May 24, 2018
BDN Maine Latest News | Poll Questions | Sharon Carrillo | Elver Season | Goat Airbnb

Predatory insect threatens hemlock trees in Maine

By Dale McGarrigle, Special Sections Writer

A tiny insect is working its way up the Maine coast and is threatening hemlock trees along the way.

Since 1999 in this state, the Hemlock Woolly Adelgid has set up its home on the new growth of hemlock, feeding off such trees and harming their vitality in the process. This insect affects only hemlocks.

The adelgid (pronounced Uh-dell-jid) was first detected in Maine on untreated nursery stock from Connecticut. By 2003, it was found in Kittery and York forests. Thanks to natural spread aided by weather, birds, and small mammals, the creature can now be found as found north as Edgecomb.

It is thought this pest will progress 5 miles a year on average through natural spread, explained Allison Kanoti, a forest entomologist for the Maine Forest Service, a branch of the Maine Department of Agriculture, Conservation and Forestry.

“We don’t have a really good handle on [how far it’s spread],” Kanoti said. “Detection always lags behind establishment, because it takes awhile to find the insect.”

Counties outside of Maine are known to have HWA infestations, and six townships in York County (Eliot, Kittery, Ogunquit, South Berwick, Wells, and York) are under quarantine. Such a designation by the Maine Forest Service prohibits live hemlock trees and certain hemlock products from being transported from those towns. An expansion of the quarantine area in Maine is expected within the next year.

While its personnel regularly survey to locate new infestations and monitor the spread of HWA, the Maine Forest Service is dependent on the public to help locate the insect, Kanoti said.

The first step is identifying which trees are hemlocks, which are fairly common in Maine, particularly along waterways in the southernmost portion of the state and in Washington

County. The tree’s leaves are needle-like and are flat and attached singly to its twig. The leaf’s sides are tapered, the twigs are flexible, and the leaf is attached to a small “peg” on the twig.

Once it’s established that a tree is a hemlock, what should a person do next?

“Check out newer growth, that which you can reach,” Kanoti said. “The adelgid is a tiny insect that you can’t see through casual observation, so you need to get really close to live branches. You’ll also need to check out dozens of branches.”

The most obvious sign of HWA is the covering of wool-like wax filaments produced as the insect matures. The woolly masses generally range from about 1/16-inch to 1/8-inch in diameter. They are most visible from this time of year through early summer on the undersides of the outermost branch tips of hemlock trees.

After establishing the presence of the pest, people should contact the Maine Forest Service. Kanoti encourages people to contact her at (207) 287-2431 or

Using chemical control by treating ornamental specimens with pesticides in high-traffic areas, such as roadsides and residential sites, can reduce accidental human spread of HWA, which is one of the Maine Forest Service’s main goals.

Treating infested trees in a forest setting is more problematic, Kanoti said.

“The best thing you can do is to support the vigor of hemlock trees, with the advice of a forester,” she said. “Adelgids aren’t going to kill a tree quickly. A healthy tree is going to do better under attack than a tree that doesn’t have a lot of resources.”

Biological control is the best hope for long-term control of HWA in Maine’s forests. Two species of beetle that feed exclusively on adelgid have been released in Maine, and release sites are monitored periodically for beetle establishment.

Hopefully, the combined efforts by the Maine Forest Service and the public will help hemlock to thrive, despite the presence of the Hemlock Woolly Adelgid.

“Hemlocks provide critical wildlife habitat, protect water quality, and are valued for their contribution to the beauty of our natural and residential landscapes,” Kanoti said.

Have feedback? Want to know more? Send us ideas for follow-up stories.

You may also like