Production and characterization of Bt Cry1Ac resistance in bollworm, Helicoverpa zea (Boddie)

Abstract

Laboratory-selected Bt-resistant colonies are important tools for elucidating Bt resistance mechanisms and helping to determine appropriate resistance management strategies for Bt crops. Here, two laboratory populations of Helicoverpa zea (AR and MR), resistant to Bt Cry1Ac, were established by selection with either Cry1Ac activated toxin (AR) or MVP II (MR) from an unselected parent strain (SC). Stable and high level resistance was achieved in AR but not in MR. AR was only partially cross-resistant to MVP II suggesting that MVP II does not have the same Cry1Ac selection pressure as Cry1Ac toxin against H. zea and that proteases may be involved with resistance. AR was highly cross-resistant to Cry1Ab toxin. AR was not cross-resistant to Cry2Aa2, Cry2Ab2-expressing corn leaf powder, Vip3A and cypermethrin. Toxin binding assays showed no significant differences, indicating that resistance was not linked to a reduction in binding. In response to selection, heritability values for AR increased in generations 4 to 7 and decreased in generations 11 to 19. While rearing on Cry1Ac treated diet, AR had significantly increased pupal mortality, a male-biased sex ratio, and lower mating success compared to SC. AR males had significantly more mating costs compared to females. AR had significantly higher fitness costs in involving larval mortality, weight, and period; pupal weight, period, and mortality compared to SC. Cry1Ac-resistance was not stable in AR in the absence of selection. In laboratory experiments with field-cultivated Bt and non-Bt cotton squares AR significantly outperformed SC. However, AR could not complete larval development on Bt cotton. Additionally, a significantly lower percentage of AR larvae reached pupation on non-Bt compared with SC. Diet incorporation bioassays indicated Cry1Ac was significantly more lethal to SC compared to AR; however, no differential susceptibility was observed in strains for gossypol. Combinations of Cry1Ac with gossypol, cotton and corn powder were synergistic against AR, but not against SC. These results may help understand the inability of AR to complete development on Bt cotton. These results 1) highlight the need to choose carefully the form of Bt protein used in experimental studies, 2) support the lack of success of selecting, and maintaining Cry1Ac-resistant populations of H. zea in the laboratory, and 3) aid in understanding why this major pest of cotton and corn has not yet evolved Bt resistance.