What is Hemlock woolly adelgid? Why should it matter to me?

Written by: Margaret Miller

Michigan has nearly 170 million hemlock trees across its forests making this tree a key player in many ecosystems. Hemlock trees are in the Pinaceae family and are medium to large evergreen. There are four common species in North America. Generally, you can identify a hemlock by its thick reddish brown bark and two-ranked flat and short needles. Hemlock trees are important to animals and humans, and you may have even sipped a tea or smelled a perfume that utilized hemlock tree needles! Deer browse the foliage of young hemlock trees and finches and small rodents eat the tree seeds. For humans, hemlock wood is important in the timber industry and is often used as wood pulp. Not to mention they have a majestic quality to them along paths and recreation trails! For this and so many other reasons, hemlock woolly adelgid is a significant concern for the Michigan Department of Agriculture and Rural Development, the Michigan Department of Nature Resources, and it should be a concern for you too!

Odocoileus virginianus

What is hemlock wooly adelgid? Hemlock woolly adelgid (HWA) is an exotic invasive pest from Asia that has killed millions of hemlock trees in North America. Hemlock woolly adelgid was first reported in the eastern United States in 1951 near Richmond, VA. Thankfully, only two of the four common hemlock species found in North America are susceptible to the disease. The two eastern North American species, Eastern hemlock (Tsuga canadensis) and Carolina hemlock (Tsuga caroliniana) are under the greatest threat while the two western American hemlock species remain moderately resistant. When a tree is infested with hemlock woolly adelgid it dies within a few years but HWA can damage more than one tree. Large hemlock trees have large, high root systems making it easy for HWA to infect more than one free once a hemlock tree falls.

The pest is small, less than 1/16 in or 1.5 mm in length and varies in color from a dark red-brown to a purple-black. As the pest matures, it produces a covering of wool-like wax filaments to protect itself from outside threats. Additionally, hemlock woolly adelgid is pathenogenic which means all pests are female. So how do you spot HWA in a hemlock tree? A great time to check your hemlock trees is right now. You should keep an eye out for tiny, cottony growths near the base of needles and on the underside of branches. Infected trees can turn a grayish-green color making them easier to spot from a distance.


Unfortunately for tree lovers everywhere, HWA is established from northeastern Georgia to southern Maine and as far west as eastern Kentucky and Tennessee. HWA has been confirmed in several Michigan locations including Muskegon and Ottawa counties and the efforts to find additional infestations are ramping up across the state.

What can we tree lovers do to help?

1) The Michigan Department of Agriculture is inviting the public to comment on possible plans to set up an invasive species quarantine in Allegan, Muskegon, and Ottawa Counties. Similar to quarantines that were set up to stop the emerald ash borer, a quarantine of the affected counties could mean regulated movement of hemlock trees, heat treatment for certain trees, or fumigation. You can find out about the possible quarantine plan here. The deadline for comment is 5 pm on Monday, February 6, 2017. 

To comment on this DRAFT quarantine you can email Mike Bryan, Nursery and Export Program Specialist at bryanm@michigan.gov, or fax to: 517-335-4540, or mail your comments to:

Michigan Department of Agriculture and Rural Development
Pesticide and Plant Pest Management Division
P.O. Box 30017
Lansing, MI 48909

2) You can join the “Michigan Eyes on the Forest sentinel tree network” and help monitor Michigan’s forest resources and help us protect against potential invasive threats including HWA. More information on the Michigan Eyes on the Forest can be found HERE, Kent County residents can also contact Nick Sanchez, District Forester, with the KENT CONSERVATION DISTRICT.

3) If you find a possible hemlock woolly adelgid infestation, take photos, note the location of the affected trees and contact MDARD at 800-292-3939 or MDA-info@michigan.gov.

The Grand Rapids Urban Forest Project is always willing to assist anyone with this process. Thank you for the help and remember do not be the vector for HWA or any other exotic pest never transport potentially infested material!

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