Tatum Dwyer, Graham P Head, Krista Hamilton, Angie Peltier, Fei Yang
The European corn borer, Ostrinia nubilalis (Hübner), is an economically important pest of corn that historically caused over $1 billion annually in yield losses and control costs. Since 1996, widespread adoption of transgenic corn producing Bacillus thuringiensis (Bt) proteins has substantially suppressed O. nubilalis populations in the US Corn Belt. However, recent cases of practical resistance reported in Nova Scotia (Canada) and Connecticut (US) between 2018 and 2023 threaten the long-term benefits of Bt corn technology. Here, we used F2 screens and diet-overlay bioassays to estimate resistance allele frequency to the Cry2Ab2 Bt protein in O. nubilalis populations collected from multiple locations across Minnesota and Wisconsin during 2023 and 2024. We established a total of 84 F2 families by crossing field-collected insects with a susceptible laboratory strain and screened 7,466 neonates against Cry2Ab2 protein. We found that 21 of the 84 F2 families carried at least 23 major resistance alleles to Cry2Ab2, with an estimated resistance allele frequency of at least 0.1509 in the region. This value exceeds the <0.001 ideal resistance allele frequency for the pyramiding strategy by >150.9-fold. The high Cry2Ab2 resistance allele frequency observed in O. nubilalis may compromise the redundant killing underlying pyramided Bt technology, resulting in pyramided Bt corn functioning as single-trait Bt hybrids against O. nubilalis and potentially accelerating resistance evolution in the field. The data generated in this study provides critical information on current resistance risk in O. nubilalis and will help develop sustainable insect resistance management (IRM) strategies for this pest.