Along the Delaware Water Gap, deep in the heart of New Jersey, there stands a line of hemlock trees that hold a significance beyond their regular role in the surrounding ecosystem: They're the last grandstand against the scourge of the woolly adelgid, a pest which has decimated populations of the evergreen tree across the eastern United States.
By now you've probably heard of the woolly adelgid, or Adelges tsugae. It has gained recognition across the U.S. since it made its way from East Asia and began sucking the sap from the eastern hemlock (Tsuga canadensis), one of the most common trees in the Great Smoky Mountains. Though the creature was accidentally introduced in America in the early to mid-1900s, it wasn't until the early 2000s that the tiny insects began to deeply impact Tennessee populations of the keystone tree species, which plays a vital role in both providing habitat for other animals and cooling stream waters.
Scary headlines followed, like The New Yorker's "A Death in the Forest: Can the trees of the Great Smoky Mountains be saved?" The National Park Service says that "without successful intervention, the hemlock woolly adelgid is likely to kill most of the hemlock trees in the park."
There are over 800 acres of old-growth hemlock trees in the Smokies — some over 500 years old — plus another 90,000 acres with forests of younger hemlocks, according to the NPS.
Now for the part you may not have heard: In November 2014, Signal Mountain residents took action to protect their own hemlocks with the introduction of Laricobius nigrinus, a type of predator beetle which feeds on the adelgid. It was hoped this would slow the adelgid's spread, though the level to which this treatment method succeeds depends on the beetles' ability to reproduce and grow in numbers substantial enough to handle the adelgid infestation.
Thanks to a private donor, $3,000 worth of the beetles (roughly 1,000) were purchased and released across Rainbow Lake. Three years later, the town's Tree Board hoped to see results in the form of resilient trees across the area. That deadline is looming, but the beetles' effect has been inconclusive, says Signal Mountain Tree Board Chairman Noah Long. Both on the mountain and in the more than dozen other eastern states where the beetles were released, the hemlock and adelgid are both so prevalent that quantifying these experimental efforts has proven to be difficult.
Signal Mountain Tree Board Chairman Noah Long says misconceptions about how to treat the issue are still a problem. For instance, he says the pesticide Imidacloprid, the recommended treatment for infected hemlocks, can be used safely without risk to bees and other native, necessary insects — provided it is used correctly and placed directly into the soil at the base of the tree.
“Hemlocks don’t have flowers,” Long explains, adding that Imidacloprid breaks down quickly, further reducing potential threat of minor impact to the ecosystem. “If you take the insecticide and use it as a sprayer, yes, it would harm them. But there are contained ground treatments.”
"Once [the woolly adelgid infestation] hits, a starter pistol goes off and you have to decide what to do," says Tennessee Forest Health Forester Nathan Hoover. "Based on the level of infestation, the adelgid can kill the tree in two to eight years."
With that in mind, the Forest Service is focusing on chemically treating trees — and asking for area landowners' help. Across the state, the Forest Service has only been able to treat and protect an estimated 12,000 trees over 1,000 acres, Hoover said.
Treatment is relatively simple — though costly. In fact, had more funds been available to the town of Signal Mountain, an insecticide treatment would have been the ideal choice to protect the mountain's hemlock population because it is more immediately effective, Long says.
At an estimated $1-$5 per tree, depending on the treatment used, neither the tree board nor the Forest Service is equipped to contain the spread, and so it continues.
"We're going to continue to lose hemlocks," Hoover says. " But if we continue to do management and engage landowners, we can preserve the future of the tree."
That stand of trees in New Jersey? Why exactly that line of trees has resisted infection, even as surrounding trees succumb, is still a mystery, forester Nathan Hoover says. And while researchers have collected seeds from the so-called “bulletproof trees” to plant and encourage new resistant growth, there is no decisive answer as to how those particular hemlocks have continued to survive and thrive.