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Hemlock Woolly Adelgid early detection in Ontario


The Hemlock Woolly Adelgid (HWA) is an introduced pest from Japan which has been rapidly spreading in the Eastern United States since the 1990’s, and is now found over more than half of the geographic range of Eastern Hemlock. Within a decade or more of its arrival HWA kills most of the hemlock trees in forest stands.

In 2012 HWA was detected for the first time in Ontario, with subsequent detections in the old-growth forest of the Niagara Gorge in 2013, 2014 and 2015. HWA can be carried by birds or humans, and may have already spread – now that it has been found in Ontario, it is important to focus on early detection of its arrival and spread (learn more about when and where to look for it).

HWA detection handout Ontario_Page_1Early detection of HWA is essential for slowing its spread and preserving high value trees and forests. Management options include the use of highly effective systemic insecticides, and release of specialized HWA predators from western North America. Waiting until signs of infestation and tree decline are obvious commonly results in many trees dying before measures can be implemented. In Ontario it’s particularly important to document the arrival and spread of HWA so we can mount an effective and rapid Provincial response to it.

Old-growth hemlock forest should be a conservation priority. Preserving old-growth forest areas is the best way to preserve the genetic diversity of the species, and maintain local seed sources for recolonization of the landscape once effective biological control is established.

How you can help

The first detection of HWA in Ontario was by an arborist, who recognized the pest on a tree in Etobicoke and reported it to authorities. This triggered a survey by the Canadian Food Inspection Agency, which subsequently detected HWA in the Niagara Gorge. We believe that citizens with a passion for trees should be working in parallel with professionals on the front lines of monitoring for HWA invasions; the more people who are looking for the pest, the better. You can start looking for it in late winter or spring before leaf-out; if you’re going for a walk in a hemlock forest, check some branches.

Use our HWA monitoring page for more information on how and when to survey. Sign up for updates and we’ll let you know if there are new resources for monitoring, HWA sightings you should know about, or other news. A great tool for learning about and reporting invasive species in Ontario, including HWA, is EDDMapS Ontario, which is available as a free phone app from Google Play or the AppStore. If you are in Halton or Peel regions, participate in the Early Detection and Rapid Response program.

A group of land managers and other stakeholders in Ontario are collaborating to prepare and plan for the arrival of HWA in Ontario. Contact Kathleen Ryan at Silvecon to learn more.

Locating old-growth hemlock forests

An important step in monitoring for HWA is to locate stands of Eastern Hemlock near likely points of arrival or spread, and also to ensure that old-growth hemlock forests are being monitored. Please let us know about old-growth hemlock forests that are not on our online map. Either send us an email at or add hemlock sites to our inaturalist project.

 The value of Eastern Hemlock

Eastern Hemlock is considered a foundation species because it is a dominant species that has a controlling influence on ecosystem function. Hemlock forests are often noted as valuable for their role as deer yards, for creating aquatic habitat by regulating stream flow and moderating temperature, and for their unique aesthetic value.

Hemlock is also one of Ontario’s longest lived trees, capable of living over 500 years, with numerous individual trees in Ontario known to exceed 400 years. Hemlock is an important component of old-growth forests on the landscape.