Subtle chemical traces in the wings of monarch butterflies have revealed that about half of the population hail from the U.S. corn belt, according to a study in the 22 December Proceedings of the National Academy of Sciences. New ways of eradicating milkweed, the butterflies' primary sustenance, could take a heavy toll on monarchs, researchers believe.

In what may be the world's most massive long-distance insect migration, each autumn tens of millions of the orange-and-black butterflies fly an average of 2500 kilometers, from southern Canada and the eastern half of the United States, to winter at a dozen hectare-sized sites in the mountains of central Mexico. Researchers have had some success at recovering tagged monarchs, but no one knew whether butterflies from particular locations in the North clumped at particular wintering sites.

Diet has now provided the key. Plant matter ingested during the caterpillar stage contains carbon and hydrogen isotope ratios linked to local rainfall patterns and climate. So chemist Leonard Wassenaar and biologist Keith Hobson of Environment Canada in Saskatoon, Saskatchewan, enlisted volunteers to hand-rear caterpillars on milkweed at dozens of sites throughout their breeding grounds. The researchers then compared the carbon and hydrogen in the wing membranes of the resulting butterflies to that from 597 dead adults at the winter roosts.

What they found was that about half of the dead monarchs grew up in a swath of the Midwest just a few hundred kilometers wide from Nebraska to Ohio. "This is a snapshot in time, so we don't know whether this reflects the historical pattern," says Wassenaar. Nevertheless, he notes, it raises concern that new herbicide-resistant corn and soy crops--which allow farmers to kill milkweed after plants have sprouted--could devastate monarch food supplies.

Entomologist Chip Taylor of the University of Kansas in Lawrence, director of the conservation group Monarch Watch, says he is "alarmed" by the finding: "We didn't know that so many monarchs come right out of the agricultural heartland, which is undergoing this tremendous change."

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** NOTICE: In accordance with Title 17 U.S.C. Section 107, this material is distributed for research and educational purposes only. **