Hemlock woolly adelgid

An adult individual body length is typically 0.8 mm, and is oval in shape. The tiny brown-colored insect has four thread-like stylets that are bundled together and function as a mouthpart. Three times the length of its body, the stylet bundle pierces the host plant's parenchymatic ray tissue to derive nutrition from stored reserves. It may also inject a toxin while feeding. The resulting desiccation causes the tree to lose needles and not produce new growth. Hemlocks stricken by HWA frequently become grayish-green rather than a healthy dark green. In the northern portion of the hemlock's range, death typically occurs 4 to 10 years after infestation. Trees that survive the direct effects of the infection are usually weakened and may die from secondary causes. The presence of HWA can be identified by its egg sacs, which resemble small tufts of cotton clinging to the underside of hemlock branches. In North America, the hemlock woolly adelgid asexually reproduces and can have two generations per year. Both generations are parthenogenetic and exclusively female. ==Control methods==

Forest level The current leading biological control method of hemlock woolly adelgid is Sasajiscymnus tsugae, [originally called Pseudoscymnus tsugae]. S. tsugae is a black lady beetle that is relatively host-specific, feeding only on three known aldegid species, including HWA. This beetle was discovered in 1992 while feeding on hemlock woolly adelgid in its natural range of Japan. Since 1995, the Pennsylvania Department of Conservation and Natural Resources's Bureau of Forestry has released hundreds of thousands of adult S. tsugae beetles into affected hemlock forests of the eastern United States to determine its effectiveness at controlling the spread of the adelgid. Laricobius nigrinus is another predatory beetle used as a biological control in response to hemlock woolly adelgid. Native to the western United States and Canada, L. nigrinus is known to prey exclusively on various woolly adelgids. Other natural predators of adelgids include aphid fly larvae (family Chamaemyiidae), certain midge larvae, green lacewings (family Chrysopidae) and brown lacewings (family Hemerobiidae). Individual trees The environmentally safest chemical control methods for treating individual trees are nontoxic insecticidal soap and horticultural oil. These are sprayed on the foliage and smother the insects as they dry. Most trees need to be treated on a yearly basis. Toxic systemic insecticides may be applied to the foliage and bark of a tree and can persist in killing the adelgid for up to four years after application. Caution must be used, Tree roots absorb and transport the product into the foliage and kill hemlock woolly adelgid. Soil drenches must be applied when soil moisture is adequate for the tree roots to absorb the product. These products should not be used in close proximity to bodies of water. Trunk injections are used for large trees that are near water or where soils are too rocky for soil injections or drenches. The chemical is injected directly into the tree and transported to the twigs and needles where the hemlock woolly adelgids are feeding. Adequate soil moisture is also necessary for the tree to take up these products. ==Significance==

Hemlock is a vital component of the New England forest system, and is the third-most prevalent tree in Vermont. It provides protection from erosion along stream banks, food for deer and wildlife, and shelter for deer in winter. The tree 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 hemlocks occur in pure stands in that region, the most commonly observed tree species to succeed it is black (sweet) birch. In the southern extreme of its range, hemlock typically occurs not 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, because 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. Loss of the eastern and Carolina hemlock from hemlock woolly adelgid infestation will likely result in many ecological shifts in eastern North America. The understory of hemlock forests is characterized as dark, damp, and cool and is an ideal habitat for various other organisms. Some species of birds have close association with the hemlock, especially during mating and nesting periods. Aquatic systems adjunct to hemlock stands are also affected by the trees' decline. Brook trout is a native fish species to the eastern United States and is known to prefer the cool, shaded streams of hemlock forests during spawning events. Vulnerable animal populations are expected to diminish as a result of loss of hemlock habitat to the invasive hemlock woolly adelgid. One factor giving hope is that the adelgid does not seem able to survive prolonged or bitter cold. Following the winter of 1999–2000, a considerable dieback of adelgids and subsequent regrowth of infested trees was observed across Connecticut. The same phenomenon was repeated after the prolonged winter of 2013–2014, in time to save numerous nearly succumbed forests. 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 ScienceDaily, 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 of HWA expansion 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. == References ==