Everybody loves monarch butterflies. Author Anurag Agrawal refers to them as “the Bambi of the insect world.” They are specifically bred to be released at weddings; their image has been pressed into service as the symbol of environmental organizations like the Union of Concerned Scientists and the Non-GMO project.
This popularity makes them a great research system for two reasons. First, funding is abundant and easy to drum up. And, unlike other darlings of conservationists—like polar bears, which look like cute, cuddly stuffed animals—monarch butterflies are bugs. So animal rights activists don’t really get worked up when scientists breed them and experiment on them, then sacrifice them and grind up their bodies for analysis.
Regular donations and a lack of harassment from PETA, however convenient though they may be, are hardly the only reasons why Agrawal has devoted his life to studying monarchs or why he 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.
A toxic relationship
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.”
Milkweeds also produce cardenolides, steroids that bind to and deactivate the sodium transport proteins present in, and essential to, all animal cells. Monarchs can tolerate this toxin because they have a mutation in their sodium pumps. Four other species—two true bugs, one leaf beetle, and a leaf mining fly, all of whom also exclusively dine on milkweed—evolved the same exact mutation.
Scientists long presumed that monarchs held on to this mutation specifically to deal with their toxic food source, but it turns out that predator avoidance, and not diet, was the selective pressure that drove this adaptation. Agrawal uses this cautionary tale to warn us that just because associations intuitively make sense does not make them true. He reiterates this warning when he unveils his grand theory of monarch disappearance at the end of Monarchs and Milkweed.
Monarchs sequester the toxic cardenolides that they eat as caterpillars in their colorful wings when they emerge as butterflies. The cardenolides are extremely unpalatable to birds, one of the monarchs’ primary predators. The ability to hold on to the toxin this way is what pushed the monarch butterfly to evolve a sodium pump impervious to cardenolides.
Evolution and migration
The butterfly’s wing patterns and colors act to warn birds away. This strategy is known as aposematic coloration, and it’s so effective that species with non-toxic wings—notably viceroy butterflies—imitate the monarch’s coloration to ward off avian predators. This imitation, in turn, is so effective that it landed a picture of a viceroy butterfly, instead of the intended monarch, on the 50-peso banknote issued by the Mexican government in 2004. The banknote was replaced in 2012.
The other thing monarch butterflies are famous for, besides being pretty, is migrating. Over the course of two months, they travel over 18,000 miles, from all over North America to their overwintering grounds in the highlands of Michoacán, in central Mexico, a location discovered only in 1976.
Not all monarch butterflies migrate. Three generations live out their short lives in the United States and Canada, while those that are born in late summer get their reproductive systems put on hold and fly south for the winter. They head back north in the spring to mate and lay eggs. Why they do this is not clear, but one hypothesis is that the movement limits parasite loads in the population.
Monarch butterflies undoubtedly are pretty, but Monarchs and Milkweed provides more information on their feeding, mating, and travel habits than most people are probably interested in, although Agrawal writes in a folksy, accessible style. And as he himself points out, we’ve known most of this for over a century. So why’d he write his book now?
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.
It is certainly the case that (1) GMO and herbicide use, which negatively impact milkweed, have been on the rise; (2) milkweed is the monarch’s sole food source; and (3) monarch butterflies have been disappearing. Yet before we hasten to connect those dots, we must heed Agrawal’s warning: “Any factor, whether it is the stock market value or the number of Twitter posts, that shows a directional year-to-year increase will show a statistically significant relationship with monarch overwintering numbers (because they have been declining in a directional way over the same years).”
Linking the decline in monarchs to increased herbicide certainly makes more sense than linking it to increased Tweets or cell phone calls. But just because something makes sense doesn’t make it true.
Agrawal noticed that the decline in the monarch population happens during the autumn migration. At this point, the population is comprised of butterflies flying south—and butterflies don’t eat milkweed. Caterpillars do. (Monarch butterflies don’t actually eat at all. They just sip nectar, delicate creatures that they are.) The last leg of the butterflies’ journey takes them over Texas and Mexico, an area that experienced a severe drought from 2010-2014. Agrawal thinks that this drought may account for the decline in monarch numbers.
If Agrawal’s right, then planting milkweed probably won’t save the monarchs, since milkweed is not limiting. But it probably can’t hurt, and it may help if he’s wrong.
In any case, monarchs are still in decline. Perhaps next time Big Bird marches into Congress to fight to retain funding for PBS, he should take a monarch butterfly with him to plug for the EPA and NSF, too.