Key factors in managing transgenic Bt cottons for the future are to have an effective resistance management strategy. An essential component of any such strategy is to establish a resistance monitoring program. The core components of this project address resistance monitoring as well as examine the performance of transgenic cotton (two genes) in relation to resistance management.

NSW Agriculture initiated the industries Bt resistance monitoring program in 1993. Intensive monitoring of Australian field populations of Helicoverpa armigera and Helicoverpa punctigera began in 1997 following the commercial release of the Bt transgenic crop, Ingard®. This report summarises results of the Bt resistance monitoring program between 1999 and 2002 and discusses possible directions for future research. In conducting the resistance monitoring program eggs are collected from all cotton growing districts throughout the season.

It is known that expression of the Bt protein, that produces the toxin Cry1Ac, declines throughout the crop cycle. The highest levels of protein expression occur during the early squaring (pre-flowering) period of growth. It is expected that this period also corresponds to maximum selection pressure against the toxin Cry1Ac. Although the decline in the Bt expression during the last half of the crop cycle reduces field efficacy against Helicoverpa spp selection for resistance is still expected to occur, thus Cry1Ac is under constant selection pressure.