Hemlock Woolly Adelgid (Adelges tsugae)


Hemlock Woolly Adelgid (HWA) is an invasive insect closely related to aphids that is extremely destructive to Eastern Hemlock (Tsuga canadensis). Native to India, China, Japan, and Taiwan, it was first reported in North America in 19191. Initially found in British Columbia, it soon spread to Alaska, Washington, Oregon, Idaho, Montana, and California, where it left western hemlock species relatively unharmed1. It was later found in the eastern United States in 1951 in Virginia. HWA is much more damaging to Eastern hemlock and spreads much faster; already 21 eastern states and Nova Scotia have established HWA populations1Hemlock woolly adelgid was detected in Etobicoke in 2012, and in the Niagara Gorge from 2013 to 2015 by the Canadian Food Inspection Agency2. After the discovery of infected Eastern hemlock trees, these trees were promptly removed and destroyed2. As New York and Michigan both have HWA infestations, it is likely that Hemlock Woolly Adelgid will continue to be introduced and found in Ontario2. Currently there are no reports of HWA in the Credit River Watershed. 

Hemlock Woolly Adelgid feeding on some Eastern hemlock needles (U.S. Department of Agriculture: Forest Service)


Hemlock Woolly Adelgid is a tiny (less than 1.5mm), aphid-like insect, and is very hard to see due to its size. HWA creates distinctive white ‘woolly’ sacs at the base of Eastern hemlock needles that are much easier to spot. To check for HWA infestations, examine the underside of hemlock branches, at the base of needles. The woolly sacs are attached to the twig and feel waxy to the touch. Eastern hemlock branches found on the ground, underneath an Eastern hemlock tree, can often provide insights regarding what is occurring higher up in the tree, and should be examined for woolly sacs3Checking the bark after a rainstorm is a good way to detect infestations early, as HWA is dispersed mainly through wind3The best times to sample are during mid-October to mid-July; as insects are very tiny and hard to spot outside this time period, and they lack the woolly white sacs5Infected trees can show other signs, such as premature needle loss, greyish-green discolouration, thinner canopy appearance, bud/shoot dieback, and twig/branch dieback3. Death occurs in as little as 4-15 years. 

The HWA life cycle is complex. All individuals are female, and they reproduce asexually in two different cycles every year. From March through May, adults lay white egg sacs containing up to 300 eggs, which can hatch into two types of nymphs: progredien and sexuparae. From May to June, sexuparae develop wings and fly in search of Eastern Hemlock trees, but as these spruce are not found in North America, the sexuparae die without reproducing. Progrediens feed on hemlock needles during this period, maturing until they lay eggs from June to July. These eggs hatch, and all nymphs in this generation are called sistens1. After crawling to the base of hemlock needles, sistens enter a period of inactivity called aestivation from July to October. During winter (October-February), this generation feeds and develops, until it lays its eggs from March-May, continuing the cycle. During their lifecycles, HWA is completely immobile, except for the early nymph stages and the beginning of the sexuparae stage.  

Simplified Hemlock Woolly Adelgid lifecycle (From “Detection Survey Protocol: Hemlock Woolly Adelgid (Adelges tsugae Annand)”. By the Plant Health Surveillance Unit, Canadian Food Inspection Agency, 2018. Reproduced the without affiliation with or endorsement from the Government of Canada. Available at https://www.invasiveinsects.ca/hwa/CFIAsurveyprotocol.pdf)1.

Ecological Threat

Eastern hemlock trees are a keystone species in many ecosystems. Growing well in shade, Eastern hemlock trees can live longer than 500 years, and grow up to 53m tall6. Unlike many coniferous trees, Eastern hemlock trees keep the needles on their lower branches, shading the forest floor and preventing many other plant species from growing. This creates a forest with unique properties that many bird species depend on, such as the Black-throated green warbler6. Animals that live on the forest floor can benefit from the cover these trees provide, and the low-hanging needles can provide nourishment for species, such as the White-tailed deer6Furthermore, Eastern hemlock stands effectively shade streams, cooling them and create, habitat for Brook trout and other cold water aquatic species. Brook trout and Brown trout are 2-3 times as likely to be found in rivers shaded by Eastern hemlock stands, versus rivers shaded by hardwood stands7.

