Hemlock woolly adelgid (Adelges tsugae), often known as HWA, is an aphid-like insect from southern Japan that infects hemlock (Tsuga spp.) trees. HWA was found for the first time in the Halifax Regional Municipality in 2023. It is estimated that HWA can kill up to 95 per cent of trees in some infested forests.
Because HWA is invasive to eastern North America, it has raised some concern. This is because HWA can spread easily, has no natural predators and is expected to harm local species and environments.
Hemlock branch with HWA egg sacs.
HWA sucks fluid from new growth on the hemlock’s branches and eventually kills the tree by preventing photosynthesis. Without urgent action, most of the municipality’s hemlock forests and the many species that live in them will be badly affected by HWA. This will also transform the parks and greenspaces where eastern hemlocks grow. Since hemlocks are a backbone species to old growth in Nova Scotia, it is crucial that we protect them.
A group of dead trees following an HWA infestation.
Identifying HWA
HWA are tiny black insects that are most easily identified by the white “woolly” egg sacs they attach to the underside of branches at the base of hemlock needles. The egg sacs can usually be seen from May to November but are most prominent in late winter and early spring. They are usually found on newer growth on the tree's branches.
Other easy-to-spot signs of infestation are:
- needle yellowing
- pieces of “wool” attached to tree trunks after rain
- dieback of twigs and branches on what appears to be a healthy tree
Addressing HWA
The municipality is learning from forest managers in other parts of Nova Scotia that chemical control is the only option available to protect eastern hemlocks from the crisis of an HWA infestation. These chemical treatments will occur two to three times over 10-15 years.
There are two forms of chemical treatment available: injections and basal bark spray. Injections are more expensive, slower to implement and more targeted. Basal bark spray is less expensive and faster, but less targeted. This means that some chemical may be detected in the environment in the area immediately around a sprayed tree.
Chemical treatment is temporary while longer term solutions are under development.
Reducing the spread of HWA
Some ways that residents can help stop the spread and manage the impacts of HWA:
- fill out the public engagement survey (open until mid-September)
- attend public engagement sessions about HWA
- July 27, 9 a.m. – 12 p.m. at Hemlock Ravine Park (parking lot near Heart Shaped Pond)
- August 22, 6:30– 9 p.m. at the Fairbanks Centre, Dartmouth
- monitor hemlock trees in your neighbourhood for infestation and report suspected HWA sightings to 311, the Nova Scotia Invasive Species Council or the Canadian Food Inspection Agency
- avoid hanging bird feeders on or near hemlock trees
- purchase firewood locally
- buy hemlock trees from local suppliers and inspect trees carefully before purchasing
- use a lint brush on your clothes and a bristled brush on shoes when moving from one forest stand to another
- visit www.nshemlock.ca for information on upcoming volunteer events to assist with treatments to prevent HWA damage
Why is treating HWA so urgent?
Eastern hemlock is among the largest growing and longest-lived tree species in Nova Scotia. They form the backbone of the small amount of old growth forest that remains in the province. Eastern hemlocks are also considered a “foundational” species because they are essential for local ecosystems.