Hemlock Woolly Adelgid

NOTE: this pest is not known to spread in or on firewood. It is included in the Gallery of Pests for general information purposes only.

The hemlock woolly adelgid (Adelges tsugae) was first reported in the 1920s, when it was observed attacking western hemlock (Tsuga heterophylla) in the Pacific Northwest (Annand, 1924). A separate introduction from the Osaka region of Japan (Montgomery and Yu 2008) established the pest in Virginia by the 1950s (https://www.na.fs.fed.us/fhp/hwa/; accessed January 2010), from whence it spread to attack eastern and Carolina hemlocks (T. canadensis and T. caroliniana; USDI NPS EIS, 2000).

While western hemlocks (Tsuga heterophylla and T. mertensiana) sustain attacks from the adelgid with little damage (https://www.dnr.state.wi.us/forestry/FH/exotics/; accessed January 2010), Eastern and Carolina hemlock populations succumb. The hemlock woolly adelgid (HWA) spread first to the Northeast, where it quickly reached southern New York and New England. By the end of the 20th Century, the adlegid had begun to spread south along the mountains; it has now reached northern Georgia.  It has also spread westward into Kentucky and Tennessee; to the Ohio borders with West Virginia and Pennsylvania; and across New York state (https://na.fs.fed.us/fhp/hwa/maps/2012.pdf; accessed June 2013). It is thought that cold temperatures in the winter are slowing the insect’s spread farther north – although warming winters in the region will presumably reduce this protection.

No-one has compiled a range-wide summary of hemlock decline in response to hemlock woolly adelgid. By the end of the 20th Century, extensive decline and mortality had been reported within 10 years of the first detection of the pest in Virginia, Pennsylvania, New Jersey, and Connecticut (Orwig and Foster, 1998). At the Delaware Water Gap National Recreation Area on the New Jersey-Pennsylvania border, HWA have not killed as many hemlocks as initially expected – 28% of the hemlock trees in the park have died over the 10 years of the infestation.  Mortality rates vary considerably from site to site, and reaches 60% at some places (Richard Evans pers. comm. June 2013). The adelgid’s impact in the southern Appalachians is widely said to be even more devastating and rapid (Ford 2008).

Michigan, Wisconsin, and Minnesota have hemlock forests that are sufficiently separated from the Appalachians that it is unlikely the adelgid could reach them through natural means. Starting in 2015, the state of Michigan started to detect established HWA populations in four counties in the western Lower Peninsula of Michigan- Allegan, Muskegon, Oceana and Ottawa counties.  A coordinated response is currently being implemented that includes extensive survey, treatment, education/outreach & communication, regulatory work and research. Michigan implemented an interior State quarantine for HWA in July of 2017 and continues to maintain its exterior state quarantine.

USFS scientists and managers developed a conservation priority-setting framework for forest tree species at risk from pests, pathogens and other threats. The Project CAPTURE (Conservation Assessment and Prioritization of Forest Trees Under Risk of Extirpation) uses FIA data and expert opinion to group tree species under threat by non-native pests into vulnerability classes and specify appropriate management and conservation strategies. The scientists prioritized 419 tree species native to the North American continent. The analysis identified 15 taxonomic groups requiring the most immediate conservation intervention because of the tree species’ exposure to an extrinsic threat, their sensitivity to the threat, and their ability to adapt to it. Each of these 15 most vulnerable species, and several additional species, should be the focus of both a comprehensive gene conservation program and a genetic resistance screening and development effort.

Eastern hemlock (Tsuga canadensis) was abundant across a range exceeding 7.5 million hectares – from northeast Minnesota to New Brunswick and Nova Scotia, south to Pennsylvania, then down the Appalachians to northwestern Alabama. Eastern hemlock is among the group facing extensive pest-caused decline but having the potential advantage of being commonly widespread with relatively high genetic variation and regeneration capacity. Despite the breathing room provided by their extensive distribution, higher regeneration capacity, and genetic variability, it is important to maintain large populations to reduce the chance of inbreeding.

