The Tiny, Toxic Caterpillar Wreaking Havoc on Maine

An outbreak of browntail moth caterpillars is vexing a large swarth of the state, and Mainers are itching to find a solution.

browntail moth caterpillars emerging from their nest
The caterpillar of the browntail moth, Euproctis chrysorrhoea, feeds on the leaves of hardwoods, particularly oak and fruit trees. Shutterstock | Muddy Knees.
By Laura Poppick

In a lab at the University of Maine, Dr. Eleanor Groden reaches into a freezer full of dead caterpillars. Inside, they’re laid out in stacks of lidded petri dishes, their carcasses shriveled and curled. When Groden offers me a dish, I hesitate.

“They won’t hurt you,” she says. 

She has just finished telling me, at length, about the dangers of touching these insects. They’re behind plastic, but still, just thinking about them makes me itch. The caterpillars are the larvae of browntail moths, notorious for the rash they cause when their tiny, toxic hairs contact human skin. It’s similar to an allergic reaction to poison ivy — blistery, irritating, and all-too-familiar to an increasing number of Mainers. 

In recent years, populations of these caterpillars have exploded in pockets of Maine, notably around the midcoast and Merrymeeting Bay, although they’ve been found along the coast from Kittery to Mount Desert Island and as far inland as Corrina. Groden, a recently retired UMaine entomologist, has collected thousands in freezers in order to study their biology and, hopefully, fight back against their proliferation. The work doesn’t come without discomforts, though: researchers on Groden’s team regularly end up with rashes after tromping through the woods or even leafing through field notes back at the lab. Grad student Karla Boyd recalls a bad case she got while collecting samples beneath a tree so infested the caterpillars had eaten every leaf and were billowing out from beneath the bark. The breeze sent microscopic hairs drifting towards her face and left her itching for days, with welts that wrapped around her neck. 

Biologists don’t know what triggered Maine’s browntail outbreak, which swept across at least 64,000 acres of forest in 2016 and ballooned to twice that by 2018. Without natural predators to keep the caterpillars in check, UMaine researchers, together with a team of collaborators from the Maine Forest Service, have become the state’s first line of defense, devising tactics to weaken the nests and thereby prevent caterpillars from emerging in the first place. In a sense, the team is the only line of defense — since the pests currently plague no other state (just a few isolated spots on Cape Cod), they aren’t a priority for many other researchers in the country. 

 “If a university in Maine isn’t researching browntail moths,” Maine Forest Service entomologist Tom Schmeelk says, “then there is not going to be anybody researching browntail moths.”

Browntail-moth caterpillars form webbed nests, often at the tips of branches in tall trees, where they’re difficult to eliminate. They’re identified by two red-orange dots on their tail end.

Though puzzling, browntail invasions aren’t new. Outbreaks have hit New England periodically since the first infestations were in recorded in the late 1890s, in Massachusetts, and the early 1900s, in Maine. Scientists aren’t certain how the moths arrived from their native habitats in Europe, Asia, and Africa, but a likely culprit is a florist in Somerville, Massachusetts, who imported rose bushes from Holland and France (though it’s possible they arrived in multiple places via multiple ships). Over time, the moths infested western New England, New York, New Brunswick, and Nova Scotia. By 1915, they spread across roughly 36 million acres, and researchers sought aggressive means to cull the pest, “which is so destructive to our deciduous trees and also so obnoxious to our people,” as a 1912 Massachusetts Forestry Department study complained. 

Towns paid bounties for the nests, which school children collected by the bushel and piled into truck beds to be transported to furnaces and burned. State and federal scientists banded together to release millions of parasitoid flies and other insects to try to thwart the moth population boom in especially hard-hit regions, while others drove around administering a lethal fungus to infested trees. 

The infestation finally began to crash in the 1940s and by 1973 tapered to a low of just 1,200 acres in patches of the New England coast. But there’s no clear explanation for why the early–20th-century browntail wave rolled back. There’s no data, Groden says, to gauge how effective any of the eradication efforts might have been or whether the populations crashed from unrelated factors, like disease. So scientists today know little more than their predecessors about what might do the caterpillars in without causing further ecological damage. 

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What scientists do know is that the caterpillars don’t sport their toxic hairs until emerging from their webs in late April, after growing through the winter. Once they’re out, they devour leaves (in high enough numbers, they can denude a single tree in weeks) and continue growing larger and more toxic until late June, when they build cocoons and prepare to transform into harmless white moths. By late July, those moths begin to lay eggs, and the cycle continues with tiny caterpillars emerging in early fall, then preparing the webs that keep them from freezing in winter. 

