5 Ways to Fight Back Against Hemlock Woolly Adelgids

hemlock wooly adelgid lead

Eastern hemlock trees are a valuable part of whitetail habitat in much of the eastern United States. There may come a day, however, when hemlock trees are not so common, and that will not be a good day for deer.

In its Northeast and Appalachian Mountain range, eastern hemlock (Tsuga canadensis) offers fairly preferable winter browse, becoming a more important part of deer diets when most hardwoods have lost their leaves and broadleaf forbs are not available. In the northern climate, where overwintering areas are a critical part of whitetail habitat, few tree species compare closely with eastern hemlock in its ability to hold a snow load, which helps create covered bedding shelter in winter. Found around lowland swamps and on dry upland sites, occurring both in pure stands and mixed in with northern hardwood forests, hemlock trees are so pervasive a member of our eastern woodlands that it becomes easy to take them for granted.

But a threat is looming. For more than half a century, the hemlock woolly adelgid (HWA) has been crawling its way through the Appalachian Mountains and beyond. This tiny insect sucks the sap from hemlock twigs, causing crown dieback and eventually death. Today, this invasive pest covers about half of the range of eastern hemlock from Georgia to New England, and it isn’t stopping. It’s ability to clone itself thousands of times results in the decimation of large stands of hemlock.

Adult hemlock wooly adelgids are tiny insects -- less than one millimeter. But the damage they are causing is huge.
Adult hemlock woolly adelgids are tiny insects — less than one millimeter. But the damage they are causing is huge.

There are a number of control measures that are being used or tested. Natural predators (other insects) have been imported and are currently being cultivated for release. The hope is that these insects will fill the role they did within their natural range and potentially keep HWA populations below damaging levels. There are also “systemic” insecticides available for individual infested hemlock trees. These essentially make the tree poisonous to the insect. These treatments can be expensive, and they may only prove effective for a few years.

There are, however, five simple things you can do:

Be Vigilant!

If you have hemlock, continually check on the known distribution of this pest to be prepared when it is near. Know how to identify HWA, looking for clumps of white, wooly wax on the underside of the needles in fall, winter, or spring. In summer, the tiny black insect is small — less than a millimeter — but visible. If you see it, report it to someone from your state’s wildlife or forestry agency. The affected hemlock branch in the photo accompanying this article was found, unfortunately, on my father’s property.

Remove Isolated Victims

Removing isolated hemlock trees that have become infested can help to prevent HWA from spreading once it’s identified.

Relocate Bird Feeders

The U.S. Forest Service suggests keeping bird feeders away from hemlocks may help to prevent HWA from being carried to new sites. If you have hemlocks in your backyard, don’t place bird feeders close to the hemlocks.

Evaluate and Replace

Take note of where you have hemlocks, and what values, like thermal cover, are offered by hemlocks. If HWA is in your area or nearby, this may help you to “shop” for another species of tree that is well suited to the site and would offer similar values. Not many species offer the same unique benefit of winter cover that hemlocks do, but some come close. They include, in order starting with the best, northern white cedar, balsam fir, white spruce, and red or blue spruce.

Go Native

Work with native plants from the range you are in. This won’t help stop HWA, but it will help to prevent future pest problems. HWA, like many other forest pests, was imported unknowingly. In the case of HWA, it is thought to have come to the United States on nursery stock from Japan. We have a diverse list of native species that are well worth planting. Let’s keep it that way!

  • We’ve had HWA in our area for a few years. On our property alone we have some large hemlock trees and a few stands that provide that much needed thermal cover. We have been planting conifer saplings for 4 years to try and mitigate the problem when the hemlocks die off from HWA. Mainly Norway spruce, Serbian spruce, blue spruce and some cedar. Hopefully this works. We have also been told that prolonged cold spells of 20 degrees and less also knock back the HWA. That is a love-hate solution. We love that the cold lessens the HWA damage, but since we run an outdoor business here the prolonged cold is bad economically. A catch-22 for sure.

    • Tim Russell

      Hi Mike. I’m sorry to hear you’ve got HWA, but glad to hear you’re being proactive. Good luck with the planting efforts!

  • Sarah Nelson

    Great post, thank-you so much! For folks who don’t live in the south and want a list of native species for their region, they can contact their local county’s Conservation District.

    • Tim Russell

      Thanks Sarah! I’m glad you enjoyed the post. County Conservation Districts are certainly great resources for landowners and land managers who are selecting a species to plant.

About Tim Russell

Tim Russell is QDMA's Young Forest Specialist in New York. Tim earned his forest resources management degree from SUNY-ESF and previously worked as a forest technician at Forecon Inc. and F&W Forestry Services, where he marked and scaled timber, inspected logging operations and conducted forest inventories. He and his wife, Amy, reside in Cortland, N.Y.