The Danger of the Hemlock Wooly Adelgid

The Hemlock Woolly Adelgid (HWA) is vigorously attacking Hemlock trees in the northeast.

Woodlot managers beware, the Hemlock Woolly Adelgid (HWA) is vigorously attacking Hemlock trees in the northeast. HWA impacts hemlocks from Maine to Georgia and has been decimating trees in the Great Smoky Mountain Region. Conservationists there are struggling to preserve 1 percent of the total hemlock population.

Mark C. Whitmore. Photo credit: Caroline Marschner
Mark C. Whitmore. Photo credit: Caroline Marschner

In the northeast, HWA is most problematic in New York, which has more hemlocks than any other eastern state.

“It’s the third most common tree in New York so we have a lot at stake with our response to HWA,” said Caroline Marschner, invasive species specialist at the NYS Hemlock Initiative.

Hemlocks are considered a foundation species for Northeastern forests. These trees provide habitat for deer and other animals during extreme summer and winter weather and they provide stabilization in steep gorges and help regulate nutrient flows into streams and lakes.

HWA is a tiny, aphid-like insect that has caused widespread mortality in hemlocks. The adelgids settle on hemlock twigs where the needle meets the twig, and suck nutrients out of the twig.

“The flow of water and nutrients in the xylem (wood) of the twig is blocked, which then kills the buds,” explained Mark C. Whitmore, extension associate in the Department of Natural Resources at Cornell University . “That’s the insidious part, the tree will still have green needles, but is unable to put out fresh foliage.”

Ultimately, the HWA insect is starving trees to death. The pest was first spotted in Virginia in the 1900s when ornamental Japanese Hemlock nursery stock was sent to an arboretum in the Richmond, Virginia area. The first reports of problems surfaced in the mid-1950s. It has been slowing spreading north and west since then.

HWA Mortality from HWA without treatment is very high. “Tree death in New York can occur as rapidly as four years in more stressed sites and can take 20 years or more in less stressed areas,” Whitmore said.

Caroline Marschner. Photo credit: Mike Henry of Canada’s Ancient Forest Exploration and Research
Caroline Marschner. Photo credit: Mike Henry of Canada’s Ancient Forest Exploration and Research

While HWA does not impact the lumber quality of infested trees, the widespread mortality it causes is impacting ecosystems throughout the East coast. The good news is you can help control the spread.

“Treatment is relatively inexpensive and lasts for several years,” Marschner said. “HWA is well controlled by imidacloprid, and it is best to treat before your trees are severely stressed.”

Monitor your trees for signs of infestation, and have a plan in place for treatment. The website provides details.

In mid-May, New York’s HWA response team received funding to create a HWA biocontrol rearing facility for the state of New York.

“This is a critical piece to the effective management of HWA, and we are thrilled to be hiring experienced post-docs and talented lab technicians to get the laboratory up and running,” Marschner said.