General Impact
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. The impact of A. piceae on the wood products industry of the north-eastern U.S. could be substantial. Hemlock trees are ecologically important and provide a unique environment. The lifespan of an eastern hemlock can reach 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. Several threatened or endangered species of flora and fauna require hemlock forests to survive. These forests are normally stable and resistant to plant invasions, so 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. For example, lepidopterans, like Semiothisa fissinotata, which feed solely on hemlocks, will be affected.
Location Specific Impacts:United States (USA) 
Agricultural: Adelges tsugae becomes an agricultural pest on trees destined for the nursery trade or wood products industries.
Economic/Livelihoods: Eastern hemlock is economically important to the nursery and wood products industries of the eastern United States.
Ecosystem change: Large-scale hemlock die-off will affect species diversity, vegetation structure, stand environmental conditions, and ecosystem processes.
Herbivory: Immature nymphs and adult Adelges tsugae damage trees by sucking sap from the twigs.
Interaction with other invasive species: Declining hemlock stands are expected to become increasingly vulnerable to invasion by alien plants in Delaware Water Gap National Recreation Area (Evans, 2002).
Modification of hydrology: It is possible that extensive hemlock mortality in the eastern USA will result in increased nitrate leaching to adjacent riparian habitats, due to increased nitrification rates and inorganic nitrogen availability. This has potential ramifications for both stream productivity and deterioration of water quality in watersheds which provide public drinking water (Jenkins et al.. 1999; in Ward et al.. 2004). eastern United States (United States (USA))
Agricultural: Adelges tsugae becomes an agricultural pest on trees destined for the nursery trade or wood products industries.
Economic/Livelihoods: Eastern hemlock is economically important to the nursery and wood products industries of the eastern United States.
Herbivory: Immature nymphs and adults damage trees by sucking sap from the twigs.
Other: Large-scale hemlock die-off will affect species diversity, vegetation structure, stand environmental conditions and ecosystem processes.
Threat to endangered species: Adelges tsugae is a threat to the eastern hemlock (see Tsuga canadensis in IUCN Red List of Threatened Species) classified as a species of "Lower Risk category" (A taxon is Lower Risk when it has been evaluated, does not satisfy the criteria for any of the categories Critically Endangered, Endangered or Vulnerable. Taxa included in the Lower Risk category can be separated into three subcategories: Conservation Dependent (cd); Near Threatened (nt); Least Concern (lc)).
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