Resistance to B.t. Toxin Surprisingly Unpredictable
Defying the expectations of scientists monitoring transgenic crops such as corn and cotton that produce insecticidal
proteins derived from Bacillus thuringiensis (Bt), target insect pests have developed little or no resistance to Bt crops thus far. These findings suggest that transgenic Bt crops could enjoy more extended, more profitable commercial life cycles and that the measures established to mitigate resistance before the crops were introduced are paying off. The diamondback moth is the only pest to have evolved resistance to Bt sprays used by organic growers, but no pest has evolved resistance to transgenic Bt crops in the field. Entomologist Bruce Tabashnik of the University of Arizona whose research group recently completed a survey of this phenomenon in collaboration with scientists from Cornell University, was quoted as saying, "If I'd gotten up seven years ago and said that there would be no evidence of increased Bt resistance after Bt crops were planted on 62 million hectares [cumulative and worldwide], I would have been hooted off the stage. No one predicted that there wouldn't even be a minor increase, which is extraordinary." Nor has Monsanto seen any signs of resistance to transgenic Bt crops, despite widespread use in a number of countries. Graham Head, who is responsible for global coordination of insect resistance management at Monsanto, agrees with Tabashnik's explanation of these findings: "The use of refuges to manage resistance that tends to be recessive and have fitness costs is a highly effective means of delaying resistance." The primary resistance-preventive measure that farmers who plant transgenic Bt crops are required to take is to set aside some acreage as refuges on which they grow varieties of the same crop devoid of Bt toxin. Population genetics models indicated that such Bt-free refuges would permit susceptible insects to survive and swamp out resistant variants that might emerge from the pest population feeding on Bt plants in nearby fields. (Nature Biotechnology, Vol. 21, No. 9, via Agnet).
