Coastal Rivers takes on Hemlock Woolly Adelgid

Coastal Rivers takes on Hemlock Woolly Adelgid

Combatting an insect threat to our region’s hemlocks

When Director of Land Conservation Joan Ray walked through the grove of ancient hemlocks at the Waldoboro Town Forest late last summer and spotted telltale white woolly masses on the underside of hemlock branches, she was filled with a sense of dread. The Hemlock Woolly Adelgid (HWA) insect, which is decimating hemlock trees throughout the eastern US, had infested this grove of enormous ancient hemlock trees, some of which pre-date the Revolutionary War.

HWA is an invasive, aphid-like insect from Asia that attacks and kills North American hemlocks. The insects themselves are very small and hard to see, but can be easily identified by the 1/16” to 1/8” white woolly masses they form on the underside of branches at the base of the needles on hemlock trees. Unlike the native woolly adelgid that one sometimes sees on alders, HWA are specific only to hemlock and often fatal to the host tree.

woolly masses of Hemlock Woolly Adelgid on hemlock

Hemlock Woolly Adelgid can be identified by the white woolly masses it forms on the underside of hemlock boughs.

This invasive pest is not just a localized problem. Hemlock trees make up a significant part of the local forest mix and are important to our coastal forest system in many ways. They help prevent erosion and provide cooling shade along streams and on the shores of our lakes and ponds. They are a source of food for deer and other wildlife, and provide winter shelter for deer. Hemlock is also a valuable source of lumber.

Back at Coastal Rivers, Joan immediately took action and notified the Waldoboro Town Manager and the Waldoboro Conservation Commission, as well as the state entomologist.

There are a number of treatment options, including several types of predatory beetles which attack the adelgid. These are often used in concert with organic horticultural oils or limited application of insecticides. The organic oils or insecticides will protect the trees until the beetle populations have grown enough to bring the numbers of adelgid under control.

the predatory lady beetle that feeds on HWA

Meet Sasajiscymnus tsugae, a host-specific black lady beetle that feeds only on three known adelgid species, including HWA. It does not affect our native woolly adelgid.

The Maine state entomologist, Colleen Teerling was able to acquire a batch of 2,000 Sasajiscymnus tsugae predatory beetles from the federal government. Later in the fall, Joan, Colleen, Waldoboro Town Manager Julie Keizer, and individuals from the Waldoboro Conservation Commission released the beetles onto select trees in the Waldoboro Town Forest.

“Coastal Rivers provided expertise throughout the process,” said Keizer. “And without their help identifying the problem and bringing in the right partners, we wouldn’t be so far along in the process of eradicating the adelgid.”

The beetles have been extensively tested, both in the lab and in the wild, to ensure they don’t attack native species and become invasive pests themselves. Once released, they will feed, reproduce, and spread as long as there is HWA present in sufficient numbers to sustain them.

In the meantime, Coastal Rivers is monitoring the spread of HWA on our own conserved properties in the Damariscotta-Pemaquid area, and targeting infested hemlock groves for treatment with the predatory beetle. Several hundred predatory beetles were released recently at Keyes Woods Preserve and the Half Moon Pond Conservation Area in Bristol.

Steven Hufnagel and Colleen Teerling releasing predatory beetles onto an infested hemlock

Executive Director Steven Hufnagel and Maine state entomologist Colleen Teerling release a batch of predatory beetles onto an infested hemlock.

Coastal Rivers is spearheading further efforts to protect our region’s hemlocks in partnership with other land trusts, towns, conservation commissions, and individuals, for example coordinating a third beetle release along the River~Link wildlife corridor and the Schmid Preserve in Newcastle and Edgecomb.

We are also assisting landowners with identifying and evaluating infestations on private property, and are collaborating with individuals and organizations to secure the expensive predatory beetles at bulk rates.

A "nest" of predatory beetles clipped to an infested hemlock bough

S .tsugae beetles arrive in batches of 100 in a small container, along with a handful of wood shavings and a moist sponge. This “nest” gets clipped onto an infested hemlock tree, and from there the beetles will spread out and up on the tree and to other nearby trees to feed on HWA. In this photo, the small black beetles are visible on the sponge and shavings.

Landowners who suspect they have Hemlock Woolly Adelgid on their property, particularly in the towns of Bremen, Bristol, Damariscotta, Edgecomb, Newcastle, Nobleboro, and South Bristol, are encouraged to contact us at to help with identification and recommend a course of action.

HWA can easily be spread by hitching a ride on birds and animals, including people. Anyone who thinks they may have come in contact with infested trees should avoid spreading the insect to healthy trees by putting all of their clothes in the dryer on high heat for at least 15 minutes.