Barn Swallow © Brittany Killingbeck

Hemlock Woolly Adelgid

NOTICE – Hemlock Woolly Adelgid (Adelges tsugae) detected at Short Hills Nature Sanctuary (Pelham, ON)

The Canadian Food Inspection Agency (CFIA) has detected Hemlock Woolly Adelgid (HWA) at HNC’s Short Hills Nature Sanctuary in Pelham, ON during their 2022 monitoring surveys.

Hemlock Woolly Adelgid (Adelges tsugae) at Short Hills Nature Sanctuary. Photo credit: Brittany Killingbeck


HWA is a tiny aphid-like insect that attacks and kills hemlock trees within 4-15 years by feeding on nutrient and water storage cells at the base of needles. The most obvious sign of infestation is the white “woolly” sacs at the base of the needles on current-year twigs. It can be spread by wind, animals, and human movement of nursery stock, logs, firewood, and other wood products.

It poses a significant threat to the ecological health of our forests – hemlocks are food sources and nesting sites, maintain water temperatures by shading streams, stabilize stream banks, and provide thermal cover to animals in the winter.

Actions Undertaken

  • HNC staff and Sanctuary Committee members met with CFIA staff on site in February to discuss the infestation, current health of the forest, and management options.
  • HNC staff met with Canadian Forest Service (CFS) staff on site in March to tour the site and install monitoring equipment for a phylogeny study.
  • HNC staff have met with various industry representatives to better understand the current silvicultural, chemical, and biological control management recommendations.
  • A small section of trail that intersects with hemlock trees has been trimmed back as a precaution to reduce the chance of trail users brushing up against the branches.

Going Forward

  • HNC will be monitoring and managing the site in consultation with the CFIA, CFS, Ontario HWA Working Group, and researchers.
  • Signage will be posted on the property shortly to inform visitors of the infestation and steps to prevent its spread. 
  • Informational signage will be installed at Spooky Hollow Nature Sanctuary, which also has a significant hemlock stand, to increase visitors’ awareness of HWA and help prevent its introduction to the site.

Prevent Its Spread

To our current knowledge the infestation is located in an off-trail area, but it will continue to expand throughout the stand over time and it is imperative as naturalists that we do not contribute to its spread.

  • Stay on the designated trail system and keep dogs on leash.
  • Avoid placing gear on or near hemlock trees.
  • Remove potential crawlers from clothing using a lint roller once you have left the forest stand, before getting into your vehicle.
  • Beat and/or lint roll hats and coats.
  • Avoid visiting additional areas with hemlock after visiting a positive area.
  • Launder clothing after visiting a positive area.


For those interested in the biology of HWA, a description of the lifecycle from the Invasive Species Centre ( is included below:

There are two generations and three forms of hemlock woolly adelgid in North America. The two generations that occur on hemlock are known as sistens and progrediens (plural sistentes and progredientes), all of which are female and reproduce asexually. The second generation of HWA can produce two forms: progrediens adults (which are wingless and stay on hemlock) or sexuparae (which have wings and migrate to spruce). Sexuparae are not known to survive on native North American spruces or on spruce trees from their native range that have been transplanted in North America. As the sexuparae form is not important to the biology of HWA in North America, we focus on the sistens and progrediens forms that attack hemlock.

The sistens generation, sometimes referred to as the overwintering generation, is present from June until March of the following year; whereas the progrediens generation (spring generation) is present from March until June (Fidgen and Turgeon, 2016). Both generations have six stages of development: eggs, four nymphal instars, and adults (Cheah et al., 2004). Once progrediens adults oviposit (lay eggs) on hemlock, the sistens generation hatches in June-July. The newly hatched first instar nymph is known as a crawler, which becomes inactive for the remainder of the summer shortly after hatching (called aestivation; CFIA, 2019). In mid-October, the crawler will resume feeding and continue to develop through the four nymphal instar stages. Following this, the sistens generation matures into full adults in early May. These adult sistentes each produce a single ovisac containing up to 300 eggs, which hatch to produce progredientes. Because there is no diapause or overwintering requirement for the progrediens generation, the life cycle occurs rapidly, usually from spring to early summer.