Adelges tsugae (hemlock woolly adelgid)
- Summary of Invasiveness
- Taxonomic Tree
- Distribution Table
- Habitat List
- Hosts/Species Affected
- Host Plants and Other Plants Affected
- Biology and Ecology
- Natural enemies
- Means of Movement and Dispersal
- Pathway Causes
- Pathway Vectors
- Plant Trade
- Impact Summary
- Threatened Species
- Risk and Impact Factors
- Prevention and Control
- Principal Source
- Distribution Maps
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PicturesTop of page
IdentityTop of page
Preferred Scientific Name
- Adelges tsugae Annand
Preferred Common Name
- hemlock woolly adelgid
Other Scientific Names
- Chermes tsugae Annand
International Common Names
- French: puceron lanigère de la pruche
Local Common Names
- Germany: Laus, Hemlockstannen; Tannenlaus, Hemlocks-
- ADLGTS (Adelges tsugae)
Summary of InvasivenessTop of page
The hemlock woolly adelgid (Adelges tsugae) is a small, aphid-like insect that has become a serious pest of eastern hemlock and Carolina hemlock. The most obvious sign of infestation is the presence of white, woolly egg masses on the underside of hemlock needles. Infested eastern North American hemlocks defoliate prematurely and will eventually die if left untreated. A. tsugae is a difficult insect to control as the white waxy secretion protects it from pesticides. It is dispersed to new habitats through the nursery trade and locally by wind, birds, mammals and humans. Hemlock trees provide important habitats for many wildlife species and A. tsugae has severe adverse ecological impacts which will become more severe as its distribution expands. Explanations for the rapid and invasive expansion of A. tsugae include a lack of natural enemies, a lack of resistance or tolerance to infestation by eastern hemlock and Carolina hemlock, and large reproductive output (Trotter and Shields, 2009).
Taxonomic TreeTop of page
- Domain: Eukaryota
- Kingdom: Metazoa
- Phylum: Arthropoda
- Subphylum: Uniramia
- Class: Insecta
- Order: Hemiptera
- Suborder: Sternorrhyncha
- Unknown: Aphidoidea
- Family: Adelgidae
- Genus: Adelges
- Species: Adelges tsugae
DescriptionTop of page
Adelges tsugae is a small (0.74mm), reddish-purple, aphid-like insect that covers itself with a white, waxy secretion. Both winged and wingless forms are present. Their mouthparts are thread-like and about 1.5mm long and used to suck sap. Eggs are brownish-orange but darken as the embryo matures. When the eggs hatch, reddish-brown crawlers move about actively in search of a suitable site to settle. The tiny crawlers can only be seen with a hand lens as they are barely visible to the naked eye. Once the crawlers settle, they insert their mouthparts into the plant at the base of the hemlock needles and remain in the same place for the duration of their life. Dormant first instar nymphs are black with a white fringe around the edge and down the centre of the back. The developing nymphs produce white, cottony, waxy tufts that cover their bodies. The white masses are 3mm or more in diameter. The presence of these masses on the twigs and bark of hemlock is a sure sign of A. tsugae.
DistributionTop of page
Native range: China and Japan. A. tsugae were first described in western North America in 1924 and may be native to western North America (Havill and Footitt, 2007; US Forest Service, 2012).
Introduced range: Unintentionally introduced to eastern USA from Japan sometime before 1951, when it was first documented on ornamental hemlocks in Richmond, Virginia. Initially a pest of ornamental hemlock, but in the 1980s, A. tsugae was recognized as a threat to eastern and Carolina hemlocks as populations moved north through New Jersey, Long Island, and southern Connecticut and ultimately all of New England (Trotter and Shields, 2009). For distribution maps see the US Forest service website on hemlock woolly adelgid.
A. tsugae have moved south almost twice as fast as they have moved north, and some have linked this with a potential role of low temperatures in limiting the expansion of populations (Trotter and Shields, 2009).
