Help for Hemlocks: Woolly Adelgid Treatment
By: Julie Day
Hemlock trees showing woolly adelgid damage.
The hemlock woolly adelgid is a tiny, aphid-like insect that is ravaging hemlocks from Maine to Georgia. It attacks large native hemlock forests as well as hemlock landscape trees in your yard. The woolly adelgid is considered a serious threat to species of eastern hemlocks as well as the overall health of forest and river ecosystems.
Hemlocks are one of the most popular trees planted in parks and backyards, so this problem can hit close to home. If the hemlocks in your yard are infested with the woolly adelgid, here’s what you need to do.
About the Woolly Adelgid
It’s hard to pronounce, and sometimes it’s even harder to see. The hemlock woolly adelgid (HWA) is a fluid feeding insect that snuck into the United States from Asia, where it’s a harmless forest resident. It was first found in the western part of the U.S. back in the 1920s, but the woolly adelgid didn’t pose a threat there either. However, as the woolly adelgid made its way east, it soon became apparent that the tiny insect was a serious threat to eastern and Carolina hemlocks.
The woolly adelgid is spreading faster than it can be controlled, resulting in extensive damage to native forests and threatening the survival of the species. If you have eastern or Carolina hemlocks in your yard, keep a close eye on your trees – the sooner you can spot and treat the problem, the better the chances of your tree recovering.
Look for the waxy nodules on the stems of hemlock trees.
How to Identify the Woolly Adelgid
The woolly adelgid is easiest to spot in spring and early summer. It targets soft new growth, setting up camp right where the needles meet the stem. The hatching insects feed on the sap at the base of the needles, eventually causing those needles (and soon, the entire branch) to die. As the infestation grows, the tree eventually starves to death. Here’s what to look for, depending on the season:
- Spring: Orangey-brown eggs.
- Early summer: Tiny reddish-brown crawling insects. They almost look like pepper sprinkled on the stems.
- Summer: The young insects spin a little white nest, made of a waxy, woolly-looking substance. The small white nodules should be visible at the base of the needles along the stems. This is the easiest way to identify the woolly adelgid.
- Fall: During the heat of summer, the woolly adelgid goes dormant. They come back out and start feeding in fall and over the winter.
How to Control the Woolly Adelgid
If you’ve spotted a woolly adelgid infestation in your trees, there are options available for treatment. Unfortunately, the most effective options aren’t the most organic ones, so exercise caution when treating your trees.
Begin by treating the trees that are the healthiest, the most integral to your landscape design, and farthest from streams and water sources. It’ll take a few months to see a change, and you’ll need to continue monitoring your trees every year. If the treatment is effective, you should see the insects disappear and new needles start to grow.
Examples of chemical treatment options for woolly adelgid on hemlock trees.
Soil treatment, or soil drenching, is considered the most effective method of treating the woolly adelgid in home landscapes. Follow these steps:
- Look for an insecticide with the active ingredient Imidacloprid. It’s sold under a variety of names, including Bayer Advanced Tree & Shrub Insect Control and Ortho Max Tree & Shrub Insect Control. You’ll need the large bottle of concentrate, rather than the ready-to-use spray bottle.
- Dig a circular trench around the tree about one-foot from the trunk of the tree and approximately 3” deep.
- Mix the concentrate according to package instructions, and pour it into the trench.
Soil drenching can take several months to take effect. If your trees are seriously threatened, you may want to pair this treatment with one of the spraying options below.
If your hemlocks are small, you can use a foliar spray to spray the needles and stems with Imidacloprid.
- Mix the concentrate yourself in a handheld sprayer, or buy ready-to-use spray bottles.
- Spray the entire tree, making sure that every branch is covered both top and bottom.
- Spray until you see excess dripping off – any missed branches can still harbor the pest!
Dormant Oil and Insecticidal Soap
You can also obtain decent results using sprays of dormant oil or insecticidal soap. These options are less toxic than the chemicals but should still not be used around water sources.
- Apply dormant oil or insecticidal soap in the fall, making sure to cover the entire tree.
- Since oils and soaps are not long-lasting, you’ll need to repeat the treatment every year.
Treating hemlock trees with oil or soap is a good companion treatment to go along with soil drenching.
If left untreated, branches infected by woolly adelgids die.
Since large forests obviously can’t be treated with insecticides, researchers are working to develop natural predators to release for large-scale control of the woolly adelgid. One predatory beetle is looking promising, and they’re commercially available from Conservation Concepts, Inc, in North Carolina. Contact your county extension office for more information.
If your trees are near water, or if the soil is too rocky to dig trenches, professional pesticide applicators have a couple more tricks up their sleeve, including high-pressure sprayers and water-safe trunk injections.
- Hemlock Woolly Adelgid Control (North Carolina Extension)
- Hemlocks at Risk (Ohio Division of Forestry)
- Save Our Hemlocks (saveourhemlocks.org)