Evidence of field-evolved resistance to Cry1Ac-expressing Bt cotton in Helicoverpa armigera (Lepidoptera: Noctuidae) in northern China.

BACKGROUND Evolution of resistance threatens the continued success of transgenic crops expressing insecticidal proteins. One of the key factors for successful resistance management is the timely implementation of monitoring programmes to detect early changes of resistance allele frequency in field populations. F(1)/F(2) screen, dose-response bioassays and field survey were used to monitor resistance to the Cry1Ac-expressing cotton in a field population of Helicoverpa armigera (Hübner), the primary target of transgenic Bt cotton in China. RESULTS Field survey showed an increased trend of egg populations of H. armigera on Bt cotton in the Qiuxian area from 2003 to 2007. By using the F(2) screening procedure, the resistance allele frequency in the Qiuxian (Hebei, China) population of H. armigera collected during 2007 was estimated to be 0.075 (95% CI: 0.053-0.100), which was 12 times greater than that estimated 9 years ago. Dose-response bioassay with the field population collected from the same area showed a significant resistance level (11-fold) to Cry1Ac toxin compared to a laboratory susceptible strain. CONCLUSION This study documented a case of field-evolved resistance in H. armigera after several years of intensive planting of Bt cotton. Proactive tactics must be adopted to prevent further increase of resistance gene frequency in the Qiuxian region.