PUTNEY — Although the Hemlock Woolly Adelgid (HWA), an invasive species whose damage to hemlock trees has been well documented in its infestations all over the East Coast, has yet to kill a tree in Vermont, Windham County residents are still worried about what might happen here.
The insect was first discovered in Vermont in 2007. According to James Esden, a Forest Protection Forester with the Vermont Department of Forests, Parks and Recreation, nearly every town in Windham County has reported an infestation.
Esden was in Putney on Jan. 15 for an HWA information session at which more than 50 area residents turned out at the Putney Firehouse to learn what they could do to keep the insect's spread in check.
As its name suggests, this bug systematically feeds on its host hemlock tree for several years, damaging the trees and, in some cases, killing them.
Hemlock trees are identifiable as slim, tall trees with lacy, glossy green foliage. They are used frequently in landscaping, but are also found natively in northeastern North America. Although they are often used to provide shade in areas of recreation, due to their minimal underbrush, these trees also have economic value in logging, especially in the floors of old post-and-beam barns as well as in the production of mulch.
The hemlock also creates habitats for porcupines, many types of birds, and deer. Hemlocks often are found in riparian zones - on the edge of rivers and streams - and prevent soil erosion with their shallow and fibrous root system.
The invasive insect HWA was discovered in the 1920s in the Pacific Northwest but was not then considered a pest; the species of hemlock found there, as well as those found in Asia and elsewhere, are not susceptible to the HWA.
But in the early 1950s, an heiress in Richmond, Va., working to build an arboretum imported some hemlock trees from Japan and unwittingly introduced the critter. With no natural predators and finding Eastern Hemlocks an easy target, the HWA took off, spreading north and south for more than a half century.
Edsen said the HWA is here to stay for the foreseeable future, as reports of infested native trees continue to roll in from all over the south of the state.
However, the fate of Vermont's hemlocks doesn't seem quite so grim as in the South. For example, in Virginia's Shenandoah National Park, the HWA wiped out 98 percent of the hemlocks since it was first observed there in 1988.
But the HWA has spread more slowly as it has ventured into colder climes. Although the HWA can kill a tree in four to six years in the South, hemlocks farther north hang on longer. Edsen said we have more time to figure out how to save our trees - though how much more has yet to be determined.
'Coolest bug'
Esden has a deep respect for the pest. “This is the coolest bug I am aware of,” he said.
He said each infestation on each tree consists of two generations per year, contributing to their rapid multiplication rate, with eggs hatching in April and July.
After hatching, the HWA loses its legs in just a few weeks, attaching itself to its host tree. The bugs that hatched in July aestivate - or “hibernate” - for the summer months until mid-October. This aligns with the hemlock's busiest season of photosynthetic production, during the winter months, and gives the HWA the advantage of having fewer predators by being one of the few insects to remain active during the winter.
In the spring, part of the population breaks off, sprouting wings and seeking out spruce trees. Interestingly, Esden said, the spruce trees in New England do not support HWA, so this group has reached its reproductive dead end.
For the population that remains, they reproduce asexually. These populations usually spread to new trees via the wind and by sticking to birds, small mammals (especially squirrels and deer), and even humans.
When the HWA attaches itself to its host and begins feeding from the tree's starchy, woody tissues, it creates a fluff called “flocculence,” from which the “woolly” part of its name is derived.
This fluff comes from pores around its mouth as it feeds, obscuring the adults' bodies and keeping its eggs hydrated. Although each bug is 1.5-2.5 millimeters long, its mouthpiece, which it inserts into the base of a needle, can be four to five times longer than the bug's body, digging deep into the tree's woody exterior. The HWA's saliva promotes scar tissue that prevents the tree from putting on new growth.
As this process continues, twigs develop more slowly, the tree turns dry and gray, the crown thins, and the tree dies.
Despite the pest's hardiness, Esden said cold winter temperatures are our biggest ally in fending off the HWA. A prolonged stretch of subzero temperatures during the winter kills 70-80 percent of HWA populations.
But last winter, which was unseasonably warm, saw no significant HWA deaths. In fact, Esden said, the area of HWA infestation doubled last year because of it.
Esden said global warming presents a bleak prospect for forests, and means more work for foresters.
“We're just figuring out how this bug works, and now [the environmental conditions are] all changing on us,” he says.
And while we can't expect to eradicate it completely anytime soon, Esden said that the HWA “doesn't move too quickly, and we can learn to manage and live with it.”
Esden and his colleagues have found several ways to do this, ranging from the application of several types of predatory beetles, chemical treatments, and even an insect-killing fungus in development at the University of Vermont.
“We're guardedly optimistic,” Esden said. “We've done lots of research and had lots of cooperation” - the latter of which he refers to as the “X Factor” in the fight against the HWA.
How to help
Esden has several suggestions of how average citizens can help combat the HWA:
• Avoid “salvaging” hemlocks prematurely, he says - that is, don't cut them down because you suspect they might be infested.
• Most discoveries are made by informed citizens; if you suspect your hemlock may be infested, call a forester. “We're paid to do house calls,” Esden says.
• Residents can keep their hemlocks healthy by monitoring them, and keeping birdfeeders at least 100 feet away from them between April and August.
Volunteers are also needed for surveying work to find the boundary of the infestation and help the forestry department get as clear a picture as possible of the current range of the HWA.
“Folks can get together to do the survey while walking, snowshoeing, skiing, snowmobiling, etcetera, and then get together afterwards for hot chocolate and chili,” Esden suggests.