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.

North America is home to four species of hemlocks: eastern hemlock (Tsuga canadensis), Carolina hemlock (T. caroliniana), mountain hemlock (T. mertensiana), and western hemlock (T. heterophylla). The first two are found in the East; they are not closely related (Havill, Montgomery, and Keena, 2011). Both have undergone severe declines caused by a single pest, the non-native insect hemlock woolly adelgid (Adelges tsugae).

Eastern hemlock was abundant across a range exceeding 7.5 million hectares, reaching from northeastern Minnesota to New Brunswick and Nova Scotia, south to Pennsylvania, then down the Appalachian Mountains to northwestern Alabama (Havill, Montgomery, and Keena, 2011). Eastern hemlocks are a major component of four forest cover types, a common member of seven types, and a minor species in 18 other forest types (Melissa Fischer, Virginia Tech). In Canada, eastern hemlock grows throughout southern Ontario and Quebec and the Maritime provinces. The species is a major component of four forest types. Still, its abundance in the Canadian range has been reduced by as much as 80% since European colonization. Hemlock forests are now found on less than four percent of its pre-Colonial range due primarily to conversion of land to agriculture and other uses, as well as the preference of commercial forestry for early-successional tree species. Remaining hemlock has high ecological and social value in eastern Canada (MacQuarrie et al. 2024).

Carolina hemlock has a much smaller range: isolated populations on cliffs, rocky slopes, and ridges along the Appalachian Mountains from southwestern Virginia to northeastern Georgia (Melissa Fischer, Ph.D. proposal, Virginia Tech; Havill, Montgomery, and Keena, 2011). Carolina hemlock has moderate levels of genetic diversity (Havill, Montgomery, and Keena, 2011).

Creator of unique systems

Eastern hemlock is one of the most common conifers in the northern hardwood forest. Before arrival of the hemlock woolly adelgid, hemlock eventually dominated forests in which temperature and moisture conditions were suitable due to the species’ slow growth, shade tolerance, and deep shade under dense canopies. The species’ importance is enhanced by the fact that it creates distinct coniferous “islands” in deciduous forests. Hemlock groves’ deep shade minimizes the presence of understory plants and saplings. The soil tends to be moist and organic-rich, with low rates of nitrogen mineralization and nitrification. Hemlock litter is slow to decompose (Lovett et al. 2006).

Hemlock-dominated riparian communities create a distinctive microclimate. Hemlock density influences light levels, temperatures, and amount of precipitation reaching the ground, 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 add to the structural diversity, and may provide cavity trees and coarse woody debris. All these factors contribute to specific wildlife associations with this forest type (Melissa Fischer, Virginia Tech).

Eastern hemlock creates important wildlife habitat for more than 100 vertebrate species in the northeastern United States (Ducey et al. 2023). Of these species, eight bird and 10 mammal species are strongly associated with the hemlock type, though none is limited to it. (Yamasaki et al.). In northern New Jersey scientists also named 14 species of amphibians (especially salamanders) and at least 152 species of terrestrial invertebrates (Brooks, 2001). Hemlock-lined streams also keep water temperatures cool enough to support populations of the native brook trout (USDI NPS EIS, 2000).

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

Arrival of the hemlock woolly adelgid

The hemlock woolly adelgid (HWA) is an aphid-like pest of eastern hemlock and Carolina hemlock. It was detected at a private arboretum in Richmond, Virginia in the 1950s (Lovett et al. 2006).