Healthy Eastern Hemlock Understory (Steven Katovich, USDA Forest Service, Bugwood.org)

Hemlock Woolly Adelgid is very destructive to Eastern hemlock trees, as infestations are able to quickly kill 100% of hemlock trees of any age in a forest and are able to prevent them from regenerating4. The loss of Eastern hemlock stands could increase erosion, reduce food availability for forest animals, remove winter cover for deer, moose, and birds, and reduce shading of streams and soil. 

Eastern hemlock stand decimated by HWA (USDA Forest Service – Region 8 – Southern, Bugwood.org)

Preventative Strategies

They best way to prevent the spread of HWA is to prevent the transportation of firewood from HWA infested areas into non-infested areas. HWA often spreads through birds and other animals, so it is a good idea not to hang bird feeders on or near Eastern hemlock trees. Furthermore, if you have Eastern hemlock trees on your property, you can promote their health through watering trees during dry spells, avoiding soil compaction near the trees, and avoid damaging any branches. If you are buying an Eastern hemlockmake sure to purchase a locally grown tree 

If any signs and symptoms of Hemlock Woolly Adelgid are found, or you would like to obtain more information, please contact CVC via email. 


https://www.youtube.com/watch?v=yQP0Tv43mxg (Created by Outsmart Project) 


1. Canadian Food Inspection Agency. 2018. “Detection Survey Protocol: Hemlock Woolly Adelgid (Adelges tsugae Annand)”. Plant Health Surveillance Unit. Web. 08 August 2018. https://www.invasiveinsects.ca/hwa/CFIAsurveyprotocol.pdf  

2. Fidgen, J.; Turgeon, J. 2016. “Detection tools for an invasive adelgid”.Frontline Technical Note 116. Natural Resources Canada, Canadian Forest Service, Great Lakes Forestry Centre, Sault Ste. Marie, Ontario. Web. 08 August 2018. http://cfs.nrcan.gc.ca/pubwarehouse/pdfs/36791.pdf  

3. Invasive Species Centre.“Hemlock Woolly Adelgid: An Invasive Species Threatening Hemlock Trees in Eastern Canada” 08 August 2018. https://www.forestinvasives.ca/Portals/0/HWA%20Fact%20Sheet%20Final%20Web.pdf 

4. Hart, J.L. 2008. Eastern hemlock decline and persistence of disjunct populations near its southern boundary. Journal of Alabama Academy of Science 79 (3-4): 174-180. 08 August 2018. http://fdl.ua.edu/uploads/5/1/0/4/51040631/jaas2008.pdf  

5. Costa, S. and Onken. 2006. “Standardizing Sampling for Detection and Monitoring of Hemlock Woolly Adelgid in Eastern Hemlock Forests.” U.S. Department of Agriculture: Forest Health Technology, Enterprise Team. Web. 26 July 2018. https://www.fs.fed.us/foresthealth/technology/pdfs/HWASampling.pdf  

6. Hemlock Restoration Initiative. 2017. “The Importance of Hemlocks”. 26 July 2018. https://savehemlocksnc.org/the-importance-of-hemlocks/  

7. Ross, R.M., Bennet, R.M., Snyder, C.D., Young, J.A., Smith, D.R., andP. Lemarie. 2003. Influence of eastern hemlock (Tsuga canadensis L.) on fish community structure and function in headwater streams of the Delaware River basin. Ecology of Freshwater Fish. 12 (1): 60-65. Web. 08 August 2018. https://onlinelibrary.wiley.com/doi/full/10.1034/j.1600-0633.2003.00006.x  

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