Relative fitness of susceptible and Cry1A.105/Cry2Ab2-single-/dual-protein-resistant Helicoverpa zea (Boddie) (Lepidoptera: Noctuidae) on non-Bt diet and a diet containing a low concentration of two proteins.

Abstract

Helicoverpa zea (Boddie) is a destructive agricultural pest species that is targeted by both Bacillus thuringiensis (Bt) maize and cotton in the United States. Cry1A.105 and Cry2Ab2 are two Bt proteins expressed in a widely planted maize event MON 89034. In this study, two tests (Test-I and Test-II) were conducted to evaluate the relative fitness of Bt-susceptible and -resistant H. zea on non-Bt diet (Test-I and Test-II) and a diet containing a mix of Cry1A.105 and Cry2Ab2 at a low concentration (Test-II only). Insect populations evaluated in Test-I were two Bt-susceptible strains and three Bt-resistant strains (a single-protein Cry1A.105-, a single-protein Cry2Ab2-, and a dual-protein Cry1A.105/Cry2Ab2-resistant strains). Test-II analyzed the same two susceptible strains, three backcrossed-and-reselected Cry1A.105/Cry2Ab2-single-/dual-protein-resistant strains, and three F1 heterozygous strains. Measurements of life table parameters showed that neither the single- nor dual-protein Cry1A.105/Cry2Ab2 resistance in H. zea was associated with fitness costs under the test conditions. The single Cry protein resistances at a concentration of a mix of Cry1A.105 and Cry2Ab2 that resulted in a zero net reproductive rate for the two susceptible strains were functionally incomplete recessive or codominant, and the dual-protein resistance was completely dominant. The lack of fitness costs could be a factor contributing to the rapid revolution of resistance to the Cry proteins in this species. Data generated from this study should aid our understanding of Cry protein resistance evolution and help in refining IRM programs for H. zea.