HWA spread first to the northeast. Severe mortality of hemlocks in the Mid-Atlantic and southern New England was observed in the 1980s (Lovett et al. 2006). This spread was probably facilitated by migrating birds, movement of infested wood or plants, and local spread by wind and mammals (MacQuarrie et al. 2024). By the end of the 20th Century, HWA had begun to spread south along the Appalachian Mountains. It reached Georgia in 2003 (https://gatrees.org/hemlock-woolly-adelgid-hwa-in-georgia/, accessed August 2024). It also spread west into Kentucky and Tennessee. It was detected in eastern Ohio in 2012 (https://agri.ohio.gov/divisions/plant-health/invasive-pests/invasive-insects/hwa, accessed August 2024). In Canada, HWA was first detected in the Niagara region of Ontario in 2012 and in Nova Scotia in 2017. As of 2024, HWA was established in Ontario as far west as Hamilton and at least one site on the north shore of Lake Ontario, as well as across southern Nova Scotia. No populations have been found in New Brunswick, Quebec, or Prince Edward Island (MacQuarrie et al. 2024)

Hemlock forests in Michigan, Wisconsin, and Minnesota are sufficiently separated from the Appalachian Mountains that it was unlikely HWA could reach them through natural means. HWA was first detected in Michigan in 2006. Eradication efforts appeared at first to be successful. However, in 2015, detection of established populations in four counties in the western Lower Peninsula led to recognition that HWA was too widespread to be eradicated. The state began a coordinated response that included surveys, treatment, research, education/outreach, and exterior and internal quarantines (Limbu, Keena and Whitmore, 2018).

The initial hope that the adelgid’s northward spread would be limited by cold winter temperatures has been countered by recent overwintering and survival data from populations in the eastern United States. MacQuarrie et al. (2024) conclude that some, if not all, of the range of eastern hemlock in Canada is at risk under both present and projected future climatic conditions.

Impacts

Mortality of hemlock trees

No-one has compiled a range-wide summary of hemlock decline in response to HWA. In the eastern United States, tree mortality becomes apparent within four to 15 years following initial attack (Orwig and Foster, 1998; Lovett et al. 2006; MacQuarrie et al. 2024). Mortality in a stand can exceed 90% (MacQuarrie et al. 2024). However, at the Delaware Water Gap National Recreation Area (on the New Jersey-Pennsylvania border), mortality was less than expected: 28% of the hemlock trees in the park died over 10 years following infestation in the region. Mortality rates vary considerably from site to site; it is up to 60% in some places (Richard Evans pers. comm., June 2013). HWA’s impact in the southern Appalachians is widely considered to be more devastating and rapid (Ford, 2008).

Ecosystem Impacts

Lovett et al. (2006) expected HWA to have large short- and long-term impacts in areas where eastern hemlock is a significant part of the forest. They based this projection on the fact that HWA is a virulent, host-specific pest attacking a unique species in the forest. Death of the hemlock trees would open up the canopy, thereby increasing light, moisture, and temperature at the forest floor. The changes in forest composition would increase nitrogen mineralization and nitrification, which might increase nitrate leaching to groundwater or surface.

Regarding associated species of terrestrial wildlife, Lovett et al. (2006) reported that black-throated green warbler (Dendroica virens) and other bird populations declined precipitously in areas where hemlocks died from adelgid attack in Connecticut. Amaral et al. (2023) detected greater than 30% decline of two other species, blackburnian warbler (Setophaga fusca) and hermit thrush (Catharus guttatus). Variations in decline of hemlock-associated species over a broader scale indicated that site factors also play a role. Declines were greater in the warmest parts of the wildlife species’ ranges. They expected that climate change will further reduce bird species’ persistence even by decreasing HWA overwintering mortality.

Another concern has been the probable impact of hemlock mortality on streams, especially water quantity and quality and aquatic species.

Early in the HWA invasion on the New Jersey-Pennsylvania border, authors of an environmental assessment (USDI NPS EIS, 2000) anticipated that loss of hemlocks would cause streams to be warmer, have lower water flows, and be more likely to dry up during summer droughts. 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. Lovett et al. (2006) concurred regaurding higher stream temperature, algal growth, and increased bank erosion. All these developments can affect fish, salamanders, and other animals in streams and riparian zones.

