Hemlock woolly adelgid


What is Hemlock woolly adelgid? What are the initial signs of infection? Are there any treatment options if I find out that my hemlocks are infected?


Thank you for contacting the Toronto Master Gardeners. Hemlock Woolly adelgid (HWA) is an aphid -like insect that attacks and eventually kills hemlock trees by feeding on plant sap. Like aphids the HWA has a piercing-sucking mouth that feeds on the water storage cells located at the base of the needles. In addition to withdrawing nutrients from the plant, it is believed that the HWA introduces a toxin into the plant that eventually damages the plant’s vascular tissue (the main water and nutrient conducting tissue), thus causing the overall death of the plant.

If you suspect that your hemlock might be suffering from HWA look for white “woolly” sacs at the base of hemlock needles on new growth. Other symptoms may include premature needle loss, premature shoot and bud dieback and discolouration of the needles. HWA can be spread by wind, animals such as birds and human movement of infected nursery stock logs and firewood.

If you think you’ve seen HWA, report your sighting! You can report Canada-wide sightings to cfia.surveillance-surveillance.acia@canada.ca or to eddmaps.org/Ontario in Ontario.

For more information on the life cycle of this pest along with photos on what to look for refer to the Invasive Species Center fact sheet on Hemlock Woolly Adelgid.

The Government of Canada also has numerous fact sheets on this destructive pest that you can find here and here.