Distribution TableTop of page
The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.Last updated: 24 Jun 2020
|Continent/Country/Region||Distribution||Last Reported||Origin||First Reported||Invasive||Reference||Notes|
|Canada||Present||Present based on regional distribution.|
|-British Columbia||Present||Introduced||Invasive||First reported: 1920s|
|-Ontario||Present, Few occurrences|
|United States||Present||Present based on regional distribution.|
|-Connecticut||Present||Introduced||Invasive||First reported: 1980s|
|-Delaware||Present||Introduced||Invasive||First reported: 1991-2002|
|-District of Columbia||Present||Introduced|
|-Georgia||Present||Introduced||Invasive||First reported: 1991-2002|
|-Idaho||Present||Introduced||Invasive||First reported: 1920s|
|-Maine||Present||Introduced||Invasive||First reported: 1920s|
|-Maryland||Present||Introduced||Invasive||First reported: 1991-2002|
|-Massachusetts||Present||Introduced||Invasive||First reported: 1980s|
|-New Hampshire||Present||Introduced||Invasive||First reported: 1985-2002|
|-New York||Present||Introduced||Invasive||First reported: 1968-1985|
|-Oregon||Present||Introduced||Invasive||First reported: 1920s|
|-Pennsylvania||Present||Introduced||Invasive||First reported: 1960s|
|-Rhode Island||Present||Introduced||Invasive||First reported: 1991-2002|
|-South Carolina||Present||Introduced||Invasive||First reported: 1991-2002|
|-Tennessee||Present||Introduced||Invasive||First reported: 1991-2002|
|-Virginia||Present||Introduced||Invasive||First reported: 1950s|
|-Washington||Present||Introduced||First reported: 1920s|
|-West Virginia||Present||Introduced||Invasive||First reported: 1991-2002|
HabitatTop of page
The secondary host of Adelges tsugae is hemlock (where the asexual cycle occurs), and spruce is the primary host where the sexual cycle occurs. Experts hope that an aversion to cold weather will slow or stop its northern movement. In New Jersey and Connecticut, a large population of Adelges tsugae died as a result of a cold period in the winter of 2000.
Habitat ListTop of page
|Terrestrial||Managed||Urban / peri-urban areas||Present, no further details||Harmful (pest or invasive)|
|Terrestrial||Natural / Semi-natural||Natural forests||Present, no further details||Harmful (pest or invasive)|
|Terrestrial||Natural / Semi-natural||Riverbanks||Present, no further details||Harmful (pest or invasive)|
Hosts/Species AffectedTop of page
Some of the adults produced during the spring generation are winged individuals that are unable to reproduce on hemlock, therefore they leave the hemlock tree in search of spruce, the alternate host. But because no suitable spruce host is available in North America, they soon die. Hemlocks growing in poor conditions (compacted soils, ledgy soils, poor drainage, drought prone, etc.) are much more likely to succumb within 3-5 years from invasion. Hemlocks growing under better growing conditions have been shown to withstand infestations longer.
Host Plants and Other Plants AffectedTop of page
|Picea abies (common spruce)||Pinaceae||Unknown|
|Picea engelmannii (Engelmann spruce)||Pinaceae||Unknown|
|Picea glauca (white spruce)||Pinaceae||Unknown|
|Picea jezoensis (Yeddo spruce)||Pinaceae||Unknown|
|Picea mariana (black spruce)||Pinaceae||Unknown|
|Picea pungens (blue spruce)||Pinaceae||Unknown|
|Picea sitchensis (Sitka spruce)||Pinaceae||Unknown|
|Tsuga canadensis (eastern hemlock)||Pinaceae||Unknown|
|Tsuga caroliniana (Carolina hemlock)||Pinaceae||Unknown|
|Tsuga sieboldii (Japanese hemlock)||Pinaceae||Unknown|
Biology and EcologyTop of page
The hemlock woolly adelgid (Adelges tsugae) utilizes hemlock as its secondary host, where the asexual cycle occurs and spruce as its primary host, where the sexual cycle occurs. In North America the sexual cycle has not been previously recorded. Young twigs are the preferred feeding sites. Immature nymphs and adults feed on trees by sucking sap from the twigs. They attach themselves just below the base (abscission layer) of newly developed needles and feed on xylem parenchyma cells (tissue that manufactures and stores plant food).
A. tsugae has both sexual and asexual (parthenogenic) reproduction. In its native range the asexual cycle occurs on hemlock while the sexual stage occurs on spruce. However, because of a lack of suitable spruce species to act as a host, in its introduced range, A. tsugae has two asexual generations per year on hemlock (Trotter and Shields, 2009). Each adult can produce between 50-300 eggs within its lifetime. The large reproductive output of hemlock woolly adelgid contributes towards its invasive expansion (see also ‘Summary of Invasiveness’ section for explanations of invasiveness).
