University of Maryland Extension

Hemlock Woolly Adelgid - Trees

hemlock woolly adelgid at base of needles

Hemlock Woolly AdelgidThe hemlock woolly adelgid is a tiny, aphid-like insect that is a serious pest of hemlock in Maryland. It is covered with a woolly or fluffy, white waxy covering for most of its life. Adelgids are found primarily on the young branches of mature hemlocks at the bases of the needles. They suck the sap from the branches and may inject a toxin into the tree during feeding. In heavy infestations, the feeding damage results in rapid desiccation and discoloration of the foliage. A heavily infested tree may die within four years.


Hemlock woolly adelgid can be controlled by thorough spraying with a registered insecticide. A dormant rate of horticultural oil may be applied in February or early March before egg laying begins to kill adult females. Sprays are not effective when egg masses are present, usually late March through early June. After all of the eggs have hatched and crawlers are present, by mid-June, a thorough spray with a summer rate of horticultural oil or insecticidal soap will give good control. The key to control is thorough spray coverage of the tree.

close up crawlers on needles
Newly hatched crawlers on needles

Additional Resource

Publication: (PDF) HG 2 Hemlock Woolly Adelgid

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