Will Hemlocks Go The Way Of The Chestnut?


The recent discovery of Hemlock Woolly Adelgid (HWA) infestations in Graham, Macon and Yancey counties is a major setback in the battle to keep Eastern and Carolina hemlocks alive in southern Appalachian forests.

“Until a few months ago, we thought the adelgid hadn’t spread beyond the northern tier [of North Carolina] counties,” says Fred Hain, a NC State entomology professor and adelgid expert. “We thought we had plenty of time. Now we know we don’t. It’s not a rosy picture.”

Scientists suspect the ornamental nursery trade is responsible for the adelgid’s devastating leapfrog south.

“People have imported nursery stock from infested areas, probably Virginia,” says Rusty Rhea, a USDA-Forest Service entomologist at the Southern Research Station in
Asheville. “Commerce transplanted the adelgid here. It’s likely that the whole Southern Appalachian region will be infested now, within five or six years. It galls and saddens me. We had hoped to build a biocontrol firewall between Virginia and North Carolina. Now we can’t.”

HWA, or Adelges tsugae, is a minute sucking insect decimating hemlock stands from Virginia to Massachusetts. Native to Japan, southwestern China, India and Taiwan, HWA is an introduced Asian pest that arrived in the Western U.S. about 80 years ago, probably on imported nursery stock. Though the adelgid is found on native western and mountain hemlocks from northern California to southeastern Alaska, it does little damage to those species.

But it’s devastating to Eastern and Carolina hemlocks. The adelgid was first discovered in the East in Richmond, VA, in the 1950s, on ornamental trees in people’s yards. Because it could be controlled with pesticides in such settings, no one worried much about it. That all changed when it showed up in native hemlock stands, where it showed its true colors.

“The adelgid hit the Blue Ridge in the early 1980s,” Rhea says. “By the mid-1980s, 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 1989-90, 95 percent of Shenandoah National Park’s hemlocks were perfectly healthy. By 1998, less than five percent were.”

Carried north by prevailing winds, hurricanes, mammals and migrating birds (neotropical songbirds’ spring migration unfortunately coincides with the HWA’s mobile crawler stage), the HWA has moved north rapidly, killing hemlocks in the mid-Atlantic states and southern New England.

Its southward progress has been much slower. It didn’t reach the northern tier NC counties until 1996, and still hasn’t been discovered in Kentucky, Tennessee or the westernmost counties of southwest Virginia. All that may quickly change, with the newly discovered infestations in western North Carolina.

Particularly troubling is the Graham Count outbreak, which brings the adelgid within a few miles of old-growth hemlock groves in the Great Smoky Mountains National Park and Joyce Kilmer Memorial Forest. Fortunately, says Hain, “there’s not much hemlock on that edge of the Smokies” [closest to Graham County]. There’s a bit of a gap there.”

But Graham County “has a widespread population of Hemlock Woolly Adelgids,” Rhea says. “They’re right across the [Fontana] lake from the Park.” He predicts
“imminent” discovery of HWA in the Smokies. Judging from the extent of damage to infested trees, he thinks the adelgids have been in Graham County five or six years.
“Some trees look pretty bad. It’s been dry for three years, which compounds the problem. Some trees that are surviving now probably won’t be able to hang on much longer.”

HWA can kill a stressed hemlock in as little as four years; non-stressed trees usually survive six or seven. Here’s the typical scenario: The adelgids spend a year or two colonizing a tree. The growing population sucks sap from base of the needles for a couple of years after that.

After three years of hosting an established population, the tree puts out little or no new growth, causing the adelgid population to crash. The tree regroups, puts out some new growth; the remaining adelgids recolonize, and eventually kill it. HWA eggs and nymphal stages are concealed in white cottony tufts — the visible sign that a tree is infested.

Graham County’s infestation is huge, compared to the 100-acre outbreak in the Celo/South Toe section of Yancey County and the 30-40 acre infestation in a retirement/second home community near Franklin. Graham’s adelgid population “exists in varying stages of development over probably 20 square miles,” says Paul Merten, a NC Division of Forest Resources pest control forester who spent three days conducting a road survey there this spring. He found adelgids on both public and private land.

