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Hemlock Woolly Adelgid: FAQ's

What is the hemlock woolly adelgid?

Hemlock woolly adelgids are non-native aphid-like insects that feed on the sap of hemlock trees, eventually killing them. They have been detected in Tompkins County since 2008.

What tree species are the hosts?

Eastern hemlocks (Tsuga canadensis) and Carolina hemlocks (Tsuga caroliniana) are host trees in the eastern United States. The hemlock woolly adelgid is also capable of alternating hosts and can feed on several species of spruce during part of its life cycle. However, this behavior is undocumented in our region.

What does the hemlock woolly adelgid look like?

The hemlock woolly adelgid is dark colored and less than 1/16 of an inch long. The insects themselves are hardly visible to the naked eye. However, they cover themselves with a woolly, white material which is especially conspicuous in the winter months.


How do hemlock woolly adelgids spread?

Hemlock woolly adelgids are spread by wind, birds, and mammals. Infested nursery stock can also be responsible for introducing this insect into a given area.


What symptoms will trees exhibit if an infestation goes unnoticed?

Hemlock needles will turn yellow and drop prematurely. Defoliation can occur, and the tree may eventually die. This can take from 3-5 years, depending on multiple factors including tree health, stress, environmental conditions (e.g. drought), and HWA density.

What should I do now?

Inspect hemlocks for the presence of this pest. Infestations may be seen throughout the year, but are most obvious from November through April when the diagnostic wool is present.

Should I cut down my hemlocks if I find an infestation?

Cutting down infested hemlocks is recommended only if treatment is not an option, and if a property owner is interested in selling the lumber. In fact, allowing hemlock stands to remain can lead to the identification of resistant individuals. In landscape settings, cutting infested trees will not completely stop the spread of this introduced insect. However, removing trees in poor health may prevent it from spreading to nearby individuals.

Can my trees be treated?

Horticultural oils or insecticide soaps can be sprayed on small trees. This treatment method must be applied annually. For large trees, systemic materials that are taken up by the tree can be administered through by a commercial pesticide applicator through trunk injection (imidacloprid, IMA-Jet 5%), time relase tablets (imidacloprid Coretect, 20%), or soil injection (imidacloprid, Criterion 2F 21.3%). Homeowners may also purchase an imidacloprid product in a local home and garden store and apply it as a soil drench, following labled instructions. Pesticide treatments typically provide 5-7 years of control, again depending on environmental and biological factors. Consult your local Cooperative Extension office for pesticide recommendations. It is also important to provide adequate water to infested trees, eliminating drought stress. Do not use nitrogen fertilizer.

How will I know if I have successfully controlled the problem?

Follow-up inspections are necessary to determine the success of control efforts.

What will happen if I don’t treat my hemlocks?

Heavy infestations can kill trees in as little as four years. However, some trees appear to be resistant or tolerant to adelgid attacks, and have been known to survive infestations.