Saving the Hemlocks
Can a UNC Asheville research partnership win the race against time to conserve a Southern Appalachian ecosystem?
The tiny parasites arrived silently, hidden among the foliage of imported nursery plants. Unnoticed, their sharp feeding tubes pierce tender bark, eventually sucking the life from their helpless hosts. They reproduce without sex, releasing untold numbers of their clones into the environment—with potentially disastrous consequences for native ecosystems in the Southern Appalachians.
The hemlock woolly adelgid, named for the tiny white, woolly masses it produces around itself on infected trees, has advanced steadily across the eastern United States. Where the infestations are worst, forests that were moist and shady even in summer—thanks to the dense green branches of hemlock overhead—are now dry and bright, the herbaceous undergrowth becoming bleached and shriveled from too much sun.
“Ecologists estimate that more than half the native range of Eastern hemlock is now affected by the hemlock woolly adelgid,” said UNC Asheville Environmental Studies Professor Kitti Reynolds, who’s teaming up in a joint project between the university and the USDA Forest Service to address the problem.
Infestations have been especially severe in the Southeast, where the pest isn’t set back by the cold winters found further north.
Ways to rescue the hemlock are needed, and quickly. And if researchers at UNC Asheville and the Forest Service are correct, one helpful hero may come in the form of a predatory beetle no bigger than the head of a pin.
Enter Laricobius nigrinus, a tiny beetle native to the Pacific Northwest, where hemlocks have coexisted with the woolly adelgid for millennia. “Its life cycle is very well synchronized with the adelgid,” said Bud Mayfield, a Forest Service entomologist working with UNC Asheville faculty and students to search for a solution to the hemlock woolly adelgid. “This predator has presumably coevolved with the adelgid in its native range.”
But it’s a different story here in the East, where no predator is present. Inserting its long, probing feeding apparatus, Reynolds explains, the adelgid fixes itself to the branch at the base of the needles, drawing nutrients from the tree’s sap. As the infestation advances, the hemlock’s foliage turns gray and falls off, and new growth ceases. Mature trees are dead within three to five years.
Reynolds and Mayfield, along with students from UNC Asheville, are exploring an integrated approach to pest management, using a biological agent—the beetles—plus the insecticide, imidacloprid. The chemical is very effective, researchers say, but its effects wear off after a few years—and the adelgid returns.
“We’re looking at the benefit to the hemlock of establishing the predatory beetles on trees that have been previously protected with insecticide,” said Reynolds, “to buy the trees some time. So when adelgids come on those trees again, the beetle can provide control.” What’s more, Reynolds explained, the trees are treated with lower rates of the chemical, reducing costs and exposure to the toxin for nearby creatures.
“Mortality in the South occurs so fast, the trees are dying before the predators have a chance,” said Mayfield. “Once the beetles are established, they’re in place as a natural control mechanism; but it takes time to build up those numbers. If we can combine these two tools, we may be able to save more trees than we could with either tactic in isolation.”
Reynolds takes a long-term view.
I doubt we’ll ever be able to eradicate the adelgid—once they’re here, they’re here,” she told UNC Asheville Magazine. “There’s no way we’ll be able to treat all the trees, but if we keep enough of them going, and reserve the genetic stock, over generations we can breed a tree that has some resistance.”
There’s good reason to believe the beetles could help turn the tide for the great trees, Mayfield explained. One positive indicator is that Laricobius nigrinus feeds only on adelgids. Another is that the beetles are active in winter—a critical point, because the adelgid is also active then. Adult beetles feed on adelgid life stages from fall through spring; best of all, they actually lay their eggs in the adelgid’s egg sacs, where their larvae feed on the pest’s eggs. The hungry beetles also feed on the adults, now fixed to their feeding spot under their protective shield of fluffy white “wool.”
Some might question the wisdom of introducing one exotic creature—a foreigner to the local ecosystem such as Laricobius nigrinus—to control another, even when the target has become a critical threat. “Species introductions are not always without surprises,” said Mayfield.
Mindful of examples like kudzu, Mayfield argued, “You have to weigh that potential against the cost of doing nothing. The hemlock woolly adelgid is such a devastating pest, and time is running so short to do something about it—especially in the Southern Appalachians, where mortality seems to be much more rapid than in the northern parts of the range. You have to weigh those costs and benefits carefully.”
Mayfield expresses confidence that Laricobius nigrinus won’t go the way of kudzu, the invasive plant that’s been dubbed “the vine that ate the South.” Before the vine was recognized as invasive in the 1970s, it was widely promoted to combat soil and stream bank erosion following a period of aggressive land clearing for agriculture.
“Laricobius nigrinus is not native to the eastern U.S., but it is not an invasive pest,” he explained. “Invasive pests, introduced from some other ecosystem, have escaped their own controls; and because their host may not have evolved any resistance or escape mechanism, their populations build rapidly and they become pests. I would not apply that term to Laricobius.
“These beetles have had several years of evaluation in quarantine, and they have met very stringent requirements to show that they are prey-specific and not a problem adding to an existing problem.”
UNC Asheville students are involved in a program of research that’s poised to make real strides in approaching the problem. The project has employed recent UNC Asheville graduates Hunter Weaver and Ashley Hancock; currently involved students include Julie Tierney, Ashley Case, and Dustin Neufeld.
Tierney, a senior in UNC Asheville’s Environmental Studies program, examined the health of previously-treated, infected hemlocks in the Chattahoochee National Forest in northern Georgia through an analysis of their growth rings.
“The overall purpose is to determine the right combination of chemical control—the insecticide—and biological control—the predatory beetle—in a stand of hemlocks,” Tierney explained. “The work involved coring trees, and bringing the sample cores back to the lab at UNC Asheville to measure the trees’ growth rates, to look for evidence of tree growth being impacted by the adelgids.”
The results indicated a significant difference for trees treated with reduced levels of pesticide—even down to 25 percent of the typical dose—with larger growth rings in trees receiving both treatments compared to those having beetles alone. “So even 25 percent of the pesticide, in tandem with the beetles, has a positive impact on the trees,” says Reynolds. “We’re putting less pesticide out, and getting improved growth.”
Tierney presented her results at the Association of Southeastern Biologists conference at the University of Georgia last year, and also at a UNC Asheville Undergraduate Research Symposium. “I was so excited to participate in real investigative research on something so important,” said Tierney, a native of Cary. “Not only did I satisfy my own curiosity for scientific investigation, but I was able to participate directly in environmental stewardship for the landscape that I love. What a great opportunity!”
Professor Kitti Reynolds points to the benefit to society. “We see potential impacts for everyone from individual homeowners to public agencies including the National Forests and the Park Service,” Reynolds said, “because these agencies spend a lot on adelgid control.”
The project has some less obvious benefits, too, according to Reynolds. Involvement by young women in programs like these is important, says Reynolds, as women have traditionally been less visible in scientific and technical arenas.
"Women are still regarded as an underrepresented minority in science," Reynolds told UNC Asheville Magazine. "According to the Association for Women in Science, women represent 50 percent of the population and capacity for innovation in the U.S., but only 24 percent of the science, technology, engineering
and math workforce." In contrast, Reynolds said, "Bud and I have worked with an equal number of men and women students."
Above all, there's the chance to save these majestic trees, while perpetuating the moist, diverse habitats they create for other species. "There's just something special about walking into a grove of hemlock trees where the trunks are two to three feet in diameter, and they're soaring a hundred feet in the air," Mayfield said. "People love hemlocks and the trees have a key role in ecosystems. People really want to be able to save hemlocks if they can."