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Hemlock Woolly Adelgid: A New Forest Invader at RBG

March 14, 2023

By Mallory Peirce, Assistant Ecologist, Royal Botanical Gardens

Early in March, while conducting routine trail inspections, RBG staff discovered a new invasive forest pest on the property. Hemlock Woolly Adelgid (HWA) is an aphid-like insect that attacks and kills native Eastern Hemlock (Tsuga canadensis) trees. The HWA was initially discovered on a single hemlock along Caleb’s Walk, on the South Shore of Cootes Paradise. Upon further investigation, the infestation appears to be widespread throughout the stand of over 100 hemlock trees within the ravine.

RBG promptly contacted the Canadian Food Inspection Agency (CFIA), who took a sample of an infested hemlock branch for further testing to confirm that indeed the specimen was HWA. The CFIA enforces the Plant Protection Act and is responsible for protecting plant life by preventing the importation, exportation, and spread of pests in Canada.

What is it?

HWA feeds on hemlock trees by inserting a feeding apparatus into the base of hemlock needles and starves the tree of its water and nutrients. This causes the eventual death of the tree sometime between four and ten years after the pest has begun its attack. Once attacked by HWA, hemlock trees do not recover.

Identification and Life Cycle

Hemlock Woolly Adelgid is easily identified through its woolly appearance, both adults and egg sacs have a cotton ball-like appearance. HWA adults are very small and often look like flakes of black pepper, measuring less than 2mm long. More often than not, the presence of HWA can be confirmed by its cotton ball-like egg sac, which is laid on the underside of hemlock branches.

Astonishingly, each egg sac can contain up to 200 eggs! Upon hatching, the first instar nymphs (aka “crawlers”) begin feeding on the hemlock needles.

Hemlock Woolly Adelgid adults at the base of needles
Hemlock Woolly Adelgid adults are very small, measuring anywhere between 0.5mm and 1.5mm in length. Photo: Mark Whitmore, Cornell University
hemlock branch with Hemlock Woolly Adelgid infestation
Both eggs and the feeding adults have a woolly, cotton ball-like appearance, as seen in the photo above.


First introduced to the U.S. through infested plant material from Japan, this forest pest has spread its way up the Eastern Seaboard over the last 70 years, with its first detection in Ontario in 2012. This population was quickly eradicated, but more satellite populations have been popping up in Southern Ontario over the last four years. The most prominent pathway of spread is through migratory birds, who pick up adult HWA on their bodies during spring migration thereby expanding the population. Humans are also guilty of spreading the pest, either by walking through infested sites and transferring HWA via their clothes or by moving firewood from an infested site to a pest-free location. Its arrival at RBG was inevitable, but it is still disheartening.

hemlock trees in a sloped forest
Hemlock stands create vital habitats for wildlife and fish.


The Eastern Hemlock is a vital tree in the forested ecosystem. The dense foliage and grove-like structure of stands creates a haven for deer to escape the snow, as well as protection for birds during the winter months. When found alongside creeks and streams, the hemlock canopies can effectively cool the water temperature resulting in unique habitat for fish and other aquatic organisms (Ross et al. 2003). Additionally, many of RBG’s hemlock stands line slopes alongside trails. Upon the death of these trees, many dead standing hemlocks will need to be removed to ensure trail user safety. Removing all dead hemlocks in a stand will increase the likelihood of erosion on the slopes, which will be cause for slope stabilization projects and plantings.


There are two options to treat a hemlock for HWA – both of which are pesticides and can be injected into the tree. This treatment is costly and requires follow-up injections. Currently, RBG is considering tree injections to protect some of our largest hemlocks but has yet to confirm treatment measures. There are current studies examining the success of potential biological controls (predators) on HWA, but these are in process, and their outcome will not be known for a few years.

Slowing the Spread

RBG is asking visitors to stay on the trail to avoid coming into contact with hemlock branches that may be infested with Hemlock Woolly Adelgid (HWA). Being a nature sanctuary, everything that can be found in nature should stay in nature. Removing anything that can be found in nature has the potential to move invasive pests from an infected area to a pest-free area. Please leave nature in nature.

Trail users can do the following to help slow the spread:

  • Use a boot brush to remove debris from boots before you visit another nature trail
  • Use a lint roller to remove debris, pick plant material off, or wash contaminated clothing
  • Keep dogs leased and brush fur to remove seed or insect hitchhikers (also a great way to check for ticks!)

Next Steps

RBG will continue to work with CFIA and track the spread of the pest across the property. If members of the public think they have seen HWA, please report the sighting to a citizen science portal, such as iNaturalist. This will help track the movement of invasive species across RBG property, as well as across the province.


Ross, R.M., Bennett, R.M., Snyder, C.D., Young, J.A., Smith, D.R., Lemarie, D.P., 2003. Influence of eastern hemlock (Tsuga canadensis L.) on fish community structure and function in headwater streams of the Delaware River basin. Ecol. Freshw. Fish 1, 60–65

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