Carolina hemlock (Tsuga caroliniana) is found along the Appalachian chain from southwestern Virginia to northeastern Georgia, primarily on cliffs, rocky slopes and ridges, usually above 914 m elevation (Melissa Fischer, Ph.D. proposal, Virginia Polytechnic Institute and State University). The CAPTURE project placed Carolina hemlock among six species that face severe pest threats, to which the hosts are relatively sensitive, but have a high capacity to adapt. The project said conservation and the facilitation of resistance through breeding are highly necessary for this species.

Hemlocks are a major component of four forest cover types, a common member of seven types and a minor species in 18 other types. Hemlocks in riparian communities create a distinctive microclimate. Hemlock density influences light levels, temperatures, amount of precipitation reaching the ground, etc., helping to create seeps and wetlands, as well as dry well-drained den sites. Hemlock density also affects mid-story and ground level light levels and hence vegetation. Even scattered individual hemlocks in forests composed largely of other species create structural components, such as cavity trees and coarse woody debris. All these factors contribute to specific wildlife associations with this forest type (Melissa Fischer, Ph.D. proposal, Virginia Polytechnic Institute and State University).

In northern New Jersey, hemlock forests shelter more than 12 species of small mammals, more than 14 species of amphibians and at least 152 species of terrestrial invertebrates. Salamanders, in particular, depend on the unique habitat under a hemlock canopy (Brooks, 2001). Hemlock-lined streams also keep water temperatures cool enough for brook trout (USDI NPS EIS, 2000).

In the northeastern United States more widely, ninety-six bird and forty-seven mammal species are associated with the hemlock type. Of these species eight bird and ten mammal species are strongly associated with the hemlock type, though none of these species is limited to it (Yamasaki et al. no date). These strongly associated species are Great Horned Owl, Long-eared Owl, Northern Saw-whet Owl, Blue-headed or Solitary Vireo, Blue Jay, Red-breasted Nuthatch, Hermit Thrush, Black-throated Green Warbler; Snowshoe Hare, Red Squirrel, Deer Mouse, Southern Red-backed Vole, Porcupine, Red Fox, Black Bear, Marten, Bobcat, and White-tailed Deer (Yamasaki et al. no date).

According to the National Park Service (USDI NPS EIS, 2000) decline of hemlock in the Delaware Water Gap National Recreation Area is likely to have “massive adverse effects on the ecological, aesthetic, and recreational values of the park.” Affected streams would be warmer, have lower water flows, and are more likely to dry up during summer droughts. Overall species diversity in hemlock-dominated habitats would probably decline by 35% or more. Decaying and downed trees would increase debris flow, interfere with water flow, and cause channel scouring that would raise the chance of extreme flood damage. Nutrient cycling would also be disturbed (Jenkins et al., 1999).

Ford (2008) studied the impact of hemlock loss on water regimes in the southern Appalachians. He found two possible outcomes. When dense thickets of Rhododendron maximum prevent tree regeneration to replace hemlock, transpiration greatly decreases, soil moisture increases, and runoff (stream flow) increases. Diurnal amplitude in streamflow decreases. When, however, rhododendrons are absent and other trees replace hemlocks – especially red maple and black birch, transpiration increases, as does seasonal variation in water regime.

Hemlocks are highly valued for their beauty and often planted in cultivated landscapes (McClure, Salom, and Shields. 2001).

Efforts to mitigate the impacts of the hemlock woolly adelgid have been significant and well-funded compared to those addressing most other tree species threatened by invasive pests or pathogens (Salom 2008). Funding from USFS has approached $4.5 million per year in recent years (USDA FS FY2011 Budget Justification). Efforts are under way in the following areas: biological control; lethal control – using both chemical insecticides and – more recently – biocides; breeding of resistant trees; managing the site to adapt to loss of hemlocks. The fact that the insect continues to spread and hemlocks remain under threat illustrates the great difficulty often encountered in responding to such invasions.

