The indirect effect of Bt maize (Cry1Ab) on Cotesia sesamiae (Hymenoptera : Braconidae)

Abstract

Busseola fusca (Fuller) (Lepidoptera: Noctuidae), Chilo partellus (Swinhoe) (Lepidoptera: Crambidae) and Sesamia calamistis Hampson (Lepidoptera: Noctuidae) are major pests of maize and sorghum in sub-Saharan Africa. The main larval endoparasitoids of these economically important lepidopteran stemborer species are Cotesia flavipes Cameron and Cotesia sesamiae (Cameron) (Hymenoptera: Braconidae). Cotesia flavipes has successfully been introduced into several countries in eastern and southern Africa, including South Africa where it could not be recovered after the first winter following its release despite its initial temporary establishment. This species was recently found in Botswana, where it was never released. Releases made in neighbouring countries are assumed to be the reason for the presence of C. flavipes in this country. The first aim of this study was to determine if C. flavipes has now also established in South Africa. Busseola fusca, C. partellus and S. calamistis larvae were collected from 15 localities in South Africa. Cotesia spp. recovered from these larvae were identified by means of morphological identification as well as molecular analyses. The only Cotesia species recovered from all localities were C. sesamiae, which confirms previous reports that no C. flavipes has been recovered to date in South Africa. Genetically modified Bt maize was planted in South Africa for the first time during the 1998/99 growing season for control of stemborers. The first Bt maize resistant B. fusca larvae in South Africa was reported during the 2006/07 growing season. Cotesia sesamiae is indirectly exposed to Bt proteins that are consumed by stemborer larvae when parasitising Bt-resistant B. fusca larvae. The second aim of this study was to determine the effect of indirect third-trophic level exposure to Cry1Ab proteins on the fitness (in terms of reproduction) and survival of C. sesamiae. Bt-resistant B. fusca larvae were reared on Bt maize stems until the 3rd/4th instar and parasitised with C. sesamiae. The number of cocoons, number of wasps emerging from cocoons and the sex ratio (females:males) of wasps were recorded during the first experiment, while the mass of host larvae and developmental time of parasitoid larvae were recorded in addition to this during the second experiment. Results obtained during both experiments showed that Bt exposure had no significant effect on C. sesamiae life history parameters. Significantly higher numbers of female wasps did, however, emerge from parasitised B. fusca larvae that fed on Bt maize compared to those that fed on non-Bt maize (t=2.93; df=55; P<0.01) during experiment 1. Males can mate more than once and the increase in the number of females emerging from larvae that fed on Bt maize are therefore beneficial to the biological control of B. fusca. The use of Cotesia specimens, both C. sesamiae and C. flavipes, is therefore recommended in integrated pest management programmes for control of B. fusca on maize.