Damage, symptoms and biology
The hemlock sawfly is a common and frequently destructive colonial defoliator.
Sawfly feeding damage is usually restricted to stripping of older needles throughout the crown of the tree. Some tree mortality and top-kill may occur in heavily defoliated stands. Hemlock sawfly populations sometimes peak in synchrony with those of the western blackheaded budworm, increasing total defoliation.
Mature larva to 18 mm long. Head, black unmarked. Body, green to grey, light greyish green middorsal stripe bordered with darker greyish green subdorsal stripes; spiracular stripe dark bordered by light greyish green supraspiracular stripe and light grey subspiracular stripe. Seven pairs of abdominal prolegs.
This species overwinters in the egg stage. Larvae feed gregariously on the old needles from late June to early August, mature larvae spin cocoons on foliage from late July to early August, and adults emerge September to early October. Females lay about 75 eggs in niches cut into the edge of needles of the current year's growth. A small proportion of the population overwinters as prepupal larvae in cocoons, some of which may diapause through a second winter.
Life cycle (West of the Rockies)
Major outbreaks have been recorded in the Bowron River to McBride section of the Upper Fraser River in 1975-77, in the Kelsey Bay area in 1978-81, along the Skeena and Kispiox Rivers in 1978-79, and on the Queen Charlotte Islands in 1985-88.
Canadian Forest Service Publications
Diet and feeding behaviour
: Feeds on the leaves of plants.
- Free-living defoliator: Feeds on and moves about freely on foliage.