damage to foliage from ash sawfly (80144 bytes)

Ash Sawfly

By Carl Wilson, Horticulturist, Denver Cooperative Extension

Brownheaded ash sawfly has spread from south to north along Front Range Colorado in recent years. The "worm" or larvae chew newly emerging ash leaves in May and early June. The good news is sawflies are only out for a month every year and they are relatively easy to control.

The larvae spend the winter in underground cocoons at the base of previously infested ash trees. In spring, they pupate and emerge as adult sawflys. The adults are small, non-stinging wasps that emerge on sunny days shortly after bud break. They quickly mate and lay eggs on young leaves.

ash sawfly larva  (67470 bytes)

Ash Sawfly larvae

The eggs hatch and feed producing "pinholes" within the leaves. As the larvae grow, they chew whole leaf sections and even whole leaves leaving only the tough leaf veins. A large infestation can defoliate a tree. Damage will diminish rapidly within 2-3 weeks after egg hatch as the larvae become full-grown. By mid-June, they drop and burrow into the ground by where they will remain until emergence the following spring.

Sawflys are easily controlled if infestations are severe. It's key to make timely, late-May applications when sawfly larvae are still small. A strong jet of water knocks many off trees. Larvae contacted by insecticidal soap sprays will be killed. Most garden insecticides are also effective except the B.t. type sprays. 

Photos: Judy Sedbrook

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Date last revised: 01/05/2010