Hemlock Woolly Adelgid (HWA) is a tiny insect that is barely visible. It is atypical of most other insect species as it is dormant for much of the growing season and active throughout the winter.
By mid-July, the immature insects settle on the stems, at the base of the needles, and become dormant, neither feeding nor developing. By mid-October, the insect resumes feeding and development. By mid-February they start producing new egg masses. Most people become aware of this pest when they notice the white, cottony egg masses lined up at the base of the needles on their Hemlocks.
The HWA 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.
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.
Once this pest has been identified on Hemlocks, it must be managed quickly. Applications of horticultural oil have been shown to be extremely effective against HWA. The oil kills the organism by suffocation and is effective against all life stages, including the eggs. Severely infested trees will need more intensive pesticide treatments.
Once the pest has been brought under control, the trees continue to require once or twice yearly treatments. Untreated trees in surrounding areas will act a reservoir for this pest and serve to re-infest treated trees. Wind and birds are primarily responsible for the movement of HWA from tree to tree.
HWA is a serious threat to our beautiful Hemlocks and should not be ignored. 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.