One of the first studies into the environmental effects of GM crops found that caterpillars of the monarch butterfly suffered serious side-effects when they fed on leaves dusted with pollen from GM maize.
Nearly half of the caterpillars died and those that managed to survive grew to only about half their normal size, the scientists report in the journal Nature.
"These results have potentially profound implications for the conservation of monarch butterflies," write John Losey, Linda Rayor and Maureen Carter, all of Cornell University in Ithaca, New York.
The monarch is one of the most beautiful butterflies in the world and its extraordinary migratory route from Mexico to the US is already threatened by habitat loss. The scientists used the pollen of maize that had been genetically engineered with a bacteria toxin called Bt which is commonly used to protect GM crops against insect pests.
They dusted the pollen over milkweed leaves - the normal food of monarch caterpillars - to see if the toxicity was transferred with the pollen, which can be blown more than 60 metres from a maize field.
It took just four days for the side-effects to appear, and the scientists concluded that it must be due to the pollen being toxic because no similar effects were seen on caterpillars that had fed on pollen-free leaves.
Dr Rayor, an entomologist at Cornell, said that biotechnology companies had not previously tested the effects of GM maize pollen on butterflies although they had shown that the pollen was non-toxic to bees and ladybirds.
"Monarchs are considered to be a flagship species for conservation. This is a warning bell. What is really new in this research is that we have shown that toxins can float in the wind," Dr Rayor said.
Although there are no plans to introduce GM maize into Britain, biotechnology companies are hoping to gain permission to grow GM oilseed rape and other crops engineered with the Bt toxin.Reuse content