Fitness costs associated with Cry1Ac-resistant Helicoverpa zea (Lepidoptera: Noctuidae): a factor countering selection for resistance to Bt cotton?

The heritability, stability, and fitness costs in a Cry1Ac-resistant Helicoverpa zea (Boddie) (Lepidoptera: Noctuidae) colony (AR) were measured in the laboratory. In response to selection, heritability values for AR increased in generations 4-7 and decreased in generations 11-19. AR had significantly increased pupal mortality, a male-biased sex ratio, and lower mating success compared with the unselected parental strain (SC). AR males had significantly more mating costs compared with females. AR reared on untreated diet had significantly increased fitness costs compared with rearing on Cry1Ac treated diet. AR had significantly higher larval mortality, lower larval weight, longer larval developmental period, lower pupal weight, longer pupal duration, and higher number of morphologically abnormal adults compared with SC. Due to fitness costs after 27 generations of selection as described above, AR was crossed with a new susceptible colony (SC1), resulting in AR1. After just two generations of selection, AR1 exhibited significant fitness costs in larval mortality, pupal weight and morphologically abnormal adults compared with SC1. Cry1Ac-resistance was not stable in AR in the absence of selection. This study demonstrates that fitness costs are strongly linked with selecting for Cry1Ac resistance in H. zea in the laboratory, and fitness costs remain, and in some cases, even increase after selection pressure is removed. These results support the lack of success of selecting, and maintaining Cry1Ac-resistant populations of H. zea in the laboratory, and may help explain why field-evolved resistance has yet to be observed in this major pest of Bacillus thuringiensis cotton, Gossypium hirsutum L.