Effects of sublethal concentrations of Bacillus thuringiensis on larval development of Sesamia nonagrioides

In the Mediterranean Basin, developed diapausing larvae of the borer Sesamia nonagrioides Lefèbvre are found consistently during autumn in transgenic corn, Zea mays L., expressing Bacillus thuringiensis Berliner (Bt) toxins derived from event 176. These larvae can feed on sublethal concentrations of Bt toxins because the expression of this corn toxin declines after flowering. To evaluate the possible consequences of this phenomenon, the effect of sublethal concentrations of the natural toxin producer, B. thuringiensis variety kurstaki (Dipel DF) on the performance of S. nonagrioides in the laboratory was examined. Larvae treated with Dipel DF showed higher mortality, longer developmental time, extra molts, and higher sensitivity to critical daylength for diapause induction than the untreated larvae. Accordingly, diapausing larvae collected in October and February in a Bt cornfield exhibited a higher number of supernumerary molts before pupating and longer diapause development than larvae collected in a non-Bt field at the same time. These results demonstrate that deployment of Bt corn in the Mediterranean may affect both the borer performance and the development of Bt resistance in addition to direct mortality caused by the Bt toxin.