Resistance Allele Frequency of Helicoverpa zea to Vip3Aa Bacillus thuringiensis Protein in the Southeastern U.S.

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 F(2) 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. ABSTRACT: 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 F(2) 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 F(2) families of H. zea collected from Arkansas, Louisiana, Mississippi, and Tennessee during 2019–2020. We found five F(2) families containing ≥3rd instar survivors on the diagnostic concentration of 3.0 µg/cm(2) Vip3Aa39. Dose-response bioassays confirmed the high levels of Vip3Aa resistance in these F(2) 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.