Evidence of field-evolved resistance in Ostrinia nubilalis to Bacillus thuringiensis Cry1Ab and Cry1A.105 + Cry2Ab2 sweet corn in Connecticut, USA

Authors: FISHER, KELSEY - Connecticut Agricultural Experiment Station; Coates, Brad; DOPMAN, ERIK - Tufts University; ROWLAND, DOMINIC - Connecticut Agricultural Experiment Station; Abel, Craig; SMITH, JOCELYN - University Of Guelph; DIVELY, GALEN - University Of Maryland

Submitted to: Journal of Economic Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/2/2025
Publication Date: 1/9/2026
Citation: Fisher, K.E., Coates, B.S., Dopman, E.E., Rowland, D., Abel, C.A., Smith, J.L., Dively, G. 2026. Evidence of field-evolved resistance in Ostrinia nubilalis to Bacillus thuringiensis Cry1Ab and Cry1A.105 + Cry2Ab2 sweet corn in Connecticut, USA. Journal of Economic Entomology. https://doi.org/10.1093/jee/toaf346.

Interpretive Summary: Corn is the largest grain crop grown in the United States, with an estimated value of $73.9 billion in 2024. The European corn borer in an invasive insect pest introduced into the United States in the early 1900s. Corn borer feeding damage had had devasted farmers, causing over $1 billion in yield losses each year. This changed when insecticidal Bacillus thuringiensis (Bt) proteins were expressed in transgenic corn to combat corn borer damage. Bt corn varies are now widely used by farmers to protect yields and increase profits, however, corn borers have developed resistance to Bt corn in southeastern Canada. Recently, ARS scientists and state agricultural researchers documented worms feeding on Bt corn at three locations in Connecticut, suggesting Bt resistance had spread to the United States. Genetic analysis of these insects determined Bt resistance was likely due to a novel gene. Since corn borer moths can move great distances to spread Bt resistance, they pose a threat to farmers across the United States. Successful early detection and monitoring by ARS provides crucial information to industry stakeholders that produce Bt corn and to regulatory agencies in the United .States. that work to contain the spread of Bt resistance. Continued monitoring and DNA testing of corn borers is important for protecting the tools corn producers use and their livelihoods.

Technical Abstract: The European corn borer, Ostrinia nubilalis (Lepidoptera: Crambidae), is an introduced pest in North America wherein populations show variation in sex pheromone communication were strains use E11- or Z11-tetradecenyl acetate, and seasonal timing of voltine ecotypes based on duration of post-diapause development (PDD). Larval O. nubilalis damage to corn has been effectively controlled for decades by planting hybrids that express Bacillus thuringiensis (Bt) crystalline (Cry) insecticidal proteins. Resistance to Bt field corn hybrids has occurred in the Maritime Provinces of Canada and subsequently found in the provinces of Ontario and Manitoba, but not in the United States. In the current report, we documented the first instance of O. nubilalis injury Bt sweet corn with Cry1Ab, and Cry1A.105 and Cry2Ab2 (Cry1A.105 + Cry2Ab2) that are active against this insect at field locations in Connecticut during 2023 and 2024. The number of larval entry holes and total tunneling length in stalks did not differ between these Bt hybrids and their non-Bt isoline hybrids during 2023 and 2024, with observed pressure reduced in 2024. Genotyping of 134 larvae collected from Bt hybrids, their isolines or wild plant hosts across years using genetic markers predictive of pheromone strain and voltinism showed that the population in Connecticut is mainly E-pheromone strain with a long-PDD time. Genotypes at single nucleotide polymorphism (SNP) markers in the ATP binding cassette (ABC) transport subfamily C2 gene (abcc2) detecting alleles previously linked to O. nubilalis resistance to Cry1F corn demonstrated that these alleles were significantly elevated in larvae from Bt corn compared to cohorts on non-Bt plants in both 2023 and 2024 for just one marker, SNP01. The marker SNP05 detecting a two-nucleotide deletion and predictive of a frameshift mutation in the mature abcc2 protein was rare, found in only 2 larvae collected from non-Bt corn. All putative resistance alleles in abcc2 detect by these SNP markers, with exception for SNP05, were also present among individuals in Cry1F and Cry1Ab susceptible laboratory strain. These results demonstrate that O. nubilalis with practical resistance to Cry1Ab and Cry1A.105 + Cry2Ab2 corn occur in Connecticut, but their frequency and geographic range in the population or genetic basis remains uncertain. The persistence or spread of Bt resistance in the O. nubilalis population threatens the sustainability of management practices used by growers.