At least 1.5 million Monarch butterflies were hit with a deadly freeze this weekend as an unusual ice and wind storm moved through the mountains of Michoacán where the butterflies roost for the winter.   The storm hit just as the spring migration was beginning.   Luckily, many butterflies exited the mountains before the freeze arrived.

The scene at Chincua two weeks after the storm. Photo by Dr. Isabel Ramíro

Exactly how many butterflies perished in the freeze remains uncertain. An Associated Press report sounded upbeat, with Mexican authorities stating that “Monarch butterflies that winter in the mountains west of Mexico City survived the severe cold snap that hit the area this week.”

But the Mexican news agency El Universal on Saturday quoted Homero Gómez González, president of the administrative council that oversees the El Rosario butterfly sanctuary, as saying that 1.5 Monarch butterflies froze to death–about 3% of the estimated 50 million roosting.

According to Gomez Gonazaléz, the recent freeze registered temperatures of -12 degrees Celsius (about 10 Fahrenheit). Other reports had winds raging up to 50 miles per hour, leaving 13 inches of snow on the ground in some areas and taking out dozens of trees.  Those living in the area were without electricity for days and hundreds of lamb and sheep were lost.
“Historic snowfall at the El Rosario sanctuary,” read the headline of the el Rosario Facebook page on Thursday, March 10. “The Monarch butterfly suffers wind, snow, rain and sleet.” The post was accompanied by photos showing several inches of snow on the ground.

The news whipsawed those who follow Monarch butterfly news.  Monarch fans had been celebrating the much-anticipated announcement in February that the population of the migrating orange-and-black insects had tripled since last year.  Reports of the devastating freeze underscored the brutal reminder that Mother Nature is in charge.
Dr. Chip Taylor, founder of citizen science group Monarch Watch, which tags the butterflies during their fall migration, weighed in from Kansas.

“Information is still sketchy about the degree of butterfly mortality,” Dr. Taylor told the  DPLEX list, an email listserv of about 800 butterfly aficionados.

“Most claims, observations and images suggest that mortality is low to moderate,” said Dr. Taylor.  “There is no evidence to date to indicate levels of catastrophic mortality (70-80%) that followed the winter storms of 2002 and 2004.” he said, adding that it will take at least a week to get more accurate information on the number of butterflies lost.

Taylor also reminded readers that “a significant portion of the population had already left” the roosting sites prior to the storm.

Dr. Lincoln Brower, who has studied Monarchs his entire life and is one of a group who submitted a petition to have the butterflies listed as “threatened” under the Endangered Species Act, seemed less optimistic.

“The current statements that the Monarchs have survived the storm are premature,” wrote Dr. Brower via email in response to the Associated Press story.  “I fear that optimistic assumptions are driving the news reports.”

Like Dr. Taylor, Brower cautioned that time will tell the accurate mortality counts.

“Based on our study of the 2002 storm, the butterflies that are killed or irreversibly damaged keep falling out of their clusters for days after the freezing event. Mortality counts need to be made at least a week after the storm.”
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