WASHINGTON (AP) - Genetically engineered corn designed to kill an insect pest spreads enough of its pollen on nearby weeds to kill monarch butterflies, researchers said Monday in the latest study on the biotech crop's environmental effects.

Iowa State University scientists found that one in five monarch larvae died after being exposed to the toxic corn pollen for two days. Three days after the initial two-day exposure more than half of the larvae died.

"In the field you may have higher mortality rates because of the cumulative effect of being exposed to the toxin throughout the larval stage," said researcher John Obrycki, whose study appears in the Internet edition of the journal Oecologia. "Coming up with a good ecological assessment of this technology probably needed to be done before planting it across the Midwest."

The biotech corn, known as Bt corn for a bacterium gene that makes it toxic to the European corn borer, became controversial last year after a laboratory study at Cornell University showed it was toxic to monarch butterflies.

The corn's pollen coats the leaves of nearby milkweed plants that butterfly caterpillars eat.

The Cornell finding produced a public outcry in Europe and calls from environmental groups to curb the spread of gene-altered crops, but biotech supporters have criticized that study and the one from Iowa State for being unrealistic.

"Much of what (the Iowa State study) reports is based on analyses taking place in laboratory manipulations rather than field conditions," said Val Giddings, vice president of food and agriculture for the Biotechnology Industry Organization.

The Iowa State researchers planted breeds of biotech corn that produced different levels of the insecticide pollen along with some corn that hadn't been engineered. They then placed potted milkweed plants at varying distances from the fields. After two or three days they took the milkweed plants into the lab and exposed the butterflies to the plant leaves.

Significant numbers of butterflies died after exposure to leaves coated with pollen, while those placed with leaves that were washed or unexposed to the Bt pollen died at a normal rate.

Giddings said butterfly caterpillars are unlikely to encounter the pollen in nature in any great numbers and that the monarch butterfly population increased almost as much as the plantings of Bt corn last year.

An estimated 19 percent of the 80 million acres of corn growing this year was Bt.

Environmental groups have petitioned the Environmental Protection Agency to have farmers surround fields of Bt corn with buffer zones of conventional corn to prevent the pollen from spreading to milkweed.

"Continued inaction would clearly be lethal to monarch butterflies," said Rebecca Goldburg, a scientist at Environmental Defense, a group critical of genetically engineered crops.

The EPA has been reviewing its standard for biotech crops and plans to release recommendations in September, said Steve Johnson, deputy assistant administrator of the office of prevention, pesticides and toxic substances.

Early this year the EPA put new planting restrictions on Bt corn to prevent insects from developing resistance to the toxin. The restrictions require farmers to plant at least 20 percent conventional corn in most regions, and 50 percent in areas where cotton is grown.

In June, University of Illinois scientists placed black swallowtail butterflies near a farmer's field and found no evidence that they were harmed by the biotech corn. The swallowtail is considered less sensitive to the corn toxin than the monarch, but is more likely to be exposed to the pollen.

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On the Net:

Environmental Protection Agency: http://www.epa.gov

USDA's biotechnology site:
http://www.aphis.usda.gov/biotechnology

Biotechnology Industry Organization: http://www.bio.org

Environmental Defense: http://www.environmentaldefense.org