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. This pest causes rapid decline and eventual death of eastern hemlock trees. 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. HWA can spread easily, has no natural predators and is expected to harm local species and environments. Without urgent action, most of the municipality’s hemlock forests and the many species that live in them will be badly affected by HWA.
HWA biocontrol introductions
In 2024, HWA was detected in Sandy Lake Regional Park, which has the highest density of eastern hemlocks of any municipal park in Halifax. It has since become a priority area for hemlock protection.
As outlined in the Hemlock Wolly Adelgid Treatment and Management Plan, approved by Regional Council in fall 2025, many hemlocks in municipal parks are being protected in the short-term with chemical (or medicinal) injection. For longer-term protection, the municipality’s Environment & Climate Change team is collaborating with the Canadian Forest Service and other partners to introduce specialist predators (also called biocontrol) to combat HWA in Sandy Lake Regional Park.
To keep residents informed about the use of biocontrol in the municipality, we will be hosting a webinar on Thursday, September 24 from 12 - 1 p.m.
Register to attend the biocontrol webinar via Microsoft Teams.
The webinar will feature presentations by Shauna Doll, Environmental Specialist with Halifax Regional Municipality and author of the municipality's Hemlock Wolly Adelgid Treatment and Management Plan, and Dr. Lucas Roscoe, Research Scientist with the Canadian Forest Service.
The first biocontrol, the Laricobius nigrinus beetle, is scheduled for release in October 2026. We also hope to introduce Leucotaraxis silverflies. These predators are used because they attack HWA without posing any known risk to local biodiversity. There are no native HWA predators currently in Nova Scotia, but these species are HWA's main predators in British Columbia, where they have helped control HWA populations.
Hemlock branch with HWA egg sacs.
Learn more about HWA on the HalifACT Hub
To explore the challenges posed by HWA, a timeline of its spread and the measures we're taking to control it, navigate over to the HalifACT Hub's HWA page.
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
On September 4, 2025, the municipality's HWA Treatment and Management Plan was approved by Regional Council.
The municipality has learned 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.
A group of dead trees following an HWA infestation.
| Park | Location | Treatment |
|---|---|---|
| Park | Location | Treatment |
| Sandy Lake | Bedford | Biocontrol release planned for October 2026. Likely to also receive some chemical treatment in spring 2027. |
| Hemlock Ravine Park | Bedford | In process of treatment via volunteer effort |
| Admiral Cove | Bedford | TBD |
| Sawgrass Drive Park and Oakfield Woods Park | Oakfield | In process of marking and slated for treatment summer 2026 |
| Shubie Park | Dartmouth | In process of marking and slated for treatment summer 2026 |
| Clarence Court Park | Fletcher’s Lake | In process of marking and slated for treatment summer 2026 |
| Brookhill Drive Park 3 | Grand Lake | In process of marking and slated for treatment summer 2026 |
| Duck Lake Brook Greenway Park | Windsor Junction | Marked and slated for treatment summer 2026 |
| Gordon R. Snow Community Centre Park | Fall River | TBD |
| Red Bridge Pond Park | Dartmouth | TBD |
| Settle Lake Park | Dartmouth | In process of marking and slated for treatment summer 2026 |
| Sir. Sandford Fleming Park | Halifax | Marked and slated for treatment summer 2026 |
| Point Pleasant Park | Halifax | None mapped |
| Shaw Wilderness Area | Halifax | Surveyed and few hemlocks found, likely to be removed from priority treatment list |
| First Lake (First Lake Regional Park, Alder Crescent Park, and Sackville River Linear Park) | Sackville | Marked and slated for treatment summer 2026 |
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.
Reducing the spread of HWA
Some ways that residents can help stop the spread and manage the impacts of HWA:
- 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