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Wrestling a wild and woolly pest

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Hemlock Woolly Adelgid infestation. Photo credit: Chris Evans, Illinois Wildlife Action Plan

By Julie Root
June 15, 2018

The hemlock woolly adelgid (HWA) is a tiny invasive insect that destroys eastern hemlock trees. Easily spread by wind, animals and human activities, HWA feeds on the fluids of hemlock trees, causing death as early as four years from first establishment. From an ecological standpoint, eastern hemlock trees are a key component of sensitive natural environments, such as riparian habitats along streams and lakeshores. First discovered in Virginia in 1951, HWA has spread up the east coast to southwestern Nova Scotia, where it was detected in summer 2017. Research scientists from Natural Resources Canada’s Atlantic Forestry Centre have developed three new projects to detect, monitor, control and manage the damage.

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Dr. Lucas Roscoe, holding one of the traps being tested this year for monitoring HWA populations in southwestern Nova Scotia


Detecting populations as early as possible gives us the best chance of minimizing tree damage and mortality. Monitoring populations where HWA has already been detected provides valuable information on how it is growing and spreading. Research scientist Dr. Lucas Roscoe and his team are developing monitoring tools involving commercially available insect traps.

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TreeAzin® treatment of an ash tree in Ontario

These traps attract HWA with attractive smells from various coniferous trees. The objective is to design trapping methods specific to HWA so researchers can detect populations as early as possible and monitor them from year to year to determine how they are developing and spreading. This information will help scientists develop treatment options to control HWA populations.


Research scientist Dr. Jon Sweeney is testing TreeAzin®, an insecticide derived from extracts of the seeds of the neem tree, to see if it is effective with the eastern hemlock. Developed in Canada, TreeAzin® is approved for protecting ash trees from the emerald ash borer. The insecticide is injected directly into the stems of the hemlock trees, travelling through the fluid system into fine branches of the tree crown, where protection is needed. Initial treatments are set to begin in fall 2018 at an infested site in Nova Scotia.

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Eastern hemlock seeds and cones


Dr. Michael Stastny studies forest insect ecology: the interactions between insects, trees and their environment that underlie the functioning of our forests. He is interested in how site conditions, particularly drought, exacerbate the impact of HWA on hemlock health. “If we know a particular stand is water-stressed or weakened by other factors,” says Dr. Stastny, “we may be able to predict the severity of hemlock decline and mortality from HWA. This information will help us to identify the most vulnerable stands and focus our response.”

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Forest damage due to Hemlock Woolly Adelgid. Photo credit: Justin Smith, Nova Scotia Department of Natural Resources

One such response could be stand-thinning – harvesting some competing trees to give the remaining hemlocks more access to water and light. This may improve their ability to tolerate HWA. Dr. Stastny is testing this forest management approach in experimental plots this year.

In addition to efforts to manage HWA, NRCAN has a backup plan. The National Tree Seed Centre has 195 eastern hemlock seed lots carefully preserved in its lab, where temperature, light and humidity are strictly controlled. Seed Centre researchers will preserve more hemlock seed over the next few years so that the species could be reintroduced in suitable environments if HWA were to be controlled or eliminated.

As scientists and partners look to slow the spread of HWA and preserve hemlock species, the public has an extremely important role to play. Knowing where HWA and other harmful insects have been detected and taking simple precautions can have a big impact on protecting our forests. For example, limiting the movement of firewood from one location to another can help slow the spread of HWA.

For more information on HWA detection in Nova Scotia, see Questions and Answers.


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