If populations are high, the current year's foliage may also be consumed. Although trees of all ages are defoliated, those in the most exposed locations or in the overstory suffer the most. As a result, branches, and sometimes an entire tree may be killed. Ornamental, windbreaks, and Christmas tree plantations are of particular concern. Identification Adults resemble flies and are 7 to 8 mm long, with four transparent wings.
Females lay small oval eggs into slits in the needles they cut with their sawlike apparatus.
They then cover the eggs with a green frothy substance. Newly emerged larvae are 6 mm long, with a shiny black head, black legs, and a yellowish-green body. By the fourth instar, the larvae have two black lines down their back with numerous yellow spots. The fifth and sixth instars have a distinct mottled pattern and can reach 25 mm in length. As with other sawflies, larvae resemble caterpillars except they have six or more pseudolegs or prolegs behind the true legs underneath the abdomen.