Hungry for Hemlocks: The Hemlock Woolly Adelgid

April 19, 2016
Hemlock Pests
S.H. LeeThe hemlock woolly adelgid, an invasive species, has no natural predators on the East Coast.

The eastern hemlock can grow to more than 150 feet tall and live more than 500 years, but the tree’s future is threatened by a tiny bug. The hemlock woolly adelgid (HWA) is an invasive species that was accidentally imported on nursery stock from Japan. First reported in the eastern United States near Richmond, Va., in 1951, it has no natural predators on the East Coast.

“Any time you have an exotic pest in the environment here without its natural enemies, you’re going to see extensive damage,” says Noel Schneeberger, an entomologist and coordinator of the U.S. Forest Service’s Hemlock Woolly Adelgid Initiative in the Northeast. “The question is, how is it going to affect the ecosystems where hemlocks provide key services?”

The eastern hemlock is a shade-tolerant coniferous tree with a dense canopy that cools nearby streams (a benefit for fish like brook trout) and provides shelter for white-tailed deer, turkeys, and other wildlife. HWA has worked its way from Virginia to Georgia and as far north as Maine, and is now present in half of the geographic range of the eastern hemlock.

“We’re trying to slow the spread but we don’t have all of the proven tools we need yet,” Schneeberger says. “We still have much to learn.”

Mighty Mite

HWA is hard to spot until a critical mass of the pests have infested an area. Their white egg sacs cling to the undersides of hemlock branches, which can be 50 feet or more off the ground. When there’s a heavy HWA population, “You’ll begin to see these tiny little cotton balls along the current year’s branches,” Schneeberger says.

HWA crawlers emerge in the spring and settle at the base of needles on a tree’s new growth. They insert a sort of feeding tube into the tree and suck its sap. Over a period of years, this weakens the tree, eventually killing it.

Saving Trees

So far, the only check on the pest’s advance has been prolonged or bitter cold spells, which kill most of the pests, but the reprieve is brief because the surviving population quickly rebounds. Foresters can save individual hemlocks by applying an insecticide, but this is not practical or affordable for large wooded areas.

Instead, scientists have been importing and lab-rearing some of HWA’s natural predators from other parts of the world. After studying these predators to ensure they won’t introduce other problems, scientists have released the bugs at monitored test sites. So far, two members of the ladybeetle family—both of which eat HWA in every stage of its life cycle—seem promising. Laricobius nigrinus, native to the Pacific Northwest, was first released on the East Coast 13 years ago, and Laricobius osakensis, from Japan, was first released four years ago. These insects have survived winters, reproduced, and dispersed from test sites in about a dozen eastern states. It’s too soon to tell how effective they will be, but Schneeberger says, “We see encouraging signs of hemlock recovery where Laricobius nigrinus is established.”

Breeding Resistance

Meanwhile, foresters are trying to develop trees that can withstand infestation. They call one group of hemlocks near the Delaware Water Gap, in northwestern New Jersey, the “bulletproof stand” because it appears healthy even though HWA has killed nearby trees. Researchers are collecting seeds of such hemlocks, and last fall they planted trees that are presumed to be resistant in half a dozen states.

Schneeberger hopes these combined efforts will save the species. “It’s a beautiful tree,” he says. “I would rather hike through hemlock stands than anything else.”


Read more about the natural world in the Wild Wisdom archives.


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Heather Stephenson

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