Greenish with longitudinal stripes - head black. Eggs are deposited in small slits cut by female along the underside of the edge of the top half of current year's needles. Usually only one egg is laid per needle. Young larvae are gregarious and always feed in groups, principally on older needles. Cocoons may be formed on the tree but most are formed in the duff beneath the defoliated tree.
Heavy defoliation causes trees to appear grey or greyish-brown and tree crowns to appear thin.
Extensive outbreaks on western hemlock have been recorded in western North America.