Monarch butterflies- the most iconic of American insects- have declined to perilously low numbers this winter:


Total Area Occupied by Monarch Colonies at Overwintering Sites in Mexico. The decline is statistically significant, and numbers that were bad 20 years ago would be considered good today.

This is horrific. We’ve lost over 80% of the butterflies.

The waning of our monarchs has lead to the inevitable speculations as to the cause. Which is fair enough. But I’d like to point out the ideas are just speculation.

No one really knows why Monarchs are declining, or why 2012-13 is such a bad year. 

Lacking the proper experiments to determine how various factors affect the butterflies, and lacking solid data about their milkweed food sources, assigning causes to the phenomenon simply isn’t possible. The monarchs are disappearing. That’s all we can say with certainty.

Candidate causes abound, though.

  • -The severe heat and drought of 2012 across much of the monarch’s summer range may have reduced the monarch’s host plants and stressed the remaining animals.
  • -The extensive use of agricultural herbicides in conjunction with herbicide-resistant crops could be eliminating the monarch’s food supply, perhaps causing the gradual decline over the past 10 years.
  • -The destruction of milkweed habitat for corn ethanol production, also a gradual new phenomenon in the past 10 years, could reduce the monarch’s food.


I had always kind of assumed we’d be able to blame the Mexicans for the eventual extinction of our treasured monarchs. After all, our eastern butterflies converge each winter on a fragile, postage-stamp patch of a forest near Mexico city. It wouldn’t take more than a couple days for los bulldozeros (or, whatever) to knock out the winter refuge for the entire monarch population. But since the Mexicans appear to be upholding their end of the migration, this loss may be on us.

People who care about North America’s most famous butterfly should be treating the decline as an emergency. One more bad year could spell the end.

In particular, we need two big things. First, we need the science. This means money to ramp up monitoring & experimentation. A lot of it. And soon. It’s hard to know how to bring the monarchs back without knowing why they are disappearing. Second, we need milkweed planted. A lot of it. And soon. While we don’t know why the monarchs are declining, the most plausible explanations involve the butterfly’s food sources, so reversing milkweed loss should be an obvious step.

I was going to add a photo of a monarch to this post, but the whole thing is just too damn depressing.