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

Ostrinia nubilalis is an introduced pest in North America. Populations show variation in sex pheromone communication where strains use predominately E11- or Z11-tetradecenyl acetate and seasonal timing of voltine ecotypes categorized by the duration of post-diapause development (PDD). Larval damage to corn has been effectively controlled for decades with hybrids that produce Bacillus thuringiensis (Bt) crystalline (Cry) insecticidal proteins. Resistance to Bt field corn hybrids occurred in Canada, but not in the United States. Here, we documented in Connecticut the first instance of O. nubilalis injury of Bt sweet corn with Cry1Ab and Cry1A.105 + Cry2Ab2 that are active against O. nubilalis during 2023 and 2024. The severity of O. nubilalis injury in stalks did not differ between Bt and their non-Bt isoline hybrids during 2023. After flail-mowing in 2023 in accordance with resistance management, pressure was reduced in Bt hybrids but remained consistent in non-Bt isolines during the 2024 growing season. Genotyping of O. nubilalis collected in this study showed that the population sample in Connecticut is mainly E-pheromone strain with a long-PDD time. Genotype results at 4 single nucleotide polymorphism (SNP) markers within abcc2 that were previously reported as linked to O. nubilalis resistance to Cry1Fa corn, may not be linked to O. nubilalis resistance to Cry1Ab and Cry1A.105 + Cry2Ab2 corn. The resistance frequency, geographic distribution, and genetic basis of practical resistance to Cry1Ab and Cry1A.105 + Cry2Ab2 corn in Connecticut remain uncertain. The persistence and potential spread of Bt resistance threaten the sustainability of management practices.