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Evaluating the frequency of resistance alleles is important for resistance management and sustainable use of transgenic crops that produce insecticidal proteins from Bacillus thuringiensis. Helicoverpa zea (Boddie) (Lepidoptera: Noctuidae) is a major crop pest in the United States that has evolved practical resistance to the crystalline (Cry) proteins in Bt corn and cotton. The standard F2 screen for estimating resistance allele frequency does not work well for H. zea because successful single-pair matings are rare. In this study, we developed and implemented a modified F2 screen for H. zea that generates F1 progeny by crossing three laboratory susceptible female moths with one feral male moth instead of single-pair crosses. During 2019–2020, we used this modified method to establish 192 F2 families from 623 matings between susceptible females and feral males from Arkansas, Louisiana, Mississippi, and Tennessee. From each F2 family, we screened 128 neonates against discriminating concentrations of Cry1Ac and Cry2Ab in diet overlay bioassays. Based on these discriminating concentration bioassays, families were considered positive for resistance if at least five larvae survived to second instar, including at least one to third instar. The percentage of positive families was 92.7% for Cry1Ac and 38.5% for Cry2Ab, which yields an estimated resistance allele frequency (with 95% confidence interval) of 0.722 (0.688–0.764) for Cry1Ac and 0.217 (0.179–0.261) for Cry2Ab. The modified F2 screen developed and implemented here may be useful for future resistance monitoring studies of H. zea and other pests. [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]

Crops expressing Bacillus thuringiensis proteins have offered valuable crop protection benefits including reduced foliar insecticide use and crop yield protection. Helicoverpa zea (Boddie) (Lepidoptera: Noctuidae) has developed resistance to one or more Bt proteins, which consequently threatens these benefits. This has resulted in a renewed emphasis on the implementation of action thresholds for H. zea management decisions in cotton. Field experiments were conducted in College Station, TX; Stoneville, MS; and Blackville, SC, during 2020–2021 to evaluate current H. zea threshold recommendations for Texas, Mississippi, and South Carolina. Three Bt cotton technologies and five thresholds were evaluated. The Bt cotton technologies evaluated included non-Bt (NBT), Bollgard® 2 (BG2; Cry1Ac + Cry2Ab), and Bollgard® 3 (BG3; Cry1Ac + Cry2Ab + Vip3Aa). Across years and locations, within NBT cotton, all thresholds, including 20% oviposition, 6% fruiting forms injury, ≥3% large larvae, and preventive sprays, resulted in greater control of H. zea and higher yield and profitability relative to non-treated NBT cotton. The threshold using 6% injury to squares and bolls was the most effective in terms of yield relative to the preventive spray. Across years and locations, the BG2 cotton sprayed based on 20% oviposition or preventively resulted in greater control of H. zea , and all treatments resulted in a yield or profitability equivalent to that of the non-treated BG2 cotton. For BG3 cotton, spraying based on any of the thresholds was not beneficial. For NBT and BG2 cotton, thresholds of 20% oviposition, 6% injury to fruiting forms, or ≥3% large larvae provided a reduction in insecticide applications relative to the preventive spray treatment, suggesting that these thresholds were effective triggers for controlling H. zea. • NBT cotton sprayed based on the oviposition, injury, or larval threshold provided effective H. zea control. • BG2 cotton sprayed based on the oviposition threshold provided effective control of H. zea. • BG3 cotton sprayed based on either the oviposition, injury, or larval threshold was not beneficial. • NBT and BG2 cotton treated using the oviposition, injury, or larval threshold provided a reduction in insecticide applications. [ABSTRACT FROM AUTHOR]