Carol Kaesuk Yoon, in the February 12 New York Times, reports that millions of Monarch butterflies lie dead in piles on the ground in their winter reserve in the mountains of Mexico. During a recent severe winter storm, between 220 and 270 million frozen butterflies fell from their roosts in the trees and now lie in piles more than a foot high.

?It was really macabre,? says butterfly biologist Dr. Lincoln Brower. ?I?ve been going down there for 25 years, and I?ve never seen anything like it.? This is the largest known die-off of Monarchs, but the loss is not expected to threaten the survival of the species, because other, smaller populations of Monarchs that do not migrate to Mexico can be found in the western United States.

Dr. Brower estimates that 74% of one major colony and 80% of another have been killed off. The butterflies now wintering in Mexico make up almost the entire breeding stock of Monarchs for the eastern U.S. and Canada.

Monarchs make the long trip to Mexico every winter, then return north in the spring to lay their eggs in the southern U.S. The butterflies that hatch out then continue the journey, and by summer they reach as far north as Canada. This migration is so unusual for such a delicate insect that it has been called an ?endangered biological phenomenon.?

The storm in the butterfly reserve dropped about 4 inches of rain on that part of Mexico on January 12 and 13, followed by freezing temperatures. Trees can act as an umbrella against the rain, and scientists have been warning against the thinning of trees in the area for years. In the last 30 years, almost half the forest in the butterfly reserve has been destroyed.

There are dead Monarchs in the trees, as well as lying on the ground. Brower says, ?Some of these clusters hanging on the trees are just all dead. It?s terrible.?

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