A Modified F2 Screen for Estimating Cry1Ac and Cry2Ab Resistance Allele Frequencies in Helicoverpa zea (Lepidoptera: Noctuidae)

Abstract

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.

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

“…Feral larvae of H. zea were sampled from the fields in four southern states, Arkansas, Louisiana, Mississippi, and Tennessee, and the fresh larvae were shipped overnight to the Entomology Research Laboratory at Texas A&M University, College Station, TX during 2019–2020. Detailed collection data were listed in Table 1 as described in Santiago-González, et al [ 22 ]. In addition, the percentage of corn acres planted containing Vip3Aa was 10.3–13.2% in Arkansas, 3.2–3.3% in Louisiana, 2.8–4.9% in Mississippi, and 7.7–11.3% in Tennessee during 2019–2020 [ 15 ].…”

“…In a previous study, we used the high probability option of formula #3 in Andow et. al, [ 29 ] to calculate the frequency of alleles conferring resistance to Cry1Ac and Cry2Ab in H. zea , because the expected allele frequency of Cry1Ac and Cry2Ab is high in the field [ 22 ]. In this study, the Vip3A resistance allele frequency is expected to be low in the field [ 30 ].…”

“…The traditional F 2 screening method does not work well in determining resistance allele frequency for H. zea because the success of single-pair mating is very low [ 19 , 20 , 21 ]. To improve the efficiency of F 2 screening for this pest, we recently developed a modified method by crossing three laboratory susceptible females with one feral male moth of H. zea to estimate its resistance allele frequency [ 22 ]. Using this method, we successfully screened 192 F 2 families of H. zea against Cry1Ac and Cry2Ab2 Bt proteins with populations sampled from Arkansas, Louisiana, Mississippi, and Tennessee during 2019–2020 in a previous study [ 22 ].…”

“…To improve the efficiency of F 2 screening for this pest, we recently developed a modified method by crossing three laboratory susceptible females with one feral male moth of H. zea to estimate its resistance allele frequency [ 22 ]. Using this method, we successfully screened 192 F 2 families of H. zea against Cry1Ac and Cry2Ab2 Bt proteins with populations sampled from Arkansas, Louisiana, Mississippi, and Tennessee during 2019–2020 in a previous study [ 22 ]. We found the resistance allele frequency of H. zea in these four southern states for Cry1Ac and Cry2Ab was 0.217 and 0.722, respectively [ 22 ].…”

“…Using this method, we successfully screened 192 F 2 families of H. zea against Cry1Ac and Cry2Ab2 Bt proteins with populations sampled from Arkansas, Louisiana, Mississippi, and Tennessee during 2019–2020 in a previous study [ 22 ]. We found the resistance allele frequency of H. zea in these four southern states for Cry1Ac and Cry2Ab was 0.217 and 0.722, respectively [ 22 ]. Here, we extended our study to report the resistance allele frequency against Vip3Aa of these 192 F 2 families of H. zea collected from Arkansas, Louisiana, Mississippi, and Tennessee during 2019–2020.…”