The adult female inserts eggs into the leaf tissue with her saw-like ovipositor. Incubation lasts 10 to 15 days. Larvae are present in late spring or early summer and immediately begin to feed on the upper surface of the leaf. It takes 3 to 4 weeks for the larvae to pass through the five stages of development. Fifth instar larvae do not feed but crawl or drop to the ground and pupate.
Second generation adults begin to emerge in July. They emerge over a shorter period of time than the first generation. Adults lay eggs soon after mating, and larvae appear in August and September. It is this generation of larvae that is the most destructive. Mature larvae drop to the ground for the overwintering pupal stage.
Pear sawfly feeds on the upper surface of leaves, skeletonizing them. On heavily infested trees, leaves turn brown, wither and drop. Defoliation on either pear or cherry can weaken the tree. Infestations can also stunt fruit and prevent maturation.
It is important to monitor continually for the slug-like larvae in August and September when large populations can build up rapidly and cause significant damage. Pay special attention to cherries, which are often neglected after harvest. A few acres within a block can easily become infested unless the trees are monitored regularly.
Little is known about biological control of the pear sawfly. Since the insect is usually not a problem in unsprayed backyard trees, natural enemies may be controlling it.
Because pear sawfly larvae are susceptible to most insecticides, they are easily controlled in commercial orchards. Generally, normal spray programs for other pests will keep this insect in check. If a problem develops, it is usually after harvest on cherries when insecticides are no longer being used. In these cases, trees can be defoliated in August or September. This seems to have little effect on the following year’s crop on mature trees, but the consequences may be more serious on young bearing trees. If control is necessary, an organophosphate insecticide would be effective.