Salom addressed the large enterprise needed to do undertake and sustain biocontrol efforts at the necessary scale – engaging state and federal agencies, universities, and others. The search for biocontrol agents targetting HWA began in 1992 (Salom 2008). Several predatory beetles have been tested and released – most recently Laricobius osakensis (Federal Register May 20, 2010) and scientists continue searching for other predators and looking at fungi that are pathogenic to insects. There is general agreement that a successful control program will require a suite of predators that attack the adelgid at various life stages and prosper under various environmental conditions (Montgomery and Yu 2008). In the meantime, a range of governmental and academic institutions have collectively reared and released tens of thousands to millions of beetles belonging to one of three species (Sasajischymnys tsugae, Schymnus sinuanodulus, Laricobius nigrinus) (Salom 2008).

Studies also continue into various types of pesticides. Imidacloprid and other neonictenoid systemic pesticides taken up by the tree through either direct injection or from treated soil have proved efficacious under some situations (Salom 2008). Another approach is naturally-occurring chemicals, such as the soil fungus Lecanicillium muscarium, to suppress the adelgid (Grassano 2008).

Other scientists are trying to understand and manipulate tree characteristics to make the tree less vulnerable to adelgid attack. In addition to researching possible resistance within Eastern hemlocks (Casagrande 2008), specific characteristics being studied include

* Structure – thickness of bark & “hairs” (trichomes) on twigs (Wallin, in 2008 symposium proceedings);

* Terpenoids and other chemicals in the tree (Lagalante, in 2008 symposium proceedings)

* Genetics (Hoover, Weston, Bentz, and Morse, all in 2008 symposium proceedings )

Camcore is carrying out a program of widespread seed collection; plantations will be established in ex situ plantings in places far from adelgid outbreaks — southern Chile, southwest Brazil, Arkansas (Jetton 2008).

Significant efforts have been made by two National parks to manage hemlock woolly adelgid: Delaware Water Gap National Recreation Area on the Pennsylvania-New Jersey border and Great Smoky Mountains National Park on the Tennessee-North Carolina border. Delaware Water Gap began studies and management efforts in 1993, 4 years after the adelgid was detected in the Park. Priorities are managing hazard trees and protecting esthetics, recreation, and native biodiversity. Park staff have released 40,000 Sasajischymnys tsugae beetles at two sites over a decade, but can detect no effect on adelgid populations. DWGNWR began releasing Scymnus and Laricobius only in 2006 (Evans 2008). By 2013, L. nigrinus could be found throughout the New Jersey side of the park – the general location of the release sites (Richard Evans, pers. comm. June 2013). Since 2004, DWGNWR has tried to protect high-value trees by injecting Imidacloprid; the results are still unclear (Evans 2008).

In Great Smoky Mountains N.P. (Johnson 2008), the priorities are protecting old-growth dominant hemlock areas and developed areas and backcountry campsites (preventing hazard trees). The integrated program includes application of foliar insecticides (oils and soaps) to accessible high-priority trees; application of systemic insecticides to 75,000 trees on 2,200 acres; and release of biocontrol agents. In recent years, biocontrol efforts have focused on Lariocobius nigrinus. Great Smoky Mountain N.P. is carefully monitoring hemlock annual growth rates & HWA densities. Some impacted trees are growing back. Park staff see huge changes in streams as hemlocks die.

For more information on this pest, please visit:

• US Forest Service Hemlock Woolly Adelgid information
• Great Smoky Mountains Hemlock Woolly Adelgid education page
• National Invasive Species Information Center Species Profile for Hemlock Woolly Adelgid

 

Sources

Annand, P. N. 1924. A new species of Adelges (Hemiptera, Phylloxeridae). Pan-Pac. Entomol. 1: 79-82.

