Susceptible bioassay base line and discrimination concentrations were determined for Bacillus thuringiensis (Bt) on a susceptible (SUS1) strain of Helicoverpa armigera using the diet infection method. Susceptibilities to commercial Bt subsp. kurstaki formulations in newly-hatched larvae of H. armigera collected from 6 counties of 5 provinces in China were tested in 1995. Results indicated that populations of H. armigera from Yanggu (Shadong), Handan (Hebei), Xinxian (Henan), Xiaoxian (Anhui) and Fengxian (Jiangsu) showed clear resistance to Bt. The LC50s increased slightly but the slope (b) decreased significantly compared with that of the susceptible strain. However, the Dongtai population (Jiangsu) remained susceptible. Resistance to Bt was diagnosed for the first time. The effects of transgenic cotton lines expressing Bt toxin on various populations of H. armigera using the leaf bioassay were also determined. The average mortality of newly hatched larvae of H. armigera (Yanggu and Xinxiang) with early resistance to Bt declined significantly (16-29%) compared with those of the susceptible strain. It is suggested that populations of H. armigera from Yanggu and Xinxiang were resistant to Bt and transgenic cotton expressing Bt toxin. A resistant management strategy for Bt is discussed.

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