Hemlock Woolly Adelgid

The hemlock woolly adelgid (Adelges tsugae or HWA) is a very small, aphid-like insect that feeds at the base of hemlock needles. Adults are red to purple-black and about 1mm long; nymphs are about the size of the period at the end of this sentence. HWA is most visible towards the end of adulthood, when they cover themselves in a white, cottony wax where they lay up to 300 eggs. Eggs hatch into nymphs which crawl or are moved by wind, birds and other animals to another hemlock needle, where they feed on the starches the needle needs to live. The adelgid stays in that spot for the rest of its life. Although many eggs are produced, mortality of the nymphs can be as high as 90% since their dispersal method is mainly dependent on chance. HWA has two generations a year so populations can build rapidly.


Hemlock woolly adelgid was first detected in the eastern US in 1951, and is now present from Georgia to southern Maine. This tiny insect from southern Japan attaches to the base of needles on eastern and Carolina hemlocks where it remains and feeds. Trees die in 3–10 years depending on the tree’s location and overall health. HWA spreads more quickly and kills trees more rapidly in areas with warmer winters, but continues to spread into cooler regions and shows evidence of adapting to colder winters.

Hemlocks are a foundation species of eastern forests, where they anchor steep slopes, keep streams cool, regulate water fluctuations across seasons, promote diverse fish assemblages, and create a unique environment for many other species of plant and animal life. Treatment with pesticides can preserve individual trees, but is impractical on a large scale. Biocontrols are available, but implementation may not arrive in time to save our valued hemlock populations.


HWA continues to expand its range without human help at a rate of up to 10 miles/year. HWA crawlers can be dispersed by wind and mammals with birds playing a role in the natural spread. HWA can also be introduced to new locations when infested nursery hemlocks are moved.


HWA develops and reproduces on all species of hemlock (Tsuga spp.). Western species are more tolerant whereas eastern hemlock and Carolina hemlock are extremely vulnerable when attacked.

There are over 280 cultivars of eastern hemlock including many weeping and prostrate forms making Hemlock a popular ornamental and landscape plant. 


Early detection of  HWA is vital for the management and preservation of hemlocks. Early detection programs are active all over the east coast and where you live will determine what action can be taken. If you live near the leading edge of the HWA infested range it is especially important to monitor hemlock trees to slow the spread of HWA to new areas. To see where this pest has been reported, visit EDDMapS hemlock woolly adelgid map. If you want to receive an email notification when HWA would be active in your area, sign up for the HWA Pheno Forecast alerts.

The best way to identify HWA is to look for the cottony balls at the base of hemlock needles in the late winter. HWA infested hemlocks also appear greyish rather than dark, glossy green.
Additionally you can look for:

  • Yellowing needles on hemlocks
  • Unseasonal needle drop, thinning foliage and twig dieback
  • Groups of dead hemlocks in a small area

HWA can be difficult to detect at low populations so it is best to look at many hemlocks and lots of branches. If you have limited time, concentrate on hemlocks near water or streams because birds play an active role in spreading HWA.

Report the Pest.

Use EDDMaps to report your sighting of hemlock woolly adelgid.


Contact Your Diagnostic Lab.

You can personally contact your state plant diagnostic lab or help collect data by submitting a sample of your plant and pest or pathogen.