BETTE HILEMAN

Most varieties of corn genetically modified to kill the European corn borer pose a negligible risk to monarch butterfly caterpillars, conclude six articles published in the Proceedings of the National Academy of Sciences. The corn, engineered to contain a gene from the bacterium Bacillus thuringiensis (Bt), produces a toxin throughout the plant.

PHOTO BY KENT LOEFFLER
PHOTO BY WAYNE SCARBERRY	NOT A HAZARD Consuming Bt pollen-dusted milkweed leaf won't prevent monarch caterpillars from turning into beautiful butterflies.

Research led by Mark K. Sears, professor of environmental biology at the University of Guelph in Ontario, finds that "in most commercial [corn] hybrids, Bt expression in pollen is low" and causes "no acute toxic effects at any pollen density that would be encountered" on milkweed, the monarch larvae's food supply.

Even if monarch larvae are feeding on milkweed in Bt corn fields, with the exception of one hybrid, "event 176," "there usually isn't much toxin in the pollen," says Sears. "You need well over 1,000 grains per cm² on the milkweed to get any effect at all from most hybrids."

Although these six papers are comprehensive, they are unlikely to end controversy over the monarch issue. "We can't conclude that the risks are negligible," says Karen S. Oberhauser, ecologist at the University of Minnesota and lead author of one of the papers. "One unresolved issue is whether or not monarchs are consuming another material," such as tassel, she says. Tassel generally has much higher levels of Bt toxin than pollen and has been found on most milkweed plants in cornfields. Larger larvae tend to consume anthers, but field studies so far have used smaller larvae, she explains.

Oberhauser is concerned that EPA will not have sufficient data to make a decision, expected in October, on reregistration of Bt corn hybrids.

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