Hemlock woolly adelgid
Hemlock woolly adelgid (Adelges tsugae), commonly called as HWA, is member of the Sternorrhyncha suborder of the Order Hemiptera and is native to East Asia that feeds by sucking sap from hemlock and spruce trees (Tsuga spp.; Picea spp.). In eastern North America, it is a destructive pest that poses a major threat to the eastern hemlock (Tsuga canadensis) and the Carolina hemlock (Tsuga caroliniana). The range of Eastern Hemlock extends north of the current range of the adelgid, but there are fears that the adelgid could spread to infect these northern areas as well. Accidentally introduced to North America from Asia in 1924, HWA was first found in the eastern United States some decades later. In Pennsylvania, for example, the earliest record is from 1966. The pest has now been established in eleven eastern states from Georgia to Massachusetts, causing widespread mortality of hemlock trees. As of 2007, 50% of the geographic range of eastern hemlock has been impacted by HWA.
The presence of HWA can be identified by its egg sacs, which resemble small tufts of cotton clinging to the underside of hemlock branches. Hemlocks stricken by HWA frequently shift to a grayish-green appearance rather than the dark green of healthy hemlocks. In North America, the hemlock woolly adelgid reproduces asexually and can have two generations per year. (In its native habitat in Asia, there is a third winged generation called Sexupera; this generation requires a species of spruce not found in the Eastern United States to reproduce, so it does not survive here.) Between 100 and 300 eggs are laid in the woolly egg sacs beneath the branches. Larvae emerge in spring and can spread on their own or with the assistance of wind, birds and/or mammals. In the nymph stage, the adelgid is immobile and settles on a single tree.
The hemlock woolly adelgid feeds on the phloem sap of tender hemlock shoots. It may also inject a toxin while feeding. The resulting desiccation causes the tree to lose needles and not produce new growth. In the northern portion of the hemlock's range, death typically occurs four to ten years after infestation. Trees that survive the direct effects of the infection are usually weakened and may die from secondary causes.
Hemlock is a vital component of the New England forest system, and is the third most prevalent tree in Vermont. Providing protection from erosion along stream banks, food for deer and wildlife, and shelter for deer in the winter, hemlock is also valued both as an ornamental and as an important source of lumber. Unlike the balsam woolly adelgid that attacked only mature balsam fir, HWA infests hemlocks of all ages. Where hemlock occurs in pure stands in that region, the most commonly observed tree species to succeed it is black (sweet) birch. In contrast, in the southern extreme of its range, hemlock does not typically occur in pure stands, but in linear riparian areas and other moist sites. Succession in these areas is affected by the presence of Rhododendron maximum which often coexists with hemlock, and, due to a combination of influences, restricts regeneration to shade and otherwise understory-tolerant plant species. Major changes in ecosystem structure and function, including hydrologic processes, are expected with the loss of hemlock.
A 2009 study conducted by scientists with the U.S. Forest Service Southern Research Station suggests the hemlock woolly adelgid is killing hemlock trees faster than expected in the southern Appalachians, and rapidly altering the carbon cycle of these forests. According to Science Daily, the pest could kill most of the region's hemlock trees within the next decade. According to the study, researchers found "hemlock woolly adelgid infestation is rapidly impacting the carbon cycle in [hemlock] tree stands", and "adelgid-infested hemlock trees in the South are declining much faster than the reported 9-year decline of some infested hemlock trees in the Northeast." In fact, as of 2007 the rate was recorded as 15.6 km/year south of Pennsylvania and 8.13 km/year (or less) in the northern section of the HWA’s range.
- "Hemlock Wooly Adelgid". Pennsylvania Department of Conservation and Natural Resources. Retrieved 2 January 2009.
- "Helping Hemlocks". Yahoo.com.
- Kok, Loke T.; Salom, Scott M., et al. "Biological Control of the Hemlock Woolly Adelgid". Virginia Tech College of Agriculture and Life Sciences, Department of Entomology.
- "Other Exotic Forest Threats - Hemlock Woolly Adelgid". Wisconsin Department of Natural Resources.
- McClure, Mark S. "Hemlock Wooly Adelgid Greenshare Factsheet". University of Rhode Island, University of Maryland Cooperative Extension.
- "Science Daily: Hemlock Trees Dying Rapidly, Affecting Forest Carbon Cycle". University of Toronto.
- Rentch, J.; Fajvan, M.A.; Evans, R.A.; Onken, B. (2008). "Using dendrochronology to model hemlock woolly adelgid effects on eastern hemlock growth and vulnerability". Biological Invasions 11 (3): 551–563. doi:10.1007/s10530-008-9270-x.