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Northern Research Station
One Gifford Pinchot Drive
Madison, WI 53726
(608) 231-9318
(608) 231-9544 TTY/TDD

Hemlock Woolly Adelgid

Effects and Impacts

[photo:] Hemlock woolly adelgiid infested stand that was preemptively harvested.The effect of Adelges tsugae, the hemlock woolly adelgid (HWA), on the two hemlocks native to the eastern United States, Tsuga canadensis and Tsuga caroliniana, is all too visible, but surprisingly difficult to quantify. It was recognized as a very serious pest around 1990 when decline and mortality of hemlocks was observed in Connecticut, New Jersey, and Virginia. Extensive stands of hemlock turned from healthy green to gray, and mortality of trees was observed 4 years following infestation by the pest. With no way to control HWA in a forest situation, some hemlock stands were preemptively harvested (photo above).  Over time, however, it was recognized that while HWA can quickly cause a dramatic change in the appearance of the hemlock crown, many stands are remarkably resilient with mortality occurring gradually over several years. Therefore, there was time to develop biological and chemical control methods and to gather information on the nature of HWA impacts and their variation in different areas.

While the Northern Research Station is a leader in research on the biology and management of HWA, other institutions have been at the forefront in defining HWA impacts.

  • Ecosystem impacts of HWA
    • Richard Evans, Ecologist, USDI National Park Service
    • David Orwig, Forest Ecologist, Harvard University, Harvard Forest
  • Hydrological impacts of HWA
    • Chelcy R. Ford, Ecologist, U.S. Forest Service, Southern Research Station
  • Economic impacts of HWA
    • Thomas P. Holmes, Research Forester, U.S. Forest Service, Southern Research Station

To learn more about research recently initiated by the Northern Research Station on impact of HWA, click on the links below.

Our Research

Last Modified: 09/28/2010