Ash Sawfly Larvae
Ash sawfly larvae are caterpillars that feed only on Ash trees. When present in large numbers they are capable of completely defoliating small trees, and partially defoliating larger trees, causing the remaining leaves to drop prematurely. There are three main types of ash sawfly larvae that you may see here in Manitoba. The Spiny ash sawfly larvae have dark heads and gray bodies with many small forked spines. The Black headed and Brown headed ash sawfly larvae both have greenish to yellow white bodies, and their appearances vary only in head color. All ash sawfly larvae are 16-19 mm long at maturity.
These larvae will feed in groups of four to twenty. They hatch in the spring from eggs that were laid in slits formed along the outer margins of young leaves. The larvae mature in late spring, and drop to the ground when mature. They then make cocoons in the soil and stay there for the rest of the summer and throughout the winter. In the spring they pupate and emerge as adults to lay their eggs.
The adult sawflies are small black colored flies, which have a light brown marking behind their heads.
As with many caterpillar type pests, we recommend that you spray your infested trees with the insecticide BT.