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Maine's browntail moth outbreak could be fueled by climate change, UMaine scientist says

browntail moth.jpg
Murray Carpenter
Maine Public
The caterpillar of the browntail moth, which has tiny hairs that give many people a rash like poison ivy.

Browntail moths have been in Maine for more than 100 years. But over the past decade, they’ve expanded their range across the state, and are often abundant enough to give Mainers irritating rashes. And one of the reasons for that recent expansion may be related to climate change.

Greg Purinton-Brown and his family own Toddy Pond Farm, a diversified farm in Monroe, with cows sheep, chickens, turkeys, goats, and some lively pigs.

The pigs often forage for acorns in a mature oak grove, but Purinton-Brown says the oak trees are also ideal hosts for browntail moths.

Just like in many areas of coastal and central Maine, the moths arrived on the farm several years ago. Purinton-Brown spends many winter hours pruning the caterpillar’s nests from the fruit orchard and nearby black cherry trees. And he's motivated by personal experience.

“I had such a rash last year, I just did not care, but we had coming for dinner, so I was out here just pinching them off the tree, cutting them out and cutting them out," Purinton-Brown said.

He’s also used inserts of systemic insecticides to treat large oak trees away from livestock foraging areas. But all of this is just enough to manage the moths near the houses and barns, where his family works and guests gather for farm dinners.

"We had a pear tree over at the cottage that I did whatever I could to, it was just too close to the house, I couldn’t beat them, so I ripped the tree right out of the ground, it was so sad," he said.

“The browntail moth is a little bit unique in that I refer to as a trifecta pest,” said Angela Mech, an assistant professor of forest entomology at the University of Maine

angela mech.jpg
Murray Carpenter
Maine Public
Angela Mech, an assistant professor of forest entomology at the University of Maine, with a browntail moth nest.

“So, most tree pests are going to affect the health of the tree, so we refer to it as an ecological pest, then we have some that are attacking valuable trees, so they are an economic pest. But the browntail moth is all three, because it is also a public health pest," she said.

The public health aspect is the long-term, irritating rash, which is not caused by the long hairs visible on the caterpillars, but by tiny, pernicious hairs that can waft around on the wind.

“The caterpillars have these very short, barbed hairs that contain a toxin, that when they come into contact with your skin it causes a poison-ivy like rash, and for some people, the rash can be pretty extreme," Mech said. "I have heard that some people are immune to it, but I’m not one of those people.”

Mech arrived in Orono two years ago, and says she didn’t notice any browntail moths on campus that year.

“Then last year there were a few dozen. And then this year we have a couple of thousand. So it is definitely growing right outside the window here on campus.”

The moths are native to Europe and were accidentally introduced near Boston in 1897. After briefly expanding across New England, their range contracted and for much of the past century they were mostly limited to Cape Cod and the Casco Bay region.

But something changed about six years ago, and they began to spread across Maine. Mech said this may be due to a changing climate, and its effects on a fungal pathogen that keeps browntail moths in check.

removing nests.jpg
Murray Carpenter
Maine Public
Monroe farmer Greg Purinton-Brown spends up to 10 hours a week in winter and early spring removing browntail moth nests from trees near the houses and barns on Toddy Pond Farm.

“And like most fungi out there, they require cool wet springs to have the right climate to proliferate. And because of climate change we haven’t had those cool wet spring temps that really allow the pathogen to knock back browntail moth and so, one of the hypotheses as to why browntail moth is exploding is because of the lack of the right climate for the fungus," Mech said.

The students and research assistants in Mech’s lab are working on varied strategies for managing the moths. Some are assessing the effectiveness of spreading female pheromones to confuse males. Another team is experimenting with biocides to see which best targets the species.

Now that the caterpillars are active, arborists like Didier Bonner-Ganter are hearing from many frustrated homeowners. But Bonner-Ganter said this time of year, the only way to get rid of them is to spray nonselective insecticides that also kill beneficial insects. He says it's best to start thinking of a plan to prune off the nests next winter.

“The biggest message is to basically try and weather through the season, and then go forward in the fall after the leaves drop, and then let’s assess, and let’s come up with a plan at that point, and let's get on a schedule, whether it’s ours or somebody else’s," Bonner-Ganter said.

Mech and other experts say it’s hard to predict the trajectory of this outbreak, and how the moths will fare in Maine’s warming climate, but they don’t expect them to go away anytime soon.