An Empty Toolbox

Horticultural oils and insecticidal soaps can control HWA on infested trees in landscape settings, but aren’t practical in woodlands because the entire tree, including crown and undersides of needles, must be drenched with insecticide. Nevertheless, Merten and a crew of 40 Forest Service personnel and South Toe volunteers used high pressure hoses and pumper trucks to douse 60 acres of infested woodland with insecticidal soap on May 12.

The spraying was the first phase of a three-part attack on the adelgids in Yancey County. Crews will return this summer to cut and burn branches of infested trees along the South Toe River (the spray can’t be used near waterways). Merten will survey the area again this fall, and perhaps order another round of spraying.

“We aren’t under any illusion we’ll eradicate the adelgids here,” he says. “We’re trying to lower their population and buy some time.”

Time is what everyone fighting the adelgid’s spread would like to buy — and a lot of it — because “our toolbox [to fight HWA] is virtually empty at the moment,” Hain says.

When researchers first realized the HWA could do to eastern and Carolina hemlocks what the blight did to American chestnuts, “we looked for native predators we could produce in great numbers to take care of the problem,” says Rhea, a member of the HWA Working Group (an ad hoc coalition of university, state, federal, private and corporate researchers working on HWA control). “We did intensive surveys and studies and found nothing.”

A few native generalist predators eat the adelgids when they come across them, but not consistently enough to have any impact on ballooning populations.

Not until 1992, when Mark McClure of the Connecticut Agricultural Experiment Station traveled to Japan to look for natural predators there, did an HWA “specialist” come to light: a tiny black lady beetle “the size of a poppy seed” whose scientific name is Pseudoscymnus tsugae. Its preferred food is the HWA, though it also eats balsam woolly adelgids, a prospect that interests researchers because those Asian aphids are killing Fraser firs here.

After several years of quarantine and study, McClure and his colleagues determined that P. tsugae was safe for release in experimental settings in the wild. In the beetle’s favor: the specificity of its dietary choices; its adaptability to a wide range of climate conditions; and a life cycle that’s in sync with HWA’s. Unlike the (also introduced) Asian ladybugs that aggregate and overwinter in our houses, P. tsugae (who winters in forest leaf litter and doesn’t aggregate) probably won’t become a human pest.

Since 1997, P. tsugae has been mass-reared by the Phillip Alampi Beneficial Insect Laboratory in Trenton, NJ. The lab has produced several hundred thousand beetles for experimental releases in almost a dozen states. This spring the lab supplied NC State with 100 beetles to launch a rearing program. Hain hopes to have some beetles available for release next year, though he’ll have a better idea of “what we can expect to accomplish in a year’s time” after he visits the New Jersey lab later this month.

Beetles Out Of The Box In NC

The first NC release of P. tsugae took place two years ago under Hain’s direction, when 10,000 beetles were released in Hanging Rock State Park. Monitoring the beetles (at first monthly, now twice a year) proved extraordinarily difficult because they’re so tiny. “There’s no good way to do it,” Hain says. “You just have to go out there and look. It’s much harder than trying to find a needle in a haystack.”

Nonetheless, some beetles were found at the site 14 months later, indicating the original beetles had either survived or reproduced. “It’s impossible to say how many are out there now,” he says. “And we can’t see any clear evidence of impact on the adelgid population, though it’s a little early to tell. A good rule of thumb is that it takes six to ten generations before you see an impact.

“In some of the releases up north, there are some hopeful signs, but the most hopeful are occurring under fairly controlled conditions — where the beetles were released onto infested branches enclosed in sleeve cages. Where they’ve been released without cages, we know they have survived and reproduced. But there’s no evidence yet that they’re having a big impact on adelgid populations. They’re not, by themselves, the silver bullet.”

Additional NC releases took place this year at a site on the Blue Ridge Parkway near the Wilkes/Alleghany county line. Then, in late May and early June, Hain and Rhea released a total of 7,500 beetles on two adjacent trees in Graham County’s Cheoah Ranger District. An experimental plot around the release site will be monitored for several years to discover whether the beetles succeed in establishing themselves and their dispersal pattern.

