Adelges tsugae

Adelges tsugae

Hemlock Woolly Adelgid

Hemlock Woolly Adelgid (Adelges tsugae) or HWA is a destructive pest that poses a major threat to eastern and Carolina hemlock trees throughout their range. Accidentally introduced from Asia in the 1920s, HWA feeds on hemlock and has now been established in 11 eastern states from North Carolina to Massachusetts, causing widespread mortality of hemlock trees. HWA has been found 20 miles from Vermont, 5 miles from New Hampshire, and 25 miles from Maine.

Description

The hemlock woolly adelgid (HWA) is an introduced aphid-like insect that feeds on the phloem sap of tender hemlock shoots.

Symptoms and Signs

In the northern portion of hemlocks' range, mortality typically occurs within 4 to 10 years following infestation.

Ecology

Hemlock is a vital component of the New England forest system, and is the third most prevalent tree in Vermont. Hemlock provides protection from erosion along stream banks, food for deer and wildlife, and shelter for deer in the winter. Major changes in ecosystem structure and function, including hydrologic processes, is expected with the loss of hemlock.

Host plants

Unlike the Balsam Woolly Adelgid that only attacks mature balsam fir, HWA infests all age classes of hemlock. In addition, where hemlock occurs in pure stands in northern regions, the most commonly observed tree species to succeed hemlock is black (sweet) birch.

Control

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 threat 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.

Actions 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 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.