Monday, December 10, 2007 - 10:53 AM

Fitness costs of Bt Cry1Ac resistance in cotton bollworm, Helicoverpa zea (Boddie)

Konasale J. Anilkumar, and William J. Moar, Auburn University, Entomology and Plant Pathology, 301 Funchess Hall, Auburn, AL

Laboratory-selected Bt-resistant insect colonies are important tools for understanding possible Bt resistance mechanisms, fitness costs, and many other characteristics of resistance. Here, a laboratory strain (AR) of Helicoverpa zea (Boddie), resistant to the Bt protein (Cry1Ac) found in all commercial Bt cotton varieties in the US, was established by selection with activated Cry1Ac toxin. While conducting Bt Cry1Ac selection experiments, percentage of mating success in AR (40.91 5.55) was significantly (p<0.01) reduced over time with selection compared with the unselected parental strain (SC) (72.76 3.68). AR (0.59 0.09) males produced significantly (p<0.01) fewer spermatophores than SC (1.24 0.11). Further, reciprocal crosses with AR and SC strains show that both sexes in AR have fitness costs. When F1 AR was reared on diet containing no Bt protein, they had a significantly (p<0.05) longer larval (AR: 15.01 0.09; SC: 13.94 0.11) and pupal (AR: 12.02 0.12; SC: 11.39 0.1) periods (days). Further AR (74.38 13.46%) produced significantly (p<0.05) more malformed adults compared with SC (13.87 1.88%) highlighting the potential importance of refuge crops in delaying field resistance development. This study shows that there are fitness costs associated with Cry1Ac resistance in H. zea when feeding on Cry1Ac, and even after selection pressure is removed, fitness costs remain. These results aid in understanding why this major pest of cotton and corn has not yet evolved Bt resistance.

Species 1: Lepidoptera Noctuidae Helicoverpa zea (cotton bollworm, corn earworm)