dennettia tripetala. Both alatae and apterae spent significantly (P<0.001) more time and made more visits to the olfactometer arm emitting host plant odour than control in single choice bioassay. In response to non-host odour perception, the aphids exhibited significantly avoidance behaviour in terms of time spent and number of visits compared to the control. When V.unguiculata and A. hypogea volatiles were compared in a dual choice test, the time spent by alatae and apterae in the two olfactometer arms were significantly greater than the time spent in the control arms. However, the aphids did not show any preference for either of the two treatment thus demonstrating that both sets of volatiles have similar attractivity. The aphids were also significantly (P<0.001) repelled by odours from A. indica and D. tripetala compared to control arms in choice tests. This study showed that A. craccivora responded with positive anemotaxis to kairomonal cues emanating from its host plants, V. unguiculata and A. hypogaea, but was repelled by non-host volatiles. The result indicate that A. indica and D. tripetala oils have potential as repellents or a part of the integrated pest management strategies in the control of A. craccivora by resource-poor farmers in the tropics.