Adelges tsugae (Hemlock Woolly Adelgid) is a small piercing-sucking insect that feeds on Tsuga spp. (hemlock ) trees. Native to Asia and the Pacific Northwest, the Hemlock Woolly Adelgid is invasive in the eastern United States where it attacks Tsuga canadensis (Eastern Hemlock) and T. caroliniana (Carolina Hemlock). It is currently found in 19 eastern states and has caused extensive mortality to hemlock forests. The ecological and economic impacts of this pest are significant, widespread, and often difficult to quantify. As the Hemlock Woolly Adelgid continues to disperse throughout the range of Eastern and Carolina Hemlocks, management techniques aimed at controlling it are being researched, implemented, and assessed. This introductory paper provides an overview of the biology, life cycle, ecology, and history of this pest in the eastern US as a foundation for this special issue.

Introduced forest pests and pathogens are a major threat to national parks. This paper reviews existing impacts, projected impacts, and management options for Tsuga canadensis (Eastern Hemlock) forests in US national parks threatened by the introduced insect Adelges tsugae (Hemlock Woolly Adelgid [HWA]). Eighty-five national parks, or 21% of all parks in the US national park system, are encompassed within the range of Eastern Hemlock. These 85 parks include iconic areas such as national battlefields and the Appalachian National Scenic Trail. Four focal parks of this study—Delaware Water Gap National Recreation Area, Great Smoky Mountains National Park, Shenandoah National Park, and New River Gorge National River—are collectively visited by 16 million people annually and Eastern Hemlock is a forest component in 2–26% of their area. Results of research on HWA impacts to forest species composition, soil nutrient cycling, watersheds and fisheries, wildlife habitat, and visitor experiences and safety have already been reported from these parks. A general principle is that after Eastern Hemlock forest decline, some species (e.g., some avian species favoring other tree species) benefit, while those associated with Eastern Hemlock must adapt or decline. Forecasting future forest-tree composition is complicated by the fact that: (i) many possible replacement tree species are themselves threatened by introduced damaging agents, (ii) changes hinge upon understory dynamics such as invading exotic plants or expansion of native shrubs, and (iii) this die-off event is occurring within a context of multiple interacting factors such as elevated herbivory, climate change, and atmospheric pollution. Some management strategies for parks include: intensive HWA chemical treatment at priority sites, biocontrol, genetic manipulation for HWA resistance in Eastern Hemlock, exotic plant treatment, facilitated establishment of native vegetation, or doing nothing, the last of which also is likely to result in appreciable forest change. Threats to US national parks posed by introduced forest pests and pathogens warrant heightened attention.

We monitored a population of Carolina Hemlocks in northwestern North Carolina for four years to examine the rate and pattern of decline in response to infestation by Adelges tsugae (Hemlock Woolly Adelgid). Our yearly census of hemlock condition and severity of the adelgid infestation included trees of all sizes. We estimated declines in condition as the portions of the leaf canopy that were lost. Initially, infestation occurred throughout the population but was severe in only a small cluster of individuals. Within 1 year, the area of severe infestation increased in size to encompass 48% of the population. In another region of the population, there was a cluster of relatively healthy individuals comprised largely of seedlings. Of the 4 size-classes of trees, sapling-sized individuals experienced the highest rates of decline in condition. Most trees declined to poor health within 3 years of an observation of moderate to severe infestation.

