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Hemlock Woolly Adelgid

Adelges tsugae
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The hemlock woolly adelgid (HWA) is an aphid-like insect that attacks and kills hemlock trees by feeding on nutrient and water storage cells at the base of needles. It is suspected HWA first arrived in North America through the importation of infested Japanese nursery stock. The pest was first discovered in British Columbia in the 1920s and later found in Richmond, Virginia in the 1950s. It has since established itself along the east coast of the United States from Maine to Georgia.


In 2012 and 2013, two detections of hemlock woolly adelgid were made in Etobicoke and Niagara Falls respectively. These populations were destroyed and both areas continue to be monitored. In 2019, the Canadian Food Inspection Agency (CFIA) confirmed the presence of HWA in Wainfleet, Ontario and in a forested area along the Niagara River near Niagara Falls, Ontario where it had previously been found. Since then, HWA has been detected in Fort Erie in 2021, Pelham and Grafton in 2022, and most recently Hamilton in 2023. Survey activities are on-going and the CFIA has implemented movement restrictions on susceptible wood products from several of these areas to slow the spread.

The lack of an effective winged generation means that the species cannot move on its own from area to area and must rely on being carried (by wind, animals, or humans) to other places.

Impacts of Hemlock Woolly Adelgid

  • Many ecological impacts would be caused by the decline of hemlock tree populations. The trees contribute to important ecological services, such as the regulation of water temperatures in streams, allowing cold-water fish like Brook Trout to survive and reproduce. They also provide a valuable thermal cover for deer and birds during cold weather.
  • Decline in hemlock trees would present economic impacts as it is used in general construction and as pulp.
  • Loss of hemlock trees will impact recreational activities, such as hiking and fishing.

How to Identify Hemlock Woolly Adelgid

  • Throughout the nymph stage, the insects are brownish-orange and can measure from 0.44 mm to 0.75mm long and 0.27 mm to 0.47 mm wide.
  • The adult hemlock woolly adelgid is an aphid-like insect, oval in shape, with four thread-like stylets that are bundled together and function as a mouthpart.
  • As an egg, they are oblong and amber in colour.
  • Eggs are contained in spherical ‘woolly’ ovisac made of white waxy threads resembling small tufts of cotton. Typically found at the base of needles on most recent twigs.
  • Signs and symptoms include premature bud and shoot dieback, premature needle loss, a thinner greyish-green crown, and dieback of twigs and branches.

What You Can Do

  • Learn how to identify hemlock woolly adelgid and what infested trees look like.
  • Don’t move infested wood material from the regulated zone to new areas; this includes all firewood, trees, logs, lumber, wood, or bark chips. Firewood should always be obtained locally and burned on site.
  • If you’ve seen hemlock woolly adelgid or another invasive species in the wild, please contact the toll-free Invading Species Hotline at 1-800-563-7711, visit EDDMapS, or search for the ‘Invasive Species in Ontario’ project on to report a sighting.

Resource Files


OFAH/OMNRF Invading Species Awareness Program. (2021). Hemlock Woolly Adelgid. Retrieved from:
This factsheet may be reproduced for non-commercial purposes.

Header photo by Steven Katovich,

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