Knowledgebase

hemlock woolly adelgid management #924984

Asked February 24, 2026, 11:47 AM EST

We have a dozen mature hemlock trees in our backyard that have an infestation of hemlock woolly adelgid. What are resources I can reach out to for treating these trees?

Anne Arundel County Maryland

Expert Response

Mature hemlocks are challenging to spray effectively (such as with lower-toxicity insecticides like horticultural oil). You may need to hire a certified pesticide applicator (we don't have a list to share, but many landscapers and arborists have this license) to apply one or more doses of systemic insecticide, which is absorbed by the roots and moved into the canopy. Adelgids ingest the toxin as they feed, though good suppression of an established adelgid population might take more than a year to achieve. The systemic pesticides most useful in controlling adelgids are not legal for the general public to buy and apply in Maryland without a pesticide license. You can learn more about the pest's life cycle on our Hemlock Woolly Adelgid web page.

Miri
Answered by the Home and Garden Information Center (HGIC), University of Maryland Extension. For timely gardening information and tips, subscribe to our Maryland Grows blog and quarterly newsletter. Follow us on Facebook, Instagram, and YouTube. Replied February 24, 2026, 4:31 PM EST

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