Loving Monarchs To Death

Posted on Feb 24, 2015


In recent years amateur conservationists have sought to replenish drastic declines in milkweed, the only plant female monarchs lay eggs on. But the most widely available milkweed for planting, is an exotic species called tropical milkweed — not the native species with which the butterflies evolved. That leads to unseasonal breeding, putting monarchs at higher risk of disease and reproductive failure.

Unlike most migrating species, monarch butterflies employ an improbable strategy that splits their round-trip migration between generations. So their life cycles must be intricately synchronized with those of the milkweed on which they lay their eggs.

“Nearly 60 percent of native Midwestern milkweeds vanished between 1999 and 2009”, the biologists Karen Oberhauser and John Pleasants reported in 2012 in the journal Insect Conservation and Diversity. The loss coincided with increased applications of the weed killer Roundup on expanded plantings of corn and soybeans genetically altered to tolerate the herbicide. Meanwhile, monarch reproduction in the Midwest dropped more than 80 percent, as did populations in Mexico.

With the loss of native milkweeds that die in the fall, monarchs are encountering tropical milkweeds that are still thriving. When monarchs encounter this lush foliage in the fall, they may become confused, start breeding and stop migrating.

It’s sad, because people think planting milkweed will help, but when milkweed is available during the winter, it changes the butterfly’s behavior. In my own work, I have found far more caterpillars on tropical plants in the winter than on natives in the summer. The close quarters place them at risk of serious infection, assuming they don’t starve or freeze first.

Butterfly enthusiasts should stop planting tropical milkweed and cut the plants they have back in fall and winter. Or even better, replace them with natives. Entomologists agree that planting the non native milkweed is risky.