Ash Sawflies (Hymenoptera: Tenthredinidae)
Ash sawfly larvae are white or cream colored and grow to about 3/4 inch in length. Although sawfly larvae are caterpillar-like, they can be distinguished by the presence of 7 pairs of prolegs on the abdominal segments and a bead-like head that seems to set apart from the body. Both the brownheaded and blackheaded ash sawflies can be found on ash trees in Iowa.
Ash sawflies spend the winter as pupae in the soil. Adults appear in very early spring and lay eggs in slits cut along the outer margin of young leaflets. Newly emerged ash sawfly larvae chew small holes in the leaflets. Older larvae consume the leaf material between the leaf veins creating damage that appears as skeletonization. Large numbers of larvae may cause considerable defoliation in a very short period of time. Ash sawflies have only one generation per year. Larvae that finish feeding in late May or early June drop to the ground and burrow a short distance into the soil to wait until the following spring.
Infested trees usually recover from defoliation as lost foliage is replaced by new foliage on healthy, vigorous trees. Therefore, control is seldom warranted. Also, by the time obvious damage is observed, it is usually too late for effective control.
The best management for ash sawflies is to maintain tree health and vigor through watering, mulching and other recommended cultural practices.
When control is justified, such as on stressed or newly transplanted trees, insecticides can be used. Timely treatment while the larvae are still small, can prevent feeding damage symptoms. Begin checking ash tree foliage for larvae in early May.