As the region experiences severe drought conditions, experts say the hot, dry weather has been a boon for nuisance caterpillars that continue to make inroads into areas of central Maine, and threaten trees and public health.

The browntail moth caterpillar had for years been relegated to areas of Massachusetts and the Maine coast, but over the last five years the caterpillar has steadily moved into areas of Waldo, Kennebec and Somerset counties. They defoliate trees in their wake and their poisonous hairs can cause skin rashes similar to poison ivy or cause respiratory difficulty if inhaled.

Exacerbating the spread has been the drought conditions over the last several weeks, which has prevented a naturally-occurring fungus from developing that can keep the browntail in check, according to Tom Schmeelk, an entomologist for the Maine Department of Agriculture, Conservation and Forestry who works in the Maine Forest Service Insect & Disease Lab.

Schmeelk said his office has been receiving reports daily from people in central Maine who are surprised to find the caterpillars in their neck of the woods. Last year’s cool, wet weather in May and June helped produce the Entomophaga Aulicae algae that attacks the fledgling caterpillar pupa, Schmeelk said, leading to noticeable population collapses in areas of southern Maine.

But “it didn’t reach the leading edge of the infestation, like in China and Lincolnville,” Schmeelk said. “We have picked up small fungus outbreaks in Liberty, Montville, Washington, but it’s sort of too little, too late this year.”

The caterpillar larva have started to pupate, meaning they mature in growth and are no longer susceptible to the algae.

Some arborists licensed by the state to prune browntail moth webs have taken notice of the infestation gaining traction. Among those is Mark Reiland, owner of Valley ArborCare Inc. in Fairfield.

“We’re definitely seeing it more than we did last year in the area. It’s been kind of all over the place; I recall being out in China a few weeks ago,” Reiland said. “Northern Kennebec and southern Somerset (counties), they’re definitely here, but I haven’t seen them in population density levels that people are going to start being too adversely affected.”

He said he’s concerned about the stress for trees, especially oaks, that are being defoliated by the browntail even as the drought conditions persist.

“It makes it harder for the trees to recover from a defoliating event,” Reiland said. “As long as we get some rainfall this season, I think the trees will bounce back pretty quick. But anytime you’re having this amount of dry weather, the trees are already under stress.”

The browntail moth, native to Europe, was mistakenly brought to Massachusetts in 1897 and it spread from there, according to the Maine Forest Service. The pest population decreased over time and became limited to Cape Cod and coastal Maine a few decades ago, but it’s been increasing a toehold in other areas of Maine in recent years.

The caterpillars are armed with defensive, barbed hairs that break off and can cause skin rashes, headaches and difficulty breathing. The dry weather means those hairs more easily float and are carried across an area, without rain to keep them stuck on the ground. That can lead to exposure to the browntail hairs simply by doing yard work, mowing the lawn or walking outside near infested trees.

On June 19, the University of Maine Cooperative Extension reported the caterpillars “have begun to pupate at all monitoring sites from north of Belfast south to Portland and west to Turner.”

“As a reminder the silky cocoons surrounding the pupae contain the last cast skin of the caterpillar and are full of toxic hairs,” the extension office wrote. “Many people become exposed to the hairs through encounters with the cocoons as they can pupate on buildings and vehicles/trailers as well as host tree foliage.”

Schmeelk said the browntail expand their range in many ways: The caterpillars and pupa can hitchhike on vehicles, boats and campers to reach other regions (and that’s believed how they have expanded from the coast to inland); while the adult moths are also capable flyers that can travel on their own before reproducing.

Schmeelk’s office conducts two rounds of population estimate surveys in several ways: aerial surveys in late spring, early summer; looking for signs of defoliation in late summer, early fall; and a web survey in winter in which the sides of major roads are checked in infested areas.

That data leads to the creation of an annual browntail moth exposure risk map, with colors showing low, moderate and high risk areas across central Maine.

Central Maine has been on the “leading edge” of the browntail expansion for several years, Schmeelk said, so the region is “experiencing it not necessarily for the first time, but high populations for the first time in a few decades.”

But not every area in central Maine is a high-risk area. Dana Lawrence of Randolph, another arborist licensed by the state to remove browntail webs who owns Dana Lawrence Tree Service, said he believes populations are down compared to last year in locations he sees.

“Last year, they weren’t secluded to just oaks; it seemed like they were in everything,” Lawrence said. “But based on what we’re running into, it may actually be down a little bit with past years. I know of numerous people who are injecting trees or spraying the trees.”

There are some parasitic flies and wasps that attack browntail, Schmeelk said, but the Entomophaga Aulicae algae “packs the most punch” in leading to collapses of the pest population. That means, he said, we’ll need some wetter conditions to produce the fungus and keep the browntail in check for the coming year.

Reiland isn’t optimistic the local populations will stop expanding.

“Unfortunately,” he said, “if they take the trajectory of most large-forest insects, of some other species, their range will keep steadily expanding.”

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