VOLUME 13, NUMBER 2
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Study Shows Bt Corn Has Little Toxic Effect On Monarch Caterpillars
This article appeared in The Grower,” December, 2001.
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A recent study by the U. S. Department of Agriculture’s Agricultural Research Service shows monarch caterpillars are not too sensitive to pollen from most types of Bt corn, and that caterpillar exposure to Bt pollen is low.
Bacillus thuringienses (Bt) is a soil bacterium used to control butterfly and moth caterpillar pests. A 1999 Cornell University study indicated monarch caterpillars suffered when they fed only on milkweed leaves dusted with Bt corn pollen. Following the study, the USDA conducted its own studies to answer two questions, says Richard Hellmich with the ARS’s Corn Insects and Crops Genetics Research Unit in Ames, Iowa. The ARS researchers wanted to find out the direct toxicity of Bt pollen for caterpillars, and the likelihood that caterpillars might be exposed to that much pollen. The ARS studies showed it took pollen levels greater than 1,000 grains of pollen per square centimeter before monarch caterpillars experienced any toxic effects. Caterpillars were found on milkweed in cornfields during the one to two weeks when corn sheds pollen. Corn pollen levels on these plants averaged about 170 grains of pollen per square centimeter. Less than 1 percent of the milkweed leaves in cornfields had pollen levels exceeding 1,000 grains of pollen per square centimeter during pollen shed. One variety of Bt corn, Bt 176, had a toxic effect with pollen doses as small as 10 grains of pollen per square centimeter. One of the earliest forms of Bt corn, Bt 176 has never been planted on more than 2 percent of the corn areas. It will be phased out by 2003. The Environmental Protection Agency, meanwhile, has approved the use of Bt corn for an additional seven years. “Bt corn has been evaluated thoroughly by EPA, and we are confident that it does not pose risks to human health or to the environment,” says Stephen Johnson, assistant administrator of the EPA’s Office of Prevention, Pesticides, and Toxic Substances. The EPA has increased environmental and compliance monitoring requirements. Companies marketing Bt corn seed will be required to monitor for the development of insect resistance, provide annual reports on the efficacy of resistance management plans, and implement remedial action plans if resistance is detected among pest populations.
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