In Ohio, West Virginia, and Virginia, the stream “periphyton” biomass (the community of organisms, including blue-green algae, fungi, microbes, bacteria, plant detritus, and animals) was greater at sites not yet invaded by HWA. At higher trophic levels – e.g., spiders – the impact was less distinct (Diesburg et al. 2021).

Ford (2008) found that in the southern Appalachians the impact of hemlock loss on water regimes depended on whether understory riparian vegetation was composed of dense thickets of Rhododendron maximum. These “rhododendron hells” prevent other tree species from replacing dead hemlocks, so transpiration greatly decreases, soil moisture increases, and runoff (stream flow) increases. Diurnal amplitude in streamflow decreases. When rhododendrons are absent and other trees replace hemlocks – especially red maple (Acer rubrum) and black birch (Betula lenta) – transpiration increases, as does seasonal variation in the water regime.

Succession

Successional dynamics following hemlock mortality is still highly uncertain. Two decades ago, Lovett et al. (2006) noted that whatever species replaces hemlock will probably not have its unique combination of evergreen foliage, long life, shade tolerance, and difficult-to-decompose litter, so the turnover in tree species will probably have a major long-term impact on the structure and functioning of the ecosystem.
Several studies (Lovett et al. 2006; Ford, 2008; Ducey et al. 2023) report that birches, especially black birch (Betula lenta L.), are the most common tree species regenerating in HWA-killed hemlock stands. Most scientists expect birch dominance to be short-lived; however, A.E. Mayfield (pers. comm.) reports that black birch can persist for 100 years. Until recently, most studies have pointed to American beech (Fagus grandifolia) increasing in abundance, even in the presence of beech bark disease. This trajectory is now more uncertain due to invasion by beech leaf disease. Maple regeneration varies by species and site history (Ducey et al. 2023).

Response

Efforts to mitigate hemlock woolly adelgid impacts have been significant, coordinated, and well-funded compared to those addressing most other tree species threatened by non-native insects or pathogens (Salom, 2008; Twery, 2004). The USDA Forest Service has provided considerably more funding for HWA than for any pest-host combinations other than spongy moth (USDA FS fiscal year Budget Justifications). In 2003, the USFS also created a mechanism to coordinate efforts by resource managers and researchers from federal, state, university, international, and private entities. Program managers meet annually; there is a separate biological control working group. It also supports the integrated pest management approach. Five-year strategic plans were adopted for 2014-2018 Strategic Plan and updated in 2021-2025. Parties to the agreement include the USDA Forest Service, National Association of State Foresters (NASF), National Plant Board (NPB), and USDA Animal and Plant Health Inspection Service (APHIS). The initiative’s goals are to control HWA at high-value sites and achieve long-term conservation of eastern and Carolina hemlocks. This is to be achieved through inter-agency technical and financial support for HWA integrated pest management (IPM), applied research, and information transfer. Components of the IPM include biocontrol, lethal control using chemical insecticides and biocides, silvicultural approaches, and managing the site to adapt to loss of hemlocks. The group is also pursuing breeding of hemlock trees resistant to the adelgid.

Biocontrol

Scientists have pursued classical biocontrol of HWA for three decades. The first agent – a predatory beetle from Japan (Sasajischymnys tsugae) was approved for release in 1995. At least two million beetles were released in 16 states. Despite hopes spurred by its impact in Japan, its mobility, and rapid buildup of densities, S. tsugae had insufficient impact (https://biocontrol.entomology.cornell.edu/predators/sasajiscymnus.php). Two additional ladybird beetles, Scymnus ningshanensis and S. camptodromus, were released but did not establish. Laricobius nigrinus is one of a suite of natural enemies that appears to regulate HWA populations in the Pacific Northwest. Beginning in 2003 it has been introduced from the Pacific coast to the East. There, L. nigrinus has established widely and is preying on winter stages of HWA. However, its activity has not reduced densities of HWA on the trees branches in spring. Laricobius osakensis, which is native to Japan, has been released since 2012. It is establishing and spreading, but it is too early to evaluate its impact (Mayfield et al. 2023; Mayfield and N. Havill, pers. comm.).

