Mountain ash sawfly

Mountain ash sawfly larvae eating a leaf.

The mountain ash sawfly, Pristiphora geniculata , is found from the northeastern United States and Canada south to West Virginia and west to Minnesota. This insect occurs in both the Old and New World, but authorities differ as to whether it is of European origin. It survives only on the foliage of the mountain ashes. The larvae devour all of the leaflet except the midvein, a symptom that identifies the pest.

Adults that produce the first generation emerge from the soil over a six week period beginning in late May, continuing into early July. Second generation adults emerge about mid August. They lay their eggs in slits cut in the epidermis with saw-like motions of the blades of their ovipositor. Adults die in about a week. Eggs of the first generation begin to hatch in early June, and some larvae may be found in the trees until early August.

Sawfly larvae resemble caterpillars, but are actually members of the insect order Hymenoptera, which includes bees and wasps. The larvae are yellow with black spots. Larvae feed in colonies, devouring the foliage on one branch before moving to another.

Second generation larvae may be found in September. In either generation, the larvae require about three weeks to complete this stage of development. The second generation is small and often inconsequential. Mature larvae drop to the ground, burrow into the soil, and prepare earthen cells where they spin paper-like cocoons. The prepupa is the over wintering stage and some individuals may remain in this stage for several years.

Though it is a defoliator, this insect seems to cause little lasting harm to mountain ash trees. It has been postulated that mountain ash sawflies do not seriously deplete the food reserve of the tree because they feed primarily during the mid-growing season. To prevent defoliation of ornamental mountain ash trees hand pick the larvae off the leaves and destroy them, or, apply an insecticide labeled for sawflies.