Susceptibility of Eldana saccharina (Lepidoptera: Pyralidae), Busseola fusca and Sesamia calamistis (Lepidoptera: Noctuidae) to Bacillus thuringiensis Cry toxins and potential side effects on the larval parasitoid Cotesia sesamiae (Hymenoptera: Braconidae)

Laboratory trials were conducted to determine the efficacy of four Bacillus thuringiensis (Bt) Cry toxins at five different concentrations (0.016, 0.08, 0.4, 2, 10 μg/mL) for controlling three lepidopteran stem-borer species (i.e., the pyralid Eldana saccharina and the noctuids Busseola fusca and Sesamia calamistis) as well as to evaluate their indirect effect on the braconid larval parasitoid Cotesia sesamiae. In addition, larvae from the treatments above, after having been parasitized, were either fed a contaminated (group 1) or a toxin-free (group 2) diet and compared with a control (i.e., parasitized larvae which have never fed on Bt-toxin). All Bt Cry toxins induced larval feeding inhibition. Compared with the control, significant mortality resulted at all concentrations and for all species of Lepidoptera. Cry1Ab was the most toxic with 10 days post treatment mortalities ranging from 81% in B. fusca and S. calamistis to 100% in E. saccharina. In contrast, Cry1Ac had comparatively low toxicity particularly for B. fusca and S. calamistis (e.g., respectively, 54 and 74% mortality at 10 days at the highest concentration). In the toxin-treated group 1, percentages of C. sesamiae cocoon-producing moth larvae were higher compared to group 2 and the control, whereas clutch size was higher than in group 2 but similar to the control. In both groups, 0.016, 0.08 μg/mL CrylAc yielded lower female-biased sex ratios than the control. It was suggested that paralyses of the moth larvae caused by the toxin may have facilitated parasitization leading to higher parasitism and clutch size.