Introduction
The hemlock woolly adelgid (HWA, Adelges tsugae) is an aphid-like, invasive insect that poses a serious threat to forest and ornamental hemlock trees (Tsuga spp.) in eastern North America. HWA are most easily recognized by the white “woolly” masses of wax, about half the size of a cotton swab, produced by females in late winter. These fuzzy white masses are readily visible at the base of hemlock needles attached to twigs and persist throughout the year, even long after the adults are dead.
Origin and 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.
HWA was first detected in New York State in the early 1980s (Souto et al. 1995). Outbreaks have expanded from initial infestations on Long Island and in the Hudson Valley to the Rochester area, the Catskill Mountains, and recently into the Finger Lakes region.
HWA was first detected on the west coast of North America in British Columbia in the 1920s, and now also has a range from northern California to southeastern Alaska. There, it occurs on both mountain hemlock (Tsuga mertensiana) and western hemlock (T. heterophylla) trees. However, HWA does not cause extensive mortality or damage on West Coast hemlocks. Recent comparative genetic analyses suggest that populations in the Pacific Northwest may actually be endemic to that region or originated from very early introductions.
Biology
The hemlock woolly adelgid has a complex life cycle, involving two different tree host species as well as asexual and sexual life stages. On eastern hemlock, HWA produces two generations a year, an overwintering generation (sistens) and a spring generation (progrediens); these two generations overlap in the spring. The progrediens has two forms, a wingless form that remains on the hemlock and a winged form (sexuparae) that flies in search of a suitable host spruce tree upon which to start a sexual reproductive cycle (McClure 1995). In New York, there are no suitable spruce, thus the winged HWA are not successful. Each generation has six stages of development: egg, four juvenile (nymph) stages, and the adult.
Overwintering adult females are black, oval, and soft-bodied (approximately 2mm long). They are usually concealed under the white woolly masses of wax (ovisacs) they secrete from special glands on their back-side. From March through May, these females lay 50 to 300 eggs in the woolly masses. The eggs are brownish-orange and very small (0.25mm long by 0.15mm wide). Depending on spring temperatures, eggs hatch from April – June.
Newly hatched nymphs – also known as crawlers – are reddish-brown with a small white fringe near the front (less than 0.5mm long). Crawlers search for suitable sites to settle, usually at the base of the hemlock needles, where they begin to feed and will remain attached to the tree with their specialized sucking mouthparts for the rest of their lives. Crawlers, an important dispersal phase of HWA on hemlocks, can be spread by wind, on the feet of birds, or in the fur of small mammals (McClure 1990). Once settled, these HWA crawlers quickly develop through the four nymph life stages, and mature in June.
Some of the adults of the spring generation (progrediens) are wingless and remain on the hemlock tree, feeding and producing eggs protected by woolly masses just like the overwintering generation, but during June-July. Their offspring hatch into crawlers, quickly settle onto hemlock branches, begin to feed and then enter a dormant period for several months until late October when feeding and development resumes. These nymphs become the next overwintering generation (sistens). The other portion of spring adults has wings and leaves the hemlock trees in June in search of spruce trees to complete the sexual phase of HWA reproduction. However, in North America, no spruce species (Picea spp.) are suitable hosts and any offspring produced die within a few days of feeding. Thus, the winged adult form can be a significant source for HWA population reduction. This is particularly important considering the number of winged adults produced in the spring generation increases with the density of overwintering adelgids, likely a result of changes in nutritional quality in the hemlock host tree.
Impacts
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.
HWA infestation resulting in thinning of hemlock Decline and mortality in infested hemlock in North Carolina Eastern hemlocks play a unique ecological role in eastern forests. Long-lived and shade tolerant, hemlocks may grow in single-species stands or in combination with deciduous hardwood species. They are frequently found growing on exposed slopes as well as protected gorges and stream bottoms. Eastern hemlocks create a cool, damp and shaded microclimate that supports unique terrestrial plant communities, maintains cool stream water temperatures for fish and stream salamanders, and provides important winter habitat structure and food resources for wildlife. Research, particularly in the hard-hit southern hemlock forests, has indicated that declines in hemlock from HWA can result in losses of unique plant and animal assemblages and drastic changes to ecosystem processes (Ellison et al. 2005).
Climate change, particularly warmer summer temperatures, will affect the suitability of habitat for eastern hemlock in the Northeast. Perhaps more troublesome are projected increases in overwintering temperatures that may promote the range expansion of HWA into more northern hemlock forests, areas previously considered unsuitable for HWA survival (Paradis et al. 2008).
Detection
Detecting new HWA infestations at the leading edge of its range is critically important for slowing the spread of HWA. Unfortunately, HWA is difficult to detect at low population levels. The first signs of HWA 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.
