Broken dark lines on sides and 4 black dots on dorsum of each segment when mature. Length is 25 to 30 mm. Moths emerge to lay eggs within 10 to 14 days. Eggs hatch the following spring. Early larval instars feed on new foliage.
Later instars feed on older foliage and clip off small twigs.
Injured foliage dries out and imparts the typical reddish colour of infestations. The preferred host is western hemlock, although Sitka spruce , Pacific silver fir , Douglas-fir and associated understory vegetation may be defoliated by this insect. See HForest .