Help for Hemlocks: Woolly Adelgid Treatment

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

Soil treatment, or soil drenching, is considered the most effective method of treating the woolly adelgid in home landscapes. Follow these steps:

  1. 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.
  2. Dig a circular trench around the tree about one-foot from the trunk of the tree and approximately 3” deep.
  3. 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.

Spraying Trees

If your hemlocks are small, you can use a foliar spray to spray the needles and stems with Imidacloprid.

  1. Mix the concentrate yourself in a handheld sprayer, or buy ready-to-use spray bottles.
  2. Spray the entire tree, making sure that every branch is covered both top and bottom.
  3. 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.

  1. Apply dormant oil or insecticidal soap in the fall, making sure to cover the entire tree.
  2. 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.

Natural Predators

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.

Professional Options

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.


  1. I am trying to save a hemlock in my yard that was infested with whooly aphids this spring. Did one soil soak treatment and have since sprayed twice with dormant oil. My questions are, how often should the soil soak treatment be done, should I prune off the dead tips of the branches and how often should I repeat the oil spray. Thank you.

  2. We have 12 Hemlocks ranging from 15 to 20 feet in height, at least 4 of which exhibit signs of Woolly Adelgids. Please more explicitly define “near water” so we can determine if the soil treatment is appropriate. We have sandy soil, a septic system and live a quarter mile up and across the street from Lake Ontario, about 100 feet higher than lake level. Another quarter mile and 50 feet lower is a stream that feeds into the lake. The local storm sewer system, in our front yard, slopes directly down about 20 feet from the trees. Thanks!

  3. I heard from some friends that you could rid your hemlocks of the aphids by using the lowest pressure on a pressure washer with soapy water in it and spray the entire tree with this fluid. This would rid your hemlocks of the aphids without the use of any harsh chemicals. Any idea if this is true or not?

  4. You really need to update your coverage of biological control strategies for hemlock woolly adelgid. Both the Conservation Concepts predator rearing lab you mention and your 3rd link are out of business. Please take a look at my science-based, non-commercial site on biocontrol of hemlock woolly adelgid : for the latest information on biological control strategies for HWA.

  5. I have a couple of hemlocks that were infected by the wholly aphids. I noticed the symptoms two years ago. The soil around the trees is very rocky and the trees are two large for spraying. After this winter I noticed the white sticky substance on the branches had disappeared. Has this ever happened before without treatment? Some of the branches that were “dead” now have new growth.

  6. Thank you for the information.. Spring will be the time to face the horrible Hemlock Wooly.. We are in southern Maine, the Wooly is creating total destruction.. more later, and THANK you again for the info.

  7. )Hello, We live in Globe, NC next to Pisgah National Forest. Our home is right on Anthony Creek and when we started seeing the adelgid on our hemlock trees we researched and found there was nothing that could be done because of the creek. I have been reading more lately…because we lost several of our old hemlocks but there are several smaller ones that are still alive.

    The entire valley section of Anthony Creek along Anthony Creek Road has many hemlock trees. We are wondering if there is anything that can be done on a larger scale to help save these trees, as well as the remaining hemlocks on our property. There are also many small seedlings that don’t appear to be affected by the adelgids yet. Should we continue to monitor these or go ahead and treat them? And if we should treat them, what should we use?

    I know I’m asking a lot of questions, but we are a bit stretched financially at this time and can’t call in tree experts.

    Thank you for providing the great website and for your work trying to save these beautiful and important trees. If you call, please leave a message.

    Thank you for your help!

  8. Hi, I care for a couple hundred hemlocks that are quite old. Large trunk short 20′ hedged to around one foot of green on low sides. It’s August and we just got through a drought in July and rain is predicted for two weeks. I thought it would be a good time to trim with hedge trimmers and loppers one hard time before fall to give them a chance to recover. Unfortunately, there are spots of hwa throughout. Little die back, but lots of spots.

    That said, would it be a good time to soil inject them all (or at least the infected ones) with Merit?

    Does this insecticidal chemical stress or pose a risk to the trees?

    I have 2 gallons of dormant oil. Will it will go far enough to do much good?

    Thank you so much!

  9. You seriously need to include mention of safe pesticide application principles in here, especially as the pesticide you’re mentioning for spraying is Imidacloprid, which is a neonic and highly toxic to bees and other pollinators. Special precautions need to be taken to prevent any drift from the spray, and it also has to be done when there’s no rain in sight for awhile. It’s also not necessary to “cover every branch”; it only needs to be sprayed on the trunk. Also, if a soil drench is going to be done, the tree should not be near any flowering plants or water sources since the pesticide can migrate in the soil.


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