Hemlock woolly adelgid
- French common name: Puceron lanigère de la pruche
- Scientific name: Adelges tsugae Annand
- Kingdom: Animalia
- Phylum: Arthropoda
- Class: Insecta
- Order: Hemiptera
- Family: Adelgidae
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Partial list of synonyms:
- Adelges funitecta (Dreyfus)
- Chermes tsugae (Annand)
General information and importance
Adelgids are a distinct group of small (adults less than 2 millimetres), soft-bodied, pear-shaped insects related to aphids. They are specialist feeders on conifers in the Pinaceae family (pine (Pinus), spruce (Picea), fir (Abies), hemlock (Tsuga), larch (Larix), and Douglas-fir (Pseudotsuga menziesii)) and inhabit boreal and temperate forests of North America and Eurasia. Feeding adelgids exude a white, waxy flocculent material that covers their bodies and gives them their common name, “woolly adelgids”. Several species have been accidentally introduced from Europe and Asia to North America where they have become forest pests.
Adelgids have complex life histories. There may be a two-year life cycle with a sexual generation on a primary host (usually spruce), followed by dispersal to an alternate, secondary host (a conifer species in a different genus) where they reproduce asexually for the second season before returning to the primary host. Alternatively, adelgids may reproduce asexually for several generations on a secondary host exclusively. This life cycle requires only one year. Newly hatched nymphs are called “crawlers” and often are the only actively mobile stage in the life cycle. Once settled at a feeding site, the adelgid nymph remains in this location for the duration of its life. All stages feed by inserting piercing mouthparts (stylets) into tree tissues, injecting saliva, and sucking up nourishment. On primary hosts, feeding induces formation of galls. On secondary hosts, feeding causes distortion and eventual loss of needles, twigs, and/or stems near the feeding site. In all cases, feeding interferes with healthy function of the tree.
Hemlock woolly adelgid is native to eastern Asia with a distinct lineage native to western North America. It has little impact on local hemlock species in its native ranges. However, a population introduced from Japan to Virginia, United States, likely in the 1920s, has invaded much of the hemlock forests in the eastern United States. It has also been detected recently in Nova Scotia and Ontario. In this introduced range, the insect has caused widespread mortality of eastern (Tsuga canadensis) and Carolina (T. caroliniana) hemlock of all ages, with significant consequences for forest and riverine ecosystems.
Distribution and hosts
In its native range of eastern Asia, tiger-tail spruce (Picea torano) is the primary host of hemlock woolly adelgid. Several species of Asian hemlock including Tsuga diversifolia and T. sieboldii serve as secondary hosts. This Asian strain of hemlock woolly adelgid has invaded eastern forests of North America from Tennessee, moving north to Maine and Ontario, west to Michigan and east to Nova Scotia. In this introduced range, there is no suitable primary spruce host, so hemlock woolly adelgid completes its lifecycle on eastern hemlock and Carolina hemlock. Hemlock woolly adelgid in western North American also has only secondary hosts; western hemlock (T. heterophylla) and mountain hemlock (T. mertensiana).
Tree parts affected
Hemlock woolly adelgids feed on new shoots of the host tree and may feed on older twigs and shoots when new shoots are not available. Through all stages of life, they feed by inserting their mouthparts into the tree, usually before the new shoot is fully formed.
Symptoms and signs
Adult hemlock woolly adelgids are very small (1.4 millimetres) and difficult to observe with the naked eye despite being covered by a white, “woolly” covering. Eggs (less than 0.4 millimetres) are laid in a protective white, waxy ovisac exuded by the parental generation at the feeding site. Active crawlers are reddish-brown, turning black with a white body fringe once settled on the undersides of twigs. Subsequent nymphal stages are most evident from their waxy coverings.
More evident than the adelgid is the symptomatic effect on the tree caused by their feeding. The combination of penetration of mouthparts and injection of saliva into tree shoots causes visible desiccation, discolouration, and loss of needles as well as deformation of shoots near the feeding site.
Life cycle
In its native Asian range, hemlock woolly adelgid has a two-year life cycle alternating between a sexual stage on the primary spruce host and its secondary hemlock hosts where it reproduces asexually. In its native western North American range, spruce species are not suitable as primary hosts, so the insect reproduces asexually exclusively on western and mountain hemlock.
