Monarchs and Milkweed: A Migrating Butterfly, A Poisonous Plant, and Their Remarkable Story of Coevolution

By Anurag Agrawal

Princeton University Press, 2017; 296 pages; $29.95

The familiar orange and black monarch (Danaus plexippus) is a natural history icon--and with good reason. Though individual butterflies weigh less than a dollar bill, monarchs by the millions migrate each autumn en masse from the eastern United States and Canada to the high country of central Mexico, a journey for some of as much as 2,600 miles [see “Monarch Migration” June/July 1977 and “Monarchs and I” July/August 2016]. Their winter refuge, which was discovered only in 1975, elicits a sense of wonder from Cornell University entomologist Anurag Agrawal, who has visited the area: “When they fly from their roosting trees, the whoosh of enumerable butterflies feels like the gentlest touch of a feather. Their bodies are like glitter in a snow globe.”

Oddly, these ethereal creatures rely on a bitter, toxic plant for their sustenance and survival. Monarch caterpillars feed exclusively on the common milkweed, which has evolved a formidable arsenal of defenses to discourage most herbivores from taking it out for lunch. Newly hatched monarchs need to penetrate leaf surfaces carpeted with prickly hairs called “trichomes” and then deal with a smothering, corrosive latex that oozes from those leaves when they are cut. More than 60 percent of monarch caterpillars never survive their first bite, but if they do, they must find a way to deal with bitter, poisonous chemicals called cardenolides that milkweed manufactures in profusion.  

Monarchs have not only evolved a tolerance for milkweed poisons, they have developed ways to turn them to their advantage. Rather than excreting cardenolides in waste pellets, monarch caterpillars and butterflies sequester the chemicals in body tissue, making them unpalatable to in-sects and birds that might otherwise find them attractive prey. Distinctive coloration helps warn prospective predators not to take even a taste, a strategy whose effectiveness has spurred flattering imitation: the viceroy butterfly masquerades as a monarch in a classic example of evolutionary mimicry, though it neither sequesters cardenolides nor dines on milkweed.

Agrawal, a chemical ecologist, writes with insight and clarity about the eons-long “arms race” between milkweed and monarchs, the plant developing defenses (monarchs provide little or no benefit in pollination), the butterfly finding ways to get around them, and other spe-cies expressing spin-off effects. The interaction between such well-observed species, he notes, not only provides an excellent model for studies of coevolution, but also for issues of conservation. Citizen scientists have been tagging monarchs and planting milkweed for years, providing a large body of data for ecologists and conservationists. Consequently, we know that there’s been a gradual decline of monarch populations in North America, a decline whose origin is not yet well understood. Fortunately for those enchanted by these delicate creatures, sustainable populations have been established around the globe. There may be imbalances in the milkweed-butterfly community here and there, but we’re likely to enjoy the magic of monarchs for a long time to come.