Hemlock woolly

Aphidlike insect

Orgin and how its spread

  • Hemlock woolly adelgid is native to Japan and possibly China where it is considered a common inhabitant of both forest and ornamental hemlock and spruce trees. It rarely achieves pest outbreak densities or inflicts significant damage to host trees in its native Asian habitat because natural enemies and host plant resistance help keep HWA populations in check.
  • Hemlock woolly adelgid was first detected on the east coast of North America in Richmond, Virginia, in the mid-1950s (Souto et al. 1995). Since its likely accidental introduction from southern Japan (Havill et al. 2006), HWA has spread to 18 eastern states from Georgia to Maine, devastating populations of native eastern (Tsuga canadensis) and Carolina (T. caroliniana) hemlock. HWA now covers nearly half the range of native hemlocks and appears to be spreading about 10 miles a year. It has reached its southern limit, but continues to expand its range to the west and north.
  • Hemlock woolly adelgids are spread by wind, birds, and mammals. Infested nursery stock can also be responsible for introducing this insect into a given area.


  • Causes up to 90% mortality in eastern hemlock species, which are important for shading trout streams, and provide habitat for about 90 species of birds and mammals.
  • Some adults have two pairs of wings. Their mouthparts are thread-like and about 1/16 inch long and used to suck sap. Sucking sap from young twigs retards or prevents tree growth and causes needles to turn grayish-green, and drop prematurely. The loss of new shoots and needles is highly detrimental to a tree's health. A tree may defoliate and die within several years.
  • The hemlock woolly adelgid feeds deep within plant tissues by inserting its long sucking mouthparts (stylets) into the underside of the base of hemlock tree needles. It taps directly into the tree’s food storage cells, not the sap. The tree responds by walling off the wound created by the insertion of the stylets. This disrupts the flow of nutrients to the needles and eventually leads to the death of the needles and twigs. Needles will dry out and lose color, turning gray and eventually dropping from the tree. Terminal buds will also die resulting in little to no new shoot growth. Dieback of major limbs can occur within two years and generally progresses from the bottom of the tree upward (McClure et al 2001)
  • The hemlock woolly adelgid has an impressive reproductive potential: consider that one female in the winter generation produces an average of 200 eggs which in turn mature and each female of this adult spring generation produces on average another 200 eggs each. That’s 40,000 eggs in one year, starting from one individual female! Thus, HWA populations can grow rapidly in a relatively short period of time. Heavy HWA infestations, particularly in the southern Appalachian Mountains, can kill hemlock trees in as little as four years, with older trees dying more quickly. However, for reasons still under investigation, some infested trees in parts of New England survive for 10 years or more. http://www.nyis.info/index.php?action=invasive_detail&id=24


Stephen Bambara, Extension Entomologist

Robert C. Baldwin, NCDA&CS Plant Industry Specialist

For assistance with a specific problem, contact your local North Carolina Cooperative Extension Service personnel

Published by North Carolina Cooperative Extension Service

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How does one get rid of or control them?

  • The hemlock woolly adelgid is a difficult insect to control because the fluffy white secretion protects its eggs from pesticides. A good time to attempt control it is in October when the second generation begins to develop. The insecticidal soap and the horticultural oil sprays seem to be very effective for adelgid control with minimal harm to natural predators and parasites of this pest. Trees that are heavily infested and are showing symptoms of decline should probably be sprayed. Horticultural spray oil can be applied during the winter and before new growth emerges in spring. Oil sprays may damage hemlock during the growing season, especially in dry weather. Registered pesticides containing imidacloprid or dinotefuran may be useful for specimen trees located away from water sources. These insecticides are systemic and are often applied as soil injection. Dinotefuran may be applied as a trunk spray.

Citations of this text and pictures







Cheah, C., M. E. Montgomery, S. Salom, B. L. Parker, S. Costa, and M. Skinner, 2004. Biological control of hemlock woolly adelgid. USDA For. Serv. FHTET-2004-04, Reardon, R. and B. Onken (Tech. Coordinators), 22pp.

Ellison, A. M., M. S. Bank, B. D. Clinton, E. A. Colburn, K. Elliott, C. R. Ford, D. R. Foster, B. D. Kloeppel, J. D. Knoepp, G. M. Lovett, J. Mohan, D. A. Orwig, N. L. Rodenhouse, W. V. Sobczak, K. A. Stinson, J. K. Stone, C. M. Swan, J. Thompson, B. V. Holle, and J. R. Webster. 2005. Loss of foundation species: consequences for the structure and dynamics of forested ecosystems. Frontiers in Ecology and the Environment 3:479-486.

Havill, N. P., M. E. Montgomery, G. Yu, S. Shiyake, and A. Caccone. 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 99:195-203.

McClure, M.S. 1990. Role of wind, birds, deer, and humans in the dispersal of hemlock woolly adelgid (Homoptera: Adelgidae). Environmental Entomology, 19(1) 36-43.

McClure, M.S. 1995. Biology of Adelges tsugae and its potential for spread in the Northeastern United States. Pages 16-25 in Proceedings of the First Hemlock Woolly Adelgid Review. USDA Forest Service Forest Health Technology Enterprise Team, FHTET 96-10. Charlottesville, VA.

McClure, M.S., S.M. Salom, and K.S. Shields. 2001. Hemlock woolly adelgid. FHTET-2001-03. Morgantown, WV: U.S. Department of Agriculture, Forest Service, Forest Health Technology Enterprise Team; 14 p.

Orwig, D. A. and D. Kittredge. 2005. Silvicultural options for managing hemlock forests threatened by hemlock woolly adelgid. Harvard Forest Fact Sheet.

Paradis, A., J. Elkinton, K. Hayhoe, and J. Buonaccorsi. 2008. Role of winter temperature and climate change on the survival and future range expansion of the hemlock woolly adelgid (Adelges tsugae) in eastern North America. Mitigation and Adaptation Strategies for Global Change 13:541-554.

Souto, D., T. Luther, and B. Chianese. 1995. Past and current status of HWA in eastern and Carolina hemlock stands. Pages 9-15 in Proceedings of the First Hemlock Woolly Adelgid Review. USDA Forest Service Forest Health Technology Enterprise Team, FHTET 96-10. Charlottesville, VA.

Whitmore, M. 2014 Invasives and Cold Feb 2014. Cornell University, Department of Natural Resources.