The spread of Adelges tsugae (Hemlock Woolly Adelgid) directly threatens the survival of Tsuga canadensis (Eastern Hemlock) and has also triggered pre-emptive and salvage logging. In this study, we took advantage of a 25-year-old accidental experiment involving Eastern Hemlock removal by logging at Smith College's MacLeish Field Station, in western Massachusetts, to investigate how microclimate, ecosystem processes, and forest-floor animal communities might change in the decades following Eastern Hemlock loss. On average, mean understory light levels in summer were 68% higher under young Black Birch (Betula lenta) canopies as compared to adjacent mature Eastern Hemlock forest. Mean daily air temperature, relative humidity, soil temperature, and organic-layer moisture content were similar between young Black Birch and mature Eastern Hemlock plots, although some of these factors were significantly more variable in the former. The soil organic horizon was significantly thicker in Eastern Hemlock plots, but net nitrification rates did not differ substantially between young Black Birch and mature Eastern Hemlock forest plots. We detected significantly greater densities of microarthropods (e.g., mites, collembolans) in the forest floors of Eastern Hemlock plots, possibly linked to the thicker organic horizon. Our results indicate substantial changes in forest structure and microarthropod communities with Eastern Hemlock removal, but little evidence of large changes in key ecosystem processes, like nitrogen cycling. Other sites that represent similar accidental experiments with Eastern Hemlock removal due to past human disturbance likely exist and should be studied before intact reference stands are lost to Hemlock Woolly Adelgid or preemptive salvage logging.

Tsuga canadensis (Eastern Hemlock) forests are experiencing widespread mortality due to the invasive insect Adelges tsugae (Hemlock Woolly Adelgid). This study sought to document the community response to HWA across latitude. I selected two sensitive response groups—plants and ants— to use as biological indicators of ecosystem change to monitor differences along a natural gradient of Eastern Hemlock mortality among 3 forest types: relatively healthy Eastern Hemlocks, dead or dying Eastern Hemlocks, and hardwood stands. I sampled understory vegetation, ants, and soils at each site and compared sites using a linear mixed-model to discern the best predictors of species density. I also compared analyses of variances across forest types among response variables. There was an average two-fold increase in understory vegetation species density between Eastern Hemlock and hardwood forests; ant species density was not influenced by forest type. Analysis of variance comparisons for understory vegetation showed that forest type affects understory vegetation, a result which was attributable to differences in a few dominant plant species. The linear mixed-model showed that Eastern Hemlock density and latitude were important predictors for both ant and vegetation species densities; soil pH and stand density were predictors for vegetation species density, and litter depth was a predictor for ant species density. My findings show that large structural changes in Eastern Hemlock forest communities (induced by the effects HWA) alter a foundation ecosystem by shifting the composition of understory plant communities, but not ant communities.

Adelges tsugae (Hemlock Woolly Adelgid [HWA]) is rapidly spreading across Kentucky, yet the potential effects on bird communities of the region are not known. We surveyed birds in Tsuga canadensis (Eastern Hemlock) and other forest types across the Appalachian mountain region of Kentucky during the early stages of infestation when HWA was absent from most sites. Based on associations between birds and forest types, we identified 4 bird species likely to be negatively affected by Eastern Hemlock decline. For one of these species, Setophaga virens (Black-throated Green Warbler), we used landscape metrics of forest composition to make predictions about the density of Eastern Hemlock necessary for persistence. This type of information can help managers establish control strategies for HWA.

The invasive insect pest Adelges tsugae (Hemlock Woolly Adelgid) threatens the ecologically unique Tsuga canadensis (Eastern Hemlock)-dominated forest type throughout its range. Relatively little is known about how the loss of this forest type will affect the relative abundance of amphibians. This study assessed the relative abundance of the juvenile phase of Notophthalmus viridescens viridescens (Eastern Red-spotted Newt, Red Eft) in Eastern Hemlock-dominated stands (n = 5) and mixed deciduous stands (n = 5) at Harvard Forest in Petersham, MA, using both transect surveys of the forest floor surface (n = 368 Red Eft observations over four seasons), and intensive searches of quadrats (n = 27 Red Eft observations over two seasons). Using data from transect surveys, the average relative abundance of Red Efts was more than two times greater in Eastern Hemlock-dominated stands than in mixed deciduous stands, however the differences were not statistically significant (P = 0.146). Quadrat surveys yielded relative abundance estimates for Red Efts that were more than 5 times greater in Eastern Hemlock-dominated stands than in mixed deciduous stands, but again the differences were not statistically significant (P = 0.213).

