Abstract:
Chilo partellus (Swinhoe) (Lepidoptera: Crambidae) is a major pest and a threat to cultivated poaceous plants in many pa1ts of Africa. The adverse ecological and environmental effects as well as the cost of chemical insecticides commonly used in its control, besides its limitations makes it necessary to explore environmentally friendly control methods. Classical biological control is one important approach in the control of exotic pests. A larval parasitoid Cotesia jlavipes Cameron (Hymenoptera: Braconidae) was introduced in Kenya from Pakistan against C partellus. Though, it has reduced borer density in overall by 32% and by 55% for C partellus, its establishment and effectiveness is limited for it does not complete development in Busseolafi1sca and Eldana saccharina which are serious stemborer pests. To increase borer suppression, Xanthopimpla stemmator (Hymenoptera: Ichneumonidae) a solitary pupal endoparasitoid which attacks lepidopteran pupae of stemborers was imported from Asia into Kenya for laboratory assessment and future release. The objectives of this study were, to detennine the location of pupae of C partellus, Sesamia oriaula, Busseola fi1sca and Sesamia calamistis Hampson (Lepidoptera: Noctuidae) in selected wild and cultivated host plants from different ecological areas, to evaluate host acceptance and preference of X stemmator to C partellus, S oriaula and Helicoverpa armigera Hubner (Lepidoptera: Noctuidae ), to assess its developmental suitability in the three borer species and to study the interaction between X stemmator and the indigenous pupal parasitoid, Pediobius fi1rvus (Gahan)(Hymenoptera; Eulophidae) in the laboratory. The results indicated that, there was no significant difference in the tunnel length between C partellus and Chilo orichalcociliellus (Strand)(Lepidoptera: Crambidae ). However, in the different plant species tested, B. fusca and S. oriaula had the longest mean tunneling length. Pupal depth and location of B. fi1sca, C orichalcociliellus, S. calamistis showed no significant difference in the different plant species tested. Pupal depth and pupal location for C. partellus differed significantly in the different plants. Chilo partellus was the most abundant borer species sampled in Kwale whereas, B. fi1sca was the most abundant borer species sampled in Trans Nzoia from the different sampled plant species. Most borer pupae were located in the upper part of the host plant stem except S. oriaula whose pupae were exclusively recovered from the lower part of Pennisetwn purpureum. Chilo orichalcociliellus and S. calamistis mainly pupated in the cob and upper part of the host plant stem. Laboratory choice and no-choice tests indicated that female X stemmator did not show any oviposition preference for C. partellus, S. oriaula and H. armigera. The female parasitoid indiscriminately accepted all the hosts. Studies on suitability revealed that the female X stemmator caused high pupal mortality in H. armigera and it successfully developed in C. partellus and S. oriaula but, did not develop in H.armigera. Xanthopimpla stemmator emerging from C. partellus developed faster than those from S. oriaula. However, X stemmator emerging from C. partellus lived longer than those from S. oriaula. Xanthopimpla stemmator progeny from S. oriaula had a wider wingspan and longer tibia length than those from C. partellus. More female X stemmator emerged from both hosts and the probability of getting female progeny increased with increase in host weight. Studies on multiparasitism showed that X stemmator successfully developed when exposed to C. partellus pupae at different time intervals and sequences. Developmental time of X stemmator progeny was slower when it stung pupae alone, but gradually shortened when P. fi1rvus was involved in the stinging of pupae at different sequences. Few P. furvus adults emerged at different sequences and the percent success of X stemmator parasitism increased with time interval in the Xs-Pf(Xanthopimpla stemmator-Pediobius .fi1rvus) sequence, but decreased m the Pf-Xs (Pediobius furvus-Xanthopimpla stemmator) sequence. Male to female proportion of P. furvus in both sequences did not differ from that of the control at the four time intervals. More X stemmator progeny were produced in the Xs-Pf sequence than in the Pf-Xs sequence. More male proportions were produced as the time interval of exposure increased in both sequences and progeny production decreased with increase in time interval of exposure. This study indicates that X stemmator could be used as a biological control agent against major and minor borer species in Kenya. The parasitoid could complement and supplement other borer control strategies.
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