Spring generation adults lay numerous eggs within large, white, woolly sacs. The eggs hatch and crawlers from the second generation move to attach themselves to new needles. Once they find a site, they settle and become dormant (no feeding) until fall (autumn). At that time, they end their dormancy and begin to feed and develop through the fall and winter. A. tsugae is atypical of most insect species in that it is inactive for much of the growing season and very active throughout the winter.
Natural enemiesTop of page
Means of Movement and DispersalTop of page
Introduction pathways to new locations
Nursery trade:Adelges tsugae was inadvertently shipped to Maine from Connecticut on untreated nursery stock in 1999.
Other:Adelges tsugae was accidentally introduced to the North American continent earlier this century (McClure 1987).
Local dispersal methods
Garden escape/garden waste:
Hikers' clothes/boots: Humans can disperse the insect through recreational activities.
Horticulture (local):Adelges tsugae can be spread via infested nursery stock (Ward et al. 2004).
On animals: The eggs and nymphs can be readily dispersed by birds, deer and other mammals.
On animals (local): The eggs and nymphs can be readily dispersed by birds.
On animals (local): The eggs and nymphs, which are not attached to the plant via mouthparts can be readily dispersed by wind.
People sharing resources (local): Humans can also disperse the insect through various activities, including logging and recreational activities.
Road vehicles: Humans can disperse the insect through recreational activities.
Pathway CausesTop of page
Pathway VectorsTop of page
Plant TradeTop of page
|Plant parts liable to carry the pest in trade/transport||Pest stages||Borne internally||Borne externally||Visibility of pest or symptoms|
Impact SummaryTop of page
ImpactTop of page
The hemlock woolly adelgid (Adelges tsugae) is damaging hemlock ecosystems in eastern North America where both eastern hemlock (see Tsuga canadensis in IUCN Red List of Threatened Species) and Carolina hemlock (T. caroliniana) serve as hosts. To date, approximately 25% of the 1.3 million hectares of host type has been infested. The entire range of eastern hemlock is at risk within the next 20 to 30 years. Immature nymphs as well as adults damage trees by sucking sap from the twigs. The trees lose vigour and prematurely drop their needles, to the point of defoliation, which may lead to death. If left uncontrolled, the adelgids can kill a tree within three to four years. Trees of all sizes and ages are attacked, but natural stands of hemlock are at greatest risk for death. The value of ornamental hemlocks is reduced by the presence of the dirty, white woolly masses attached to the twigs or base of needles. Eastern hemlock is economically important in several areas of the eastern United States. The nursery industry in North Carolina and Tennessee currently maintains approximately $34 million in hemlock growing stock. This industry is feeling the effects of A. tsugae in reduced sales of native hemlock for ornamental use.
Hemlock trees are ecologically important and provide a unique environment. Eastern hemlock trees can reach a lifespan of 900 years and this tree is a component of many old growth communities. The hemlock forest also provides nesting sites and a foraging habitat for Neotropical migratory bird species. For example, the Acadian flycatcher (Empidonax virescens) uses Eastern hemlock for nesting and populations are thought to be negatively affected by A. tsugae infestation in Appalachian riparian forests (Allen et al., 2009). In addition, lepidopterans, such as the hemlock angle (Semiothisa fissinotata), which feed solely on hemlocks, will be affected. Several threatened or endangered species of flora and fauna require hemlock forests to survive. These forests are normally stable and resistant to plant invasions, but the loss of hemlocks from such forests will greatly affect the microclimate and soil conditions. Large-scale hemlock die-offs will affect species diversity, vegetation structure, stand environmental conditions and ecosystem processes. Numerous studies have suggested that A. tsugae and associated loss of hemlock trees can impact upon ecosystem processes such as nitrogen and carbon cycling (see Cobb, 2010; Nuckolls et al., 2009; Orwig et al., 2008; Hadley et al., 2008). The loss of hemlock trees from the forests can lead to increased dominance of deciduous hardwood species, which may in turn have ecosystem effects (Spalding and Rieske, 2010).
Threatened SpeciesTop of page
|Threatened Species||Conservation Status||Where Threatened||Mechanism||References||Notes|
|Tsuga canadensis (eastern hemlock)||LC (IUCN red list: Least concern)||Canada; USA||Herbivory/grazing/browsing||ISSG (2011)|
|Plethodon shenandoah (Shenandoah salamander)||VU (IUCN red list: Vulnerable); USA ESA listing as endangered species||Virginia||Ecosystem change / habitat alteration||US Fish and Wildlife Service (1994)|
Risk and Impact FactorsTop of page
- Proved invasive outside its native range
- Ecosystem change/ habitat alteration
- Host damage
- Negatively impacts forestry
- Threat to/ loss of endangered species
- Threat to/ loss of native species
Prevention and ControlTop of page
Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.
