What is it?
The Hemlock Wooly Adelgid (Adelges tsugae) is an aphid-like insect that has infested Eastern and Carolina hemlock trees in the eastern and southeastern United States. It is native to Japan, was first recorded in the US in the 1920s, and was first reported in Virginia in the early 1950s. By 2005, it had spread from Maine to Georgia and has now infested at least half of the hemlock's range.
It is the nymph form of the adelgid that attaches and feeds on the twig tissue, depriving it of stored nutrients and eventually killing it. When enough damage is done, the whole tree dies. In the picture above you can see how a couple of the needles have already turned brown where the adelgid has been feeding.
As the adelgid matures it develops a white, waxy, almost wooly-looking substance around it. This substance helps protect the adelgid and its eggs from predators. A quick look at the underside of the branches from late fall to early summer can tell you quickly whether adelgids are present.
If left untreated, the trees are defenseless and mortality can occur in as little as three years, particularly if the trees are already stressed by drought or other disease. The worst affected areas have been in Virginia, parts of West Virginia, Pennsylvania, New Jersey, and Connecticut.
The picture below is a migrograph of the adelgid itself.
What can be done about it?
The National Park Service and US Forest Service have been working aggressively since 2006 to try to control the population of adelgids, but, for now at least, the prognosis isn't good. They have been treating as many trees as possible with insecticides injected at the base of the tree. Unfortunately, the focus has been on saving the trees in widely visited areas and not as much has been done for trees farther into the canopy.
The fear is that, unless researchers can find a cure for this infestation, the hemlock will eventually join the ranks of the American Chestnut and American Elm as just another majestic tree we've lost.
What can I do about the trees in my backyard?
There is one Eastern Hemlock tree on my property and, unfortunately, I discovered that it was infested several years ago. I went to Lowe's and bought a commercially available insecticide (Bayer Advanced 12 Month Tree and Shrub Insect Control - now called Bio Advanced 12 Month Tree and Shrub Insect Control) that you bury at the base of the tree. I treated it for a couple of seasons and, so far, the tree is doing fine. Typically, I don't endorse the use of pesticides or insecticides, but given the damage this adelgid can do, I decided it was the lesser of two evils.
There's a great document from the University of Michigan for homeowners who are needing help with the adelgid problem. Here's a link to download it:
And if you'd like a fact sheet on the Hemlock Woody Adelgid, you can download that here:
Hemlock forests make up over 90,000 acres in Great Smoky Mountains National Park where this valuable tree helps to keep streams cool and provides habitat for a number of species. With the park being so close to where I live, naturally I wondered what they're doing about the problem, and as it turns out, quite a bit.
They're using predator beetles that prey exclusively on adelgids, but it takes some time for these beetles to reproduce enough to be effective. Between 2002 and 2011 the park released over half-a-million beetles. The results are still being studied, but they look promising so far.
They're also using systemic treatments of imidacloprid which is the ingredient in the Bayer Advanced treatment I used myself. This has shown to be a highly effective method for several years, and trees whose foliage had already turned gray have flourished and been able to produce new growth.
The last thing they're using is foliar treatments where they spray the foliage with insecticidal soap and horticultural oils that smother and dry out the adelgids on contact. Unfortunately, it only kills the bugs that are on the tree at that time.
The park's adelgid control is being funded through the Save the Hemlocks initiative of the Friends of Great Smoky Mountains National Park. For additional information, or to learn more about how you can help, visit the Friends of the Great Smoky Mountains National Park‘s website or call (865) 932-4794.
Until next time.
Madbird Biologist - Mark A. Dunaway
I am a wildlife biologist and nature recording expert who is passionate about our planet and the species we share it with. I am an advocate for conservation and enjoy educating others about the plants and animals around us. Tag along with me as I document what I'm up do or talk about things I think others may be interested in. Thanks for joining me on this journey into the natural world.