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Protect Our Parks

Saving the Hemlock Trees in Great Smoky Mountains National Park

This shade-loving tree that keeps the park cool in the summers faces a silent killer.

There’s a ruthless silent killer in Great Smoky Mountains National Park, and its target for years has been some of the park’s oldest inhabitants.

A number of the casualties have been as old as 500 and a whopping six feet in diameter. Others are much younger and thinner, just getting their start in this ecological paradise. If you have been to the park, you’ve seen them running up and down the park’s hills and perhaps even sought them out to sit under their shade by a stream. They are eastern hemlock trees, known as the “redwood of the East,” and they are being attacked by a pernicious army of tiny insects.

First discovered in the park in 2002, the hemlock woolly adelgid (pronounced a-dell-jid) has spent decades since its arrival in the United States, working its way south from Richmond, Va., decimating hemlock populations in its path. It’s believed to have landed in the States in the 1950s on ornamental plants brought from Japan. Since only a small percentage of them develop wings, the majority hitchhike to new forests via nursery plants, deer, birds, wind, garden waste, hikers’ clothes and boots and even firewood. Incredibly, the hemlock wooly adelgid made its way north from Virginia to southern New England riding the winds of a hurricane in the mid-1980s, according to University of Massachusetts Amherst researchers.

Jesse Webster at Leconte Creek in Great Smoky Mountains National Park
Jesse Webster at Leconte Creek in the park. Photo: NPS/Great Smoky Mountain National Park

“We knew it was on its way to the South since the early 1990s,” says Jesse Webster, a forester at Great Smoky Mountains National Park. “It, coupled with drought in 2006 and 2007, really impacted the trees because they were under drought stress. These extremes that can be exacerbated by climate change can be out of our control. It’s kind of like the pine beetle in the West (that tore through forests in the Rocky Mountains) — drought really exacerbated its impact.”

Hemlock Trees Under Attack in Great Smoky Mountains National Park

Left untreated, the adelgids can kill a tree in as little as 3-4 years, according to the University of Tennessee Institute of Agriculture. And without hemlocks, Great Smoky Mountains National Park’s forests could be losing one of its most important tree species and the ecosystems that they support.

If the hemlock wooly adelgid was left to its own devices, it’s likely the park could lose all of its more than 50,000 acres of hemlock trees. That acreage is equivalent to a staggering 37,878 football fields. It nearly defies the imagination that an insect only about 1 millimeter long that sucks tree sap could do so much damage. It’s believed the insects inject a toxic saliva while feeding, killing needles, shoots, branches and eventually the tree. But with teamwork of the U.S. Park Service, U.S.D.A. Forest Service and non-profit partners such as Friends of the Smokies and the Great Smoky Mountains Association, the park and its hemlocks seem to be winning the war against it.

“The effort to manage the hemlock wooly adelgids and the hemlocks is a multi-organizational effort,” says Bud Mayfield, a research entomologist with the USDA Forest Service, who’s worked with Webster and Great Smoky Mountains National Park for close to a decade. “The great thing about it is so many people care and so many agencies, including university partners and non-profits, are setting priorities and plans for the whole effort.”

In areas of the park like Cosby, you’ll see the fruits of their labor. The hemlock wooly adelgids got to the area later than others and as Webster notes, “We had our game on by then.” The park has been able to save 95% of the hemlocks there.

A hemlock tree on the Alum Cave Trail in Great Smoky Mountains National Park
A hemlock tree on the Alum Cave Trail in the national park. Photo: NPS/Great Smoky Mountain National Park

Why Save the Hemlock Trees in Great Smoky Mountains National Park?

So, why is it important to save hemlock trees? Afterall, there’s no shortage of trees in the Great Smoky Mountains. To get right down to it, there’s so much more to hemlocks than just providing essential shade for tourists during hot, humid days in the park. That shade also keeps stream temperatures down, which makes the water livable for the native brook trout that swim in them.

“If you take the hemlocks out, there are ripple effects that change the character of the ecosystem,” Webster explains. “Whether you are looking for and love shady groves of trees or you love fishing for brook trout or love black-throated green warblers, there are a number of reasons why keeping the ecosystem intact is so important.”

In fact, people aren’t the only ones visiting Great Smoky Mountain National Park. And those other visitors — neotropical migrants — could be greatly impacted by the loss of hemlock trees.  While this obscure term may conjure up images of folks from Florida wielding giant sunscreen bottles and descending on the park, “neotropical migrants” actually refers to birds. Songbirds, more specifically.

There’s the black-throated green warbler and the blackburnian warbler, a small bird that’s under 5 inches long, that migrate to the park from far-flung places like Peru and Colombia. The park’s hemlock trees are like the warbler-equivalent of Las Vegas, providing delectable insect buffets and comfortable, roomy places to nest high in the forest canopy.

