The commercial release in Australia, in 1996, of transgenic cotton containing the Cry1Ac gene of Bacillus thruingiensis Berliner var. Kurstaki (Bt), was a significant step towards reducing the use of conventional pesticides on cotton crops, for the control of Helicoverpa armigera. It is possible, however, that H. armigera could develop resistance to the Bt toxin, as has occurred with the use of other insecticides. The seasonal variations in the efficacy of Bt transgenic cotton, reported from field trials (Fitt et al 1994), could exacerbate the development of resistance (Daly, 1994), so there is a need for these variations to be better understood.