Invasive Bug Poses Threat For Hemlocks

Springfield, Vt. — An invasive pest with a nasty habit of killing trees has established a toehold in the Upper Valley, and could bring devastation to the area’s forests in coming years, wildlife officials say.

The hemlock woolly adelgid is a tiny insect, only about one-third the size of a female flea, but Jim Esden, a forester with the Vermont Department of Forests, Parks and Recreation, says it breeds in such massive numbers that it has the potential to threaten not only the hemlocks it feeds on, but the species that depend on those hemlocks — a list that includes deer, trout and northern goshawks.

“Hemlock tree health is declining,” Esden said, “and hemlocks play such an important role in forest ecosystems that the (state foresters) are concerned and monitoring” the situation.

Right now, most of Vermont is free of the woolly adelgid, but there are documented ongoing infestations in the southwest corner of the state. There’s an infested acre in Pownal, with other sites in Windham County. The state has been monitoring populations in Vernon, Brattleboro, Townshend and Jamaica.

The northernmost infestation is at the bottom of the Upper Valley, in the southern edge of Springfield, Vt., and in Charlestown.

The infestation there was found in 2014, but most of Sullivan County and all of Grafton County thus far is free, according to the New Hampshire Division of Forests and Lands’ Forest Health Program. The insect first was discovered in 2000 in Portsmouth, N.H., and has spread through most of southern New Hampshire, as well as into Belknap and Carroll counties.

Michael Quinn, a tree advocate in Windsor, said he has seen firsthand the widespread destruction the bug can cause in his home state of Pennsylvania, where, in areas including Shenandoah National Park, hemlock mortality approached 95 percent.

“I think what happened to the Pennsylvania hemlock could happen here,” Quinn said. “Once it hits, once the insects are in our region, then we’re going to be talking about it.”

Hartford Tree Warden Brad Goedkoop said the threat of the woolly adelgid has discouraged landscapers from using what would otherwise be an excellent tree for certain types of plantings.

“This is, in itself, tragic because of the few true natives to our region, another is now subtracted due to an introduced pest,” Goedkoop said.

Others are worried about the prospect of losing hemlocks that already have been integrated into ambitious cultivated landscapes.

Craig Carmody owns the Path of Life Garden in Windsor; the garden’s centerpiece attraction is a life-size maze made up of a dense hedge.

“The maze here is made of 1,000 hemlocks,” Carmody said. “If it did come here, then obviously it would be quite devastating.”

He said the prospect of replacing the trees with another species would be both costly and time-consuming but that, until the pest finds its way to Windsor, “we’re keeping an eye on it.”

Hartford Conservation Commission Chairman Jon Bouton, who retired recently as Windsor County forester, explained that hemlocks play a unique role in the local ecosystem because of their ability to put out dense needle foliage in the heavily shaded areas beneath the canopy created by taller deciduous trees.

“Most snow doesn’t get to the forest floor because it gets hung up on needles and branches, so snow depth is much less under hemlock trees,” Bouton said. “Also, because you’ve got this insulating layer between the top of the trees and the ground, the temperatures underneath the hemlock trees don’t fluctuate as much as it might in the open or in a hardwood forest.”

The insulation and lack of snow combine to form ideal wintering yards for deer; subtract the trees for the landscape, Bouton said, and “my feeling is, in some areas, it would be devastating.”

In addition to the goshawk, Esden said, birds such as the black-throated green warbler, the Acadian flycatcher, the blue-headed vireo and the Blackburnian warbler also depend on hemlocks.

Stands of hemlock on the riverbank provide a water-cooling shade that is needed by several aquatic species, including trout. In other areas, they allow vernal pools to form, which gives amphibious species a chance to meet and breed.

The bugs damage trees when they drill into the woody tissue of twigs and feed on the tree’s starchy energy reserves; the trees suffer from the loss of energy, gradually lose the ability to replace needles that fall off, and die.

But Esden says forestry officials are adding new weapons to an arsenal that currently includes chemicals, harvesting and a predatory beetle named Laricobius nigrinus; combined with Vermont’s harsh winter climate, it might be possible to indefinitely hold off the woolly adelgid.

“It’s still possible we may manage and coexist,” Esden said.

In addition to the predatory beetle, Esden said, researchers from the University of Vermont also are working with other biological controls — a species of silverfly is showing promise, as is a fungi that takes advantage of the adelgids’ limited mobility to consume them.

Still, the Twin States’ best current defense against the adelgids may be their harsh winters.

But if even one adelgid survives a winter, its unusually prolific reproduction pattern allows it to replenish its numbers with frightening rapidity. They reproduce asexually, laying such large egg broods twice a year that a single surviving adelgid can spawn 30,000 grandchildren in one year.

After two harsh winters that prevented the adelgid from spreading, the most recent mild winter had local experts braced for a quantum leap in the range of infestation.

But Esden and other foresters learned that the expected population surge didn’t happen — mortality rates were as high as 99 percent.

The die-off is another mystery, but one that has an element of hope to it.

“There was one cold snap right around Valentine’s Day that we think may have had an impact on the mortality rate,” Esden said, but he’s not quite sure. “Some extreme fluctuations in temperatures may have just kept them so off-balance that they weren’t able to survive.”

Even in cases where a tree is doing a reasonably good job of fighting off its tiny predators, Esden said, their presence weakens their host, making it more susceptible to a one-two punch from another parasitic species or water shortages.

With climate change leading to forecasts of increasingly mild winters, Esden said, keeping the bug in check will be increasingly dependent on human intervention.

Matt Hongoltz-Hetling can be reached at or 603-727-3211.

Article credit: Matt Hongolz-Hetting. Valley News Staff Writer