Adelges tsugae (Hemlock Woolly Adelgid; HWA) is a non-native forest insect that causes defoliation and mortality of hemlock in the eastern US. We quantified the extent to which people are potentially affected by the spread of HWA infestation where they live and where they recreate. We also considered how these impacts might change through time using data from 2007, 2009, and 2011. The study area included hemlock stands in a 7500-km² region of central Connecticut and central Massachusetts. We used sample-plot data on live basal area and vigor of hemlock stands to interpolate hemlock health characteristics for all hemlock stands in the study area. We estimated a loss of property values in the region of approximately $24.6 million USD. This estimate was conservative because there were insufficient data to fully quantify the economic losses associated with the death of hemlock trees and the degradation of recreational opportunities. The spatial extent of the HWA infestation suggests that both of the latter categories of economic losses are likely substantial. These data can be used to consider the economic efficacy of actions taken to ameliorate the effects of the HWA infestation.

In light of the increasing need for long-term, sustainable management for Adelges tsugae (Hemlock Woolly Adelgid), researchers are investigating host-plant resistance as part of an integrated approach to combat the pest. This paper reviews the progress made towards developing a resistant hemlock in the southern Appalachians and highlights the importance of investing research and development resources in this field. Along with describing inter- and intraspecific resistance in hemlocks, this paper reviews investigations into resistance mechanisms and outlines the many options for actions that could be or are being taken to increase resistance and restore hemlock forests.

Populations of Tsuga canadensis (Eastern Hemlock) and Tsuga caroliniana (Carolina Hemlock) are declining due to infestation by Adelges tsugae (Hemlock Woolly Adelgid), an exotic insect pest. A better understanding of the environmental conditions required for seed germination is needed to more efficiently utilize the seeds collected for genetic-resource conservation and the establishment of seed orchards. This study examined the effect of cold—moist stratification treatments of varying duration (0, 1, 15, 30, 60, 90, and 120 days) on total germination (%) and the number of days to first and peak germination (germination speed) on seeds of both species in experiments conducted at 22 °C and 16 h:8 h, light:dark photoperiod. Overall total germination for Eastern Hemlock was 33.3% and increased with increasing duration of the stratification treatments. Carolina Hemlock total germination was 17.1% and varied little among the treatments, although fewer seeds tended to germinate following longer durations of stratification. Stratification increased germination speed of Eastern Hemlock but not Carolina Hemlock. The results indicate that Eastern Hemlock seeds should be cold—moist stratified at 4 °C for at least 30–60 days prior to sowing to promote higher total germination. Carolina Hemlock seeds can be sown directly following a 24-h soak with no additional cold—moist stratification.

Laricobius osakensis (Coleoptera: Derodontidae) was imported from Japan to the United States in 2006 for study in quarantine facilities as a potential biological control of Hemlock Woolly Adelgid. Laricobius osakensis was released from quarantine in 2010, but it was soon discovered that the colony also contained a cryptic species, Laricobius naganoensis. This led to the placement of L. osakensis back into quarantine and development of a method to purify the colony. To distinguish the two species, we designed a restriction fragment length polymorphism (RFLP) assay using mitochondrial DNA sequences and developed a non-lethal testing method. Twenty-one diagnostic nucleotide sites separated the two species, and they both exhibited extraordinary intra-specific haplotype diversity. Sequencing the ITS2 nuclear region did not produce evidence of hybridization between the species in the field or in the lab colony. Splitting the colony into small groups and testing their species composition was successful in isolating L. osakensis. Efforts should be made to maintain high genetic diversity in L. osakensis colonies. Continued genotyping of new colony stock will be necessary to fully characterize the diversity within both species.

