Graceful, majestic and long-lived native hemlocks of the eastern United States are currently besieged from Georgia to northern New Hampshire by a tiny insect which was inadvertently introduced from Asia : the hemlock woolly adelgid, (HWA), Adelges tsugae. Hemlocks are extraordinarily shade tolerant and shaded seedlings or saplings can survive with suppressed growth for two hundred years until overstorey removal allows release and resumption of normal growth. Stands with dense canopies provide important watershed protection and thermoregulation year-round of trout streams, while also affording essential winter habitat for many mammal and avian species. The natural range of eastern hemlock, Tsuga canadensis , extends from Minnesota across southern Ontario and the Maritime states, down through the Appalachian mountains into Georgia and Alabama. In landscapes and woodlands alike, vulnerable hemlocks are in decline from the ever-spreading adelgid, which currently infests over half the natural range. Also under threat is the Carolina hemlock, Tsuga caroliana , endemic to the southern Appalachians . While western hemlock, Tsuga heterophylla and mountain hemlock, Tsuga mertensiana , are reportedly resistant to HWA attack, our native eastern species are very susceptible and can die within a few years from unchecked, heavy infestations of HWA (Fig. 1). Hemlock decline is characterized by a reduction of new shoot production in infested parts of the crown followed by needle drop and a thinning of the foliage (Fig. 2). Although first discovered and described by Annand in Oregon in 1924, HWA was initially detected in eastern USA in a private plant collection in Richmond , Virginia in the early 1950s. In just half a century, infestations now range from the Carolinas and Georgia to recent establishments in New Hampshire and Monroe County , New York , on the shores of Lake Ontario . Rates of hemlock decline are also influenced by stressors such as drought, site conditions and other major and secondary pests or diseases. Hemlock mortality is most severe in parts of Connecticut , New Jersey and Virginia.
Life History of HWA
The life history of HWA in eastern North America consists of two complete parthenogenetic (all female) generations on hemlock: the sistens generation whose development spans 9-10 months from July through to April or May of the following year, and the progrediens generation which occurs in May through June (Fig. 3). A winged adult stage also develops in early summer but cannot reproduce on hemlock or any other conifer species in North America (Fig. 3). While most insects are dormant in the winter, the sistens generation of HWA actually feeds and develops slowly to adult from October through April. Feeding via the insertion of long mouthparts through the plant tissue targets the xylem ray parenchyma cells, depleting the tree's storage reserves and reduces the ability to put out new shoots. With the exception of the mobile first instar nymph (crawler), the other damaging stages on hemlock are sessile feeders with non-functional legs. Because there is no need for sexual reproduction, HWA is capable of explosive rates of population increase, especially during the initial stages of infestation on healthy hemlocks. In both generations, waxy woolly filaments envelope and protect the insect (Fig. 4) and its eggs and give the species its characteristic appearance. The wool is most prominent in late winter, early spring through mid-summer. Eggs and crawlers (Fig. 5) are easily spread passively by wind, wildlife, especially birds, humans and infested nursery stock movement. These dispersive stages from the two generations are generally present from late March through July in the Northeast and this represents the most critical period for the spread of infestations. Crawlers which hatch in June and July, settle at feeding sites on hemlock shoots and undergo a summer dormancy where there is no development. This stage is inconspicuous and easily missed because of the lack of the pronounced woolly covering (Fig. 6).
Although native lacewing and predatory fly larvae are commonly seen feeding on HWA, these have not been effective in regulating HWA populations in the long term. The large, colorful and exotic “Halloween” ladybeetle, Harmonia axyridis (Fig. 7) has also been reported as a frequent opportunistic predator on HWA but its impact has not been quantified. Populations of HWA can also be dramatically reduced by extreme extended winter temperatures or sharp temperature fluctuations during abnormally mild winters. Winter mortality of HWA in 2003 in Connecticut has generally been 85-90% over the whole state.
Horticultural oil (2 % or lower) or insecticidal soap sprays can be applied at anytime but preferably in spring, to suffocate HWA on accessible trees but applications are costly and impractical for large scale control. Coverage needs to be thorough to be effective and needs to be repeated annually to prevent reintroductions from taking hold. These least toxic options are preferable to other pesticides as there are minimal impacts on beneficial or non-target organisms. Chemical control is also limited in its use in the forest setting, although newer chemicals such as imidacloprid (Bayer Corporation) are increasingly used for systemic treatments by arborists to control HWA. Soil drench imidacloprid applications are now available for homeowner use. However, the chief hope for long term HWA management remains biological control using introduced specialist predators from Japan , China and British Columbia , Canada . The only mass-reared predator from Japan currently available for implementation in infested eastern states is Pseudoscymnus tsugae (Coleoptera:Coccinellidae), a tiny black ladybeetle, about 2mm in length, of the Tribe Scymnini from Japan (Fig. 8). Over eight years of careful research at the Connecticut Agricultural Experiment Station, largely supported by the USDA Forest Service, has documented its life history, biology and potential as a biological control agent, including experimental field releases and short-term impact on HWA in Connecticut and Virginia . Field release sites are still being monitored and assessed three to five years after release. The predator is considered established in Connecticut where overwintering adults have been recorded yearly since its first release in 1995 and reproduction has been documented in the field. To date, nearly a million P. tsugae have been released in over 100 sites in 15 eastern states from Georgia to Maine and this species has also been recovered from release sites in other states. Other related ladybeetle predators from China (various Scymnus species), and a derodontid beetle, Laricobius nigrinus , from British Columbia, are under investigation as additional biological control agents at the USDA Forest Service, Hamden, Connecticut, and at Virginia Polytechnic and State University, respectively. Yet another avenue being explored is that of entomopathogenic fungi for biological control of HWA at the University of Vermont.
An excellent source for more information, current HWA and P. tsugae distribution maps, links to state quarantine regulations, and synopses of related studies can be found at the USDA Forest Service website at: www.fs.fed.us/na/morgantown/fhp/hwa/hwasite.
All photos and diagram by Dr. Cheah.
This article originally appeared in the June 2003 ECFLA/WDLT Newsletter.