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Freak Mexican Winter Kills Millions of Monarch Butterflies

Severe winter weather to blame for monarch deaths
weather.com, March 13, 2001

An unusually harsh winter storm in Mexico may have spurred rumors of environmental terrorism.

Environmental groups in Mexico have publicly accused loggers of spraying millions of monarch butterflies with pesticides in order to gain access to government-protected forests where the butterflies spend the winter.

But, Dr. Orley Taylor, professor of entomology at the University of Kansas-Lawrence and director of Monarch Watch, says such rumors are unfortunate, because there is currently no evidence to support the claims.

The storm in early March may have killed 70-90 percent of the monarch population in some areas, Taylor says.

Taylor says more tests on the dead butterflies are warranted, but so far, there is no physical or chemical evidence of pesticides. And because there are no eyewitnesses, Taylor says these rumors may just be speculation and miscommunication.

"Stories about poisonings started to appear in February," Taylor says. "Those stories were followed by another story that claimed massive mortality because of winter storms, which created confusion."

But only the story about the storm on March 2 and 3 is verifiable. "Temperatures dropped dramatically and there was snow even at low elevations in the mountains near Mexico City," says Gladys Rubio, meteorologist at The Weather Channel.

Monarch over-wintering sites have a specific set of conditions that help the butterflies survive through the season. The ideal conditions are found in forests on mountaintops in Central Mexico, nearly 2 miles above sea level. Under normal conditions, these forests provide the cool temperatures and moisture monarchs need, and serve as a buffer against snow and wind, according to Monarch Watch.

The storm of early March, however, created abnormal conditions.

"It's possible those spraying stories are misinterpretations of the natural mortalities of the butterflies," Taylor says. "Lots of monarchs die in over-wintering sites. When the weather warms up, they may move to a new location. Then, there are no live butterflies at the original location, but a carpet of dead ones, and people assume pesticides."