Mountain ash sawfly

You will see caterpillar-like larvae with green bodies and rear ends curled upward feeding in groups along leaf margins in early summer. Larvae skeletonize leaves from the edges inward, leaving veins and midribs intact. Damage first appears on lower branches and can seem overwhelming overnight. Sorbus scopulina and sitchensis in Western Washington are susceptible. This is a relatively recent pest, first documented in 2009.

Scout new growth on lower branches beginning in spring and look for blister-like pockets on leaf edges indicating egg-laying. Hand-pick larvae where practical and remove heavily infested twigs. First generation control reduces subsequent generations. Spray when larvae are small and clustered.