Field tests on managing resistance to Bt-engineered plants

Several important crops have been engineered to express toxins of Bacillus thuringiensis (Bt) for insect control. In 1999, US farmers planted nearly 8 million hectares (nearly 20 million acres) of transgenic St crops approved by the EPA. Bf-transgenic plants can greatly reduce the use of broader spe ctrum insecticides, but insect resistance may hinder this technology. Prese nt resistance management strategies rely on a "refuge" composed of non-Bt p lants to conserve susceptible alleles. We have used Bf-transgenic broccoli plants and the diamondback moth as a model system to examine resistance man agement strategies. The higher number of larvae on refuge plants in our fie ld tests indicate that a "separate refuge" will be more effective at conser ving susceptible larvae than a "mixed refuge" and would thereby reduce the number of homozygous resistant (RR) offspring. Our field tests also examine d the strategy of spraying the refuge to prevent economic loss to the crop while maintaining susceptible alleles in the population. Results indicate t hat great care must be taken to ensure that refuges, particularly those spr ayed with efficacious insecticides, produce adequate numbers of susceptible alleles. Each insect/Bt crop system may have unique management requirement s because of the biology of the insect, but our studies validate the need f or a refuge. As we learn more about how to refine our present resistance ma nagement strategies, it is important to also develop the next generation of technology and implementation strategies.