Homeless Butterflies.

Each winter, hundreds of millions of monarch butterflies migrate from chilly regions in the U.S. to warm forests in southern Mexico. Now an alarming study claims the monarchs' winter home--which once covered thousands of acres of lush mountains--may be destroyed within 50 years.

Biologists have long voiced concern over the rapid destruction of the monarchs' winter habitat (home) by loggers and farmers--even in areas protected as butterfly sanctuaries since 1986. But the new study, which includes aerial photos snapped over three decades, has shocked scientists: Little more than half of the original forest remains. "From what I've seen there year after year, I predicted it would get worse," says Lincoln Brower, a monarch biologist at Sweet Briar College in Virginia. "But I didn't predict it would be this bad."

The Mexican government hopes to save the butterflies' roosts by expanding the protected area to more than three times its current size. But back in the U.S., scientists worry monarchs' spring and summer habitats may also be in peril.

Monarchs breed in the Midwest--just as young corn plants shed pollen (dust-like fertilizing cells). But one-quarter of U.S. cornfields are planted with genetically altered or transgenic seed; to protect corn crops from pests, scientists insert a gene from the bacterium (single-celled organism) Bacillus thuringiensis into the plant cells. The gene makes the corn produce its own "pesticide"--a toxin (poison) called Bt.

Some recent research shows that Bt-corn pollen could be deadly to the butterflies. Many monarchs ingest the toxin when they feast, on pollen-dusted milkweed--their main food source--growing at the edges of cornfields. But other studies show that Bt-corn has no affect on monarchs. "No one study is really definitive; you need an accumulation of evidence over time," says Jane Rissler, senior scientist at the Union of Concerned Scientists in Cambridge, Mass.

Little is known about the ecological effect of genetically altered corn, and more research is needed to determine the real threat it poses to monarch butterflies--and the environment.