Light infestation of hemlock woolly adelgid Heavy infestation of hemlock woolly adelgid Winter is the optimal time to detect HWA, 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 HWA. 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. For more information about examining hemlock trees and surveying hemlock stands, please see Whitmore (2009) "Early Detection of the Hemlock Woolly Adelgid (Adelges tsugae) in Small Northeastern Hemlock (Tsuga canadensis) Woodlots"
Management
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 HWA makes 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 HWA at 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 HWA 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 HWA-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 HWA survival and reproduction. Take care moving plants, logs, and mulch from infested to uninfested areas, particularly when HWA eggs 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.
The hemlock woolly adelgid (/əˈdɛl.dʒɪd/;[1] Adelges tsugae), or HWA, is an insect of the order Hemiptera (true bugs) native to East Asia. It feeds by sucking sap from hemlock and spruce trees (Tsuga spp.; Picea spp.). In its native range, HWA is not a serious pest because populations are managed by natural predators and parasitoids and by host resistance.[2] In eastern North America it is a destructive pest that threatens the eastern hemlock (Tsuga canadensis) and the Carolina hemlock (Tsuga caroliniana). HWA is also found in western North America, where it has likely been present for thousands of years. In western North America, it primarily attacks western hemlock Tsuga heterophylla and has only caused minor damage due to natural predators and host resistance.[2] Accidentally introduced to North America from Japan, HWA was first found in the eastern United States near Richmond, Virginia, in 1951.[2] The pest is now found from northern Georgia to coastal Maine and southwestern Nova Scotia.[2] As of 2015, 90% of the geographic range of eastern hemlock in North America has been affected by HWA.[3]
An adult individual body length is typically 0.8 mm, and is oval in shape.[4] 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.[5] It may also inject a toxin while feeding.[6] 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.[7]
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.[4] In its native Asian habitat, a third winged generation called sexupera occurs; although this generation's sexual reproduction requires a species of spruce not found in the Eastern United States, and therefore dies, Between 100 and 300 eggs are laid by each individual 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, or mammals. In the nymph stage, the adelgid is immobile and settles on a single tree.[8][9]
The current leading biological control method of hemlock woolly adelgid is Sasajiscymnus tsugae, [originally called Pseudoscymnus tsugae].[10] 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.[11] From 1995 to 1997, experiments in Connecticut and Virginia found that releasing adult Sasajiscymnus tsugae beetles into infested hemlock stands resulted in a 47 to 88% reduction in adelgid densities within 5 months of introduction.[11] The beetle's lifecycle is in parallel to the lifecycle of the hemlock woolly adelgid. Both lay eggs in the spring and hatching occurs nearly simultaneously. When hatched, S. tsugae larvae are highly mobile and feed on hemlock woolly adelgid eggs and larvae. Each S. tsugae larva can effectively consume about 500 adelgid eggs or nearly 100 developing adelgid nymphs.[11]
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.[10] L. nigrinus adults lay their eggs on top of wintering adelgid larvae in early spring, and upon hatching, the larval beetles feed on hemlock woolly adelgid.[10]
Also under study is Laricobius osakensis from Japan, a relative of L. nigrinus. They have shown promise in field trials.[12]
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.[13]
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,[13] and restraint exercised around bodies of water.
Soil drenches/soil injections/bark sprays are used in larger trees that cannot be completely sprayed with insecticidal soaps or foliage insecticides. The most common insecticide is imidacloprid, which can be effective for several years if absorbed through the soil.[14] 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.
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.[15] 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, and because of 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.[5] The moist environment is preferred by many native amphibian species, particularly newts and salamanders.[16] Some species of birds have close association with the hemlock, especially during mating and nesting periods.[17] 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.[18] 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.[19] 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 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."[20] 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.[21]
Bays Mountain Park, Sullivan County, Tennessee. With lacewing (Chrysopidae egg) 
closeup The hemlock woolly adelgid (/əˈdɛl.dʒɪd/; Adelges tsugae), or HWA, is an insect of the order Hemiptera (true bugs) native to East Asia. It feeds by sucking sap from hemlock and spruce trees (Tsuga spp.; Picea spp.). In its native range, HWA is not a serious pest because populations are managed by natural predators and parasitoids and by host resistance. In eastern North America it is a destructive pest that threatens the eastern hemlock (Tsuga canadensis) and the Carolina hemlock (Tsuga caroliniana). HWA is also found in western North America, where it has likely been present for thousands of years. In western North America, it primarily attacks western hemlock Tsuga heterophylla and has only caused minor damage due to natural predators and host resistance. Accidentally introduced to North America from Japan, HWA was first found in the eastern United States near Richmond, Virginia, in 1951. The pest is now found from northern Georgia to coastal Maine and southwestern Nova Scotia. As of 2015, 90% of the geographic range of eastern hemlock in North America has been affected by HWA.
