Hemlock Woolly Adelgid

Hemlock woolly adelgid (Adelges tsugae), commonly abbreviated as HWA, is a true bug native to East Asia that feeds by sucking sap from hemlock trees (Tsuga 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 to these northern areas too. 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 1967. 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 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.

Few options are available for control of this pest. The adelgids can be physically removed from the tree, either by vigorous washing or by cutting off infected branches. Various petrochemical pesticides are available for home use or by a licensed arborist, including diazinon, fluvalinate, imidacloprid and malathion. The use of pesticides is limited because hemlocks tend to grow within close proximity to aquatic environments such as rivers or streams, and the risk of contaminating these vulnerable ecosystems is great. Furthermore, the large-scale use of pesticides is often denounced in forest settings. Horticultural oil and insecticidal soap are another option.

Since 1997, the main approach has turned to biological control. The use of entomopathogenic fungi and certain insects as predators specific to the adelgid have potential. Several species of the coleopteran genus Laricobius, a kind of winged insect, are being tested, and the coccinellid Pseudoscymnus tsugae (a beetle) an effective predator of the Hemlock woolly adelgid in Honshū, Japan. Pseudoscymnus has been released in several states in the northeastern US. There is also evidence that another beetle, Harmonia axyridis, may be a predator of adelgids.

Scientists have also investigated the ability of Asian hemlocks to be resistant or deterrent to HWA. Mitochondrial DNA analysis suggests the western species of hemlock (Tsuga heterophylla and T. mertensiana), as well as Asian species (T. chinensis, T. dumosa, T. diversifolia and T. sieboldii), show long evolutionary history with Adelges tsugae (HWA) and therefore are not damaged by the adelgid. Scientists have successfully hybridized foreign hemlock species with the eastern North American hemlocks, maintaining the aesthetic look and canopy structure similar to the native eastern hemlocks, but retaining the resistance to HWA seen in foreign hemlocks. Scientists found that by hybridizing Tsuga chinensis with T. caroliniana, the resistance traits were inherited, thus adelgids were less likely to settle on the plants, where they expressed a slower growth and survival rate.