WSU Extension


Caption: Mountain ash sawfly larvae feeding on leaf
Photo by: S. Katovich, USDA Forest Service,
print version| pdf version| email url    

Mountain Ash : Mountain ash sawfly
(revision date: 2/14/2019)

Mountain ash sawfly is a relatively new pest to western Washington. First reported in 2009, the larval stage of this insect feeds on leaves of both European and American mountain ash (Sorbus aucuparia and S. americana). Adult sawflies are a stout, thick-bodied wasp. The females lay eggs in slits cut along leaf margins. Caterpillar-like larvae emerge in early spring and feed in groups or rows along the leaf margins, often with their rear ends curled. These voracious feeders skeletonize leaves, leaving only the stem, midrib, and occasionally some larger veins. Young larvae are greenish with black spots and black heads. As they mature over a period of 3-4 weeks, the larvae reach about 5/8 to 3/4 inch long and turn yellow-orange with black spots. They drop from the plant to the ground, where they pupate in soil or leaf litter. A second generation of larvae begins to feed in August. It is believed that a third generation may occur in some years. Damage is first seen on lower branches and may seem to appear overnight. Early detection is important to limit defoliation. While mountain ash sawfly is considered to be mainly an aesthetic problem, repeated severe defoliation may injure trees.
Management Options

Non-Chemical Management
  • Monitor trees beginning in early spring. New growth in the lower canopy is particularly susceptible. Watch for the characteristic blister-like pockets on leaf margins which indicate egg-laying activity.
  • Hand-pick and destroy sawfly larvae where practical. Also, remove and destroy heavily infested leaves, twigs, or branches.
Select non-chemical management options as your first choice!

Chemical Management

Mountain ash sawfly is primarily an aesthetic problem. Applications should only be made when repeated, severe infestations pose a risk to tree health. Homeowners should not make foliar applications to trees over 10 ft tall. Consult a commercial pesticide applicator for treatment of trees and shrubs over 10 ft tall.

Listed below are examples of pesticides that are legal in Washington. Always read and follow all label directions.
  • Bonide Captain Jack's Deadbug Brew RTSpray [Organic]
    Active ingredient: spinosad  |  EPA reg no: 4-471
  • Bull's-Eye Bioinsecticide
    Active ingredient: spinosad (spinosyn A+D)  |  EPA reg no: 62719-314-56872
  • Monterey Garden Insect Spray [Organic]
    Active ingredient: spinosad (spinosyn A+D)  |  EPA reg no: 62719-314-54705
  • Safer Brand BioNEEM Multi-Purpose Insecticide & Repellent Conc [Organic]
    Active ingredient: azadirachtin  |  EPA reg no: 70051-6-42697
  • This list may not include all products registered for this use.
    - hide images

+ Show larger images

Caption: Mountain ash sawfly larvae feeding on leaf
Photo by: S. Katovich, USDA Forest Service,
Caption: Mountain ash sawfly skeletonizing damage to leaves
Photo by: D.D. O'Brien, Cornell Univ,
Caption: Mountain ash sawfly eggs in leaf margin
Photo by: L.J. Lipovsky, Maine Forest Svc,