No abstract

Th e development of resistance to Bacillus thuringiensis (Bt) in insect populations in agriculture not only depends on the level of resistance conferred by a selected resistance mechanism, but also on the fi tness cost associated with the resistance mechanism under specifi c ecological and environmental conditions. Bt resistance in the cabbage looper (Trichoplusia ni), which was identifi ed by Janmaat and Myers (2003), is a case of Bt resistance evolved in an agricultural system, and is used in this chapter to review and discuss the mechanism of Cry1Ac resistance that is selected in an agricultural environment.

Transgenic cotton that expresses a gene derived from the bacterium Bacillus thuringiensis (Bt) has been deployed for combating the cotton pests Helicoverpa armigera and Pectinophora gossypiella since 1997 in China. The pest management tactics associated with Bt cotton have resulted in a drastic reduction in insecticide use. However, the evolution of resistance by the pests threatens the continued success of Bt cotton. The development of resistance to Bt is of great concern, and there is a vast body of research in this area aimed at ensuring the continued success of Bt cotton. Here, we review studies on the evolution of Bt resistance in these two bollworms, focusing on the commercial release of Bt cotton varieties in China, and the biochemical and molecular basis of Bt resistance. We also discuss resistance management strategies, and monitoring programmes implemented in China and other countries.

The evolution of resistance to Bacillus thuringiensis (Bt) maize by the African stem borer, Busseola fusca, in South Africa highlighted the importance of the development of appropriate integrated resistance management (IRM) strategies for stem borers in Africa. Landscape heterogeneity is characteristic of African agroecosystems. This heterogeneity, in addition to between field and within-field spatial mosaics resulting from variable gene expression in Bt maize, will provide challenges to managing resistance evolution of the lepidopteran stem borers that attack maize. Adding to this landscape heterogeneity is the cultivation of open-pollinated maize varieties (OPVs) and bimodal rainfall patterns that allow two maize cropping seasons each year in many subtropical and tropical areas. The role that these factors, as well as aspects such as low-dose expression events, refuge compliance, the genetic bases of resistance, pest behaviour, host plant range and farming practices, may play in the evolution of stem borers to Bt maize in Africa are addressed in this chapter.

Resistance to the microbial insecticide Btk occurred in a number of laboratory populations of Lepidoptera before it was discovered in field populations. Therefore, it is not surprising that cabbage loopers (Trichoplusia ni) in semi-contained vegetable greenhouses were among the first examples of Btk resistance in agricultural situations. We have studied the occurrence of Btk resistance in cabbage loopers in greenhouse populations and the movement of resistance among greenhouse populations of moths. Migratory populations of cabbage loopers remained highly susceptible to Btk sprays in fields in the vicinity of greenhouses. Complete clean-up of greenhouses to remove any overwintering moths is necessary to reduce selection for resistance. Cabbage loopers represent a model system for the study of Btk resistance in contained, seasonal environments.