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Evolution of resistance by Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) can reduce the efficacy of transgenic cotton producing Bacillus thuringiensis (Bt) toxin.The frequency of CrylAc resistance alleles and quantitative shifts in susceptibility of field H.armigera to Cry1Ac had been monitored by screening F1 and F2 isofemale offspring since 2002 in Xiajin County of Shandong Province (an intensive Bt cotton planting area) and Anci County of Hebei Province (a multiple-crop system including corn, soybean, peanut, and Bt cotton).