Keeping the Legacy Alive


What is Hemlock Woolly Adelgid?

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. HWA can be spread by wind, animals, and human movement of nursery stock, logs, and other wood products.

Where was the initial detection of Hemlock Woolly Adelgid?

Hemlock woolly adelgid was first reported in Canada in British Columbia in the 1920s, and it was detected in Southwestern Nova Scotia in 2017. The first Eastern North American detection was in Richmond Virginia USA in 1951, on an ornamental hemlock from Asia.

How did Hemlock Woolly Adelgid arrive in Canada?

It is not known at this time how the hemlock woolly adelgid came into Canada, and the exact source will be difficult to determine. Dispersal of HWA occurs by wind, storms, hurricanes, birds, animals, and human movement of nursery stock, logs, and other wood products, including firewood.

What is the life cycle of the HWA?

The HWA is different than most of the insects found in the Northeast in that it is dormant through much of the growing season and active throughout much of the winter. HWA immatures (crawlers) settle onto twigs by mid-August and dormant. They neither feed nor develop during this period, until mid-October, when they come out of dormancy and resume feeding until approximately early March. As adults, they will now begin to produce eggs surrounded by ribbons of a white, waxy material that appears as small cotton balls lined up at the base of the needles. This is when most people become aware of their presence. These eggs will hatch, and a new generation will begin feeding. These crawlers will mature in late-May to early-June, another batch of eggs is produced, and the cycle begins again. All HWA are females and the vast majority of these are wingless. Head to the Phenology page to learn more.

What is the potential for hemlock woolly adelgid to spread?

Once established, HWA will spread naturally via wind, birds, animals, and human movement of nursery stock, logs, and other wood products, including firewood. To help prevent the spread of this pest, the public is encouraged not to move potentially infested firewood and other hemlock forest products.

Is hemlock woolly adelgid considered a regulated pest?

Yes — import and domestic movement requirements are in place to prevent the introduction and minimize the spread of the hemlock woolly adelgid. To see the complete program, please refer to the Canadian Food Inspection Agency Directive D-07-05. This directive is being revised to reflect the recent Hemlock Woolly Adelgid Infested Place Order (issued on May 28, 2020) and therefore, there are parts of this version of the directive that are not yet consistent with that order. For cases in which requirements for regulated commodities have been established by the Order, the Plant Protection Act or the Plant Protection Regulations, those requirements take precedence over the requirements outlined in section 1.4 in this directive. This directive will be updated in the upcoming months.

What are the potential economic and ecological impacts of hemlock woolly adelgid?

The economic value of hemlock to the forest industry is not as high as other trees species, however, eastern hemlock can be processed for use in general construction or as pulp. Hemlock woolly adelgid, and the resulting loss of hemlock trees, has the potential to cause major ecological impacts in Canada. In many forests, hemlock serves as a foundation tree in the environment. Loss of eastern hemlock could negatively affect the health of vegetation, birds, aquatic organisms and mammals, as hemlock trees serve a vital role in protecting watersheds and streams in natural forest ecosystems.

What are the movement restrictions of hemlock in Nova Scotia?

The Canadian Food Inspection Agency (CFIA) regulates all movement of hemlock in the infected counties of Nova Scotia. Please refer to the Amended Hemlock Woolly Adelgid Infested Place Order for up-to-date movement regulations.

What can I do if I find HWA on my property?

You have several options to proactively manage HWA on your property. Before you consider harvesting your timber, please review these options and contact us to talk it through. Your forest is an important part of the broader landscape and together we can safeguard biodiversity and forestry for future generations.

What are my options?


No management:

At least 80% of infested hemlock trees are expected to die within 4–15 years. Individual trees may possess some level of resistance to HWA and will be extremely important for the survival of the species. The loss of hemlock will gradually lead to hardwood-dominated stands or a mix of hardwoods, red spruce, and white pine. During this transition, the dead standing, and downed hemlock will provide valuable habitat for a variety of birds, mammals, and invertebrates such as pollinators.


Chemical control:

Hemlock can be protected from HWA by treatment with systemic insecticides. In Canada, only one option is currently available: direct stem-injection of imidacloprid, a neonicotinoid pesticide which spreads via the vascular system of the tree and kills HWA as it feeds. Trees remain protected against HWA for up to 5 years. The injection process is costly, so prioritizing individual trees or valuable stands is strongly recommended. To learn more about imidacloprid and its uses, availability, application regulations and certified applicators visit the Chemical Treatment page.


Silvicultural management:

Long-term stand management should be ongoing to promote overall stand health and resilience against stressors. Prior to HWA impacts, stand thinning may promote hemlock growth, boosting its tolerance to HWA infestation, although this approach alone is unlikely to be sufficient in the long term. Following HWA detection in your stand, removal of highly infested trees (hotspots) may slow down the further spread of the pest. Pre-emptive cutting of healthy hemlock stands in anticipation of HWA is not recommended, as it could result in the loss of resistant hemlock, and dramatically impact the associated habitat.


Biological control:

Biological control is on the horizon! The introduction of specialist natural enemies of HWA is the only longterm, region-wide strategy to ensure persistence of hemlock on the landscape. Canadian Forest Service, Parks Canada, and other members of the HWA Working Group – Maritimes are actively researching the potential for releasing predators of HWA to help control the populations of the pest. Biocontrol is not a simple or quick process; however, we remain optimistic that it will begin to play a key role in the integrated pest management of HWA over the next 10-20 years. Another reason to leave uninfested hemlock in the forest.

How can I slow the spread of HWA?

Keep in mind that HWA spreads very easily to establish in new areas! If you know or suspect that HWA is present in a given stand, please follow the checklist below to help prevent further spread:


  • If possible, avoid visiting hemlock stands in other, uninfested areas for several days
  • Do not collect and move hemlock foliage – ten adults can produce up to 30 million new adelgid in two years; take photos instead
  • Use a lint roller to remove potential crawlers from clothing
  • Do not bring your pet into infested stands
  • Do not park your vehicle near or under hemlock trees
  • Launder all clothing prior to re-entering the field
  • Avoid placing bird feeders near hemlock trees – birds can accidentally transport HWA to new locations
  • Source firewood locally; don’t bring it from home if you’re going camping
  • Buy certified heat-treated (kiln-dried) firewood where available
  • Check with parks or campgrounds before you go for their rules about firewood


Doing the above will help to reduce the risks of spreading HWA. Please help by doing your part in keeping this invasive species in check.

If I find HWA, how can I report it?

If you think you have found HWA, please take a picture and report it to our team. You can either submit the observation to iNaturalist or send an email directly to us at

Where can I find out more?

If you would like to talk more about what your options are, or get involved in HWA surveys on your land as a volunteer, please contact:

Mersey Tobeatic Research Institute (MTRI)

9 Mt Merritt Rd, Kempt, NS

B0T 1B0

(902) 682-2371