Hemlock woolly adelgid

Hemlock woolly adelgid (Adelges tsugae), commonly abbreviated as HWA is a true bug native to East Asia that feeds by sucking sap from hemlock trees (Tsuga sp.). In eastern North America it is a destructive pest that poses a major threat to the Canadian hemlock (Tsuga canadensis) and the Carolina hemlock (Tsuga caroliniana). The former extends north of the current range of the adelgid, but there are fears that it could spread to these areas. Accidentally introduced from Asia in the 1920s, HWA has now been established in eleven eastern states from Georgia[1] to Massachusetts, causing widespread mortality of hemlock trees.

Additional recommended knowledge

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Daily Sensitivity Test

Hemlock is a vital component of the New England forest system, and is the third most prevalent tree in Vermont. Providing protection from erosion along stream banks, food for deer and wildlife, and shelter for deer in the winter, hemlock is also valued as both an ornamental and as an important source of lumber. HWA has been found just inside the Vermont border, five miles (8 km) from New Hampshire, and twenty five miles (40 km) from Maine. Few options are available for control of this pest. The use of pesticides is limited because in forested lands, hemlocks often grow along streams, swamps, or lakes and the risk of aquatic pollution is great. In urban communities the general public does not favor the use of these materials. Alternative management options are needed and the potential use of entomopathogenic fungi is a viable tool for consideration.

The hemlock woolly adelgid feeds on the phloem sap of tender Hemlock shoots. Unlike the balsam woolly adelgid that only attacked mature Balsam Fir, HWA infests all age classes of hemlock. In the northern portion of the Hemlock's range, death typically occurs four to ten years after infestation. Where Hemlock occurs in pure stands in that region, the most commonly observed tree species to succeed it is black (sweet) birch. In contrast, in the southern extreme of its range, Hemlock does not typically occur in pure stands but in linear riparian areas and other moist sites. Succession in these areas is vastly different for many reasons, one being the presence of Rhododendron maximum which often co-exists with hemlock, and due to a combination of influences, stricts regeneration to shade and otherwise understory-tolerant plant species. Major changes in ecosystem structure and function, including hydrologic processes, is expected with the loss of hemlock.

Action taken

Steps that have been taken to eradicate this pest:

• A quarantine has been established in the tristate area of Maine, New Hampshire, and Vermont. Homeowners and nurseries cannot bring in hemlock seedlings or trees from adelgid-infested states into these states without an inspection permit certifying that they are pest-free.
• Pathogenicity trials: isolates have been screened for pathogenicity against thrips, aphids and HWA.
• Isolation and identification: all fungi discovered were isolated, established in pure culture and identified.
• Exploration: cooperative efforts in ME, MA, CT, NJ and VA states led to the isolation of many insect-killing fungi associated with HWA populations. Exploratory activities were also conducted in China where HWA originated.
• Characterization: isolates have been evaluated to determine their growth, sporulation and germination characteristics.
• Biological studies: hypothesizing that range expansion of HWA is limited, trials were designed to determine the low lethal temperatures of this pest. Tests are also being done to ascertain if HWA populations from different plant coldhardiness zones respond differently to low temperature exposures.