Save the Hemlock Program

Coming Soon To A Hemlock Near You

By Elizabeth Hunter

That sucking sound? Well, you can't hear it at all, but the massive scale of the woolly adelgid's work on the hemlock trees of the mountains of the South is carried out by tiny insects via even tinier sucking tubes. And perhaps the worst part is the news of just the past several months: The small, relentless, deadly non-native pest is showing up in places where it's never been seen before, on majestic host trees that were expected not to be hit for at least several years. Experts say the region faces an ecological tragedy that parallels the Chestnut Blight.

Sixty-seven years ago, a Depression-era make-work project was launched near the Virginia/North Carolina state line that would ultimately link Shenandoah and Great Smoky Mountains national parks. More than a half century elapsed before the 469-mile Blue Ridge Parkway was completed. It's taken less than a third of that time - a mere 14 years - for the two parks and the scenic corridor between them to be connected in a far less happy way. In May, a minute, non-native, sucking insect that has destroyed most of Shenandoah's hemlock stands was discovered for the first time in the Smokies. Simultaneously, parkway personnel found hemlock woolly adelgid infestations along a new 40-mile stretch of the parkway in North Carolina - from Deep Gap to Linville Falls.

The arrival of the hemlock woolly adelgid in the Smokies is "a major ecological disaster," says GSMNP supervisory forester Kris Johnson. "We have 5,000 acres in which hemlock is the dominant tree - over 50 percent of the forest - and many more where it's a major forest component. The impact of the adelgid in this park will be the equivalent to the chestnut blight, visually and ecologically. I don't think people realize how bad it's going to be. They should be taking their children to see old growth hemlock now - while they still have a chance."

"It was a dark, dark day," says parkway resource management specialist Lillian McElrath, of her discovery of adelgids "pretty much everywhere" at Linville Falls: in the picnic area, along the river, around the visitor center, on trails to overlooks, and in Linville Gorge. "I think we'll be seeing tree death at Linville Falls within the next two years. Once the adelgid hits, it's a pretty quick thing."

It was certainly quick in Shenandoah, where the adelgid was first discovered in 1988. Between 1990 and 1994, the percentage of hemlocks with crowns in good health nose-dived from 77 percent to 1 percent. A cold, snowy winter in 1995-96 allowed some trees to recover temporarily, but drought and a string of warm winters since 1999 erased that gain. Today, says Shenandoah's forest ecologist James Akerson, "you can walk into every hemlock stand in the park and see gray crowns. At higher elevations, there are still some needles attached. But at lower elevations, most of the needles have fallen. All you have left are dead sentinels. Along our very popular lower trails, like White Oak Canyon, you have virtually no live hemlocks. Five years ago, I predicted the park would lose its hemlock stands within the next five to 10 years. Indeed, the loss has occurred at the shorter end of that. I take no comfort in seeing my prediction come true."

To think that an insect the size of a pinhead can kill a 600-year-old hemlock in the space of a half-dozen years is breathtaking. But the hemlock woolly adelgid is a formidable pest, with a complicated biology, and a dizzying ability to reproduce. It's parthenogenetic, which means that all individuals are female, so don't have to find mates. Each adelgid can lay 300 eggs; there are multiple generations a year. With their capacity for exponential growth, offspring of a single adelgid can kill a hemlock in as little as four years if the tree is stressed by drought or other factors, in six to eight years if it's not.

A Tragedy in the Making

Damage. Needles on infested branches dry up and drop.

"The hemlock woolly adelgid is native to Asia, but has been in the United States for 80 years. It first showed up in the Pacific Northwest, probably imported on infested nursery stock from Japan. No one worried much about it there, because western and mountain hemlocks, like Asian species, showed resistance to it and suffered little mortality. In the 1950s, it turned up in Richmond, Va. No one paid much attention to it there either, because it was infesting hemlocks in landscape settings easily treated with pesticides. Trees that died were simply replaced. It wasn't until the adelgid - carried by prevailing winds, hurricanes, birds and mammals - reached the great natural hemlock stands of Eastern America that the scope of the problem emerged. Like other exotic pests (gypsy moths, for example), the diseases and predators the adelgid co-evolved with had been left behind in its native land. With no natural controls - and a huge new food source to exploit - its population exploded. Tragically, eastern and Carolina hemlocks - a rare hemlock species that occurs only in isolated pockets in Virginia, North and South Carolina, and Georgia - showed no resistance to adelgids. Trees began dying - fast."

Infestations. Native range of hemlock (green) and range of hemlock woolly adelgid (red) in March 2002. They have shown up in more places since then.

The adelgid hit the Blue Ridge in the early 1980s," says USDA-Forest Service entomologist Rusty Rhea, a member of the Hemlock Woolly Adelgid Working Group, an ad hoc coalition of university, state, federal, private and corporate researchers working on adelgid control. "By the mid-'80s, it had moved down into the western side of the watershed. By the early '90s, there was significant mortality. The damage it does is highly variable within a stand. It picks off one tree, then another. But over time, I've watched it move down a watershed until all the hemlocks - every one, large and small - are gone. Gray ghosts in the landscape.

"In the half-century since they appeared in Richmond, adelgids have impacted hemlocks in 15 states, from Georgia to New Hampshire. More than half the hemlock's natural range in the East is now infested; the entire range is at risk. The adelgid is showing up in new places so rapidly now that any story (including this one) is soon out of date. The Smokies news office issued a press release May 16 announcing discovery of adelgids in two park locations; two weeks later, that number had jumped to eight. Included on the expanded list were Stony Branch, Laurel Falls, Greenbrier and Cataloochee - all areas harboring old growth hemlock stands.

