Increasing tolerance to Cry1Ac cotton from cotton bollworm, Helicoverpa armigera

1. Changes in the frequency of Cry1Ac resistance genes and shifts in tolerance of cotton bollworm, Helicoverpa armigera , to the Cry1Ac toxin were assessed using bioassays of F 1 and F 2 offspring of isofemale lines from Anci County of Hebei Province (a multiple-crop system including corn, soybean, peanut, and Bt cotton) and Xiajin County of Shandong Province (an intensive Bt cotton planting area) in Northern China during 2002 – 2005. 2. A conservative analysis of the overall results indicated that there was a small increase in the frequency of major, non-recessive resistance genes over time. 3. The relative average development ratings [RADR – growth rate of a line on a Bt diet in proportion to the growth rate on a non-Bt (NBT) diet] of the bollworm larvae in F 1 tests increased significantly from year to year, indicating a gradual trend towards higher tolerance to Cry1Ac in the field populations. 4. There were also significant positive correlations between RADR of the lines in the F 1 generation and the RADR of their F 2 offspring, indicating that the tolerance was genetically based. 5. Quantitative genetic simulation analysis showed that resistance of H . armigera to Bt cotton in Xiajin could evolve to a high level in 11 – 15 years if no effective resistance management measures are carried out.