 

Barr, M. W. 2002. Eastern Hemlock (Tsuga canadensis) Mortality in Shenandoah National Park. Proceedings: Hemlock Woolly Adelgid in the Eastern United States Symposium, February 5 – 7, 2002, East Brunswick, New Jersey.

 

Brooks, R. T. 2001. Effects of the removal of overstory hemlock from hemlock-dominated forests on eastern redback salamanders. For. Ecol. Managem. 149: 197-204.

 

Evans, R. and J. Shreiner 2008. Research, Monitoring, and Management of Eastern Hemlock Forests at Delaware Water Gap National Recreation Area; in United States Department of Agriculture Forest Service. Forest Health Technology Enterprise Team. 2008. FHTET-2008-01. Brad Onken and Richard Reardon, Compilers. Fourth Symposium on Hemlock Woolly Adelgid in the Eastern United States. Hartford, Connecticut. February 12-14, 2008. Accessible at https://na.fs.fed.us/fhp/hwa/pubs/proceedings/2008_proceedings/fhtet_2008.pdf

 

Ford, C.R. and J.M. Vose. 2008. Water Use by Eastern Hemlock: From Implications for Using Systemic Insecticides to Ecosystem Function; in United States Department of Agriculture Forest Service. Forest Health Technology Enterprise Team. 2008. FHTET-2008-01. Brad Onken and Richard Reardon, Compilers. Fourth Symposium on Hemlock Woolly Adelgid in the Eastern United States. Hartford, Connecticut. February 12-14, 2008. Accessible at https://na.fs.fed.us/fhp/hwa/pubs/proceedings/2008_proceedings/fhtet_2008.pdf

 

Jenkins, J. C., J. D. Aber, and C. D. Canham. 1999. Hemlock woolly adelgid impacts on community structure and N cycling rates in eastern hemlock forests. Can. J. For. Res. 29: 630-645

 

Jetton, R.M. W.A. Jetton, W.S. Dvorak, and K.M. Potter. 2008. Ex situ conservation of eastern and Carolina hemlocks in the southern Appalachian Mountains. in United States Department of Agriculture Forest Service. Forest Health Technology Enterprise Team. 2008. FHTET-2008-01. Brad Onken and Richard Reardon, Compilers. Fourth Symposium on Hemlock Woolly Adelgid in the Eastern United States. Hartford, Connecticut. February 12-14, 2008. Accessible at https://na.fs.fed.us/fhp/hwa/pubs/proceedings/2008_proceedings/fhtet_2008.pdf

 

Johnson, K. T. Ramaly and G. Taylor. 2008. Managing Hemlock Woolly Adelgid at Great Smoky Mountains National Park: Situation and Response; in United States Department of Agriculture Forest Service. Forest Health Technology Enterprise Team. 2008. FHTET-2008-01. Brad Onken and Richard Reardon, Compilers. Fourth Symposium on Hemlock Woolly Adelgid in the Eastern United States. Hartford, Connecticut. February 12-14, 2008. Accessible at https://na.fs.fed.us/fhp/hwa/pubs/proceedings/2008_proceedings/fhtet_2008.pdf

 

Lamb, A., S. Shiyake, S. Salom, M. Montgomery, and L. Kok. 2008. Evaluation of the Japanese Laricobius sp. n. and other Natural Enemies of HWA in Japan; in United States Department of Agriculture Forest Service. Forest Health Technology Enterprise Team. 2008. FHTET-2008-01. Brad Onken and Richard Reardon, Compilers. Fourth Symposium on Hemlock Woolly Adelgid in the Eastern United States. Hartford, Connecticut. February 12-14, 2008. Accessible at https://na.fs.fed.us/fhp/hwa/pubs/proceedings/2008_proceedings/fhtet_2008.pdf

 

Lu, W. H., and M. E. Montgomery. 2001. Oviposition, development, and feeding of Scymnus (Neopullus) sinuanodulus (Coleoptera: Coccinellidae): a predator of Adelges tsugae (Homoptera: Adelgidae). Ann. Entomol. Soc. Amer. 94: 64-70.

