Location of stemborer pupae in various host plants and implications for the performance of natural enemies with emphasis on the pupal parasitoid Xanthopimpla stemmator (Hymenoptera: Ichneumonidae)

Abstract

In order to predict host accessibility by the pupal parasitoid Xanthopimpla stemmator (Thunberg), four grass species (Sorghum bicolor, Pennisetum purpureum, Sorghum arundinaceum and Zea mays) were sampled for stemborer pupae in Kwale, in the low altitudes of southern Kenya, and in Trans-Nzoia, in the high altitudes of western Kenya. The pupal position of Chilo orichalcociliellus (Strand), Chilo partellus (Swinhoe), Sesamia calamistis Hampson, Sesamia sp. nr oriaula (Tams and Bowden) and Busseola fusca (Fuller) in the plant were determined in relation to (a) the distance of pupae from the edge of the stem (depth), (b) the distance between the moth-exit hole and the head of the pupa (location) and (c) the length of the tunnel from the moth-exit hole to the base of the tunnel. Pupal depth and location for C. partellus and B. fusca varied significantly in the different plant species tested, and the pupae tended to be embedded deeper in cultivated than wild hosts. On all host species, the borers were located at a depth less than 0.35 cm. Most C. orichalcociliellus and S. calamistis pupae were found pupating in the ears of maize or the upper part of the wild hosts' stem. Sesamia sp. nr oriaula was only collected from the lower parts of P. purpureum. For B. fusca, tunnel length varied significantly among plant species and was longer in cultivated hosts. Xanthopimpla stemmator has an ovipositor length of about 0.52 cm, thus it is anticipated that the parasitoid could easily reach and parasitize the pupae in these host species.