SNOWFALL IN CENTRAL MEXICO IMPERILS MONARCH BUTTERFLIES.

Byline: Julie Preston The New York Times

The monarch butterfly may look fragile, but underneath those delicate orange-and-black wings is one tough customer. Every year monarchs brave gales and downpours to fly from their summer haunts in Canada all the way across the United States to a winter refuge in central Mexico.

But monarchs are not made to survive snow. And Dec. 30, in a rare cold spell, almost 12 inches of snow fell in the fir-topped mountain reserves where tens of millions of monarch butterflies were clustered for their winter sojourn.

Environment officials of the Mexican government estimate the freeze killed up to 15 percent of the monarch population. But Homero Aridjis, a leading Mexican environmentalist, said the death rate could be as high as 35 percent of the colony.

The new monarch freeze is especially serious because it comes in the wake of an adverse migratory season for butterflies moving north from Mexico. There was a chilly spring and a dry, baking summer in the eastern United States. American butterfly watchers noticed a thinning in the monarch flocks.

"A combination of things this past year did not favor their reproduction," said Professor Orley Tayler, an entomologist at the University of Kansas and founder of a group called Monarch Watch whose volunteers around the United States track the butterflies' migrations.

"The problem with natural history is that a couple of disasters one after the other can suddenly become devastating to a whole population."

In an astonishing pilgrimage, monarch butterflies that spend the summer across Canada, Maine and the Great Lakes states all concentrate during the winter months in one region west of Mexico City, which straddles the states of Michoacan and Mexico.

In 1986 the Mexican government created five separate nature reserves in the area, on mountaintops at an altitude of about 11,000 feet, covered with the fir trees the monarchs prefer.

On Tuesday morning, in spite of rising temperatures, government forest rangers and biologists who toured part of the reserves said there were snow banks littered with thousands upon thousands of frozen monarchs, with many butterflies buried under the snow. According to a 1994 census, at least 60 million monarchs live in the reserves.

A similar snowstorm in 1992 killed as much as 80 percent of the butterflies in some spots in the reserves, according to monarch expert Lincoln Brower, a zoologist at the University of Florida at Gainesville. The loss of life caused an ebb in the monarch migrations to the northeastern United States the following spring. But the monarchs bounced back in 1993, Brower said.

But butterfly scientists are concerned that the slow logging of the fir trees in the Mexican reserves, despite government prohibitions, is leaving the monarchs more vulnerable to severe storms and cold. Local farmers who make a meager living by selling lumber and firewood have continued to quietly cut the trees.

Brower said his studies show that rain-dampened butterflies are much more likely to die in freezing weather than dry ones.

The loss of trees and their canopies makes it more likely that the butterflies will be exposed to rain and snow.

"The more these forests are degraded, the more holes there are in their blanket," Bower said.

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Photo (1) A park guard tries to warm a monarch butterfly in the El Rosario refuge. Associated Press (2) Dead monarch butterflies litter a snowy field in the El Rosario refuge. A cold snap has killed millions of monarchs at their winter haven. Associated Press