In the American Pacific Northwest region, HWA is a native, minor pest on western hemlock (Tsuga heterophylla) (Annand, 1924; Havill et al. 2011).

Scientists now believe that control of HWA will depend on deploying a suite of predators that attack the adelgid year round, throughout its entire life cycle. They have focused on predators that feed on the eggs and nymphs during spring and early summer. This led them to silver flies in the Leucotaraxis genus. Two species with genetic lineages that specialize on Adelges tsugae have been found in the Pacific Northwest: Leucotaraxis argenticollis and L. piniperda (Havill et al. 2023). Releases began in the eastern US in 2016. However, as of 2023, no field establishment had been documented (Mayfield et al. 2023). In the current strategic plan – for 2021-2024 – scientists hope to rely on field-collected Laricobius spp. Consortium members are also developing regional release strategies for high-value sites.

Other components of the integrated pest management program

At high-value sites, the strategic plan is to integrate biocontrol (Laricobius nigrinus or Laricobius osakensis paired with both Leucotaraxis species), chemical control, and silvicultural practices.

Systemic pesticides, (i.e., imidacloprid and other neonictenoid systemic pesticides) have proved efficacious under some situations. Imidacloprid infusions protect a tree for five to seven years; dinotefuran protects the tree for one to two years (Mayfield et al. 2021). The Plan calls for treating trees at high-value sites, and recording those treatments in a database. They are also evaluating the impacts of neonictenoid chemical on non-target organisms. (Hemlocks are wind-pollinated, so they expect little transfer to pollinators.) They are also evaluating efficacy of less-toxic pesticides as they become available. Hope to use chemicals to protect trees temporarily while expanding biocontrols.

Silvicultural practices being explored include thinning, group selection, and planting. A variety of goals are being pursued: improve hemlock health, forest sustainability, restore ecosystems, aesthetics, recreation, wildlife habitat, revenue generation, utilization, or various combinations. Since young hemlock saplings in more open, sunnier, sites are attacked by fewer adelgids; experimental plantings are being placed in small gaps which are maintained by herbicide applications to control competition. In the southern Appalachians (North Carolina), trees in these canopy gaps also exhibited improved carbon balance and foliage density (Brantley et al. 2017; Mayfield and Jetton, 2020; Mayfield et al. 2023; Miniat et al. 2020).

Understanding whether these efforts are successful in reducing the impact of HWA and protecting hemlocks depends on other components of the strategy such as survey and monitoring programs that document HWA spread, HWA winter mortality, biocontrol agent establishment, dispersal, abundance, and impacts. Plan participants monitor hemlock health and change at the regional level using various tools such as existing long-term hemlock monitoring plots, the USFS Forest Inventory and Analysis, and remote sensing.

Maintaining the effort and minimizing duplication requires information transfer. This coordination is done through the annual HWA managers’ meeting, technical committee meetings, databases containing information on biocontrol and insecticide release and response/recovery, development of focused coalitions (i.e., Lake States, tribes, Canada) as well as maintaining a current HWA website.

Resistance Breeding

The USDA Forest Service CAPTURE project (Potter et al. 2019) determined that eastern hemlock faces extensive decline as a result of pest attack, but potentially is protected by its large range and relatively high genetic variation and regeneration capacity. The authors recommended maintaining large populations to reduce the chance of inbreeding. The statement on genetic variation is contradicted by Havill, Montgomery, and Keena (2011), who report that Tsuga canadensis has low genetic variation. They note that this will complicate efforts to breed trees that are more resistant to the adelgid.

The USDA Forest Service CAPTURE project (Potter et al. 2019) placed Carolina hemlock among six species facing severe pest threats but having a high capacity to adapt. The authors called for conservation and facilitation of resistance through breeding.

