How to Get Rid of Hemlock Woolly Adelgid

Hemlock woolly adelgid

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Woolly adelgid (Adelges tsugae) is an aphid-like insect that feeds mainly on Eastern hemlock trees, therefore is it also commonly referred to as hemlock woolly adelgid (HWA). The pest was introduced to the United States from Japan in 1951 on infected hemlock trees. It its native environment, woolly adelgid is kept under control by predators, mostly beetles. While we do have the predator beetle, Laricobius nigrinus, that feeds on adelgids it resides in the Pacific Northwest and is currently being assessed as a possible biocontrol agent. The infestation of woolly adelgid in the eastern part of the United States and also parts of Canada, has already caused a high mortality among hemlock trees.

If not controlled, woolly adelgid can kill a healthy hemlock within three to five years, and a tree under drought or other form of stress even faster. Therefore, it is important to recognize the early signs of an infestation. And while it is not possible to completely eradicate woolly adelgid, taking prompt action when you spot the pest allows you to at least contain it.

 Woolly white masses at the base of the needles

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What Does Hemlock Woolly Adelgid Look Like? 

Woolly adelgids are very small and it takes a strong magnifying glass to identify them in any of their life stages. What makes them even more elusive is that unlike most other pests, they are inactive during most of the growing season and very active during the winter. The one telltale sign of woolly adelgid presence are the woolly white masses they produce. These are the eggs; they look like tiny, pin-head sized white cotton balls at the base of the needles. They are present all year but are the most prominent in the early spring.

Two Ways to Get Rid of Hemlock Woolly Adelgid 

While there is ongoing research to find predatory insects that can be used for biological control of woolly adelgid, these methods are not yet available to home gardeners. But there are other ways to get rid of woolly adelgid. 

Organic Insecticides

Both horticultural oil (dormant oil or summer spray) and insecticidal soap can be used to control woolly adelgids, following the product label for mixing concentration. As these insecticides must be applied thoroughly to the entire tree so no woolly adelgid is left, this method is only feasible for smaller trees. If the pest reappears, a second application later in the season might be necessary.

Chemical Insecticides

The next step up is to use a systemic chemical pesticide with imidacloprid or dinotefuran as the active ingredient. They are commonly injected into the stem in mid-May and will kill all the HWA as well as protect the tree against future attacks for a few years. The two drawbacks are that these are broad-spectrum insecticides that act on the nervous system of insects, including beneficial ones, so they must be used with extra caution. Also, they can only be used on healthy trees that properly take up water and nutrients; they won’t work in an ailing tree because the systemic insecticide won’t get transported to the location where it is supposed to be ingested by the HWA. 

Signs of a Hemlock Woolly Adelgid Infestation 

In addition to the woolly white egg sacs at the base of the needles, dieback of twigs and branches and eventually the death of the tree are other signs of a heavy HWA infestation. 

From mid-July until mid-October, the immature nymphs stay put on the stems at the base of the needles. They are in a state of aestivation (the opposite of hibernation) and don’t feed during that time. Once cool fall weather sets in, they start feeding by using their piercing-sucking mouthparts to remove plant sap and from the twigs of the host plant. The nymphs develop into adults which lay new woolly egg masses in the early spring. There are two generations every year and spring is a time of heavy feeding when an infested tree suffers a lot of damage. However, you might not notice it right away, only when the tree starts losing its needles and fails to grow new needles and branches because the nutrients have been sucked out of it.

What Causes Hemlock Woolly Adelgid?

If you live in one of the 20 states where hemlock woolly adelgid has been reported, there is always the risk that it can spread to your hemlock tree(s). If you also have a spruce tree, you might be even at greater risk, as the complex life cycle of the hemlock woolly adelgid involves spruce trees—in the late summer, some winged woolly adelgid adults move to nearby spruce trees where they produce eggs. 

Although the pest spends most of its life cycle firmly attached to the hemlock twigs and does not move on its own, especially its eggs can be spread widely and rapidly by vectors such as wind, birds, deer and other mammals. Humans can also inadvertently spread it by moving plants, logs, firewood, or wood mulch from infested areas to non-infested areas between May and June, which is prime time for woolly adelgid activity, which is why it is prohibited in several states.

Healthy eastern hemlock tree
Healthy eastern hemlock tree

Jillian Matte / Getty Images

How to Prevent Hemlock Woolly Adelgid

There are several measures you can take to prevent woolly adelgid from infesting your hemlock trees:

  • Keep the tree healthy by watering it regularly during periods of drought, as a drought-stressed tree is more prone to an attack.
  • Only fertilize the tree when you are sure there is no infestation, as the extra nitrogen can give HWA populations a boost when they suck the sap.
  • Prune dead and dying branches. Even though you might not see any signs of woolly adelgid on them, they might be infested.
  • Plant resistant hemlock varieties. The western hemlock (Tsuga heterophylla), a species native to the northwestern United States, is highly resistant to HWA but make sure that it can be grown in your climate zone.

Hemlock Woolly Adelgid vs. Balsam Woolly Adelgid

Balsam woolly adelgid (Adelges piceae) is another pest, equally tiny and difficult to spot as HWA. It was introduced to the United States from Europe. It sucks the sap out of true firs. North American fir trees are especially susceptible. The pest is host-specific; you won’t find balsam woolly adelgid on a hemlock.

Stand of dead eastern hemlocks killed by HWA

mophojo / Getty Images

  • Can a hemlock tree survive woolly adelgid?

    Some trees appear to be resistant and survive an infestation while others, even healthy trees, succumb to the pest in as little as four years.

  • How does the hemlock woolly adelgid affect the environment?

    When eastern hemlock trees in a forest stand die due to hemlock woolly adelgid, it means a loss of biodiversity. Insects, birds, and mammals that are feeding on the tree, or using it for shelter, disappear as well as they don’t find sustenance any longer.

  • Is woolly adelgid an invasive species?

    Hemlock woolly adelgid is an invasive insect. Just as invasive plants choke out native vegetation, this pest eradicates native trees.

Article Sources
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  1. Aphid-Munching Beetle Could Help Save Hemlock Forests. Science.

  2. Hemlock Woolly Adelgid (HWA). Penn State Extension.

  3. Hemlock Woolly Adelgid. University of Massachusetts, Center for Agriculture, Food, and the Environment.

  4. Balsam Woolly Adelgid. USDA, Forest Insect & Disease Leaflet.