Hitching Rides

Remarkably, the adelgids popping up everywhere aren't very mobile. They can't fly. Once they settle in at the base of a hemlock needle and insert their feeding tubes into the plant tissue, they're there for life. They kill their host by sucking its nutrients, literally starving the tree to death. Typically, it takes adelgids a year or two to colonize a tree. After hosting an established population for three years, a tree puts out little or no new growth. The adelgid population crashes. The tree recovers enough to put out a little new growth. The remaining adelgids recolonize, and the tree soon dies. Adelgid eggs and nymphal stages are concealed in white cottony tufts. This "wool" is the visible sign that a tree is infested. It's most noticeable in spring, wearing away as the seasons progress.

Nymphs. They settle at the base of a hemlock needle, where they usually remain for the rest of their life.

To move from their natal tree, adelgids have to hitch rides. Most often, they're carried by birds. Adelgids go through a highly mobile crawler stage that coincides with the spring migration of neotropical songbirds, who ply hemlocks for hidden insects to fuel their journey north. "The birds eat a few adelgids," Johnson says, "and get them all over their feet." Flitting from tree to tree, they deposit crawlers as they go. Crawler-stage adelgids can survive 10-15 days off their hemlock host, which helps explain why adelgid infestations have been moving inexorably northward year after year.

The adelgid's movement south has occurred much more slowly. It wasn't until 1996 that it reached North Carolina's northernmost counties. Then, last spring, isolated adelgid infestations were discovered in Yancey, Macon and Graham counties. "People have imported nursery stock from infested areas, probably from Virginia," Rhea says flatly. "Commerce transplanted the adelgids here. It's likely that the whole Southern Appalachians will be infested within five or six years." The Yancey and Macon infestations were relatively small, but the population in Graham County was widespread. Damage to trees - some of them right across Fontana Lake from the Smokies - was advanced. "As soon as I heard that the adelgid was south and east of us," says Johnson, "I knew it was just a matter of time before it showed up in the park."

What We're About to Lose

Eastern hemlocks, while not an abundant component of eastern forest ecosystems, nonetheless occupy a vital ecological niche. Many grow along streams, where their dark, dense, year-round foliage moderates stream and ground temperatures year-round. Hemlock stands are cooler in summer - and warmer in winter - than hardwood forests. Stream monitoring in Shenandoah has shown that ground not shaded by hemlocks undergoes extreme spikes of temperature in March, before deciduous trees leaf out. "The true significance of hemlocks' ability to moderate temperature is in winter and early spring," Akerson says.

Eggs. An easy way to identify infested trees, the woolly ovisacs can contain up to 300 eggs.

What effect the loss of the hemlock will have on native brook trout, who are found more commonly in hemlock-shaded streams, or on blackburnian and black-throated blue warblers, blue-headed vireos and northern goshawks, who require hemlock forest habitats, remains to be seen. Dozens more bird species use hemlock as a food source, and for nesting and roost sites and for shelter. Hemlock stands provide important cover for ruffed grouse, deer, wild turkey, rabbits - and homes for innumerable insects and other invertebrates, for salamanders and whole complexes of plants.

Then there's our loss. Though hemlock is not a valuable timber species, it is widely used as a landscape tree (274 cultivars of eastern hemlock have been developed), and glorious in natural settings. In spring, bright new growth at the tip of every twig offsets older needles' deep and somber green. To walk beneath old growth giants - on the way to an overlook at Linville Falls, in Cataloochee, or in other places where remnants of the forest primeval still exist - is to glimpse, however dimly, what natural cathedrals our woodlands once were.

What Hope Is There for the Hemlock?

The search is under way for ways to control the adelgid. Seed banking is planned, and biologists are looking for resistant trees in the wild (so far, no one has found any). Researchers hope one day to be able to breed western hemlock's resistance into eastern and Carolina hemlocks while maintaining as many characteristics of the eastern species as possible.

Landscape trees can be protected using insecticide sprays, though every needle must be thoroughly drenched annually to be effective. Chemical control, in its current form, is impractical to impossible in forest settings. Hemlock stands are scattered and often inaccessible; full insecticide coverage is difficult to achieve; many trees can't be sprayed because of their proximity to streams. Other chemical applications are being used in some areas, including systemic insecticides applied through stem injection, implantation and soil drenches. Shenandoah began experimenting this year with an injectable systemic that may allow protection of some large isolated trees not yet seriously infested with adelgids.

Predator. Tiny but voracious, both the larva and adult (shown here) of Pseudoscymnus tsugae attack all stages of the adelgid.

Work is also under way on biocontrols, the most promising of which is a tiny black coccinellid beetle (Pseudoscymnus tsugae) discovered feeding on adelgids in Japan in 1992. Years of quarantine and study convinced researchers that the beetle was safe for release in the wild. (Other predatory beetles and mites that prey on the adelgids in Japan and China are also being studied, but haven't been approved for release.) A lab in New Jersey has been mass-rearing P. tsugae since 1997. Several hundred thousand beetles have been experimentally released in nearly a dozen states. Among the places the beetles have been released are a few parkway locations north of Boone; in Ellicott Rock Wilderness Area; and, in early June, in the Smokies at Cataloochee, Laurel Falls and Panther Creek.

No one knows yet whether the beetles will reproduce in numbers in the wild, or make a dent on the adelgid population. Even so, demand for the beetles far outstrips supply - and will, for years to come. "I wish I had a whole shoebox - or 10 shoeboxes - of those beetles for Linville Falls," McElrath says sadly. "But I don't."

Article written by: Elizabeth Hunter, as seen in the Sept/Oct 2002 issue of Blue Ridge Country

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