 

McClure, M. S. 1995. Diapterobates humeralis (Oribatida: Ceratozetidae) – an effective control agent of hemlock woolly adegid (Homoptera: Adelgidae) in Japan. Environ. Entomol. 24: 1207-1215.

 

McClure, M.S., S.M. Salom, K.S. Shields. Hemlock Woolly Adelgid Forest Health Technology Enterprise Team . USDA Forest Service FHTET-2001 – 03 March 2001

 

Mausel, D., S. Salom and L. Kok. 2008. Laricobius negrinus Establishment and Impact in Native and Introduced Habitats; in United States Department of Agriculture Forest Service. Forest Health Technology Enterprise Team. 2008. FHTET-2008-01. Brad Onken and Richard Reardon, Compilers. Fourth Symposium on Hemlock Woolly Adelgid in the Eastern United States. Hartford, Connecticut. February 12-14, 2008. Accessible at https://na.fs.fed.us/fhp/hwa/pubs/proceedings/2008_proceedings/fhtet_2008.pdf

 

Michigan Department of Agriculture and Rural Development. Michigan’s Hemlock Trees Face Big Threat from a Tiny Insect (Press Release). May 20, 2013.

 

Montgomery, M.E. and G. Yu. 2008. An Overview of Lady Beetles in Relation to Their Potential as Biological Controls for Hemlock Woolly Adelgid; in United States Department of Agriculture Forest Service. Forest Health Technology Enterprise Team. 2008. FHTET-2008-01. Brad Onken and Richard Reardon, Compilers. Fourth Symposium on Hemlock Woolly Adelgid in the Eastern United States. Hartford, Connecticut. February 12-14, 2008. Accessible at https://na.fs.fed.us/fhp/hwa/pubs/proceedings/2008_proceedings/fhtet_2008.pdf

 

Orwig, D. A., and D. R. Foster. 1998. Forest response to the introduced hemlock woolly adelgid in southern New England, USA. J. Torrey Bot. Soc. 125: 60-73.

 

Potter, K.M., Escanferla, M.E., Jetton, R.M., Man, G., Crane, B.S., Prioritizing the conservation needs of US tree spp: Evaluating vulnerability to forest insect and disease threats, Global Ecology and Conservation (2019), doi: https://doi.org/10.1016/

 

Salom, S.M., L.T. Kok, A. Lamb, C. Jubb, and B. Onken. 2008. Biological Control of Hemlock Woolly Adelgid: What is it Going to Take to Make it Work?; in United States Department of Agriculture Forest Service. Forest Health Technology Enterprise Team. 2008. FHTET-2008-01. Brad Onken and Richard Reardon, Compilers. Fourth Symposium on Hemlock Woolly Adelgid in the Eastern United States. Hartford, Connecticut. February 12-14, 2008. Accessible at https://na.fs.fed.us/fhp/hwa/pubs/proceedings/2008_proceedings/fhtet_2008.pdf

 

United States Department of Agriculture Forest Service. FY2011 President’s Budget Budget Justification

 

United States Department of Interior National Park Service Northeast Region 2000. Environmental Assessment for the Release and Establishment of Pseudoscymnus tsugae Adelges tsugae) (Coleoptera: Coccinellideae) as a Biological Control Agent for Hemlock Woolly Adelgid at Delaware Water Gap National Recreation Area.

 

Wallace, M. S., and F. P. Hain. 2000. Field surveys and evaluation of native and established predators of the hemlock woolly adelgid (Homoptera: Adelgidae) in the southeastern United States. Environ. Entomol. 29: 638-644.

 

Yamasaki, M., R.M. DeGraaf, and J.W. Lanier. (date?) Wildlife Habitat Associations in Eastern Hemlock — Birds, Smaller Mammals, and Forest Carnivores. Proceedings: Symposium on Sustainable Management of Hemlock Ecosystems in Eastern North America. USDA Forest Service Northeastern Region GTR-NE-267