Hemlock woolly adelgid

Identification

Size: Less than 1.5 mm long

Color: Dark reddish-brown to purple, covered in white woolly wax

Description: The Hemlock woolly adelgid is an invasive, aphid-like insect that feeds exclusively on hemlock trees and can kill them within 4–10 years. Not yet established in Alberta, but ornamental hemlocks purchased through nursery trade represent a potential introduction pathway.

The Hemlock woolly adelgid (Adelges tsugae) is an invasive insect native to Asia that feeds exclusively on hemlock trees (Tsuga spp.). It is currently established in eastern North America and parts of the Pacific Northwest and has not been confirmed as established in Alberta. However, hemlock is grown as an ornamental tree in Alberta and is available through the nursery trade, creating a potential introduction pathway that warrants awareness.

Unlike most sap-feeding insects, HWA feeds year-round by inserting its stylet at the base of hemlock needles to extract stored starches. This continuous feeding prevents trees from recovering between seasons and leads to progressive needle loss, branch dieback, and eventually tree death — typically within 4–10 years of infestation without treatment.

HWA is a regulated pest in Canada. Any suspected detection should be reported to the Canadian Food Inspection Agency (CFIA) or Alberta Agriculture and Irrigation.

The most reliable identification sign is the presence of small, white, woolly egg sacs (ovisacs) attached at the base of hemlock needles on the underside of branches. These are approximately 1.5 mm in diameter and are present year-round, giving infested branches a distinctive ‘snowy’ or cotton-dusted appearance.

Additional identification features:

The adelgids themselves are very small (< 1.5 mm), dark reddish-brown to purple, and found beneath the white wool
Infested needles turn from dark green to grayish-green before dropping prematurely
Damage begins on the lower branches and progresses upward as infestation intensifies
Winged forms may be observed in late spring migrating from the hemlock