Save the Very Hungry Caterpillars

Common insecticides are killing monarch butterfly larvae, says a USDA study.

Photo: Brett Whaley

Monophagy—relying entirely on a single food source—is a risky survival strategy for a species. Some make it work, like the plentiful snail kite, a hawk that subsists on freshwater snails. But other monophagous species find themselves in trouble when humans interfere with their sole form of sustenance. Just ask a giant panda—if you can find one—about the declining stock of bamboo.

Monarch butterflies have also gambled on monophagy. They lay their eggs on milkweed, a wide-ranging perennial, and after hatching, monarch caterpillars feed exclusively on the plant. A few decades ago, milkweed blanketed much of the American landscape, so putting all your evolutionary eggs in the milkweed basket wasn’t such a terrible idea. The advent of crops genetically modified to tolerate weed-killing herbicides, however, has enabled farmers to spray huge amounts of herbicide on their fields. Milkweed has suffered greatly. A widely cited 2012 study found that 58 percent of Midwestern milkweed disappeared between 1999 and 2010, leading to an 81 percent decline in monarch reproduction.

Photo: USGS

The conventional wisdom is that herbicides, such as Roundup, are the main threat to monarchs in the United States—if we stopped killing milkweed with glyphosate and its ilk, we could restore milkweed to the vast U.S. breadbasket and the path for one of the world’s most impressive migrations would be safe.

A recent study, however, suggests that herbicides are just the jab in a one-two punch that American farmers are delivering to monarch butterflies. In addition to herbicides, farmers spray large amounts of a family of pesticides known as neonicotinoids, named for their chemical similarity to nicotine. Neonics are effective because, after the chemical is taken up by a crop, it becomes part of its pollen, nectar, and leaves. The insecticide effectively makes the plant itself toxic to pests.

Neonicotinoids already have a bad name among conservationists because they have contributed to the alarming decline in bee populations. The chemicals are extremely toxic to certain bee species, like the blue orchard and alfalfa leafcutter bees and several varieties of honeybees. Over the last eight years, bee colonies have declined by nearly 30 percent annually.

Neonicotinoids now appear to be wounding monarch butterflies as well. Earlier this month, a pair of scientists, including one from the U.S. Department of Agriculture, published a study showing that the concentration of neonics in milkweed found adjacent to fields sprayed with the insecticide is high enough to stunt caterpillar growth.

Jonathan Lundgren sampled milkweed near South Dakota cornfields and found that the majority of plants contained detectable levels of clothianidin, a type of neonicotinoid. The average concentration in those plants was slightly more than one part per billion, which sounds like a negligible amount. When it comes to poison, however, assumptions can be dangerous. Lundgren’s laboratory research on butterflies showed that a dose of clothianidin at one part per billion at the wrong time can hinder a caterpillar’s development. Individuals exposed at that level were shorter, lighter, and had smaller heads than their uncontaminated peers. Lundgren also speculated that the high rate of infant mortality—er, larva mortality—among monarchs in his study might also be related to clothianidin.

Critics of this study might point out that the plants were extremely close to the cornfields—about 1.5 meters on average—and that the concentration of neonics might drop sharply as you travel away from farmland. The problem with that argument is that you can’t get very far away from farmland in the middle of the United States, which is a major transit route for migrating monarchs. In the Corn Belt, 58 percent of land is dedicated to crops. In the Northern Plains it’s 53 percent. Those amber waves roll on for a very long time.

Photo: Randy Wick

Making matters worse, monarchs have a preference for milkweed in and around agricultural fields, for reasons we don’t fully understand. Scientists think the monoculture of cornfields may work as a cue to the butterflies, making the milkweed easier to identify, or perhaps, fertilized farm milkweed is more appealing than its wild relatives. Regardless of the explanation, it’s possible—although not yet proven—that we could be drawing butterflies to plants that are poisonous.

“We need to find alternative solutions to manage agriculture,” says Sylvia Fallon, director of NRDC’s Wildlife Conservation Project (disclosure). “We’re spraying herbicides and insecticides everywhere. It’s no wonder we’re seeing declines in all kinds of beneficial insects and pollinators that weren’t the intended target.”

The U.S. Environmental Protection Agency instituted a moratorium on new applications to use neonicotinoids earlier this month, but the agency will honor existing permits—the ones that caused the current problems for bees and butterflies. So we continue to blanket the American landscape with these chemicals that harm butterflies and their food, while time is running out on the monarch. The population is now so small that a single storm, like the 2002 snowstorm that killed 80 million butterflies in Mexico, could wipe the world’s most interesting migrators off the planet.

onEarth provides reporting and analysis about environmental science, policy, and culture. All opinions expressed are those of the authors and do not necessarily reflect the policies or positions of NRDC. Learn more or follow us on Facebook and Twitter.

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