Detecting new hemlock woolly adelgid infestations at the leading edge of its range is critically important for slowing the spread of hemlock woolly adelgid. Unfortunately, hemlock woolly adelgid is difficult to detect at low population levels. The first signs of hemlock woolly adelgid are the presence of the white, woolly ovisacs on the underside of twigs, most often on the newest growth. This white, waxy wool is most easy to observe with the naked eye or through binoculars January through June. Other signs of infestation include graying and dropped needles and limb dieback.
Winter is the optimal time to detect hemlock woolly adelgid, as the ovisacs are most apparent and the leaves from adjacent deciduous trees that could interfere with observations are absent. An inexperienced observer may confuse several look-alikes with hemlock woolly adelgid. Spider sacs may look superficially similar but are constructed of much stronger fibers and are usually not closely pressed to hemlock twigs. Spittlebugs, never found in the winter, produce watery, white foam, not wooly and waxy fibers. Scale insects are common, but are found directly on the hemlock needles, not the twigs. Pine pitch and bird droppings may also confuse an untrained observer.
Currently, the two approaches for managing HWA infestations are chemical insecticides and the use of natural enemy predator species as biological control. Infested hemlock trees can be protected individually with chemical, systemic insecticides. These insecticides, typically applied as a soil drench or an injection into the soil below the organic layer or as a basal brak spray, are incorporated by sap flow into the tree’s tissues and can provide multiple years of protection from a single treatment. However, the costs associated with application, environmental safety concerns about applying insecticides near water resources, and the tremendous reproductive potential of hemlock woolly adelgidmakes this approach less feasible on a broad scale in natural areas. For insecticide guidlines for New York State see Cornell University's Crop and Pest Management Guidelines http://ipmguidelines.org/. And, consult a certified pesticide applicator.
To manage hemlock woolly adelgidat the landscape scale, researchers have been investigating the use of biological control agents. Over the last 10 years, scientists have evaluated the effectiveness of several hemlock woolly adelgid predators from Japan and the Pacific Northwest including the beetles, Sasajiscymnus tsugae, Scymnus spp., and Laricobius nigrinus as well as fungal pathogens. Some promising evidence has emerged, but further study is needed to test the effectiveness of biological control at larger geographical scales and over the long-term (Cheah et al. 2004).
Homeowners would be wise to take an integrated management approach for hemlock woolly adelgid-infested hemlock trees on their property. In lieu of systemic insecticides, spraying hemlock foliage with properly labeled horticultural oils and insecticidal soaps may be effective when trees are small enough to be saturated in order to ensure that the insecticide comes in contact with the adelgid. Owners can reduce hemlock tree stress by watering during drought periods and pruning dead and dying limbs and branches. Avoid the use of nitrogen fertilizers on infested hemlocks as it will actually enhance hemlock woolly adelgid survival and reproduction. Take care moving plants, logs, and mulch from infested to uninfested areas, particularly when hemlock woolly adelgideggs and crawlers are present (March – June). Actions such as moving bird feeders away from hemlocks and removing isolated infested trees from a woodlot may also help prevent further infestations. Please read NC State Extension's Recommendations for Hemlock Woolly Adelgid Control in the Landscape.
Woodlot owners should consult Orwig & Kittredge (2005) for available silvicultural options.
Remember, when using a pesticide, first consult your local CCE office or State pesticide guide to identify insecticides that are registered for use in your state and the proper timing for chemical application.
- 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.