Diana Gitig: New Hypothesis about Declining Monarch Butterflies

Ars Technica

Anurag Agrawal ... has written a book about them called Monarchs and Milkweed: A Migrating Butterfly, a Poisonous Plant, and Their Remarkable Story of Coevolution. Monarches and their sole food source, the toxic milkweed plant, provide a great example of coevolution. Monarch caterpillars are born on milkweed leaves, where their mothers deposited them as eggs. They grow fat eating the plant. They are not pollinators; the milkweed gets nothing out of the relationship. In fact, the plant goes to great lengths to fight the monarchs. Milkweeds exude latex (the “milk” responsible for the eponym), which contains a noxious chemical cocktail suspended in the sticky rubber. A newly hatched monarch caterpillar’s teeth and feet can, and often do, get easily mired. “More than 60 percent of monarchs died in the burst of latex that accompanied their first bites into the plant,” Agrawal tells us. “Less than 10 percent make it to full size.”

Because monarch butterflies are disappearing. They have experienced a 75-percent drop in their numbers over the past 25 years, and a number of reasons for this decline have been suggested. One is that their overwintering grounds, already quite small, are being threatened by logging, hunting, cattle grazing, and climate change.

But the prevailing idea is that monarchs are disappearing because milkweed is disappearing due to urbanization, the expansion of agriculture, and especially the indiscriminate overuse of herbicides enabled by the advent of herbicide-resistant crops. Monarch caterpillars have nothing to eat, this idea goes, and so they die.

Despite the popularity and the appeal of this hypothesis, Agrawal does not buy it.