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Abstract: The sugarcane borer, Diatraea saccharalis (F.), is a major maize borer pest and a target of transgenic maize expressing Bacillus thuringiensis (Bt) proteins in South America and the mid‐southern region of the United States. Evolution of resistance in target pest populations is a great threat to the long‐term efficacy of Bt crops. In this study, we compared the genetic basis of resistance to Cry1Ab protein in 3 resistant colonies of sugarcane borer established from field populations in Louisiana, USA. Responses of larvae to the Cry1Ab protein for the parental and 10 other cross colonies were assayed in a diet‐incorporated bioassay. All 3 resistant colonies were highly resistant to the Cry1Ab protein with a resistance ratio of >555.6 fold. No maternal effect or sex linkage was evident for the resistance in the 3 colonies; and the resistance was functionally nonrecessive at the Cry1Ab concentrations of ≤ 3.16 μg/g, but it became recessive at ≥10 μg/g. In an interstrain complementation test for allelism, the F1 progeny from crosses between any 2 of the 3 resistant colonies exhibited the similar resistance levels as their parental colonies, indicating that the 3 colonies most likely shared a locus of Cry1Ab resistance. Results generated from this study should provide useful information in developing effective strategies for managing Bt resistance in the insect. [ABSTRACT FROM AUTHOR]

Simple Summary: Evolution of insect resistance is the main threat to the sustainability of the Bt technology. Helicoverpa zea (Boddie) (Lepidoptera: Noctuidae) is a major target pest of Bt corn and cotton. The purpose of this study is to determine the frequency of alleles conferring resistance to Vip3Aa in H. zea in the mid-south region of the U.S. Using a modified F2 screen method, we found the major resistance allele frequency for Vip3Aa in H. zea collected from four southern states during 2019–2020 was 0.0155, suggesting that the frequency of Vip3Aa resistance alleles in H. zea is not rare in the field. Helicoverpa zea is a major target pest of Bt crops expressing Cry and/or Vip3Aa proteins in the U.S.A. Widespread practical resistance of H. zea to the Cry1 and Cry2 proteins makes Vip3Aa the only effective Bt protein against this pest. Understanding the frequency of resistance alleles against Vip3Aa in field populations of H. zea is crucial for resistance management and the sustainability of Vip3Aa technology. Using a modified F2 screen method by crossing susceptible laboratory female moth with feral male moth of H. zea, we successfully screened a total of 24,576 neonates from 192 F2 families of H. zea collected from Arkansas, Louisiana, Mississippi, and Tennessee during 2019–2020. We found five F2 families containing ≥3rd instar survivors on the diagnostic concentration of 3.0 µg/cm2 Vip3Aa39. Dose-response bioassays confirmed the high levels of Vip3Aa resistance in these F2 families, with an estimated resistance ratio of >909.1-fold relative to the susceptible strain. The estimated resistance allele frequency against Vip3Aa in H. zea for these four southern states is 0.0155 with a 95% CI of 0.0057–0.0297. These data should provide critical information for understanding the risks of Vip3Aa resistance in H. zea and help design appropriate resistance management strategies for the sustainability of the Vip3Aa technology. [ABSTRACT FROM AUTHOR]