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 recently been identified in Tompkins County.

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.

What should I do now?

Inspect hemlocks for the presence of this pest. Infestations may be seen throughout the year, but are most obvious in the winter months.

How should I report hemlock woolly adelgid infestations?

The New York Invasive Species Research Institute (NYISRI) was established at Cornell University’s College of Agriculture and Life Sciences in the Department of Natural Resources, with support from the New York Department of Environmental Conservation. You can report sightings on their website here. If you have questions, please contact the Tompkins County Cooperative Extension at (607) 272-2292.

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, culling 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. For large trees, systemic materials that are taken up by the tree can be administered through soil injection, soil drenches, or tree injection. Homeowners may use approved pesticides or hire a NYS-certified pesticide applicator to treat their trees. 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.