Keeping the Legacy Alive

What is HWA?

© Connecticut Agricultural Experiment Station Archive
A Creeping Conflict

Hemlock Woolly Adelgid (HWA, Adelges tsugae) is an invasive, aphid-like insect that threatens hemlock trees in North America. HWA came to the U.S. from Japan where it is a native hemlock pest. It does not pose a serious threat in areas where HWA is native, as there are a suite of HWA predators and hemlocks in these areas are adapted to living with HWA. However in North America, HWA is considered an invasive species and poses a serious risk to eastern Hemlocks. In North America, HWA was first reported in Virginia in 1951, and it has moved northward up the eastern seaboard in the last 70 years, throughout much of the hemlock range. Areas of extensive tree mortality and decline are found throughout the infested region, which now includes parts of Southwestern Nova Scotia.

© Ron Neville, CFIA

© Ron Neville, CFIA

© C.Gray

© C.Gray


HWA was first discovered in Nova Scotia in July of 2017 in Yarmouth County, although it had likely arrived several years prior based on the presence of dead and dying hemlocks. Areas of extensive tree mortality and decline are now found throughout the infested region which, as of 2020, includes 5 counties: Yarmouth, Shelburne, Digby, Annapolis, and Queens.

© MacAulay LIbrary, Cornell Lab of Ornithology
Some HWA Facts

HWA can cause defoliation, twig dieback and mortality in as few as 4 to 10 years, though it can take up to 20 years.  All hemlock sizes and ages are vulnerable to attack. HWA is naturally spread by wind, birds and mammals. Long distance dispersal can occur via infested nursery stocks, logs and firewood.

© New York Hemlock Initiative
Life Cycle

HWA are all female and reproduce asexually. They have two generations per year, living their lives entirely on hemlock. In its native range, HWA has a winged sexual generation that flies to spruce trees, but this stage does not survive on North American spruce species.

Click here to learn more about the life cycle of the HWA and how that impacts management strategies.

Impact on Trees

 The HWA feeds on the storage cells at the base of the needle, eventually killing the needles and buds and causing needle loss and stem dieback. In the late summer months the newly settled HWA crawlers (sistens) go through a period of dormancy called aestivation, where its mouthparts have been inserted but they are not actively feeding and developing. Once HWA breaks aestivation, they are active throughout the fall, winter, and spring, accumulating the wool that gives it its name. The wool protects the developing HWA, insulating it from cool winter temperatures and sheltering it from natural enemies, and ultimately creating an ovisac in which to lay eggs.

Additional Resources
A Documentary on Hemlock Woolly Adelgid
Hemlock Woolly Adelgid in Nova Scotia