Native spruce species are not suitable for sexual reproduction of hemlock woolly adelgid introduced to eastern North America, so the insect completes two asexual generations per year by parthenogenesis. First generation adelgids (sistens) hatch from eggs in June and disperse to feeding sites where they settle and enter summer dormancy (aestivation) until autumn. Nymphs resume development in October and pass through four stages, reaching the adult stage by February in the mid-Atlantic states and by April farther north in Nova Scotia. These adult females are the largest (1.4 millimetres) and darkest of the two generations. Eggs of the second generation (progrediens) are laid from April through to end of May, depending on the location. Crawlers hatching from these eggs disperse and settle on twigs near the base of needles. This generation does not enter dormancy but develops rapidly (4 to 6 weeks) to become adults and begins laying eggs in June. These adults are smaller (0.9 millimetres) and lighter in colour than their parents. Some winged sexual forms are produced by this second generation. However, North American hosts are not suitable and these forms die without reproducing.
Damage
Eastern and Carolina hemlock in eastern North America are especially vulnerable to hemlock woolly adelgid. This is because the introduced lineage attacks new shoots, in contrast to the western lineage, which feeds on older twigs. Feeding by adelgids depletes plant reserves and interferes with photosynthesis and translocation of nutrients and water. Salivary compounds injected by the adelgid induce abnormal growth that further compromises tree function. The result is an initial loss of new shoots followed by loss of old needles and eventual dieback of branches and discolouration of the tree canopy. Death of the tree may occur within three years, depending on availability of moisture and other site-related factors. Trees in northern and high-elevation locations may gain some reprieve if cold winters cause significant mortality in the sistens generation. However, reproduction in the progrediens generation the following spring can compensate for winter-kill and quickly restore population density.
Hemlock is a less significant commercial species in eastern North America today than it was in the past. However, where it remains abundant it is considered a foundation species and is critical to forest structure and ecosystem function. Its presence in riparian areas provides shade for streams and the forest floor, influencing carbon and nutrient cycling and providing critical wildlife habitat. Once gone, hemlock frequently does not re-establish naturally and is often replaced by hardwood species, which do not provide the same ecological function.
Prevention and management
Pest management strategies for a particular pest vary depending on several factors. These include:
- the population level of the pest (i.e., how numerous the pest is on the affected host[s]);
- the expected damage or other negative consequences of the pest’s activity and population level (either to the host, property, or the environment);
- an understanding of the pest’s life cycle, its various life stages, and the various natural or abiotic agents that affect population levels;
- how many individual host specimens are affected (an individual tree, small groups of trees, plantations, forests);
- the value of the host(s) versus the costs of pest management approaches; and
- consideration of the various silvicultural, mechanical, chemical, biological, and natural control approaches available and their various advantages and disadvantages.
Decisions about pest management strategies require information about each of these factors for informed decision-making. These various factors should then be weighed carefully in terms of costs and benefits before action is taken against any particular pest.
A biological control program to import and release natural enemies has existed in the United States since 1992. There are no known insect parasitoids associated with adelgids, so biological control has focussed on predators and pathogens. Specialized predatory beetles and flies from the insect’s native range have shown the most promise.
Government agencies in both the United States and Canada regulate hemlock woolly adelgid as an invasive species. This includes defining regulated areas where the insect is known to be present and restricting the movement of any foliage, nursery stock and firewood from infested areas.
Hemlock woolly adelgid is a challenging insect to manage. Populations on small, nursery stock trees can be reduced with foliar insecticides, but this practice is not practical for large trees or natural forest situations. Systemic insecticides injected into the tree or sprayed on the trunk and then absorbed through the bark are widely used in the United States. In general, these products demonstrate efficacy, but their use can be limited by cost and the environmental tolerance for the active ingredient.
Silvicultural manipulation of hemlock stands has also shown promise as a management technique. Crossing susceptible North American hemlock with more resistant Chinese hemlock (Tsuga chinensis) has not been successful. Some stands of eastern hemlock have shown partial resistance to hemlock woolly adelgid and genetic material from these stands has been incorporated into breeding programs. Seed banks from native trees throughout the range of hemlock have been established to conserve the genetic variability of the species.
Pesticides registered for use against hemlock woolly adelgid under specific situations may change from year to year. Therefore, please search Health Canada’s Pesticide Product Information Database for currently registered pesticides and product information for use against this insect. The application of any registered product should be based on population size and applied only when necessary and against the approved life stage. It is also recommended to consult a local tree care professional. Pesticides may be toxic to humans, animals, birds, fish, and other beneficial insects. Apply registered products only as necessary and follow all directions and precautions noted on the manufacturer’s label. In some jurisdictions and situations, only a licensed professional can apply pesticides. Consulting relevant local authorities to determine local regulations that are in place is recommended.
Photos
Hemlock twig infested by hemlock woolly adelgid.
Robert L. Anderson, USDA Forest Service, www.forestryimages.org
Hemlock woolly adelgid adult with wax removed.
