Resistance to Transgenic Bacillus thuringiensis Crops in Target Insect Pests: Current Status and Prospect

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The ability to transfer foreign genes among unrelated species represents a major technological advance in modern agriculture. Since commercialized in 1996, transgenic Bacillus thuringiensis (Bt) crops (e.g. corn and cotton) have been widely used for managing insect pests. During 2008, a total of >40 million ha of Bt corn and Bt cotton were planted in 22 countries worldwide. Bt crops have been excellent in controlling their target insect pests. Growers of both industrial and developing countries have recognized great economic, ecological, and social benefits by planting Bt crops. Resistance development is considered to be a major concern to the long-term success of Bt crops. Major resistance genes capable of survival on commercial Bt crops have been identified in several target species. However, frequency of major Bt resistance alleles in field insect populations has been estimated to be low in most target insect species. In most cases, resistance to Bt crops is incomplete and controlled by a single recessive or partially recessive gene. Bt resistance is often associated with fitness costs in absence of selection pressures. The most common mechanism of Bt resistance may be due to a reduction in binding of toxins to cadherin receptors in the midgut. Field resistance development has been suspected in four target species. However, up to date, field control failures or reduced control efficacies due to resistance development have been documented in only two isolated cases, the fall armyworm, Spodoptera frugiperda, to Cry1F corn in Puerto Rico in 2006 and the African stem borer, Busseola fusca, to Cry1Ab corn in South Africa in 2007. Mandatory Bt resistance management plans have been successfully implemented in the United States, Canada, and Australia. Because of the substantial benefits achieved from planting Bt crops, it is widely expected that adoption of Bt crops will consistently increase rapidly, especially in the developing countries. Bt resistance management, therefore, will continue to be a big challenge for the future success of Bt crop technologies.