It is late spring, a bright afternoon full of the sunshine of approaching summer and I am driving slowly up a graveled fire road approach to the summit of Reddish Knob. This lonesome mountaintop in the George Washington National Forest straddles the border between Virginia and West Virginia to the west of Harrisonburg and offers 360-degree vistas of the surrounding Allegheny Mountains. But today, as my car climbs the summit, all around me is death. By the hundreds, perhaps thousands, along the road and cascading down the steep ridgelines around me are the gray ghosts of hemlocks lost to the hemlock woolly adelgid (HWA).
This has become an all too common sight in our southern mountain forests, as our beloved native trees fall under siege from non-native exotics like HWA, creating the potential for an overhaul of the Appalachian ecosystem. Biologists and foresters are scrambling for a solution, but HWA, like other exotic pests before it, came on the scene slowly and methodically, capturing the monitors of our forests off guard.
A tiny aphid-like insect, generally recognizable only by the woolly white sac in which it coats itself on the underside of hemlock needles, HWA was first discovered on the East Coast in the 1950s. Its spread has been labored, but it eventually showed up in Shenandoah National Park in 1988. “I don’t think most folks in natural resources at that point speculated it would cause so much devastation,” says Shenandoah National Park biologist Rolf Gubler. Today, 80% of the park’s hemlocks have succumbed to HWA.
“There are some relatively healthy hemlocks left,” says Gubler, “about two percent of the original population. Another 10% have greater than 50% crown health.” Right now Shenandoah’s main focus is on preserving a hemlock gene pool for the future. “Our current management practices,” Gubler explains, “are geared toward spraying accessible hemlocks and doing soil injections in more remote areas.”
The spray consists of an insecticidal soap known as M-PedeR, which suffocates the adelgid but has no negative impact on other species. Soil injections consist of a pesticide called MeritR. Biologists have to be more careful with this combat method, as it can impact stream health if injected too close to water sources.
But a biological solution to HWA is well in the works. Several southern universities, including Clemson, Virginia Tech, the University of North Carolina, and the University of Tennessee, are all working diligently to raise and distribute predator beetles, another non-native species which feeds exclusively on HWA. “The trick has been to find a predator that feeds on the adelgid and nothing else,” explains Terry Seyden with the Nantahala National Forest in North Carolina. “The screening process is pretty rigorous.”
HWA was first discovered in the Nantahala National Forest about five years ago. Forest biologists released the first predator beetles into the forest last spring at 18 sites, including the popular Joyce Kilmer Memorial Forest and in the Linville Gorge area. “It’s too early to tell if it’s working,” says Seyden. “It will probably take 10 years before we know. A sign of success is seeing the beetle reproduce in the next season.”
The predator beetle solution has come too late for Shenandoah, however. “We got hit harder and faster than everyone else,” Gubler explains. “It takes awhile for the predator beetle population to build, and they need a healthy population of hemlocks, which we don’t have.” Shenandoah was also hit with two years of severe drought in 2001 and 2002, resulting in further stress on the park’s hemlocks. “All of the adult trees were virtually gone by 2003,” he says. Thus, Shenandoah must rely on spraying and injections to preserve its remaining hemlocks.
“The primary long-term solution has to be biological,” says Seyden. “Chemical solutions are a stopgap.”
Unlike its sister Shenandoah, the Great Smoky Mountains National Park has fared much better. “Every major drainage in the park is infested,” says Smokies forestry supervisor Kris Johnson, “but as soon as we saw the adelgid, we immediately began working with the U.S. Forest Service to treat the trees with predator beetles, insecticide injections, and spraying.” The park has received funding for the efforts through the nonprofit group Friends of the Smokies, as well as through the National Park Service and U.S. Forest Service.
“Without too much trouble, we can treat front country areas like roadsides, picnic areas and campgrounds, but we’ve been releasing predator beetles into the backcountry,” Johnson adds. The park’s first release of predator beetles occurred in May 2002, shortly after the initial discovery of HWA in the park north of Fontana Dam. But it will be at least 10 years, most biologists say, for the predator beetles to reproduce on a scale large enough to suppress HWA on their own.
But predator beetles are expensive and difficult to raise in the lab. The Blue Ridge Parkway, for instance, continues to rely on spraying and injections to control HWA. “We did a predator beetle release two years ago,” says Lillian McElrath, a parkway resource management specialist, “but we just can’t get the beetles anymore. It’s both an availability and expense issue.”
“The Smokies are a more politically prestigious park,” she adds, “so they can get funding more easily.”
Whether or not predator beetles end up being the solution to HWA infestations remains to be seen, but they’ve provided the first glimpse of long-term hope. While hemlocks may not be an economically valuable tree species, their value to the forest ecosystem is substantial. A common sight along streams, hemlocks regulate ground and water temperatures year-round with their thick canopies. Native brook trout, for example, are quite dependent on hemlocks for shade and cool water. Some species of warblers are known to nest only in hemlocks.
“Other species are not going to provide the same type of closed canopy that hemlocks do,” warns Gubler. “Hemlock die-off is going to change the dynamics of the areas where they have been prevalent. We’re seeing the hemlocks be replaced by more exotics like garlic mustard and woody invasives. Infestation of exotics is, of course, a concern.”
“Some fear we could lose 70 to 80% of the hemlocks from the southern Appalachians,” says Seyden. “That would be a shame, as we have beautiful stands that are 400-600 years old.”
While Shenandoah’s only hope may be preservation of the hemlock gene pool, other areas that have been infested more recently may still bounce back. “The Smokies are in a situation of having an early start on this,” Gubler notes, “and good funding.”
What is the Hemlock Wooly Adelgid?
Native to Japan and China, the hemlock woolly adelgid (HWA) is a miniscule aphid-like insect that sucks the sap from branches of the hemlock tree. Its name comes from the fuzzy white covering in which it wraps itself on the underside of hemlock needles. By sucking the sap from the tree, HWA deprives the needles of nourishment, causing them to dry out and fall off. HWA produces two new generations each year, making it quite prolific. It is generally carried from one host tree to another by wind or animals. It can cause a tree to die within two to three years of initial infestation.
HWA was first discovered on the West Coast in the 1920s and has since migrated east, having shown up in Richmond, Virginia, in the 1950s. It began infecting the Blue Ridge Mountains in the 1980s, nearly wiping out the hemlock population in Shenandoah National Park.
What Can You Do in Your Own Backyard?
To help prevent hemlock woolly adelgid infestations in your own backyard, keep birdfeeders away from hemlock trees, as birds are a primary carrier of HWA. While HWA is nearly invisible to the naked eye, you can usually spot an infestation if you find white woolly sacs on the underside of hemlock needles.
Spraying small trees from top to bottom and on the underside of needles with insecticidal soap (available from garden stores) will often present an effective treatment. If your trees are large, however, or already severely debilitated, contact your local county extension office or state forester for information on stem injections you may be able to use to treat your trees.
For more information about the hemlock woolly adelgid and how you can help prevent its further spread, visit www.saveourhemlocks.org.