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The hybrid crop, known as Bt-corn, is safe for human consumption and it does not seem to harm honey bees or ladybirds but it produces a pollen, dispersed by the wind, that can be harmful to monarch larvae.
``It's certainly a serious potential problem,'' John Losey, of Cornell University, said in a telephone interview.
``If it's really having an impact on a large proportion of the population (of monarch butterflies) I think it is a very serious problem.''
Bt-corn has genes from the bacterium Bacillus thuringiensis spliced into the plant genes, making it resistant to a hard-to-control pest called the European corn borer. Last year more than 7 million acres of the crop were planted by U.S. farmers.
The genetically modified (GM) plants produce a pollen containing crystalline exdotoxin from the bacterium genes. The pollen can be blown more than 60 yards onto plants outside the cornfields, including the milkweed that monarchs feed on.
Losey and his team fed monarchs milkweed dusted with the pollen from Bt-corn. Their research, published in the science journal Nature, showed the butterflies ate less than those fed on normal milkweed and nearly half of the larvae died.
Although the research is limited to laboratory tests and there is no evidence of what effect the transformed pollen has on monarch butterflies in the field, the study highlights some of the worst fears about the effects of GMOs on the environment.
``Monarchs are considered to be a flagship species for conservation. This is a warning bell,'' said Linda Rayor, a co-author of the study.
``Monarch themselves are not an endangered species right now, but as their habitat is disrupted or destroyed, their migratory phenomena is becoming endangered,'' she added.
Losey emphasized the need for more data and the need to look at the big picture. Although he does not support a moratorium on the planting of GM crops, he said the proven benefits in terms of increased yields and reduced rates of pesticides needs to be weighed against any potential risks.
``If we are going to allow them to go forward what we need is a commitment from the industries and the regulatory agencies and academia to get the data to be able to tell the effects of GM crops on the population,'' he said.
The British Medical Association Monday called for a moratorium on the planting of GMOs until scientists know more about their impact on the environment. Britain's Labor government said there is no evidence to justify a ban.
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