Submitted to: Southwestern Entomologist
Publication Type: Peer reviewed journal
Publication Acceptance Date: 8/1/2006
Publication Date: 12/1/2006
Citation: Prasifka, J.R., Sumerford, D.V., Hellmich II, R.L., Lewis, L.C., Calvin, D.D. 2006. Sampling European Corn Borer (Lepidoptera: Crambidae) Larvae from Seed Corn Drying Bins for Bt Resistance Monitoring. Southwestern Entomologist. 31(4):269-279. Interpretive Summary: Crops genetically modified to produce insect toxins from the bacteria known as “Bt” (Bacillus thuringiensis) help prevent damage from serious pests, such as the European corn borer. However, as has happened with conventional insecticides, pests could develop resistance and overcome the advantage of Bt crops. To monitor for development of resistant pest populations, several techniques are used to collect and test potentially resistant insects. This study tests a new method of collection for larvae of the European corn borer (Ostrinia nubilalis), which can be found beneath drying ears of Bt corn in commercial seed production facilities. Collections of corn borer larvae beneath drying Bt corn did not appear to contain any individuals with genes that make resistance to Bt possible, though a relatively small number were tested effectively. The high rate of death of collected larvae (likely due to heat stress and disease) and the tendency of corn borer larvae to move between bins limit the efficiency of this sampling method. However, several changes to this technique could improve its potential as an additional tool for detecting the development of Bt resistance. This information is useful for all stakeholders interested in preserving the effectiveness of Bt crops (and Bt sprays) as pest control tools for agriculture.
Technical Abstract: 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 the inbred O. nubilalis 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 from 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, though 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.