Preventative measures: Certain species of Asian hemlocks are resistant to A. tsugae, and nurseries are beginning to grow and sell those in the United states. The western hemlock (Tsuga heterophylla) is highly resistant to A. tsugae, but it does not grow well outside of its native range of the northwestern United States. Care should be taken when moving trees, logs or firewood from infested areas to uninfested areas (see 'Cultural measures'). Selective removal of heavily infested trees from the immediate vicinity will retard the establishment of new infestations.
Remote sensing may aid earlier detection of A. tsugae infestation; spectral signatures of infestation become apparent before needle discolouration is visible (Cook et al. 2010).
Legal measures: A number of states in the USA impose strict quarantine measures to prevent the further spread of A. tsugae. States with existing quarantines include Maine, New Hampshire, Vermont and Michigan (Ward et al. 2004).
Cultural measures: It is imperative to maintain the health of threatened trees, as a healthy tree can tolerate higher densities of A. tsugae (McClure, 1995; in Ward et al. 2004). This can be done by simple steps such as mulching to maintain soil moisture and irrigation during dry periods. Quarantines should be observed to limit movement of hemlock products such as logs, firewood, and seedlings from infested areas (Ward et al. 2004). Nitrogen fertilizer should not be applied as it enhances A. tsugae survival and reproduction (McClure, 1995; in Ward et al. 2004).
Silvicultural measures: There are a range of silvicultural options available for management of infested hemlock-dominated forests: 1) do nothing, 2) light selection cut/shelterwood cut, 3) high intensity cutting. For hemlock-hardwood or hemlock-conifer mixes, the options include 1) do nothing, or 2) cut hemlock in groups or throughout the stand. Orwig and Kittredge (2005) provide a comprehensive guide for silvicultural management options.
Fajvan (2007) is developing silvicultural thinning guidelines – it is hoped that thinning can play a part in increasing hemlock survivability, since with reduced stand density, the hemlock may re-allocate resources towards increased vigour, which could reduce impacts of a subsequent invasion of A. tsugae.
Salvage cutting is sometimes used - harvesting trees which are dead or dying to recover their economic value (Ward et al. 2004).
Chemical treatment: The high cost of treatment limits insecticide use to the protection of individual high value trees. Applications need to be repeated at regular intervals to be effective (Ward et al. 2004). However, because A. tsugae can damage trees so quickly, it is important to detect infestations early and to implement a management program immediately. In the landscape and nursery industry, A. tsugae is very manageable if the problem is addressed early and aggressively. The black nymphs are most susceptible to control tactics when they are exposed on new growth, before they secrete their white, protective wax.
One of the best control tactics is the use of horticultural oils. These can be applied as either a dormant oil spray or as a summer spray. In order for these to be effective, they must be applied on the entire plant and may need to be reapplied later in the growing season. Oil is also effective against rust mites, spider mites and armoured scales, while being nontoxic to certain predators (Cowles and Abbey, 1999; in Ward et al. 2004). However, this is impractical for tall trees or large areas (Ward et al. 2004).
Another product that has been shown to achieve very good results is a chemical pesticide known as imidacloprid (Merit). It works best as a systemic and can be introduced into the plant via soil or trunk injection. Webb et al. (2003) discovered that hemlocks recovered quickly following applications of imidacloprid. Cowles et al. (2005) found that trunk injection methods were less effective for control of A. tsugae than near-trunk soil placement of imidacloprid, and they state that insecticide treatment should be considered a "stop-gap measure to preserve trees of exceptional value until such time that biological control becomes established". Soil application can provide multiple years benefit, but there is concern about the impact on aquatic and soil organisms. Trunk injection is better suited for areas where there is special concern for minimising imidacloprid release to the environment, but protection is shorter-lived than soil application (Ward et al. 2004). Cowles provides recommendations for soil applications of imidacloprid: "apply imidacloprid (1) as a shallow subsurface injection, (2) dispersed near and around the tree’s base, (3) using the lowest effective labelled rate (0.75 g active ingredient per inch d.b.h.), and, most importantly, (4) when the soil is moist but not saturated. Currently, some managers keep a 50-foot buffer zone around streams, within which trees may only be treated via trunk injection".
