Bt corn damages butterflies in wild
New Scientist, 11 September 2001
Studies in North America's grain belt have for the first time shown that genetically-engineered maize can damage the growth of caterpillars in the field.
Laboratory studies in 1999 raised alarm when caterpillars of monarch butterflies died after being force-fed on leaves dusted with the pollen of maize engineered to produce the Bt insecticide. But the new studies are the first to show damaging ecological effects in and close to real corn fields.
Diane Stanley-Horn of the University of Guelph, Ontario, found that monarch caterpillars were damaged by typical levels of pollen falling onto the leaves of milkweed plants, their favourite food, within cornfields. They "exhibited 60 per cent lower survivorship and 42 per cent less weight-gain compared to those exposed to leaves from outside the field," she reports.
However, the experiment involved one of the most powerful versions of Bt maize, which is now being withdrawn from sale by Syngenta. This product type, known as "event-176", expresses up to 40 times more toxin than some other brands.
Other Bt maize types appeared to have little impact. "The only transgenic corn pollen that consistently affected monarch larvae was from event-176 hybrids", said Richard Hellmich of the US government's Agricultural Research Service in Ames, Iowa.
Another caterpillar, black swallowtail, was also affected by the event-176 hybrids. Entomologist May Berenbaum of the University of Illinois in Urbana reports "a significant reduction in growth rates of black swallowtail larvae that was likely caused by [Bt] pollen exposure".
Berenbaum says her results appear worrying because the effect showed up despite rainfall during the experiment that "removed much of the pollen from the leaves".
Berenbaum told New Scientist that she was not sure how policymakers should respond to the findings. She said such ecological impacts are "potentially manageable" but that "at the very least, event-176 should probably not be re-registered" with the US Environmental Protection Agency.
Syngenta, the company that makes event-176 hybrids, says they comprised only two percent of all the US corn grown in 2000. The company says all forms of agriculture have environmental impacts.Bt maize is the most widely grown transgenic crop in the US and was developed to fight the European corn borer, one of the crop's most damaging pests. But there has been continuing concern that it might be toxic to other insects.