Multiple and non-recessive resistance to Bt proteins in a Cry2Ab2-resistant population of Helicoverpa zea

Transgenic corn and cotton expressing Cry and/or Vip3Aa proteins from Bacillus thuringiensis (Bt) have been planted for managing lepidopteran and coleopteran insect pests for more than two decades in the U.S. Evolution of insect resistance to these Bt proteins is a major threat for the continued efficacy of Bt technology. Understanding patterns of cross-resistance and dominance of resistance in target insects is essential for resistance management and pest control. The bollworm, also called corn earworm, Helicoverpa zea (Boddie), is a major target pest of pyramided Bt crops in the U.S. In this study, we evaluated the responses of Cry2Ab2-resistant (RR), susceptible (SS), and heterozygous (RS) strains of H. zea to the Cry1Ac, Cry1A.105, Cry2Ab2 and Vip3Aa39 proteins using diet-overlay bioassays. The Cry2Ab2-resistant H. zea also possessed high levels of resistance to Cry1Ac and Cry1A.105, but were very susceptible to Vip3Aa. The resistance to Cry1 and Cry2 proteins in RR H. zea varied from incompletely recessive to completely dominant depending on the protein concentration. These data suggest that the RR H. zea possess multiple non-recessive resistance to Cry1/Cry2 proteins. These results have significant implications for understanding field-evolved resistance of H. zea to Cry1 and Cry2 proteins in U.S. Bt corn and cotton fields and should contribute to developing effective management strategies for control of H. zea.