Information about Bt corn's impact on monarch butterflies is now available on a Web page www.ars.usda.gov/is/br/btcorn from the Agricultural Research Service. The core of the Web page is research coordinated by ARS and recently published in the Proceedings of the National Academy of Sciences.
The fact that Bt corn might present a risk to the monarch butterflies became a matter of scientific and public concern when a small study in 1999 indicated caterpillars suffered when given no choice but to feed on milkweed leaves heavily dusted with Bt corn pollen. Bacillus thuringiensis (Bt) is a soil bacterium used as an effective alternative to chemical insecticides for controlling moth pests.
According to entomologist Richard L. Hellmich, researcher with the ARS Corn Insects and Crops Genetics Research Unit in Ames, Iowa, two major questions needed to be scientifically answered to establish whether Bt corn actually posed a threat to monarch caterpillars: the direct toxicity of Bt pollen for caterpillars and the likelihood that caterpillars might be exposed to that much pollen.
The studies found monarch caterpillars are not very sensitive to pollen from most types of Bt corn, and that caterpillar exposure to Bt pollen is low. It took pollen levels greater than 1,000 grains of pollen per square centimeter (cm2) before there were any toxic effects in monarch caterpillars, and even greater levels before the effect was significant.
Caterpillars were found on milkweed in cornfields during the 1-2 weeks pollen is shed by corn, but corn pollen levels on these plants were found to average only about 170 pollen grains per cm2. Less than 1 percent of the milkweed leaves in cornfields had pollen levels exceeding 1000 grains per cm2 during pollen shed.
One variety of Bt corn--Bt 176--did have a toxic effect with pollen doses as small as 10 pollen grains per cm2. Bt 176 is one of the earliest forms of Bt corn and has never been planted on more than 2 percent of the corn acres. It will be completely phased out by 2003.