Scientists started resistance breeding efforts early in HWA invasion. They quickly determined that T. caroliniana is more closely related to the Asian hemlock species and can be crossed with them. Hybrids of T. caroliniana and T. chinensis are resistant to HWA. T. canadensis diverged from Asian hemlocks millions of years earlier and cannot be crossed with them. Havill, Montgomery, and Keena (2011) identified other challenges to breeding eastern hemlock: its low genetic variation and relatively narrow site requirements.

The importance of conserving genetic variability from throughout eastern hemlock’s large range was recognized early. Camcore (Central America and Mexico Coniferous Resources Cooperative; formed 1980; part of the Department of Forestry and Environmental Resources at North Carolina State University) began collecting from carefully chosen locations/populations throughout hemlock ranges in 2003. As of 2021, seeds representing 593 families and 81 provenances of eastern hemlock and 220 families from 29 provenances have been placed in storage at both Camcore facilities and the USDA Center for Genetic Resources Preservation in Colorado. In addition, two seed orchards for each species have been established in western North Carolina (Mayfield et al. 2021).

Hemlock participants have also been identifying surviving trees in the field, collecting cuttings / seeds, developing a tool to test for resistance/tolerance, and potentially move towards production & planting.
In 2022, several USFS scientists announced formation of Forest Health Collaborative to be located at Holden Forests & Gardens in northern Ohio. Holden has been working with the USFS since 2005. Furthermore, it has hemlock woolly adelgid (as well as beech bark disease, beech leaf disease, and emerald ash borer) on their property. The purpose of the new consortium is to establish participatory breeding networks and transfer technology developed through research into the hands of those willing and able to implement it (Jennifer Koch, pers. comm.)

Consortium partners will assist in

1. pre-breeding activities — identification of candidate resistant trees, testing to confirm resistance, scion collection, seed collection,
2. breeding activities — establishing seed orchards, clone tests and progeny tests, and
3. restoration activities — planting improved seedlings to restore impacted forest lands.

The collaborative will support resistance breeding work and facilitate transfer of developed technologies when appropriate (i.e., breeding strategies, guidelines for selecting candidate resistant trees, methods to test for resistance, methods of clonal propagation, seed germination, pollen collection, etc.) to partners through training and workshops. In its annual report for October 2023–September 2024, (https://holdenfg.org/wp-content/uploads/2024/11/GLBFHC-Annual-Partners-Report-2024.pdf) accessed November 2024) the Collaborative notes the following activities pertaining to eastern hemlock:

• Outreach and education collaborations with several partners to help find lingering hemlock trees or set up long term monitoring.
• Studies to improve understanding of genomic variation in eastern hemlock across its range in the region.
• Studies to improve hemlock propagation methods
• Application of eDNA collection method for HWA early detection.
• Hosting a workshop re: training for lingering hemlock detection and plot monitoring

In 2022 The Nature Conservancy (TNC) announced receipt of financial support for this breeding effort as part of a broader project named Tree Species in Peril (TSIP), which also includes green, white and black ash and American beech. TNC’s TSIP engagement focuses on strategies to find and protect existing resilience to these pests, while maintaining range-wide genetic diversity of these imminently imperiled tree species, and advocacy for long-lasting funding, research, and proactive research programs. Much of this work will be through adding capacity of all sorts (staff, post-doc, PhDs, lab time, nursery maintenance, lab techs, travel funds, etc.) to existing high quality projects.

National Park Service activities

The ecological importance of hemlocks has prompted an unusual level of management efforts by the National Park Service (NPS).

The first park to be affected was Shenandoah National Park in Virginia. HWA was detected in the park in the late 1980s. By 2003, roughly 95% of the Park’s hemlocks had succumbed to the adelgid combined with drought. In the mid-1990s, the park attempted to suppress HWA in developed areas using horticultural oil and insecticidal soap. Those treatment were only partially effective. In 2005, the park switched to systemic soil treatments of imidacloprid applied every 7 years. Since 2005, staff have treated 2,500 – 3,500 trees per year, a total of over 30,000 trees. Crown health has improved at all treated sites (https://www.nps.gov/shen/learn/nature/eastern_hemlock.htm accessed August 2024).

