Millions of Migrating Mexican Monarchs

Subtle chemical traces in the wings of monarch butterflies have revealed where they dine on their beloved milkweed before fluttering to Mexico for the winter. About half of the monarchs come from the midwestern U.S. corn belt, according to a study in tomorrow's Proceedings of the National Academy of Sciences. New ways of eradicating milkweed from those crop fields could take a heavy toll on monarchs, researchers believe.

In what may be the world's most extraordinary insect migration, each autumn tens of millions of the orange-and-black butterflies travel an average of 2500 kilometers, from eastern North America to a dozen hectare-sized sites in the remote wooded mountains of central Mexico. Researchers have tried to map these migrations by sticking tiny markers on the butterflies, but the odds of recovering a tagged monarch are notoriously low. So, no one knew whether each wintering site in Mexico housed a distinct part of the U.S. population, or whether the butterflies flew randomly to one of the enclaves.

Diet, however, can provide a clue. Because milkweed and other plants eaten by the caterpillars before they become adults contain carbon and hydrogen isotope ratios linked to rainfall patterns and climate, chemist Leonard Wassenaar and biologist Keith Hobson of Environment Canada in Saskatoon, Saskatchewan, checked these ratios in monarch wing membranes. They enlisted volunteers to hand-rear caterpillars on milkweed at sites throughout their breeding grounds, then compared the carbon and hydrogen in the wings of the adult butterflies to those gathered from 597 dead adults at the winter roosts.

The team found that most Mexican sites hosted butterflies from a broad range of birthplaces. However, about half of the monarchs came from a swath of the Midwest just several hundred kilometers wide, stretching from Kansas and Nebraska through Iowa, Illinois, and into Ohio. "This is a snapshot in time for 1997, 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 later in the growing season--could ultimately starve millions of monarchs.

"When I saw these results, I was alarmed," says entomologist Chip Taylor of the University of Kansas in Lawrence, director of the conservation group Monarch Watch. "We didn't know that so many monarchs come right out of the agricultural heartland, which is undergoing this tremendous change." Taylor and Wassenaar concur that an even graver threat is potential loss of the Mexican wintering sites from illegal logging.