MONARCHS AND MILKWEED

The relationship between monarch butterflies and milkweed plants, a story that is “much more…than bright coloration and a penchant for epic journeys.”

The monarch has an abiding fascination for scientists and nature lovers alike. An individual North America monarch may fly up to 3,000 miles from Mexico to Canada annually. Along the way, it will lay eggs on milkweed, which provide sustenance for the next generation of emerging caterpillars. Milkweed is toxic to sheep and horses but crucial to butterflies. Agrawal (Ecology, Evolutionary Biology, Entomology/Cornell Univ.) explains how the monarch and milkweed, both native to North America and likely dating back millions of years, “share a deep evolutionary history.” Their relationship is an example of “coevolution,” and the author shows how they “have spent millions of years evolving chemical traits and reciprocally coevolving in a manner that puts chemistry at the center of their arms race.” Birds that would otherwise feed on monarchs are made nauseous if the butterflies have fed on milkweed and therefore quickly learn to avoid them. In the course of their annual, cross-country flight, the monarchs lay their eggs on the plants, providing shelter, food, and safety for their caterpillars as they emerge. The author describes the extraordinary appetite of these monarch caterpillars, whose birth weight is comparable to that of a bread crumb but whose mass quickly increases more than 200 times in the first two weeks of its life. Over time, monarch butterflies have become impervious to the toxins released by milkweed to deter pests. In response, the plant has evolved an alternate strategy, releasing a blend of volatile compounds to attract wasps that feed on the caterpillars. As Agrawal accessibly demonstrates, this is exemplary of the arms race between predator and prey, which is an important driver of evolution.

A lively, highly informative introduction to significant research in ecology that highlights the importance of conserving our natural habitats.