Mountain ash sawfly (2009)

Michael R. Bush, Christopher Adams, and Sven Spichiger

Pristiphora geniculata

Mountain ash sawfly is a recent addition to pests of American and European mountain ash, (Sorbus americana and S. aucuparia respectively), which are commonly planted ornamental landscape trees and naturalized in parks and woodlands. The insect arrived in the U.S. from Europe in the 1920s. It was found north of Seattle in 2009 and has spread to several counties since.

Description and damage The mountain ash sawfly is a gregarious caterpillar-like larvae that feeds on the leaves of its hosts. They characteristically consume all but the rachis and midveins of leaflets leaving only a skeleton. The young larvae are yellowish and somewhat translucent. As they mature, they become orange-yellow with distinct black spots. The larvae feed gregariously, lining up along the edge of the leaflets nearly head to head, and swing their tail end away from the leaf. In this position, they eat their way to the midrib, then move on to new leaves. The larvae spin a brownish-tan capsule and remain as a prepupa during the winter. When conditions are right they become pupae and then transform to the adult stage. Adults look like small dark flies (but with four wings, they are actually in the order Hymenoptera: bees, ants and wasps).

Biology and life cycle Winter is spent in the pupal stage in the soil. In late spring, the adults emerge, mate, and insert eggs into slits in the leaf. These hatch and the tiny first instar larvae line up along the leaf edge and feed. They molt several times until they are fully mature, then pupate in mid-summer. There are two and possibly three generations. Pupae are reported to be able to delay emergence for two or three years.

Pest monitoring Scouting should include a periodic visual scan for larvae on leaves. Sticky traps hung from branches can to provide evidence of adult emergence. Assess the severity of the infestation. A few bunches of larvae might be ignored early in the spring, but re-evaluate the abundance and damage caused by the second generation. Scouting is encouraged each year as heavily infested trees one year may be free of sawfly larvae the next year.

Management—cultural control

Prune off the clusters of sawflies as soon as they are noticed to reduce to minimize damage to the tree. Early removal reduces the potential for defoliation in mid-summer by the second generation. If the tree is defoliated, provide water and light fertilizer to stimulate new growth.

Management—chemical control

See “Sawfly” in Table 2:

For further information:

Granger, B. 2016. Mountain Ash Sawfly Persists in the Pacific Northwest. Pacific Northwest ISA.