How Cold Is Too Cold For Monarch Butterflies?

One of my more popular posts, at least in terms of comments, was the post titled “It’s cold, can Monarch’s survive.” I listed a reference that said Monarch Butterflies can crawl at 41 degrees Fahrenheit, and can fly at 55 degrees Fahrenheit. I’ve said from my own experience the caterpillars can survive freezing temperatures, but not for long.

If you’ve kept up with the Monarch populations that travel from Canada to Mexico to overwinter you already know that the population has been getting significantly smaller every year. However, this year, 2016, the Monarch’s overwintering in Mexico rose to 4.01 hectares compared to 1.13 hectares in the previous year (and even worse the year before that).  Sounds great right?

Well bad news. The Monarch Butterfly population just can’t seem to get a break. On March 10-11, 2016 a severe winter storm struck the Monarch Sanctuaries (Storm Decimates Monarch Sanctuaries). The wind was blowing up to 60 miles per hour, it delivered 2.85 inches of rain, and worst of all, the temperatures dropped to 23.9 degrees Fahrenheit.

At first, despite that severe storm, Mexico’s Environmental Protection Agency said “The monarch butterflies held up under the snowfall.” WRONG!  Read about the real impact from those on the ground and not in the government, Impact of the Storm, Observations from Cerro Pelon, A Sad Day in El Rosario by Estela Romero and Likely Storm Mortality in Mexico. Look at these pictures posted by Estela Romero Pictures. Dead Monarch Butterflies just cover the forest floor.

Let me go over the main points of what I learned and the important part of this post. If Monarch Butterflies are exposed to rain (or wetness) at 24.08 degrees Fahrenheit, 50% of those will die. If they are wet and exposed to temperatures of 17.6 degrees Fahrenheit, then 100% will die. Monarch Butterflies fare a little bit better if they are not wet and experience cold temperatures. If they are dry, only 50% will die if temperatures are 17.6 degrees Fahrenheit (remember 100% if wet), and 100% will die if temperatures reach 5 degrees Fahrenheit and they are dry. My references for all this information is from Likely Storm Mortality in Mexico.

In summary, things were starting to look a bit positive for the Monarch Butterflies this year where the population was at first estimated at 200 million. Then on March 10-11, a freak winter storm blew through the reserves with heavy rain, blistering winds, and freezing temperatures. This is literally right when the Monarchs prepare to leave the reserves and mate. It couldn’t have come at a worse time. Their fat reserves from hanging around in the trees all winter were at their lowest levels. Then the storm hits, and it’s estimated that 50% of the population died from that storm (100 million). So much for the good news:-(.

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