Good and bad news for migrating monarchs.

Each winter the Oyamel forests of central Mexico blaze orange with hundreds of millions of monarch butterflies that come to escape colder areas. Their stunning wings carry natural chemical tags that are enabling scientists to trace the insects to their birthplaces across eastern North America, giving clues about where monarch conservation is most needed.

Leonard I. Wassenaar and Keith A. Hobson of Environment Canada in Saskatoon, Saskatchewan, have used natural chemical tags to study songbird migration (SN: 12/5/98, p. 356). Their report in the Dec. 22, 1998 PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES marks the first time the technique has been applied to migrating insects.

"It will allow us to definitively answer questions that we have been just nibbling at for decades," says monarch specialist Sandra M. Perez of the University of Arizona in Tucson.

Until now, researchers' primary method for tracking monarch migration has been to paste tiny adhesive tags on the butterflies' wings while the insects are up north and hope to find the same insects later. This technique revealed the monarchs' general migration pattern, but only 125 out of hundreds of thousands of tagged butterflies have turned up in Mexico since 1975, when the winter colonies were discovered.

"With the [new] technique, every monarch butterfly you see has a `tag,'" Perez says. The chemical tags reflect different concentrations of hydrogen isotopes in rainwater across the continent.

The isotope called deuterium clutches an extra neutron in its nucleus and so is heavier than ordinary hydrogen. As a result, water molecules containing deuterium tend to fall out of clouds before those bearing hydrogen. For example, storm systems that begin over the Gulf of Mexico and work their way inland drop more deuterium along the coast than in mid-continent regions.

The tag is passed along as the water moves from soil to milkweed plants, the monarch caterpillar's main food. "You are what you eat, isotopically," Wassenaar says. The isotope ratio is constant from the plant to the caterpillar and, finally, to the adult butterfly.

Wassenaar and Hobson matched the isotope tags of 100 monarchs from each of the 13 winter colonies with those of butterflies captured at the breeding grounds the previous summer. The summer origins that the researchers identified revealed that each winter colony draws butterflies from all across the breeding range, "which is good news for the monarch," Wassenaar says. Logging threatens most of the Mexican forest colonies, and researchers had feared that loss of a single winter site could strike a fatal blow to an entire breeding population.

The bad news is that Mexico is not the only country with a monarch-conservation problem, says Orley R. Taylor of the University of Kansas in Lawrence. At least half of the winter migration stems from the U.S. agricultural heartland, between eastern Nebraska and western Pennsylvania. Open fields of corn and soybeans are popular spots for milkweed, but the plant may soon be wiped out as crops are bio-engineered to survive pesticides designed to kill all weeds, says Taylor. Loss of the milkweed would threaten the monarch's survival.