The adelgid is a tiny insect, no bigger than the “e” in lethal, and of Asian origin. It turned up in our Pacific Northwest in the 1920s and by the 1950s had made its way to the Eastern Seaboard, first detected here in a Virginia nursery. It gets its name from the trademark white “wooly” egg sacs it leaves on the branches of hemlocks.
Though not particularly harmful to hemlocks in its native Asia, the adelgid arrived here to a system with no natural predators and no natural resistance. It is a lethal threat to the two species of hemlock we see in our Southern Appalachian forests: the eastern hemlock, (Tsuga canadensis) and the less-common Carolina hemlock (Tsuga caroliniana).
The adelgid reached western North Carolina’s Nantahala and Pisgah National Forests in 2001 and some experts fear that today it infests half the hemlock forests in the Eastern U.S. The recital is a grim one. The National Park Service estimates that fully 80 percent of the hemlocks in Great Smoky Mountains National Park are already dead from the ravages of the insect and maybe as many as 90 percent have perished in Virginia’s Shenandoah National Park.
It is difficult to overstate the significance of hemlock forests in our region. Eastern hemlocks grow on 19 million acres in forests from Georgia to Canada and are the predominant tree species on 2.3 million acres across their range. Those who have walked through a hemlock forest will know without being told the beauty and aesthetic value of this remarkable species.
Among other things, the hemlock is the longest-lived tree in our forests, some reaching 800 years. The largest ever recorded soared 175 feet. It is amazingly shade tolerant, allowing new hemlocks to emerge in forest gaps to maintain hemlock stands.
Foresters refer to the hemlock as a “keystone species,” one whose role in a system is disproportionately large compared to its abundance. Predictably, the disappearance of such a species is likely to result in the disappearance or diminution of many species associated with it.
Hemlocks, the U.S. Forest Service’s Southern Research Station notes, play an important role in the ecology and hydrology of mountain ecosystems. They offer critical habitat for birds. As many as 90 species can be found in hemlock forests. A few, mostly songbirds, seem particularly associated with hemlocks: the black-throated green warbler, the Blackburnian warbler and the Acadian flycatcher. Some warblers nest nowhere else.
Hemlocks do best in moist ground and, while they are found from swamps to ridges, they are especially common along streams where they play a major role in stream ecology, helping to soften temperature extremes, creating structure in stream courses, offering a rich breeding ground for the invertebrates that fish feed on. In a study the U.S. Geological Service did for the National Park Service, researchers found:
Hemlock-dominated watersheds supported more aquatic invertebrate (insect) species than streams draining hardwood forests. Fifteen aquatic insect species were strongly associated with hemlock and three species were found only in hemlock streams. Brook trout were two and a half times as likely to occur in hemlock streams as in hardwood streams, and were twice as abundant in hemlock streams.
So if there are brook trout in your favorite mountain stream, thank a tree–most likely a hemlock. In all, according to some experts, hemlock forests are home to upwards of 120 vertebrate species as well. Hemlocks are a favored browse species for white-tailed deer, a dietary preference that can inhibit hemlock regeneration as deer nip off new growth.
How Does the Adelgid Spread and Kill?
Winds can carry the pest. So can migratory birds, mammals and humans. Another source apparently is infected nursery stock. And the pest itself is prolific. All hemlock woolly adelgids are females and continue the cycle asexually. The annual increase in numbers is startling: one adelgid can lay up to 300 eggs and that will produce as many as 90,000 new adelgids in a single year. Generations of the pest inhabit the same tree.
The hemlock woolly adelgid attaches itself to the base of hemlock needles and feeds on starch. That slows the growth of needles and causes them to drop, which in turn deprives the tree of nutrients. The crown thins and branches die back. Infected trees die in a surprisingly short time–5 to 10 years, some researchers say, as few as 3 to 5 years according to others.
Dire Comparisons…and Some Hope
The disappearance of hemlocks promises both aesthetic and potentially devastating environmental consequences. Some analysts are comparing it to the chestnut blight, a fungus that struck in the early part of the last century and by 1940 wiped from the landscape an estimated 3.5 billion chestnut trees. The loss transformed the forests of the Southern Appalachians and the hemlock woolly adelgid is on a path to wreak a similar transformation.
But forest science is much advanced since the chestnut blight struck and researchers from a variety of state and federal agencies and universities are aggressively at work to stop the spread of the adelgid. Prospects range from pesticides, which are practical on individual trees or in small stands but are likely not much use in deep forests, to biological methods that could well work in the backcountry. In particular, two nonnative beetles that prey on adelgids–and only on adelgids–show promise.
Adelgids and the Nantahala-Pisgah Forest Plans
As the research continues, though, hemlocks will continue to die. The question is what will replace them. To a surprisingly large degree, that is a matter of human choice: what do we want our forests to look like–to be and do–in the wake of the loss of hemlocks. Forest ecologists doubt that there is another single coniferous species that is ideally suited to replace the hemlock and its role in ecosystem health in western North Carolina. A likelier option is some combination of conifers chosen fairly deliberately to fill the void. Absent some such action, hardwoods are likely to claim the vacuum.
These matters of choice are very much the province of the revised forest plans for the Nantahala and Pisgah National Forests.