A screen of the F2 generation in discriminatory concentration bioassays to determine the frequency of alleles conferring resistance to the Cry1Ac, Cry2Ab2, and Vip3Aa39 Bt proteins in Helicoverpa zea

F₂ screening was conducted on H. zea populations in Texas and the Midsouth from 2018 to 2020. H. zea-F₂ families were screened on diet overlayed bioassays at the diagnostic concentrations of 10µg/cm² Cry1Ac, 10 µg/cm² Cry2Ab2, and 3µg/cm² Vip3Aa39. 128 neonates per family were exposed to each Bt protein for 7 days. Families resulting in 5 survivors (≥ 2^nd instar) with at least one at 3^rd instar were considered resistant. Chi-square analysis was used to compare the observed vs. expected survivorship of the F₂ families under monogenic inheritance to determine the number of resistant alleles per family. In two consecutive years, 99 out of 106 H. zea-F₂ families from Texas (93.4%) and 178 out of 192 from the Midsouth (92.7%) survived the diagnostic concentration of Cry1Ac, resulting in a resistance allele frequency of 0.4150-0.4975 and 0.6458-0.6953, respectively. 42 out of 120 from Texas (35%) and 74 out of 192 (39%) from the Midsouth survived the diagnostic concentration of Cry2Ab2, with a resistance allele frequency of 0.1097-0.1228 and 0.2526-0.2656, respectively. 2 out of 126 (1.6%) from Texas and 5 out of 192 in the Midsouth (2.60%) survived the diagnostic concentration of Vip3Aa39 and the resistance allele frequency was 0.0042 and 0.0130, respectively. A high frequency of Cry resistant alleles in H. zea jeopardizes the sustainable use of 3^rd generation Bt technologies. Adaptive resistance management is recommended to delay further increases in the Vip3A resistant allele frequency in this pest.