Though retired, Groden continues to work on the browntail project, alongside newly hired UMaine entomologist Angela Mech. Their team is conducting experiments on how different environmentally friendly options — from essential oils to detergents to tiny pathogenic worms called nematodes — might weaken those protective winter webs, stopping the devastation before it emerges with the caterpillars in the spring.

Another option is to manually remove nests and burn them, as school kids did a century ago. But with a single tree capable of holding as many as 5,000 nests (and each nest capable of holding hundreds of caterpillars), this proves time-consuming, costly, and even dangerous. 

“You’re up 55 or 60 feet in the air,” says Shannon Mahan, an arborist with MAC Tree Service in Pemaquid, who prunes browntail nests. “You’re leaning out of the bucket to get each nest, and all the nests tend to be on the tips of branches.”

The breeze sent microscopic hairs drifting towards her face and left her itching for days, with welts that wrapped around her neck. 

Eben Mann, an arborist with Mann’s Lumber & Tree in Litchfield, says a subcontractor he works with once had to go to the hospital after developing a severe browntail rash. Moreover, Mann says, manual removal just isn’t practical in heavily infested areas. 

But it can offer relief where caterpillars have just begun to creep in. Jesse Wheeler, exotic plant program coordinator at Acadia National Park, took this approach after a visitor spotted a few webs last year, the first of this outbreak. He sent his team out searching in areas of the park that receive the most visitor traffic, and they ended up manually removing 141 nests from 29 different sites. That was manageable, Wheeler says. But what if the population were to double, which could happen in a single season if it were left unchecked? “That would definitely take a toll on our capacity to manage,” Wheeler says. 

In Portland, still on the edges of the outbreak, nests were found last winter in Deering Oaks park. This prompted the city to prune and spray nests with pesticides, both as a public safety precaution and to protect the city’s heritage oaks, says Ethan Hipple, deputy director of Portland’s Department of Parks, Recreation and Facilities. Trees can withstand two or three years of infestation, but after that, they weaken and can die from leaf loss. The caterpillars’ appearance last year doesn’t bode well for the state’s most densely populated area, Hipple says. “Now we know they are here, and they are probably not going away, unfortunately, until the natural cycle wanes.”

But scientists don’t know how long that cycle might last, Groden says, or what factors — from climate change to natural shifts in parasitoid populations — might affect it.

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In towns at the heart of the outbreak, browntail caterpillars have left some Mainers nervous about venturing outside in the spring and early summer. Some doctors have reported an uptick in patients complaining of severe rashes or respiratory symptoms from inhaling the hairs.

“There were definitely a few weeks last summer when everyone who came in was talking about them,” says Theola Gelerman, co-owner of Rockport’s 47 West espresso bar. “We heard one story from customers who had friends moving up from out of state and said they were on the verge of signing on a house near Megunticook Lake, then backed out because of the moth issue.”  

In high enough numbers, the caterpillars can denude a single tree in weeks.

The most promising countermeasure to emerge from the UMaine team’s research is a naturally occurring fungus, Entomophaga aulicae, that proves lethal to browntail caterpillars but relatively benign to the rest of the ecosystem. It’s the same fungus New England arborists and scientists tried to spread a century ago. Under the right conditions, it kills caterpillar bodies from the inside out and spreads rapidly by forming “fungal cannons,” inverted nipples that pop out when exposed to moisture, flinging spores into the surrounding environment. Tens of thousands of microscopic fungal cannons can infect a single caterpillar — and when they explode, they spread spores onto other caterpillars in proximity. “When you have a really high density [of caterpillars],” Groden says, “it just rips through the population.”

A wet spring in 2019 provided the right conditions for Entomophaga aulicae to develop and spread naturally, and in some pockets along the coast, it killed thousands of caterpillars. But relying on natural fungal blooms isn’t a long-term solution, Groden says. Their team is looking into commercially available fungi that may have a similar effect but be easier for arborists to purchase wholesale. Groden says they’ll need several more years of lab tests to determine the viability of this option, and even if it proves successful, it will be only one in a suite of tools required to combat the spread. She’s optimistic about their ability to get more tools into the hands of arborists, who can help homeowners in critical areas, but she doesn’t see an end in sight for the caterpillars.  

“I don’t know that we have any potential long-term strategies that are going to solve the outbreak,” she says.