Survey and assessment of Cry1F resistance alleles in US European corn borer populations

Bacillus thuringiensis (Bt) corn is genetically engineered to produce proteins with toxic and insecticidal characteristics towards specific insect pests. However, insects are rapidly evolving resistance to Bt crops, including European corn borer (ECB). The first case of practical Cry1F resistance to Bt corn by ECB was documented in 2018 in Nova Scotia, Canada. DNA mutations in ABC transporter genes have been linked to Bt resistance in other species. In ECB, six molecular markers (i.e. single nucleotide polymorphism, or SNPs) associated with resistance have been identified within the ABCC2 gene. These SNPs cause nonsynonymous changes, but one marker, SNP5, causes a two-base pair deletion, frameshift, and a premature stop leading to significant changes in the protein structure. We generated SNP genotypes from 364 ECB individuals collected across the USA in 2022 and 2023. The frequency of resistance-linked alleles was compared across all sites, and at three sites with multiple collections over 2 consecutive years. We observed high frequencies of resistant-linked alleles in our collections, ranging from 50% to 100%, except for SNP5. SNP5 was only detected in 2023 samples, with low frequencies of 7% in NH, 4% in OH, and 4% in IA. The low frequency of SNP5 in the U.S. aligns with the lack of reported field failures, suggesting that it could be the causal allele. This study provides data that will help further understand the evolution of Bt resistance in ECB, and how likely it will spread across the USA.