Hemlock woolly adelgid (HWA) is native to Asia, and established in western North America prior to the last glacial period; it is not a significant pest in either of these regions. First detected in eastern North America in the 1950's, the eastern HWA population originated from southern Japan, and is now established from southern Canada through to Georgia. The adelgid has no effective natural enemies in eastern North America to help keep its population under control.
Biology: The adelgid is tiny insect, less than 1.5 mm in length. In North America there are only female HWA, which reproduce asexually. There are two generations of HWA on hemlock each season, with the more fecund generation laying up to 300 eggs per female. The newly emerged nymph, called a crawler, is the dispersing stage of the insect moving to new trees by wind, on the feet of birds, or in the fur of small mammals; it spreads 12 km or more per year. Once the crawler finds a location to feed, it inserts its mouthparts into the twig and becomes immobile. The nymph feeds on the tree's stored starches, depleting its energy stores. Feeding by HWA also induces a systemic defensive response, which further damages the tree. Unlike other insects, the adelgid is inactive through much of the summer, resuming feeding and development in the fall. During this time, the nymph produces its distinctive woolly white covering.
Hemlock woolly adelgid (Adelges tsugae), or HWA, is an invasive insect that is a serious pest in eastern North America, damaging and killing eastern North American hemlock species. Established populations are known in Nova Scotia (since 2017) and parts of Ontario (since 2019). Because it is difficult to detect, HWA could be established in your area. Be on the lookout, early detection allows more options for mitigating the insect's impact.
Trees at risk: Hemlock woolly adelgid is a threat to eastern hemlock, Ontario's only hemlock species. In the southeastern US, hemlock has shown little resistance to HWA, and die within a few years of infestation. In the north part of HWA's range, some infested hemlock survive longer, ten years or more. Trees growing in more stressful sites or conditions are more vulnerable, as are older trees. The ecological impact of HWA in hemlock-rich areas can be considerable – hemlock cover modulates stream temperatures and buffers nutrient inputs, it stabilizes soils and provides habitat for animals and plants.
Current range: In eastern Canada, the adelgid is considered established in southwestern Nova Scotia and parts of Ontario. In the U.S., the eastern population of HWA is distributed from Georgia and Tennessee northeast to southwestern Maine.
Detection: On infested trees, white woolly patches are found on the underside of twigs near the base of the needles. They are easiest to see in late winter or spring when they are larger (March to May in southern Ontario). Carefully inspect the underside of the branches especially the younger twigs. The adelgid is difficult to detect at early stages of the infestation when there are few insects, which may be located in the upper branches of the tree. At later stages of infestation there will be dieback of foliage and twigs. The distribution of HWA infested trees within a forest can be patchy - start by looking at supercanopy hemlock or hemlock trees along streams where HWA are often transported by wind or birds respectively. Read more about HWA detection. Need help sampling? Contact us for a quote.
What you can do: If you think you have found hemlock woolly adelgid in Canada outside of its known range, contact the CFIA's Plant Health Surveillance Unit.
HWA land manager working group for Ontario A group of stakeholders in Ontario are collaborating to prepare and plan for the arrival of HWA in their forests. Contact us to find out more.