Development of the pupal parasitoid Xanthopimpla stemmator (Thunberg)(Hymenoptera: Ichneumonidae) in various cereal stem borers (Lepidoptera)
Gitau, Ann Catherine Wanjiru
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Classical biological control (CBC) is a management strategy that employs natural enemies against exotic pests. The method has been used against chilo partellus (Swinhoe) (Lepidoptera: Crambidae), an introducted pest of maize in Africa, using the introduced larval parasitoid Cotesia flavipes (Cameron) (Hymenoptera: Braconidae). However, C. flavipes is not able to attack all stem borer species in the targeted areas. To complement its work, Xanthopimpla stemmator (Thunberg) (Hymenoptera: Ichneumonidae) a solitary pupal endoparasitoid, which attacks pupae of Lepidoptera stem borers, was imported from Sri-Lanka via Mauritius and South Africa to Kenya for laboratory trials. Xanthopimpla stemmator has successfully been established in Mauritius on Chilo sacchariphagus (Bojer) (Lepidoptera: Crambidae). The objectives of this study were to, examine preference of X.stemmator for C. partellus, Eldana saccharina Walker (Lepidoptera: Pyralidae), Busseola fusca Fuller and Sesamia clamistis Hampson (Lepidoptera: Noctuidae), to asses suitability of the four stem borers for the development of X.stemmator, to compare life tables and intrinsic rates of natural increase of X. Stemmator females mate more than once and whether multiple mating has an effect on longevity and sex ratio of progeny produced. Xanthropimpla stemmator females emerging from the pupae of the four stem borer species did not discriminate between the four stem borer pupae in dual and four choice tests. Pre-adult learning experience did not affect choice of the pupae by X.Stemmator females. Exposure of X. stemmator females to pupae of the four stem borer species showed that all hosts and host ages were acceptable. Eldana saccharina had a significantly lower progeny emergence as compared to the other three hosts. Parasitoids emerging from C. partellus developed faster while progeny from B. fusca had the widest wingsapn. Progeny production was significantly higher for X.stemmator females emerging from B.fusca, C.partellus and S.calamistis compared to E.saccharina pupae. Life table studies were carried out at 27± 2°C by giving pupae of B.fusca and C. partellus every day, to newly emerged X. stemmator females until the parasitoides natural death. The intrinsic rates of natural increase, net reproductive rate, mean generation time, finite rate of increase and doubling time of X.stemmator female parasitoides were statistically the same. The intrinsic rate of natural increase was 0.105 and 0.106 and the population multiplied 27.9 and 24.0 times in 31.4 and 30.2 days for females emerging from B.fusca and C. partellus respectively. Studies on mating frequency revealed that multiple mating is not common in X.stemmator females. The number of times females mated did not significantly affect longevity and sex ration of progeny. Male emergence was significantly high in females mated more than once. The study shows that X.stemmator could be used as a biological control agent against three major Lepidoptera stem borers in Kenya. The parasitoid could play a complementary role to C. flavipes but may not establish in areas where E. saccharina is the dominant species.