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Climate change is keeping temperatures higher in the fall, setting up browntail-moth caterpillars to boom in summer.
The caterpillar is roughly an inch and a half long with a fuzzy coat, brown but for two white stripes that flank its back and two red-orange dots near its rear. It has a soft visual texture that makes it seem harmless, charming even, tempting enough to stroke.
But touch an adult browntail-moth caterpillar at your own peril.
“Browntail-moth-caterpillar hairs are barbed and hollow. And inside that hollow tube, there’s a reservoir of a toxin,” says Allison Kanoti, the state entomologist for Maine, which is in the middle of a massive browntail-moth outbreak. Until recently, the insect was constrained to a few coastal areas of the state. This year, they’ve been spotted in nearly every one of Maine’s 16 counties.
Portland, Maine’s largest city, has had to temporarily relocate its farmers’ market to avoid the insects’ uncanny habit of dropping down from the trees where they nest. In June, Waterville, a city of 16,000, declared a public emergency because of the caterpillars. And all across the state, shoppers covered in itchy abrasions have stripped drugstore shelves bare of witch hazel and cortisone, key ingredients in a DIY compound designed to soothe the downy beast’s ferocious burn. Brushing up against a browntail-moth caterpillar or otherwise encountering its hairs—on a picnic table, on a dock, on a bit of clothes hung out to dry—can leave a person itching for days as a poison-ivy-like rash creeps across the flesh. And as the caterpillar sheds its hairs, they can go airborne, causing wheezing if they’re inhaled. Both reactions can be so severe it necessitates a trip to the emergency room. Research suggests that getting a hair of a browntail-moth caterpillar in your eye can even cause blindness (though this is exceedingly rare). The hairs can remain toxic for up to three years.
How did Maine, a state that thrives on its reputation for lobster, lighthouses, and blueberries, become ground zero for such a poisonous caterpillar? The state traces its present predicament to two calamities: a warming climate in the present moment, and an accident that happened more than a century ago.
The browntail-moth caterpillar hails from Eurasia, where it has eked out an existence from the warmth of the Mediterranean Sea to chilly Scandinavia. It likely arrived in the U.S. on cargo via Massachusetts sometime in the 1800s. Its arrival went unnoticed at the time, but by 1897 the caterpillar’s presence was plain. Historical accounts describe towns in eastern Massachusetts as “swarming with caterpillars” and being “overrun with these pests.”
“It spread pretty rapidly over 15 years from Massachusetts, to Long Island up into Vermont, New Hampshire, Maine, all the way over to New Brunswick, Nova Scotia, and into Canada,” Angela Mech, an assistant professor of forest entomology at the University of Maine, told me. “It was a huge nuisance.”
Now, in Maine, signs warning people about the caterpillar’s presence are posted all over the place. The insects are an outdoor hazard, and Maine is a state of outdoor attractions—hiking, camping, cycling—for both Mainers and tens of millions of annual visitors.
“Once people get wind of the browntail moth, they cancel their plans to come visit Maine. And so it’s affecting tourism, it’s affecting jobs,” Mech said. “I’ve heard that tree-care companies are having difficulty finding employees because they’re constantly breaking out in the rash. Nobody wants to break out in a rash.” And because the browntail-moth caterpillars are voracious leaf eaters—their preferred hardwoods include oaks, birches, cherries, and apples—trees and forests are also suffering. If there’s a large outbreak in the fall, the just-hatched caterpillars eat through enough leaves “that they cause a bronzing of the foliage on the tops of trees,” says Eleanor Groden, a professor of entomology at the University of Maine.
Mech told me that “it is a very rare insect that is this sort of trifecta pest,” impacting ecological, economic, and human health.
The health risk, at least, is seasonal. By late June the caterpillars have entered their cocoons in preparation of becoming moths, and the caterpillars that hatch from eggs in the late summer or early fall aren’t yet poisonous. Their toxins emerge over months as the caterpillars grow in their winter webs, shedding old skin, and revealing a larger new skin underneath, before becoming toxic shortly after they emerge from their webs in April. Maine has been dealing with this severe outbreak since 2016, with the epicenter centered around the midcoast communities of Bath and Brunswick, Maine, but each spring the caterpillars’ reach grows.
This doesn’t mean they’re unstoppable. They’ve declined before. During the initial outbreak in the 1890s, the browntail-moth caterpillars caused so much havoc that, according to Mech, kids were paid to clip down their tents in winter. In a single year more than 24 million browntail-moth webs were clipped off trees and burned, while federal and local governments introduced dozens of different biocontrol agents, including predator insects from Europe. Whether because of human efforts, the caterpillars’ natural outbreak cycle, or some combination, within 20 years of their appearance the caterpillars were on the decline. By the 1960s they were found only on the tip of Cape Cod in Massachusetts and a couple of islands in Maine’s southern Casco Bay area.
The caterpillars started to regain ground in 1989, with moderate outbreaks occurring periodically before the current dramatic surge. To understand what might have brought the comeback of this scourge, Groden, the University of Maine entomologist, and her colleagues compared the rate of tree damage caused by the browntail-moth caterpillar to environmental factors, including summer temperatures, spring temperatures, and spring rainfall. They found that warming temperatures might be impacting the outbreak in two ways. The first involves a fungus known as Entomophaga aulicae. Because the caterpillar is not native to the United States, it has few natural adversaries, but E.aulicae is one of them. Once it takes hold, it can wipe out entire caterpillar colonies. But the fungus needs cool, wet springs to thrive. And Maine, which is located in the fastest-warming region in the lower 48 states, has experienced warm springs in the past few years.
“Last year, we had a warm, dry spring. And this year, we’ve had a warm, dry spring,” Kanoti, the state entomologist, told me.
At the same time, the researchers found that fall temperatures were a greater predictor of the caterpillar’s population, because in warmer years, more of the insects survive. And because of climate change, fall temperatures are rising.
Caterpillar populations “seem to be able to bounce back much, much faster than they would have in the past,” Groden said. “The favorable fall temperatures are having a positive trajectory. And that trajectory gets knocked back when we have a good year for the fungus. But they’re able to bounce back.”
This doesn’t mean that the caterpillar’s future is locked in. Because the caterpillars were contained for so long, figuring out how to deal with them, through, say, targeted pesticides, wasn’t a priority. Now it is. The state is also calling on residents to keep an eye on trees in the winter for nests—so they can be destroyed before the caterpillars emerge, when it is more effective.
But the browntail-moth caterpillar is not the only insect Mainers should be on the lookout for. Because of climate change, southern pine beetles, which are native to the southeastern U.S. and among the most destructive of the pine-bark beetles, have moved northward. Already they have infested trees in New York, Rhode Island, and Connecticut. According to Mech, the climate projections have them on pace to reach Maine any day now.
Kendra Pierre-Louis is a climate reporter currently working with the Gimlet/Spotify podcast How to Save a Planet.