Ento-musings from the University of Kentucky Department of Entomology

Saturday, March 23, 2013

We're Starving the Monarch Butterflies

A recent blog post by Chip Taylor, director of Monarch Watch, said: "All in all, it was not a good year for monarchs." 

You can say that again. A March 13th press conference held by the World Wildlife Fund-Mexico and CONANP (Comisíon Nacional de Áreas Naturales Protegidas) revealed that the total area occupied by Danaus plexippus in the Trans-Mexican Volcanic Belt over the winter of 2012-2013 declined precisely 59% since the previous season.

Granted, the monarch butterfly population has a tendency to fluctuate annually—even greater shifts have been observed in the past—but this past winter's numbers are the lowest since the monarchs' overwintering locality was discovered by science in 1975. Moreover, a very statistically significant decline has occurred overall in the past 19 years. 

Of course, we Americans' first instinct would be to point fingers at the Mexicans, who have chronically logged the subtropical pine-oak forest upon which D. plexippus depends; but, although conditions at the Monarch Butterfly Biosphere Reserve remain sub-optimal, according to the press release, "by protecting its sanctuaries and practically eliminating large-scale deforestation, Mexico is doing its part."

Concurrently, 25.5 million more acres of corn and soybeans were planted last year in the United States and Canada than as recently as 2006. What's the connection? Monarch caterpillars feed only on milkweed (Asclepias sp.) (along with a number of other insects), a plant that grows in unkempt grasslands, often occurring along the edges of roadways and fields; but this habitat is disappearing as cropland expands—not, apparently, due to a greater need for food, but because of our (yes, our) increasing demand for the biofuels that are vaunted as "green". 

The proximal cause of the 2012-2013 monarch decline was probably the severe drought of 2012: temperatures above 95°F are often lethal to D. plexippus caterpillars. In the long run, however, the decline of Asclepias is to blame. So: what's to be done? For Monarch Watch, the answer is straightforward: plant milkweed. Lots of it. It is urgently needed.

And no, this is not a plug for the politically powerful milkweed-grower's lobby. 


  1. That is a good point, Zach. Actually, several milkweed species make great garden plants, so people should consider planting them.

    1. Thanks for commenting! Hope you are doing well...