Blackburnian Warbler
Blackburnian Warbler Photo: Wikimedia Commons/Paul Hartado

Blackburnians are known to pick insects and larvae from high in hemlock trees, the males’ bright orange throats and triangular-shaped markings near their eyes making them easily identifiable. The Louisiana waterthrush also flies to the park from Central America and the Caribbean. Like the brook trout in the park’s streams, it feeds on the insects on the water’s edge.

“With the loss of the hemlock forests, these species will disappear,” Webster says.”It’s a cascading effect.”

And that would be the end of Las Vegas-style buffets for warblers. And possibly the end of warblers in the park. And that would, in turn, affect insect populations. The loss of trees would also lead to a rise in stream temperatures, which could negatively impact brook trout. Which could end fishing for those who cast lines in the park’s waters.

Fighting the Hemlock Wooly Adelgid

That’s why Webster and others are taking a three-prong approach to beat the hemlock wooly adelgid. Park foresters have found that spraying the canopy and branches of smaller trees with a horticultural oil helps control the invasive insect. They have also found that treating larger hemlocks with an insecticide injected directly like a vaccine into the tree trunk or soil protects it for between 5-7 years. While the effects of insecticides on neighboring species is something that foresters are concerned about, Webster says rigorous studies have found no negative effect on other valuable resources. He notes both are “stop gap” controls to keep the hemlock wooly adelgid levels down until biological controls can take over.

The alternative? The decimation of the hemlock population, which would have a disastrous effect on the ecosystem.

But the most interesting method of fighting the hemlock wooly adelgid is, hands-down, the use of “Larry.” Larry is the nickname for Laricobius nigrinus, a beetle native to the Northwestern United States that actually eats the hemlock wooly adelgid. Studied and observed in quarantine from 1997-2003 at Virginia Tech, it began to be released in the park in 2003. There’s a second beetle Laricobius osakensis that is native to Japan and a natural predator to the hemlock wooly adelgid. Between it and its Northwestern cousin, more than a half million Larrys have been released into the park. And many more have been released from southern Appalachia all the way to Canada.

Is it safe to introduce non-native insects to feed on non-native insects? As part of USDA compliance, entomology labs conduct rigorous trials to determine if these new predators are a safe option for controlling pests. These beetles, like most insects, are species specific, Webster says, and only complete their life cycle if there are hemlock wooly adelgids to eat.  If they don’t find the insect, they will not establish themselves on the landscape.

Scientists like Mayfield are also exploring something called silviculture, opening up canopies in the forest to more sunlight. He says there’s evidence that the presence of sunlight is helping hemlocks tolerate hemlock wooly adelgid invasions and can therefore live longer.

“We’re trying to bring a toolbox of strategies against the pest,” Mayfield says. “It’s interesting because the hemlock is one of the most shade-tolerant trees and is able to persist and grow in shade. But it succumbs easily to the hemlock wooly adelgid when it’s heavily shaded.”

Mayfield and foresters like Webster feel all the tools they are using in the park and beyond its boundaries to save the hemlocks are making a huge difference.

“I’m an optimist,” says Webster. “It’s a complicated system and we’ve definitely seen changes in the landscape that can be overwhelming. But seeing the good we’re doing is really important. I don’t feel it’s going to be the next chestnut blight.

A hillside of dead Eastern Hemlock trees in Great Smoky Mountains National Park
A hillside of dead Eastern Hemlock trees in Great Smoky Mountains National Park Photo: National Park Service

Avoiding Becoming the Next Chestnut Blight

There’s not a tree in the forest that wants to be the next chestnut blight. What befell the chestnut tree last century has been called “the worst ecological disaster to strike the world’s forests in all of history,” according to The American Chestnut Foundation.

For 40 million years, the chestnut tree grew and adapted in North America, stretching from Alabama all the way to Maine and Ontario, Canada, becoming the largest, tallest and fastest-growing trees in the eastern United States. Its wood was used to build log cabins, railroad ties, fences, flooring and furniture. Its chestnuts were roasted and consumed by Americans during the holidays and eaten by cattle that needed fattening.

But in 1904, a non-native chestnut blight fungus called Chryphonectria parasitica started attacking chestnut trees. By 1950, nearly all American chestnut trees in the eastern United States were dead.

“I believe we are looking at a much more positive story,” Webster says. “If we hadn’t done anything and stepped in, within 20 years we’d be looking back and asking ourselves why we hadn’t done more.”

End Note: You can help save the hemlocks by donating to the hemlock wooly adelgid project via Friends of the Smokies, a non-profit fundraising partner for the park.