Fact Sheet

Hemlock Woolly Adelgid


Adelges tsugae


Hemlock Woolly Adelgid is an aphid-like insect that parasitizes Eastern Hemlock (Tsuga canadensis) trees. The first generation of the bug are 1.4 mm long, and are present from June to March. Second-generation crawlers are 0.9 mm long, and are present March to June. Both generations are black, wingless, have heavy waxy coats, and lay clusters of oblong amber eggs (0.3–0.4 mm long) in white woolly ovisacs. The nymphal stage is the only mobile stage. During this period, they are less than 1.5 mm long, and are brownish-orange in colour.

Habitat & Impact

This pest affects the Eastern Hemlock – a foundational tree in Nova Scotian forests – especially on riverbanks and lakeshores. It feeds on the base of hemlock needles, causing premature dieback, needle loss, foliage thinning, and tree death. Wide-scale Hemlock death causes changes in forest nutrient cycling, and deprives birds, moose, and deer of winter shelter and food. If adjacent to water, Hemlock death increases erosion rates, and decreases the shade available to aquatic organisms.


In North America, Hemlock Woolly Adelgid was first recorded in Virginia in the 1950s, likely imported unintentionally with Japanese nursery stock. From there, it slowly spread up the Eastern seaboard. This species was first recorded in south-west Nova Scotia in 2017. It has been spreading steadily since.

Dead Hemlocks
Dead Hemlocks in a distant forest

Similar Species

Balsam Wooly Adelgid Adelges piceae

This tiny insect is native to central Europe, and was likely introduced accidentally through the nursery trade. It attacks both stems and shoots of our native Balsam Fir (Abies balsamea), which weakens and kills trees. This species occurs throughout the province, however, it does not kill trees as quickly or as extensively as Hemlock Woolly Adelgid.

Balsam Woolly
Infestation of Balsam Woolly Adelgid on tree trunk

Key Identification Features

Woolly sacs with amber eggs, damage to trees, adult insects.

Hemlock Woolly Adelgid
Hemlock Woolly Adelgid eggs
Hemlock Woolly Adelgid eggs
Hemlock Woolly Adelgid Egg Sacks
Woolly egg sacks cling to branches
Infestation on Hemlock
Hemlock Woolly Adelgid infestation on Eastern Hemlock
Woolly Sacks on Hemlock
Woolly sacs on Hemlock

Interesting Fact

There are two generations of Hemlock Woolly Adelgid born per year. Each generation can produce over 150 eggs. Once this species  has become established in a habitat, its population grows rapidly.

Stewardship Actions

Monitor Hemlock Trees for indicators of infection. Because adult Hemlock Woolly Adelgid are wingless, they spread mostly via wind, birds, and human activities. Avoid moving firewood or wood scraps over long distances. If you spot this invasive species, report the location to iNaturalist, or directly to the NSISC.