Insecticidal soap, malathion or diazinon can be used effectively for control if the infested tree is thoroughly drenched with the pesticide once or twice a year. Pyrethroids are effective against A. tsugae but may prevent establishment of specialist predatory beetles released for biological control (Ward et al. 2004).
Biological control: Because chemical control is not practical in a forest setting, research is underway to evaluate predators imported to the United States from the native habitat of A. tsugae.
The ladybird beetle (Sasajiscymnus (Pseudoscymnus) tsugae) and the oribatid mite (Diapterobates humeralis) are effective predators in Japan. The ladybird beetle has been reared and released in Connecticut, New Jersey, Virginia and others, and demonstrates excellent potential. The USDA Forest Service is spearheading a mass-rearing and release program in National Forests in the eastern United States. As of 2004, nearly one million beetles had been released in more than 100 sites in 15 states from Georgia to Maine (Ward et al. 2004). It has been correlated with a recovery of eastern hemlocks in Connecticut (Cheah et al. 2005).
Another ladybird beetle, Scymnus sinuanodulus, collected in China is also being evaluated. A third beetle species (Laricobius nigrinus), from British Columbia (Ward et al. 2004), is under study at the Virginia Tech Department of Entomology as a host-specific predator of A. tsugae in western North America. Both S. sinuanodulus and L. nigrinus are currently being evaluated in field studies and show good potential (see USDA, 2012; Zilahi-Balogh et al., 2005). L. nigrinus is a predator endemic to western North America; the beetle's life history is synchronous with the spring generation of A. tsugae. After analysing results from releases, Mausel et al. (2010) recommended larger release sizes in cold areas where establishment probability was low. It has also been found that L. nigrinus individuals from inland areas appear to be more cold tolerant and would therefore be preferable for release in the colder portions of A. tsugae's invaded range compared with coastal beetles (Mausel et al., 2011). Another Lariocobus species (L. osakensis) has been investigated - a host range study (Vieira et al., 2011) has indicated that L. osakensis is a specific predator of A. tsugae, and is not a threat to non-target species, supporting its potential as a biological control agent.
Other possibilities include the use of an insect-killing fungi, which has shown some promise (see Costa et al. (undated)), and a study by Shields and Hirth (2005), which reports the existence of endosymbiotic bacteria in A. tsugae, and the death of the insect following treatment with antibiotic.
Integrated management: Rehabilitation of affected areas: Hemlock forests affected by A. tsugae are often replaced by species such as maple and birch, resulting in a quite different forest. For example, Dhungel et al. (2011) suggested that partial canopy removal resulting from the loss of T. canadensis due to hemlock woolly adelgid may benefit aggressive tree species such as striped maple (A. pensylvanicum). Restoration efforts can begin before salvage operations by encouraging regeneration of desirable species, such as white pine. It is also important to limit the spread of invasive alien species when restoring damaged stands. Of particular concern are burning bush (Euonymus alata) and Japanese barberry (Berberis thunbergii). Control of invasive species is best done by eradicating established plants before hemlock mortality. See Ward et al. 2004 for more detail.
BibliographyTop of page
References from GISD
Asaro, C., Berisford, C.W., Montgomery, M.E., Rhea, J. and Hanula, J. 2005. Biological control of the hemlock woolly adelgid in the Southern Appalachians. In: Onken, B. and Reardon, R. (Compilers). 2005. Third Symposium on Hemlock Woolly Adelgid in the Eastern United States. Asheville, North Carolina. February 1-3, 2005.
Cheah, C.A.S-J., Mayer, M.A., Palmer, D., Scudder, T. and Chianese, R. 2005. Assessments of biological control of hemlock woolly adelgid with Sasajiscymnus tsugae in Connecticut and New Jersey. In: Onken, B. and Reardon, R. (Compilers). 2005. Third Symposium on Hemlock Woolly Adelgid in the Eastern United States. Asheville, North Carolina. February 1-3, 2005.
Conway, H., Culin, J.D. and Hedden, R. 2004. Introduced biological control agents for hemlock woolly adelgid (HWA). Clemson University Entomology Insect Information Series.
Cowles, R.S. Soil application of imidacloprid to control hemlock woolly adelgid: best management practices. USDA Forest Service, Hemlock Woolly Adelgid website: Chemical control.
Cowles, R.S., Cheah, C. S-J. and Montgomery, M.E. 2005. Comparing systemic imidacloprid application methods for controlling hemlock woolly adelgid. In: Onken, B. and Reardon, R. (Compilers). 2005. Third Symposium on Hemlock Woolly Adelgid in the Eastern United States. Asheville, North Carolina. February 1-3, 2005.
