by Courtney Llewellyn

An invasive species, browntail moth, poses human health risks as well as concerns for forest and orchard health. This species is currently experiencing a population boom in Maine, and unfortunately, cold winter temperatures do not kill these pests.

Tom Schmeelk, forest entomologist, Maine Department of Agriculture, Conservation and Forestry, recently hosted a webinar about browntail moth, speaking about what its impacts are and how those in New England can try to contain it.

Browntail moth has been established in Maine since 1904, but its populations ebb and flow. Their maximum infestation peaked in 1922, when a huge biological control program was instituted including parasitoids and predators. The population collapsed in the late 1910s/early ‘20s, possibly due to combination of cool and moist weather, the fungus E. aulicae and parasitoids, Schmeelk explained. Occasional outbreaks occurred over the next six decades. “Currently in Maine, we’re in the middle of the most recent fairly severe outbreak,” he said.

There are many fuzzy caterpillars in the Northeast. The distinguishing characteristics of browntail moth are the red-orange dots on its tail end and the white tufts along their sides. They have a few natural enemies, with Townsendiellomyia nidicola (a species of tachinid fly) probably being the most specific.

“They have a wide range of hosts: oak, birch, cherry, elm, poplar, apples, pears, crab apples and other hardwoods,” Schmeelk said. “Basically any fruit tree.”

A menace to Maine society

Browntail moth webs are distinguishable by their bright white silk and size – about the size of a human hand. Photo courtesy of Jan Samanek, Phytosanitary Administration,

Caterpillar feeding results in tree damage by causing branch dieback at greater than 30% defoliation, with tree mortality occurring after multiple years of severe defoliation and stress. “In our seventh year of outbreak, we’re starting to see mortality due to browntail moth,” he said.

On the human side of things, the moth is a health risk because of the toxic barbed hairs on caterpillars which result in rashes similar to those from poison ivy, lasting hours to weeks. The issue is most common from April through July. The tiny barbed hairs become airborne, and their toxin lasts in the environment for one to three years.

Caterpillars emerge in late April and May, communally feeding on buds and foliage until late June. They molt five to eight times, casting off the skins that have the toxic hairs on them. In late June/early July, they will make cocoons anywhere they feel safe, but often right on the host foliage. The moths emerge from their cocoons in July.

If you’re working outdoors in known infested areas, wet down the area with a hose or work during damp times of day, as moisture helps keep the hairs from becoming airborne, thereby minimizing contact. You can also use a HEPA vacuum to remove caterpillars from buildings, driveways and equipment.

They overwinter in palm-sized webs, which they start creating in August. Using bright white silk, they connect loose leaves to the tips of branches, with 25 – 400 caterpillars per web.

“The first step to management is education,” Schmeelk said. February – and winter in general – is a good time to find these webs and plan for pest management for the coming year. Web clipping and destruction is ideally completed by early April.

The preferred method for eliminating browntail moth is pruning out winter webs, which can then be burned or soaked in soapy water. The risk of contact with toxic hairs is low in winter, but gloves should still be used.

Chemical control of larvae may help, but it needs to be applied before the end of May (but after there is enough leaf surface to spray on). “We can’t spray our way out of this problem,” Schmeelk cautioned.

Heavily impacted counties in Maine are Androscoggin, Kennebec, Knox and Waldo. Schmeelk reported the Forest Service received well over 500 calls about browntail moth in 2021. The moth damaged more than 108,000 acres in Kennebec County alone, and nearly 199,000 acres across the state, last year.

Now is the time to scout your property and help reduce the Maine menace that is browntail moth.