Susceptibility of field populations of Ostrinia nubilalis from Minnesota and Wisconsin to Cry1Ab, Cry1Fa, and Cry1A.105 Bt proteins

The European corn borer, Ostrinia nubilalis, is a major target pest of transgenic corn expressing insecticidal Cry proteins from Bacillus thuringiensis (Bt). The primary threat to the long-term sustainability of Bt corn is insect resistance. Between 2018 and 2024, cases of practical O. nubilalis resistance to Cry1 and/or Cry2 Bt proteins have been documented in Canada and United States. Additionally, high frequencies of Cry2Ab2 resistance alleles have been reported in regions of Wisconsin and Minnesota in 2023-2024. To assess the risk of O. nubilalis resistance to pyramided Bt corn, we need to understand the status of O. nubilalis resistance to the remaining Bt proteins currently used for their management. The resistance allele frequencies of Cry1Ab, Cry1Fa, and Cry1A.105 were evaluated using 84 F₂ families of O. nubilalis from Wisconsin and Minnesota during 2023-2024. We found no major but numerous minor resistance alleles to the three Cry1 Bt proteins in these 84 families. The frequency of major resistance alleles in the field population for each Cry1 protein was 0-0.0175 with 95% confidence interval, suggesting that resistance risk to Cry1 proteins in O. nubilalis is currently low but not negligible. However, the frequencies of minor resistance alleles in the field population were 0.0338-0.1088, 0.0625-0.1559, and 0.0383-0.1167 with 95% confidence intervals for Cry1Ab, Cry1Fa, and Cry1A.105, respectively. The potential role of minor resistance alleles in the evolution of Bt resistance is discussed. These findings are significant for understanding Bt resistance in O. nubilalis and the sustainability of Bt corn for managing this pest.