The trees the researchers chose are “mature, 70- to 80-foot tall hemlocks with building populations of adelgids,” Rhea says. P. tsugae has had the greatest impact in newly infested forests where adelgid densities are low and trees are generally healthy.

“I don’t have any illusions we’re going to save the trees we released the beetles on,” Rhea says. “But if we can build a population that will play check and balance with the adelgids, so they aren’t causing so much damage, I’ll be happy. That will occur, if it does, somewhere adjacent to the release trees. And it will take time.”

Other Bug-Zappers May Help

Currently, P. tsugae is the only non-native biocontrol that has been released in the field. “Others are in the pipeline, but they’re only at the experimental stage now,” Rhea says. Three lady beetle species from the genus Scymnus found feeding on HWA in China by Michael Montgomery, of the USDA-FS Northeastern Center for Forest Health Research in Hamden CT, are being studied at several university labs. A derontid beetle, Laricobius nigrinus, that preys on HWA in the western U.S., is being studied at Virginia Polytechnic Institute. Researchers are also looking for fungi that attack the adelgid.

Others are looking for individual trees resistant to the adelgid (similar to that shown by its western U.S. and Asian relatives). So far none have turned up, though Hain says some infested trees have hung on as long as 8 years, perhaps because of site conditions. Whether the resistance the western species enjoy can be bred into eastern hemlocks, no one yet knows. “With clonal propagation, we can cut some corners, but it’s still a long term project,” Rhea says.

Researchers know now that drought increases hemlock mortality, and that cool, moist springs and summers help infested trees regain ground. Mild winters aid and abet growth of adelgid populations. Very cold temperatures can kill off large numbers of adelgids (a string of sub-zero nights in northern Connecticut in January 2000 killed 90 percent of the adelgids there, though populations can quickly rebound), as can spring cold snaps following warm weather.

“Hemlocks ‘wake up’ in the spring earlier than lots of other trees do,” Rhea says. “That triggers something in the adelgid. If you get cold weather after that, the adelgid population can get hammered.”

To prevent the kind of leapfrogging that just occurred in Western North Carolina, Maine, New Hampshire and Vermont have imposed quarantines on hemlock seedlings, nursery stock, logs, etc., that come from, or have been transported through, HWA-infested areas. And the HWA Working Group, which thus far has “operated on a shoestring,” has submitted a multi-year proposal to the USDA for “some major research dollars” to combat the HWA, Rhea says.

The first year’s $673,000 request, if approved, would go to projects at a half-dozen universities and the USFS. Though the money is desperately needed in the fight to save hemlocks, Hain thinks the proposal has “no better than a 50-50 chance” of being approved. “We’re competing with lots of other projects, projects have to do with food supply and agriculture,” he says.

Though it’s not an important timber tree, hemlock’s ecologic importance is incalculable. A dominant component of some old-growth forests and an important riparian species, hemlocks can live 800 years, shading streams, keeping their waters cool enough for trout, providing food and cover for dozens of birds and other species. Mortality is already increasing siltation, erosion and stream temperatures.

“What’s going to happen to our streams if the hemlocks go, is an open question,” Hain says. “We don’t know the answer to that one.”

Scientists do know that “one biocontrol won’t do the job. It’s going to take several things impacting the adelgid to control it,” Rhea says. “My hope is that in my career, we’ll find something that will make a difference. The adelgid doesn’t march across the landscape. It will take it awhile to search out all the cracks and crevices. We won’t lose the hemlock in the next 20 years, though the [eastern] hemlock’s entire range could be infested by then.”

“I was at a meeting the other day, where one scientist called the gypsy moth ‘an eradicable pest.’ I’m not willing to go that far,” Hain says. “But we do have a pretty complete toolbox to fight them, after 30 years of research. The difference between them and the adelgid is that the gypsy moth isn’t really a tree killer — and the adelgid is. You only have about 10 percent mortality with gypsy moths; that’s not the case here. And we don’t have 30 years. I would say, if we don’t come up within something in the next decade, there’s not much hope for the hemlock.”

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