As an adult, she is a millimeter long, so minute that to see her clearly you need a microscope. She has a voracious appetite, feasting on trees for most of her two-month lifespan. And there are millions and millions of her—the Hemlock Woolly Adelgid—decimating vast acres of Eastern and Carolina hemlock trees from northern Georgia to southern Maine, including the North Carolina mountains. The pest was also found in 2010 at Hemlock Bluffs Nature Preserve in Cary.
The ramifications of the adelgid invasion cannot be overstated. When large stands of hemlocks die, as they are in Western North Carolina, it ignites a chain reaction that damages, sometimes irreparably, the ecosystem. Fish, birds and other wildlife lose their habitats, which are already threatened by development, climate change and other natural and man-made forces. Scientists have found these vast tree die-offs actually change the carbon cycle in parts of the North Carolina mountains, where hemlocks offer an endless buffet for the hungry adelgids. Here, the infestations are killing trees in greater numbers than even in the Northeast.
Throughout the world, leading entomologists and biologists, including several at N.C. State University, are researching ways to combat the adelgids. Unfortunately, so far, the adelgids are winning.
In the entomology laboratory at N.C. State, it smells like Christmas. Sections of hemlocks and Fraser firs, which are attacked by the equally damaging Balsam Woolly Adelgid, lean against the wall. A sign is posted on a refrigerator: "HWA and BWA diets." Several branches, their needles stippled with white woolly masses, are tagged with orange labels that read, "infested."
The woolly adelgid is an exotic insect native to China and Japan, where hemlock trees are immune to it. "Most hemlocks throughout the world have some degree of resistance in their native ranges," said Fred Hain, professor emeritus of entomology at N.C. State. But in the eastern U.S., including North Carolina, he explained, the trees are vulnerable. Here, they are invaders with no natural predators. And without natural predators, the adelgids spread unchecked.
As eggs and crawlers, adelgids can hitchhike on things that move—shipping cartons, birds, people—to new destinations. Adelgids were initially found on the West Coast, where they attack Western hemlocks, but don't kill them, according to a University of North Carolina thesis by environmental science student Jessica Long.
The first spotting of the adelgid in the Southeast occurred in 1951 in a Richmond, Va., nursery. From there, Long reports, it spread for a half-century at a rate of 10 miles per year. It has infested nearly all Appalachian hemlock stands south of Virginia in just a decade.
All adelgids are females. They reproduce asexually and lay hundreds of eggs, usually two generations per year. Their first egg-laying phase occurs in spring when the adelgids encase themselves in a white, protective woolly mass on needles of the hemlock tree. When the eggs hatch, the baby adelgids, known as crawlers, march off in search of a spot to feed. In a manner that recalls the movie Alien, a stylet, a pointed feeding tube one-and-a-half times larger than adelgid's body, uncoils from inside her thorax and inserts it between the needle and the twig. She sits there for the rest of her life and feeds, sucking sap and, scientists speculate, injecting toxic saliva into the tree.
"A lot is unknown about their feeding," said Erin Mester, an N.C. State graduate student studying the adelgid. "They are hard to study because they are so tiny."
In the way a person's immune system can overreact to a foreign body, such as with asthma, the tree's immune system also overreacts. The needles turn grayish-green and fall off. Buds die, extinguishing any hope of the hemlock's regrowth. Weakened, an infested tree can die within four years, sometimes sooner, depending on other stressors, such as drought.
When the hemlocks die, the understory—plants that grown on the forest floor—change. Soil composition—its acidity, for example, can be altered, which determines what plants, including invasive varieties, can grow.
"Hemlocks play a huge role in our ecosystem," said Mester, who recently create the Tiny Terrors project to engage the public in adelgid-related research (see sidebar at right). Hemlocks grow along stream banks and prevent erosion. Trout thrive in streams cooled by the shade of hemlock trees. When those trees die, sunlight warms the stream, making it less hospitable to the fish.
Winter in North Carolina, even in the western elevations, is still warmer than the brutal climes of the far Northeast. And it's cooler here than in northern Georgia. In other words, Hain said, "In North Carolina, the temperature is good for Hemlock Woolly Adelgids."
The 158-acre Hemlock Bluffs Nature Preserve in Cary has a distinct microclimate found nowhere else in central North Carolina. Cold air sinks and flows down from the bluffs. The soil and the comparatively cool climate is ideal for Eastern hemlocks. At Hemlock Bluffs, the largest trees are 300 to 400 years old.
In 2010, Cary officials spotted the signs of the adelgid: tiny white spots that resemble cotton balls—the egg sacs—had formed on the needles.
Cary and state parks officials (Hemlock Bluffs is a joint effort between the town and state) considered introducing beneficial insects to eat the adelgids. "That wasn't feasible here, and it's unsure whether they work in all situations," said Mark Johns, recreation program specialist.
With that option dashed, they considered injecting the trees with chemicals, but as Johns noted, "it's slow-acting, less effective and tree mortality is high." Coating the trees with toxic soaps or oils could immediately kill the adelgids but it would be difficult to reach the trees on the bluff, and there were concerns about residual effects on the environment.
After a lot of research, officials decided to temporarily close the bluffs to the public and hired a crew of eight workers to paint the base of each of the park's 235 hemlock trees—their locations had already been mapped—with a chemical that would be released into the bark and kill the adelgids, but would not drift into the air and run off into waterways or onto the soil. The cost for the treatment was $7,000, with an additional $3,000 to monitor its effectiveness.
"We could treat our trees because we have a finite amount of them," said Laura White, nature center supervisor.
The town then developed a management plan of inspecting the trees twice a year. So far, the cure seems to have worked; no adelgids have been seen since. "It's a big success story," said Dwayne Jones, recreation manager.
When scientists discuss how to defeat the adelgids, they sound like oncologists waging the war on cancer. The can treat the tree with chemicals. They can introduce pests that hopefully will kill the pests. And they hope to breed new hybrids of hemlocks that are immune to the invaders.
In Cary, the hemlocks could be treated chemically because there were so few of them—and they had been mapped, making them easy to find. But in the thickets of the North Carolina mountains, it's financially and logistically impossible to chemically treat thousands of hemlocks. There are also environmental issues with spraying, in that it is difficult to keep the chemicals out of streams and rivers.
Researchers have also tried importing natural insect predators from Asia, but giving those beneficial bugs a foothold in North Carolina is problematic. The trees have to maintain a critical mass of adelgids so the predator insects have enough to eat. These predators also require time to establish colonies—time during which trees keep dying.
"We've not seen dramatic results with biological controls," Hain of N.C. State said. "I can't say we saved any trees."
Considering these limited options, scientists have moved on to genetics, crossing Chinese hemlocks with Eastern and Carolina versions. They hope a new breed of trees can develop resistance to the pest.
Finding unaffected hemlocks is key to Hain's research and his effort to save the trees with the Alliance for Saving Our Forests, which he co-founded. Researchers at the University of Georgia are also working on techniques that would produce adelgid-resistant seedlings.
"We know what a huge issue this is, but it's alarming how many people don't know about it," said Mester. "And it's happening to our forests and hurting our economy."
This article appeared in print with the headline "Kill this pest."