Evans, R.A. 2002. Hemlock ecosystems and Hemlock Woolly Adlegid at Delaware Water Gap National Recreation Area.
Felton, K. and Onken, B. 2006. Life stages of the hemlock woolly adelgid in the northern range. Prepared by USDA Forest Service, Forest Health Protection, Morgantown, West Virginia.
Havill, N.P. 2005. Using mitochondrial DNA to determine the native range of the Hemlock Woolly Adelgid. In: Onken, B. and Reardon, R. (Compilers). 2005. Third Symposium on Hemlock Woolly Adelgid in the Eastern United States. Asheville, North Carolina. February 1-3, 2005.
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Mark S. McClure. 1998. Hemlock Woolly Adelgid, Adelges tsugae (Annand), The Connecticut Agricultural Experiment Station.
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McClure, M.S. Undated. Hemlock Woolly Adelgid, Adelges tsugae (Annand). The Connecticut Agricultural Experiment Station.
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Orwig, D.A. and Kittredge, D. 2005. Silvicultural options for managing hemlock forests threatened by hemlock woolly adelgid. In: Onken, B. and Reardon, R. (Compilers). 2005. Third Symposium on Hemlock Woolly Adelgid in the Eastern United States. Asheville, North Carolina. February 1-3, 2005.
Pais, R.C. and Polster, K.M. Undated. Strategies for Managing Hemlock Woolly Adelgid (Adelges tsugae) in Forests. EcoScientific Solutions LLC, 930 Meadow Avenue, Suite 2B, Scranton, PA 18505.
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Salom, S. M., 1999. Virginia Tech Department of Entomology. Hemlock Woolly Adelgid: A Major Threat to Eastern Hemlock, Virginia Forest Landowner Update. Spring 1999 -- Volume 13, No. 2 .
Salom, S.M. 1999. Hemlock Woolly Adelgid: A Major Threat to Eastern Hemlock. Virginia Forest Landowner Update. Spring 1999. 13 (2). Virginia Tech Department of Entomology.
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Salom, S.M., McClure, M.S., Montgomery, M.E., Zilahi-Balogh, G., and Kok, L.T. 2001. Hemlock woolly adelgid in the United States: status of on-going biological control efforts. pp. 87-97. In: IUFRO World Series Vol. 11. Protection of world forests from insect pests: advances in research. Proceedings of the XXI IUFRO World Congress 7-12 August 2000, Kuala Lumpur, Malaysia
Snyder, C.D., Young, J.A., Ross, R.M. and Smith, D.R. 2005. Long-term effects of hemlock forest decline on headwater stream communities. In: Onken, B. and Reardon, R. (Compilers). 2005. Third Symposium on Hemlock Woolly Adelgid in the Eastern United States. Asheville, North Carolina. February 1-3, 2005.
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ReferencesTop of page
Allen MC, Sheehan J Jr, Master TL, Mulvihill RS, 2009. Responses of Acadian Flycatchers (Empidonax virescens) to Hemlock Woolly Adelgid (Adelges tsucae) infestation in Appalachian riparian forests. Auk, 126(3):543-553. http://www.bioone.org/doi/full/10.1525/auk.2009.08073
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Dhungel EK, Groninger JW, Holzmueller EJ, 2010. Tree species and environment associations within hemlock-silverbell stands treated for hemlock woolly adelgid in Great Smoky Mountains National Park. Journal of the Torrey Botanical Society, 137(4):401-409. http://www.torreybotanical.org/journal.html
Fajvan MA, 2008. The role of silvicultural thinning in eastern forests threatened by hemlock woolly adelgid (Adelges tsugae). General Technical Report - Pacific Northwest Research Station, USDA Forest Service [Integrated restoration of forested ecosystems to achieve multiresource benefits: Proceedings of the 2007 National Silviculture Workshop, Ketchikan, Alaska, USA, 7-10 May 2007.], No.PNW-GTR-733:247-256.
Forest Service US, 2012. Northeastern Area. Forest Health Protection - Hemlock Woolly Adelgid. Distribution Maps. http://na.fs.fed.us/fhp/hwa/maps/distribution.shtm.
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ContributorsTop of page
Reviewed by: Gabriella (Riella) Zilahi-Balogh, PhD Entomologist/Entomologiste Programme Officer, Plant Health/agent de programme, protection des végétaux Canada
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