In contrast, caterpillars never have more than five prolegs. Older larvae are 25 mm long, and have glossy black heads and olive green bodies. They have two longitudinal black stripes on their back, with a row of black spots on each side, and a large spot on the last abdominal segment. Mature larvae spin golden-brown cocoons that are 3.2 x 7.5 mm in size. Cocoons can be found in the soil, on twigs, in bark crevices, and beneath loose bark scales.
Adults emerge from the cocoons and look similar to flies.
Note that sawflies have two pair of wings and true flies only have one pair. Eggs are deposited at the bases of needles on the tips of shoots. Newly hatched larvae are similar to older larvae, yet smaller in size. Eggs overwinter and hatch from early to mid- April. Larvae feed gregariously for five weeks and cocoons are formed in mid-May, with adults emerging at the end of May. Larvae from the second generation emerge in June and form cocoons during late July to early August.