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What Is The Hemlock Woolly Adelgid

What Is the Hemlock Woolly Adelgid? All You Need to Know

You may have already heard of the Hemlock Woolly Adelgid. Neighbors may have mentioned this nuisance, local arborists may be talking about it on social media, and your local nursery may warn about it. And for good reason. The Hemlock Woolly Adelgid is something to stand up and pay attention to, as there is no avoiding it and a real push to get rid of it. 

So, what is the Hemlock Wooly Adelgid and what is all the fuss about?

Hemlock Woolly Adelgid Adults at the base of needles

HWA adults at the base of needles
Photo: Mark Whitmore, Cornell University

Hemlock Woolly Adelgid White Whoolly Egg Masses

White woolly egg masses
Photo: Connecticut Agriculture Experiment Station,

What Is the Hemlock Woolly Adelgid?

A teeny tiny insect about the size of a period at the end of a sentence, the Hemlock Woolly Adelgid is native to Asia. It is widely thought that the bug was brought here accidentally to the Western US from either Japan or China. However, it is not considered a pest in California and Oregon where it can be found, likely because of a lack of Hemlock trees in comparison to the East and South. The eastern and southern distribution of this insect spreads across the eastern seaboard and down into South Carolina, North Carolina, Tennessee, Virginia, and West Virginia. 

The bug is reddish brown and oval shaped, but it is covered in a dry, white, woolly substance on its body and egg masses. It is most identifiable by this cottony substance, which is how it got its name.  Once the Adelgid hatches it becomes very active in hemlock trees of all sizes.

Adelgid Activity

The Hemlock Woolly Adelgid, or HWA, has thread-like mouthparts, which are three times the length of the HWA body. This mouthpart extends deep into the plant tissue as it feeds, and it sucks out sap and injects a toxic substance that is lethal to the plant. 

The HWA is highly reproductive, making it difficult to die off, particularly because in the US it has no known predator. It is spread from tree to tree easily through wind, birds and other forest animals, and even people. It survives extreme temperature changes, from lows down to -35 degrees Celsius and highs up to 40 degrees Celsius. 

In Asia, it has natural predators that consume it and keep its growth in check, but in the Eastern US, where Hemlock forests are prevalent and it has no natural predators, it is rapidly killing off Hemlock trees. It has as of today killed off 90% of Hemlock trees in the Shenandoah National Park in Virginia.

Identification and Removal of the Hemlock Woolly Adelgid

If you suspect you may have an HWA infestation in your trees, your best bet is to contact your local arborist immediately. Multiple approaches may need to be taken, and the steps are extensive and involved.

Close-up view of HWA ovisacs and a juvenile HWA on a hemlock branch Photo: Pennsylvania Dept. of Conservation and Natural Resources - Forestry,

Close-up view of HWA ovisacs and a juvenile HWA
on a hemlock branch
Photo: Pennsylvania Dept. of Conservation
and Natural Resources – Forestry,

Hemlock woolly adelgid egg masses Photo: Chris Evans, The University of Georgia

Hemlock woolly adelgid egg masses
Photo: Chris Evans, The University of Georgia

Identification of the Hemlock Woolly Adelgid

A tree with an Adelgid infestation that goes unchecked will not last longer than ten years, with the nutrients slowly being drained from the tree. 

You will notice the infestation by its trademark woolly substance lying at the base of the needles, typically on the underside of the branch on the tree. You will also notice, after time, that the needles on your tree begin to lose their vibrant color, becoming a duller green and even a grayish color. That change in color is because the HWA is consuming the nutrients the tree usually sends out to its needles, as well as inserting a toxin that slowly poisons the tree.

Removal of the Hemlock Woolly Adelgid

Two ways exist to rid a tree of the HWA, and both require attention to detail, diligence, and access to the entire tree from top to bottom.


Several sprays exist at your local garden center or nursery that can be applied to the infested trees. The sprays are not toxic, so multiple applications must be used. Further, the entire tree must be sprayed from base to top, including all surfaces and foliage. If your tree is too tall for you to get to, the infestation will endure, and all of your work will go to waste. The tree must also be sprayed to the point of runoff, extensively coating every surface to ensure you get every single insect. 

The spray is only effective during two seasons, when the HWA do not have their protective coating on their bodies. These windows are from March to April and from September to October.

Systemic Treatments

The other approach to rid yourself of this infestation is systemic, meaning feeding a toxin directly into your tree so that the bugs will eat it and die. The toxin, fortunately, is not dangerous to the plant. There are three ways to feed this toxin to your tree.

  • Soil Drenching: Soil drenching involves pouring the insecticide directly into the soil surrounding the tree and then watering the soil extensively to ensure the insecticide gets into the roots of the tree to be delivered throughout.
  • Soil Injection: Soil injection is much more concentrated than soil drenching, wherein an arborist will inject the insecticide directly to the soil, using a special tool, deep enough to target the roots of the tree.
  • Trunk Spray: Trunk sprays with a concentrated systemic treatment is an effective way to treat the tree. 

Do you have questions about hemlock woolly adelgid?

In the case of the Hemlock Woolly Adelgid, it is imperative that you take action as quickly as possible. Mayer Tree Service has a long history and a solid reputation for saving trees in our community, and we have the experience to both diagnose and treat your trees. Contact us today if you suspect you may have an HWA infestation.

If you have questions about tree service or plant health care or would like an estimate, give us a call at 978-768-6999 or click here to contact us.

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