Michael Montgomery, USDA Forest Service, www.forestryimages.org
Hemlock woolly adelgid adult and eggs within an ovisac.
Michael Montgomery, USDA Forest Service, www.forestryimages.org
Hemlock with a yellowing crown and little to no new growth indicative of severe hemlock woolly adelgid infestation.
James Johnson, Georgia Forestry Commission, www.forestryimages.org
Hemlock with severely thinned crown indicative of severe hemlock woolly adelgid infestation.
Robert L. Anderson, USDA Forest Service, www.forestryimages.org
Selected references
Canadian Food Inspection Agency. 2020. D-07-05: Phytosanitary requirements to prevent the introduction and spread of hemlock woolly adelgid (Adelges tsugae Annand) from the United States and within Canada. 5th revision. Canadian Food Inspection Agency. Available at: https://inspection.canada.ca/plant-health/invasive-species/directives/forest-products/d-07-05/eng/1323754212918/1323754664992
Emilson, C.; Bullas-Appleton, E.; McPhee, D.; Ryan, K.; Stastny, M.; Whitmore, M.; MacQuarrie, C.J.K. 2018. Hemlock woolly adelgid management plan for Canada. Natural Resources Canada. Canadian Forest Service. Great Lakes Forestry Centre, Sault Ste. Marie, ON. Information Report GLC-X-21. 31 p.
Havill, N.P.; Montgomery, M.E.; Keena, M. 2011. Chapter 1: Hemlock woolly adelgid and its hemlock hosts: a global perspective. Pages 3–14 in B. Onken and R. Reardon, editors. Implementation and status of biological control of the hemlock woolly adelgid. U.S. Department of Agriculture, Forest Service, Forest Health Technology Enterprise Team. Morgantown, West Virginia. FHTET-2011-04.
Havill, N.P.; Montgomery, M.E.; Yu, G.; Shigehiko, S.; Caccone, A. 2006. Mitochondrial DNA from hemlock woolly adelgid (Hemiptera: Adelgidae) suggests cryptic speciation and pinpoints the source of the introduction to eastern North America. Annals of the Entomological Society of America. 99(2): 195–203. https://doi.org/10.1603/0013-8746(2006)099[0195:MDFHWA]2.0.CO;2
Havill, N.P.; Vieira, L.C.; Salom, S.M. 2014. Biology and control of hemlock woolly adelgid. U.S. Department of Agriculture, Forest Service, Forest Health Technology Enterprise Team. Morgantown, West Virginia. FHTET-2014-05. 21 p.
Limbu, S.; Keena, M.A.; Whitmore, M.C. 2018. Hemlock woolly adelgid (Hemiptera: Adelgidae): a non-native pest of hemlocks in eastern North America. Journal of Integrated Pest Management 9(1): 1–16. https://doi.org/10.1093/jipm/pmy018
MacQuarrie, C.J.K.; Gray, M.; Bullas-Appleton, E.; Kimoto, T.; Mielewczyk, N.; Neville, R.; Ogden, J.B.; Fidgen, J.G.; Turgeon, J.J. 2025. The distribution of the hemlock woolly adelgid (Hemiptera: Adelgidae) in Canada. The Canadian Entomologist. 157(11): 1–20. https://doi.org/10.4039/tce.2025.2
McAvoy, T.J.; Régnière, J.; St-Amant, R.; Schneeberger, N.F.; Salom, S.M. 2017. Mortality and recovery of hemlock woolly adelgid (Adelges tsugae) in response to winter temperatures and predictions for the future. Forests 8(12): 497. https://doi.org/10.3390/f8120497
Oten, K.L.F.; Merkle, S.A.; Jetton, R.M.; Smith, B.C.; Talley, M.E.; Hain, F.P. 2014. Understanding and developing resistance in hemlocks to the hemlock woolly adelgid. Southeastern Naturalist 13(6): 147–167. https://doi.org/10.1656/058.013.s610
Spaulding, H.L.; Rieske, L.K. 2010. The aftermath of an invasion: structure and composition of central Appalachian hemlock forests following establishment of the hemlock woolly adelgid, Adelges tsugae. Biological Invasions 12: 3135–3143. https://doi.org/10.1007/s10530-010-9704-0
Zilahi-Balogh, G.M.G.; Humble, L.M.; Lamb, A.B.; Salom, S.M.; Kok, L.T. 2003. Seasonal abundance and synchrony between Laricobius nigrinus (Coleoptera: Derodontidae) and its prey, the hemlock woolly adelgid (Hemiptera: Adelgidae). The Canadian Entomologist 135(1): 103–115. https://doi.org/10.4039/n02-059