Prior to the loss of Tsuga canadensis (Eastern Hemlock) stands in the Great Smoky Mountains National Park (GRSM), we collected baseline data during 2006–2009 at two locations (Copeland Creek and Gabes Mountain) regarding macrofungi that occur under this tree species. We studied macrofungi in order to understand the current and changing ectomycorrhizal and saprobic fungal community structure associated with healthy (imidicloprid-treated) and dying Eastern Hemlock stands and to contribute data for the All Taxa Biodiversity Inventory (ATBI) in GRSM. A total of 121 taxa representing 75 ectomycorrhizal, 1 pathogenic, and 45 saprobic species, were collected from 92, 59, and 106 sampling locations in 2006, 2007, and 2008, respectively. Macrofungal species richness, diversity, and evenness (E) in Copeland Creek were significantly greater (P = 0.05) in 2006 than 2007. Eighty percent of all fungi collected were found in Copeland Creek (487 m elevation) where trees are <75 years old; the remaining 20% were collected at Gabes Mountain (1158 m elevation) where trees are >150 years old. The most common taxa found across sampling locations included Russula fragrantissima (22.0%), Amanita citrina var. lavendula (Lavender False Death Cap; 17.1%), Austroboletus gracilis (Graceful Bolete; 14.6%), Laccaria laccata (Deceiver; 9.8%), and Russula russuloides (9.8%). Amanita cinereopannosa occurred at a low frequency overall (2.4% of trees sampled) but when present had high abundance. Total macrofungi and ectomycorrhizal fungi at Copeland Creek had significantly greater species richness, density, and evenness than at the Gabes Mountain site. Ectomycorrhizal fungi evenness values following imidicloprid soil-drenching treatments were significantly less uniform for macrofungi collected from within 5 m of trees treated in subplots with full rates of the chemical than for those near untreated trees (E = 0.2 and 0.4, respectively). In addition, there was a numerical trend towards significantly less diversity and evenness at the full chemical application rate compared to the half rate or untreated control plots. There were no differences in the occurrence of saprobic fungi between chemically treated and control trees. Associated vegetation had a significant impact on the occurrence of macrofungi. Across both locations, a total of 37 plant, shrub, or tree species were identified; Acer pensylvanicum (Striped Maple) with an overall abundance of 11.0%, Pyrularia pubera (Buffalo Nut; 10.5%), Ilex opaca (American Holly; 8.2%), Calycanthus floridus (Eastern Sweetshrub; 4.8%), and Rhododendron maximum (Great Rhododendron; 4.7%), were the most common associated species. Species richness, diversity, and evenness of the associated vegetation were significantly different between locations. Evenness data for plant species abundance was not equal, but varied within and across locations.

Below- and above-ground microbial communities are expected to dramatically change following the loss of healthy Tsuga canadensis (Eastern Hemlock) ecosystems due to the effects of Adelges tsugae (Hemlock Woolly Adelgid [HWA]) infestation. In 2006, few healthy Eastern Hemlock stands remained in Great Smoky Mountains National Park (GRSM), and we sought to obtain baseline soil microbial data from these undisturbed areas to better understand them. This study surveyed root fungal communities (e.g., mycorrhizae) using molecular cloning and cultural methods at 2 remaining healthy Eastern Hemlock stands in GRSM. We sampled roots from 40 trees representing 2 age classes (<75 and >150 years old) and 2 elevations (486 m and 1158 m) to obtain molecular clones and isolated cultures. Molecular clones from the ITS region indicated the presence of 124 taxa in 23 genera. The dominant genera were Piloderma (8.2%), Russula (6.8%), Tomentella (4.0%), Meliniomyces (3.4%), and Clavulina (2.9%). Ninety-seven of the taxa identified occurred at <1.0% relative abundance. The most frequent fungal species were Meliniomyces variabilis (22.6%), Russula granulata (16.1%), Phialocephala fortinii (9.7%), and Cladophialophora chaetospira (16.1%). Species richness, diversity, and evenness for total fungi, ectomycorrhizal species, and saprobic species were similar at different elevations and stand ages. Using ITS sequence data, we identified 9 species from 1800 fungal isolates from Eastern Hemlock root tissue, the most common of which were Trichoderma harzianum (24.4%), Trichoderma viride (24.4%), Trichoderma asperellum (17.1%), and Ilyonectria radicicola (12.2%). Species richness, diversity, and evenness were not significantly different across locations for the molecular data. Baseline data for the fungal taxa identified in the study may be important for understanding ecological changes that can occur post-HWA damage and HWA-caused stand losses. These data could also be used to support Eastern Hemlock reforestation efforts within current or former habitats in GRSM and surrounding ecosystems.