Decline Of Monarch Butterflies Linked To Modern Agriculture

Kate Prengaman | Ars Technica | June 5, 2014

Loss of milkweed plants in the midwest reduces caterpillar survival.

The massive migration of monarch butterflies is amazing—the insects go from grazing on milkweed plants as caterpillars in the midwest to spending winters in Mexico. But Monarch populations have been on the decline for some time, with a variety of factors being considered: lost habitat in Mexico, damage from pesticides, or climate change.

Conservation strategy for a species that traverses thousands of miles is complicated business, so a team of scientists from the University of Guelph decided to sort out which factors were the most responsible for the monarch's population declines—changes at the breeding grounds, the wintering sites, or climate changes.

Their conclusions suggest that we can't blame deforestation in Mexico for this environmental problem. The monarchs are suffering from a lack of milkweed, the only plant the caterpillars eat. In fact, a model built by the researchers suggested that monarch populations were four times more sensitive to the loss of milkweed on their breeding grounds than the loss of the forested habitat in which they spend the winters.