Frequency of alleles linked to Cry1F resistance in European corn borer (Lepidoptera: Crambidae) from the United States.

Corn genetically engineered to produce insecticidal proteins from Bacillus thuringiensis Berliner (Bacillales: Bacillaceae) (ie Bt corn) has toxicity against specific insect pests. However, field-evolved resistance to Bt crops has emerged in several species, including the European corn borer (ECB, Ostrinia nubilalis Hübner). The first case of practical resistance in ECB to Bt corn producing the Cry1F insecticidal protein was documented in 2018 in Nova Scotia, Canada. Six mutations in an ABC transporter gene (OnABCC2) were associated with this resistance; five of these caused nonsynonymous amino acid changes and one causes a two base pair deletion (TA12076-) resulting in a frameshift and premature stop codon. We genotyped 364 ECB individuals collected between 2022 and 2023 across the USA at these six loci and complemented the analysis with targeted OnABCC2 exome sequencing and ECB strain association analysis. Allele frequencies significantly varied across markers and locations. The TA12076-allele was detected in 2023 at a low frequency (0.01) across all sites. Targeted OnABCC2 exome sequencing identified 130 alleles, 41 of them which cause nonsynonymous changes, but no additional indels. Strain genotyping matched previously known ECB strain distributions but showed no association with Bt resistance Our data indicated that the TA12076- allele occurred at low frequency in U.S. populations, consistent with the absence of reported Cry1F field failures, as resistance is recessive. This indel was detected in heterozygous individuals from 3 populations (New Hampshire, Ohio, and Iowa). These observations underscore the importance of efforts monitoring for Bt resistance evolution and spread in ECB populations.