Dr. Ernest Williams, Hamilton College – Weeds and Monarch Butterflies

The storied mass migration of monarch butterflies is remarkable and well known. In the fall they fly from their breeding range in the eastern United States and Canada to overwintering sites in the mountains of central Mexico and then fly back the next spring.

Grim news about monarchs has reached the airwaves recently, however. Their abundance shows a significant downward trend, and measurements during the winter of 2012-13 revealed the fewest overwintering monarchs ever recorded. The monarch story illustrates how humans are altering the landscape in ways that harm other species.

Monarchs fly back each summer because of the abundant growth of milkweeds in the central U.S. and southern Canada. These are the only plants they lay their eggs on. But expanded planting of corn and soybean crops genetically modified to tolerate herbicides has resulted in increased use of these chemicals, and as a result, milkweeds are disappearing throughout the agricultural areas of the Midwest.

Monarchs are also losing their overwintering habitat. Descended from tropical ancestors, they cannot survive freezing, but they find just the right climatic conditions to survive the winter in the high elevation forests of central Mexico. There they congregate densely on fir trees. However, logging has thinned the insulating capacity of the forest canopy, increasing the risk of their freezing.

Monarchs are unique in their endangered migration across three countries. Just imagine: a butterfly that weighs only as much as a paper clip can fly more than 2000 miles to pass the winter away from freezing storms, and then its descendants fly back the next year to repopulate most of eastern North America. For this phenomenon to continue, people must protect the fir forests on Mexican mountains and plant and maintain milkweed patches in the summer breeding range.