D3298: An extended investigation of Bt resistance allele frequency in southwestern corn borer and sugarcane borer in Tennessee and Louisiana

The southwestern corn borer (SWCB: Diatraea grandiosella) and sugarcane borer (SCB: D. saccharalis) are two economically important pest species targeted by Bt corn in the Americas. Bt resistance allele frequency (RAF) to common Bt transgenes including Cry1A.105, Cry2Ab2, and Cry1F in the two pests is still largely unknown in the United States. In this study, Bt corn leaf tissue was used to screen F₂ progeny from single-pairing or group mating of 97-148 SWCB individuals collected in corn fields in Tennessee and 178-208 SCB collected from sugarcane plants in Louisiana during 2019-2022. The object was to identify resistance alleles (RAs) to Cry1A.105, Cry2Ab2, Cry1F, Cry1Ab (SCB only), and a dual-protein Cry1A/Cry2A (SWCB only). The screen with SWCB identified one field individual likely possessing a RA to Cry1A.105, while no RAs to Cry2Ab2, Cry1F, and Cry1A/Cry2A were detected. The estimated RAF in the SWCB populations was 0.0068 with a 95% confidence interval of 0.0008 to 0.019 to Cry1A.105, < 0.0099 to Cry2Ab2, < 0.0155 to Cry1F, and < 0.015 to Cry1A/Cry2A. The leaf tissue screen with SCB did not detect RAs to Cry1A.105, Cry2Ab2, and Cry1Ab, and thus, the RAF to the three Bt proteins was estimated to be < 0.0075, < 0.0071 and < 0.0081, respectively. In contrast, most SCB F₂ progeny survived well on Cry1F corn leaf tissue. These findings suggest that, except for SCB against Cry1F, RAF to common Bt corn transgenes for controlling lepidopteran moths are relatively low in the two pest species in the region.