Sampling European corn borer (Lepidoptera: Crambidae) larvae from seed corn drying bins for Bt resistance monitoring

Monitoring for resistance to Bacillus thuringiensis Berliner (Bt) toxins in transgenic crops is challenging, in part because alleles conferring resistance appear to be rare. Consequently, several complementary methods are used to identify, collect and test putatively resistant individuals. A series of experiments conducted at commercial seed production facilities explored an alternative sampling method. Larvae of the European corn borer, Ostrinia nubilalis Hübner, were collected from bins containing Bt hybrid seed corn and their inbred progeny (both F₂ and backcross-F₂ larvae) were tested for resistance to the Bt toxin Cry1Ab. Marked, laboratory-reared O. nubilalis larvae also were placed beneath drying corn ears to evaluate potential contamination of samples by larvae developing on non-Bt corn. Both feral and laboratory-reared larvae were used to examine the causes and levels of mortality of larvae in drying bins. Screening of larvae on diet containing Cry1Ab failed to provide evidence of resistance, although insufficient inbred lines survived to make conclusions about the presence of resistance alleles in larvae originally collected beneath Bt corn. Both larvae from previously dried non-Bt corn and O. nubilalis moving between adjacent bins are potential sources of contamination of larvae collected beneath drying Bt corn. Exposure to conditions inside seed corn drying bins for 3 d significantly increased O. nubilalis mortality. Larvae collected beneath seed corn also showed infection by the pathogens Nosema pyrausta (Paillot) and Beauveria bassiana (Balsamo) Vuillemin, with significant mortality apparently caused by B. bassiana. While contamination and mortality may limit the application of sampling beneath drying bins, several modifications could improve the potential utility of the technique.