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 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
Back to Insects on Trees
Back to Pests
Back to Home