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Larger Refuges Needed for Bt Corn

Crop and Soil Environmental News, September 1998

Charles Hagedorn
Extension Biotechnology Specialist

The Environmental Protection Agency (EPA) has, for the first time, conditioned Bt corn approvals on the establishment of large refuges. Bt crops are transgenic crops that produce an insecticidal toxin by using genes from the bacterium Bacillus thringiensis (or Bt). The Agency's most recent approvals of Bt crops (Novartis's Bt popcorn and Aggrieve's Bt field corn) include requirements that farmers plant either 40 percent or 20 to 30 percent of their corn acreage in refuges of non-Bt corn, depending upon whether or not they spray with insecticides (40% if not sprayed and 20 to 30% if sprayed). Earlier agency approvals of several Bt corn varieties did not require refuges of specific sizes. By comparison, the refuges required for Bt cotton were only 4 percent unsprayed or 20 percent sprayed.

EPA's refuge requirements follow the recommendations developed by a group of federal and university entomologists working on insect resistance in corn systems. Their recommendations offer two refuge options. Farmers who prefer treating their non-Bt refuges with insecticides must establish non-Bt field corn or popcorn on at least 40 percent of their corn acreage. Farmers who choose not to spray are allowed to plant a smaller refuge. In either case, non-Bt refuges must be close to the Bt crop (within 1500 to 2000 feet for field corn and within one-half mile for popcorn).

The new EPA requirements are similar to recommendations in an earlier publication from the Union of Concerned Scientists entitled "Now or Never: Serious New Plans to Save a Natural Pest Control". The scientists who wrote the report recommended that 50 percent of a farmer's corn acres be planted in non-Bt corn refuges if treated with insecticides and 25 percent if not treated.

Non-Bt refuges are needed to delay the evolution of Bt resistance in pest populations. The refuges are havens where Bt susceptible insects are expected to survive. By mating with Bt resistant insects that develop on Bt corn, the susceptibles help dilute the resistance trait in the insect populations, thus making a larger insect population that is susceptible to the Bt toxin. The controversy has been over how much acreage should be devoted to non-Bt refuges, and where the refuges should be located in relationship to the Bt acreage.

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