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Controlling and Preventing Hemlock Woolly Adelgid Infestation

Ensure your Hemlocks are safe from this invasive pest 

Hemlocks are wonderful trees to have in your landscape, and their density makes them a great choice for planting in privacy screens. However, without proper care, Hemlocks are very susceptible to Hemlock Woolly Adelgid, which can seriously damage and eventually kill your Hemlocks.

What is Hemlock Woolly Adelgid?


Hemlock Woolly Adelgid (HWA) are tiny, aphid-like insects that are barely visible. It is atypical of most other insect species as it is in a dormant stage for much of the growing season and starts to be active mid-October through the winter.

When is Hemlock Woolly Adelgid Active?

As temperatures begin to cool in the fall, Hemlock Woolly Adelgid leaves its summer dormancy to feast on the Eastern Hemlocks. Depending on the temperature, this usually occurs between late September and October.

By mid-February HWA starts producing new egg masses. This is when most people become aware of this pest because of the white, cottony egg masses that are lined up at the base of the needles of their Hemlocks. Then by mid-July, the immature insects settle on the stems, at the base of the needles, and become dormant, neither feeding nor developing. Preventative or controlling treatments at this time are best as they will keep the pests at bay all fall, winter, and into the spring – when they are most devastating to Hemlocks.

How Does Hemlock Woolly Adelgid Damage Trees?

Unlike other insects, such as Winter Moths, that feed on the leaves, needles, twigs, or new buds of trees, the Hemlock Woolly Adelgid (HWA) does not damage Hemlocks by feeding. The Hemlock Woolly Adelgid sucks the sap from Hemlock needles but also introduces a chemical from its saliva which acts as a toxin. This toxin accelerates the demise of the tree. The loss of sap will cause needles to brown and drop, leaving trees unable to produce food and energy via photosynthesis.

Healthy trees, in good growing sites, may withstand infestations for 5-10 years before being seriously affected. Trees stressed from drought, soil compaction or those in poor growing sites may succumb to HWA within 3 years. Hemlocks infested with Hemlock Woolly Adelgid, usually stick out like a sore thumb, as the rich green color is replaced by a grayish or yellowish hue. In a group of Hemlocks, you can usually pick out one infested with HWA fairly easily.

Hemlock Woolly Adelgid Affecting Mass amount of Trees

This pest attacks both the Eastern (Canada) Hemlock and the Carolina Hemlock, two species common to New England. The Western Hemlock is resistant to this pest, but does not grow well in the Northeast.

Preventing and Managing Hemlock Woolly Adelgid

Once this pest has been identified on Hemlocks, it must be handled quickly. Applications of horticultural oil have been shown to be extremely effective against Hemlock Woolly Adelgid. For the best preventative measures, the foliar applications should be in both early spring and fall to keep your Hemlocks free of this pest.

The horticultural oil kills the organism by suffocation and is effective against all life stages, including the eggs. Hemlocks that have already been infested may require more treatments to kill the pest, and keep it from returning.

For trees that have already been seriously infested, we recommend pairing a foliar application with a soil injection treatment for the best chance of saving the Hemlock tree. However, if the toxin from their saliva has already infiltrated the tree and it may take a while for the tree to rebound. It can take an entire year or more until the toxicity in the tree is reduced.

The common misconception among homeowners with treated Hemlocks is that they think the tree is still infested because they see the small “white spots” on their trees. What really happens when treatments are applied correctly is that the insects are killed, but their woolly protective covering remains on the tree. Over time the woolly covering (white spots) will fall off.

Once the pest has been brought under control, the trees continue to require once or twice yearly treatments as well as regular pruning to prevent infestation. Untreated trees in surrounding areas will act a reservoir for this pest and serve to re-infest treated trees. HWA spreads through wind and birds which are primarily responsible for the movement of Hemlock Woolly Adelgid from tree to tree.


Controlling and preventing Hemlock Woolly Adelgid is relatively straight forward, and the insects can be killed quickly. Hemlocks should be treated for HWA every year. Even if your Hemlocks have not been infested, these pests have spread so rapidly that there is no way to ensure your trees will remain free of infestation. The most popular question regarding HWA treatments is, “What does it cost?” Depending on the number and size of trees to be treated. The cost of removing and/or replacing the dead Hemlocks is significantly greater than the investment in HWA preventative treatments, so keeping your Hemlocks safe and healthy with preventative treatments is always a wise idea.

If you have Hemlocks on your property it is a good idea to schedule a free inspection with a Certified Arborist to ensure the health and safety of your Hemlock trees. Request a free evaluation below.


Editors Note: This post was originally published in April 2012 and has been completely revamped and updated for accuracy and comprehensiveness.



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