Resistance management options for conventional Bacillus thuringiensis and transgenic plants in Australian summer field crops

Conventional Bacillus thuringiensis (Bt) is being increasingly used for control of Helicoverpa spp. in Australian cotton. Five years ago the annual usage of Bt in cotton was less than 10 000 l. It is now about 200 000 l but still represents only about 0.5 of a spray in cotton which normally receives 6-8 conventional insecticide sprays for Helicoverpa control each season. Bt use in other field crops has been minimal. The present risk for development of resistance to Bt in Helicoverpa spp. is, therefore, low. However, any increased use of conventional Bt on cotton or extended registrations to alternative Helicoverpa host crops could increase the risk significantly. Susceptible bioassay baselines and discriminating doses for conventional Bt on both H. armigera and H. punctigera are currently being established. The aim is to assess the natural variability in bioassay response (diet incorporation technique using early third instars) in a number of strains of both species from a number of geographical areas. This will allow monitoring of the impact of increased Bt use and/or the introduction of transgenic cotton on potential resistance problems. The ecology of H. armigera and H. punctigera is discussed in this paper, particularly in relation to the differential resistance risk of the two species where H. punctigera essentially 'manages its own resistance'. This is extended to the development of a possible resistance management strategy for H. armigera on transgenic cotton which will rely on continual dilution of rare resistant mutants by large numbers of unsprayed susceptibles allowed to breed in designated refugia areas. This high-dose/high-immigration resistance management approach will essentially maintain resistant H. armigera individuals as rare, hopefully functionally recessive heterozygotes. Two other factors will be critical for the success of this strategy: (1) strains of conventional Bt with different toxin profiles (e.g. Bt subsp. aizawai) should be used on non-transgenic alternative crop hosts of H. armigera (e.g. sorghum, sunflowers, grain legumes, maize, tomatoes and oilseeds), and (2) development of transgenic alternative crop hosts of H. armigera (especially sorghum and maize) should concentrate on toxins other than those from Bt.