Hemlock wooly adelgid infestations are easily identifiable underneath hemlock branches, particularly during winter and early spring months. New York Invasive Species Information

An invasive insect that weakens and kills hemlock trees is steadily moving north into central Maine as climate change makes the environment warmer and more welcoming.

Originally from East Asia, hemlock wooly adelgids are tiny insects that survive by sucking the sap of hemlock trees in North America. Under the right conditions, the pest can kill hemlock trees in as little as four years.

Hemlock wooly adelgid infestations span from Georgia to Nova Scotia. In Maine, it has primarily been identified along the coast where temperatures are more mild, but is has steadily been creeping north and inland due to climate change.

The invasive pest lacks a natural predator in North America, posing a significant threat to the eastern hemlock found in Maine and the Carolina hemlock.

The International Union for Conservation of Nature has listed both species of hemlock as “near threatened” due to the conifers’ decline.

The adelgid was identified last year in three towns that border Androscoggin County: Casco, Pownal and Litchfield. Infestations have also previously been identified in Freeport, Brunswick, Topsham and Raymond.

Hemlock wooly adelgids are relatively easy to spot, particularly during the winter months when the tiny insects cover themselves with a protective waxy wool-like material. Little white dots can be found in clusters underneath the branches and needles of infested hemlocks.

Map of the hemlock wooly adelgid extent in Maine as of December 2022. The pest have been identified as far north as Mt. Desert Island and as far inland as Litchfield and Naples. Maine Forest Service

As of March, the hemlock wooly adelgid had yet to be found in Androscoggin County. But Colleen Teerling, an entomologist with the Maine Forest Service, said it’s only a matter of time.

“It could well be there, and it’s either at a level that we aren’t able to see yet or we just haven’t been looking at the exact right spot,” she said.

Entomologists have yet to find the pest in Oxford County either, however infestations have been identified in Casco, Naples and Sebago.

The type of hemlock wooly adelgid that survives in North America cannot fly, meaning it is primarily transported by animals and human actions.

One of the best ways to slow the spread of the hemlock wooly adelgid is restricting the movement of foliage materials, mainly leaves and needles, between towns. Currently, eight Androscoggin County towns are under quarantine for the pest, and state approval is necessary to move foliage from these towns into nonquarantine towns. The eight quarantined towns are Auburn, Durham, Lewiston, Lisbon, Mechanic Falls, Minot, Poland and Sabattus.

“It is here, it is in Maine,” Teerling said. “We’ll never get rid of it. The best that we can ever hope for is to learn to live with it in some way.”

The hemlock wooly adelgic creates a waxy wool-like substance to cover itself and its eggs. U.S. Forest Service

Hemlocks are important for shading aquatic habitats and providing shelter for deer during snowstorms, Teering said. Due to their dense canopies, hemlocks help keep water temperatures cool, important for many native species of fish.

Some species of birds are also dependent on hemlocks, including the blue-headed vireo, hermit thrush, blackburnian warbler and black-throated green warbler, all of which nest in Maine.

Maine has some protection from the hemlock wooly adelgid due to its cold winters, but this protection is lessening as winters continue to warm, Teerling said.

“We expect that the cold winter, the cold snap that we had over the winter probably led to higher mortality than we’ve had in the last few years,” she said, referring to the record-low temperatures in February.

Once established, there’s little anyone can do to combat hemlock wooly adelgid. Pesticides can be applied to individual trees to kill infestations, but broader removal is significantly more difficult, if not impossible.

Some Maine land trusts have turned to biologic control to reduce the impact of hemlock wooly adelgid on their forests. One beetle, Sasajiscymnus tsugae, is raised in Pennsylvania and sold commercially to reduce hemlock wooly adelgid populations.

A Sasajiscymnus tsugae larva feeds on hemlock wooly adelgid. The non-native beetle is not known to harm any native species in Maine. Carole Cheah photo

Joan Ray, the director of land conservation at Coastal Rivers Conservation Trust in Damariscotta has helped local landowners release beetles on their property for several years. Their goal is to establish a local, self-sustaining population to combat the invasive pest.

“The beetles are controversial because we’re introducing a nonnative species,” Ray said, “but it’s been tested extensively up and down the East Coast for dozens of years, and it hasn’t been shown to be a problem on any other trees. It seems to just attack the hemlock wooly adelgid.”

It’s too early to know whether it’s working, but other similar programs in the state have been successful, Ray said.

Androscoggin County is at a critical period, Teerling said.

“Of all the southern counties, Androscoggin is still at a pretty good spot because if (people) start taking taking steps to reduce the spread and prune back those high-risk hemlock trees you may actually slow down the spread of adelgid it into the county.”

She recommends land owners prune back hemlock trees that may often brush up against people or vehicles, including those that line nature trails.

When visiting the coast or other infected areas, people should take extra caution not to brush up against hemlock trees. The insects, which spread most readily from March through July, are tiny and can crawl onto clothing and vehicles.

Those who believe they have found hemlock wooly adelgid can email a photo to foresthealth@maine.gov. More information about the pest can be found on the Forest Service’s website.