Adelges tsugae (hemlock woolly adelgid)

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).

  The USDA Forest Service has produced a handbook which summarizes current scientific knowledge of the impact of Adelges tsugae on eastern hemlock forests. It is designed to help resource managers make informed decisions in preparing management plans to deal with the hemlock woolly adelgid. In addition, Onken (2005) provides a summary of management options and future directions for the control of

.Preventative measures: Certain species of Asian hemlocks are resistant to

, and nurseries are beginning to grow and sell those in the United states. The western hemlock (

) is highly resistant to

, 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.