Status of Cry1Ac and Cry2Ab2 resistance in field populations of Helicoverpa zea in Texas, USA.
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Helicoverpa zea is a major target pest of Bt corn and Bt cotton. Field-evolved resistance of H. zea to Cry1 and Cry2 proteins has been widely reported in the United States. Understanding the frequency of resistance alleles in a target insect is critical for Bt resistance management. Despite multiple cases of practical resistance to Cry proteins having been documented in H. zea, there are no data on the current status of alleles conferring resistance to Cry1Ac and Cry2Ab2 in field populations of this pest. During 2018-2019, a total of 106 F2 families for Cry1Ac and 120 F2 families for Cry2Ab2 were established using mass mating and light trap strategy. We screened 13,568 and 15,360 neonates using a discriminatory dose of Cry1Ac and Cry2Ab2, respectively. The results showed that 93.4% and 35.0% of the F2 families could survive on the discriminatory dose of Cry1Ac and Cry2Ab2, respectively. The estimated resistance allele frequency for Cry1Ac in H. zea ranged from 0.4150 to 0.4975 and for Cry2Ab2 ranged from 0.1097 and 0.1228. These data indicate that the frequency of alleles conferring resistance to Cry1 and Cry2 proteins in H. zea in Texas are high. In addition, our data suggest the resistance to Cry1Ac and Cry2Ab2 in the screened families of H. zea varies from recessive to dominant. The information in this study provides precise estimates of Cry resistance allele frequencies in H. zea and increases our understanding of the risks to the sustainability of Bt crops.