Shenandoah began releasing biocontrol agents (Laricobius spp.) in 2015. To date, these releases have been made at six sites. The Laricobius beetles have shown promising establishment and spread. Park staff plan to continue releases of Laricobius beetles and eventually the predatory silver flies in remaining hemlock sites and to phase out the imidacloprid treatments (https://www.nps.gov/shen/learn/nature/eastern_hemlock.htm, accessed August 2024). Both projects are supported by the Shenandoah National Park Trust, which is the official philanthropic partner of the Park (www.snptrust.org).

Next to be invaded was the Delaware Water Gap National Recreation Area (DWG NRA) on the New Jersey-Pennsylvania border. DWG NRA began studies and management efforts in 1993, four years after HWA was detected in the Park. Priorities were managing hazard trees and protecting the esthetic, recreation, and native biodiversity resources of the Park. DWG NRA released biocontrol agents (Sasajischymnys tsugae beetles) as soon as they were approved; however, they could detect no effect on HWA populations. The Park also released the next agents (Scymnus spp. and Laricobius spp.) in 2006 (Evans and Shreiner, 2008). By 2013, L. nigrinus could be found throughout the NJ side of the park – the general location of these releases (Richard Evans, pers. comm., June 2013). In 2004, DWGNWR also began injecting Imidacloprid into high-value trees. The results were said to be still unclear four years later (Evans and Shreiner, 2008).

The hemlock woolly adelgid moved to the south somewhat later than to the northeast. Great Smoky Mountains National Park (GRSM), had a similar program: protect old-growth dominant hemlock areas and manage hazard trees in developed areas and backcountry campsites (Johnson, Ramaly and Taylor, 2008). Initially GRSM applied insecticidal oils and soaps to accessible high-priority trees. Later they began applying systemic insecticides to 75,000 trees on 2,200 acres. GRSM also began releasing of biocontrol agents in later years, especially Lariocobius nigrinus. GRSM is carefully monitoring hemlock annual growth rates and HWA densities. Some impacted trees are growing back. Park staff see huge changes in streams as hemlocks die.

Partners

These governmental programs are supplemented by joint government-volunteer initiatives. For example, the Virginia Department of Forestry implements a cost-share program for landowners or organizations wanting to treat their hemlocks. In New York, Cornell University invites volunteers to sign up for one of four projects that report on HWA presence/absence encountered in their outings, monitor a specific site, assist in determining when the HWA is breaking aestivation or ovipositing.

North Carolina has a particularly strong program. The Hemlock Restoration Initiative (HRI) was founded in 2004 with the objective of restoring both eastern and Carolina hemlocks to their native habitats in North Carolina through working with partners. The HRI strategic plan involves:

• Identifying and establishing hemlock conservation areas
• Educating landowners on how to treat and manage the hemlocks on their properties
• Increasing the number of trees being treated on public lands
• Implementing Integrated Pest Management and long-term biological control of HWA
• Advancing the development of other control strategies and restoration techniques, including the search for HWA-resistant trees and the growing conditions that hemlocks like best

In 2023, the HRI and its partners:

1. Treated almost 6,600 hemlocks at many sites across Western North Carolina. Because of drought, treatments focused on Carolina hemlocks growing on rocky outcrops and ridgetops.
2. Monitored presence of HWA-predator beetles and effects of treatment.
3. Carried out many outreach actions, such as forest festivals, hikes, professional conferences & guest instructors for various college classes.
4. Assisted research partners through such activities as collecting branch clippings used to map the eastern hemlock genome, developing protocols to identify native hemlocks that might have some heritable resistance to HWA, to examining the diversity of soil organisms in chemically treated stands, to measuring planted hemlocks to determine efficacy of silvicultural practices to benefit hemlocks in the understory.

Sources

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