Scientists Turning Tide In Battle Against Invasive Hemlock Pest

Jul 30, 2015

A Hemlock Woolly Adelgid, magnified.
Credit NC State University

The hunt for the hemlock woolly adelgid begins in an unexpected place, tucked between a golf-course community and the Koka Booth Amphitheater in Cary. It's hardly the setting for a tree and a pest that prefers cool mountain air.

But the trees are here, clinging to a small cup-shaped notch in a north-facing cliff in the Hemlock Bluffs Nature Preserve.

“So this is one of those little pockets like this, right in here,” Mark Johns, the Preserve’s operations supervisor, explains, standing on a wooden deck attached to the bluff. “You go over to west overlook, which we could almost throw a rock to, you just can’t see it through all the trees, same thing. Pocket. And there’s a bunch of them there and a bunch of them here and not much in between.”

About five years ago, the woolly adelgid showed up on one of these hemlocks. It’s a sixteenth-of-an-inch long insect that sort of looks like a tiny football with legs. When it feeds on the tree’s sap, it creates a white foam, like a cotton ball. That explains the “woolly” part.

The woolly adelgis is an invasive insect that is killing thousands of hemlock trees in the North Carolina mountains. It arrived in the United States from Asia in 1951, appearing first on a private nursery in Richmond. It now afflicts hemlocks up and down the Appalachians, from Massachusetts to the Great Smoky Mountains.

Once the adelgid shows up, a hemlock can die in as few as five years. Scientists estimate that half of the hemlocks in North Carolina are already dead.

Because the hemlocks in Cary are hundreds of miles from any other natural location of the trees – and because there are only about 230 of them – treating the hemlocks was simple, if expensive.

“They took the wand and the insecticide came out and sprayed directly onto the trunk of the tree,” says Johns, pointing to a spot on a tree a few feet off the ground to explain how a private tree company did its work. “It dried in 30 seconds to a minute and was absorbed into the tree’s system.”

Because of the vast numbers of hemlocks in North Carolina and across the Appalachians, injecting trees was not an option. That goes for spraying or any other type of insecticide delivery.

“There is no economically feasible way of treating the natural stands,” says Fred Hain, a professor emeritus in entomology at NC State. “So once it gets established in the natural stands, it just takes off.”

Hain is involved with the Alliance for Saving Threatened Forests - one of several ongoing efforts to save the hemlocks. He is working on growing hemlocks that are genetically resistant to woolly adelgids, like those found in Asia.

At the same time, other researchers are introducing natural enemies of the woolly adelgid. Bugs like the Lari beetle. It’s native to the Pacific Northwest and can eat up to six adelgids per day.

“Our work is to try to breed trees that have some tolerance or resistance to the adelgid,” says Hain. “So in an integrated approach the natural enemies will have a major impact. They will be a big part of the story.”

It’s a story being written in many places, from research being done in Rhode Island to a mysterious stand of resistant hemlocks in Wilkes County to greenhouses in Raleigh and Waynesville.

Hemlocks, infested with woolly adelgids, being grown in a greenhouse at NC State.
Credit Dave DeWitt

At a facility at NC State, Hain is cultivating woolly adelgids to serve as test subjects that will help determine the best natural predators and genetic defenses.

Hemlocks, like everything in the natural world, are part of an ecosystem – connected to other forms of life. Many hemlocks, for example, grow on river banks and keep the water cool for trout. As hemlocks die out, so will overheated trout, and, maybe anything that eats them.

It’s an ecosystem that’s been around for a long, long time, and each individual, full-grown hemlock represents a few centuries of it.

“They’re a very long-lived tree, before the adelgid arrived,” says Hain. “Four-hundred-, 500-, 600-years old is pretty typical of how long they can live.”

Earlier this year, the North Carolina Department of Agriculture announced a $250,000 grant to help save the hemlocks.

It will further fund the efforts that are slowly but surely turning the tide, and might assure that North Carolina’s hemlocks are still here for centuries to come.