Pest description and damage This is a new pest in western Washington and possibly elsewhere. It was noticed in the spring of 2009 in Everett, Lynnwood and Monroe areas, so it is likely to have arrived earlier. Typical of many sawflies, small gregarious larvae hatch from eggs in early spring and begin feeding in groups. Larvae are at first greenish with black dots down the side and a black head and legs. Before they pupate, the head and body turn orange with black spots. Initially, the larvae consummeonly soft leaf tissue leaving a fine network of leaf veins, but as larvae mature, larger veins are consumed along with the other leafy tissue. Finally, only the petiole, midrib, and the bases of a few secondary veins remain. Known hosts include the European and American mountain ash (Sorbus aucuparia and S. americana). Sawfly larvae are caterpillars with 5-7 prolegs (soft fleshy gripping hind legs) while moth larvae have five or fewer prolegs. Moth larvae also have little crochets on their prolegs while sawfly larvae do not. This is important because Bacillus thuringiensis is effective on moth and butterfly caterpillars, but not on sawfly caterpillars. A characteristic of sawflies is to feed in groups along a leaf edge with their hind end curled upward. The adult is a black wasp from 0.2 to 0.4 inch in length.
Biology and life history Adult sawflies emerge from the cocoons overwintering in the soil and leaf litter. Females cut slits in the marginal leaf surfaces and insert eggs in blister-like "pockets" in the leaf tissue. Larvae emerge and feed gregariously. By three to four weeks, the larvae are mature and drop to the ground to spin their cocoons. In Monroe, Washington, adults emerged in August and a second generation of larvae began to feed immediately suggesting a third generation.
Pest monitoring Sawflies that are new to an area tend to build up large numbers and can cause significant defoliation. Early detection by watching as the new growth in the lower canopy develops is important because sawflies typically eat continuously and then drop out of sight (to pupate in the soil). Damage appears to occur overnight. Control of the first generation will reduce the number and severity of defoliation by the second and third generations.
Prune out infested branches, though that seems more radical than allowing defoliation to occur with branches releafing. Often the first generation of larvae that emerge in the spring concentrate their feeding damage on the lower branches, while later generations feed on the higher branches.
See Table 2 in:
Chemical Control of Landscape Pests
For more information
See "Sawfly" in:
Rosetta, R. 2009. Mountain ash sawfly. OSU Nursery IPM (https://agsci.oregonstate.edu/nurspest/insects/mountain-ash-sawfly)