Maple petiole borer
Published in Yard & Garden Brief, March 2002
Life cycle and damage
Maple petiole borers are small sawflies, a type of non-stinging wasp. After overwintering as pupae in the soil, they emerge as adults in spring to lay eggs in the petioles (stems) of maple leaves. Maple petiole borers attack all maple species, especially sugar maples.
When larvae hatch from the eggs, they tunnel into leaf petioles. They feed within the petioles for 20-30 days. Larvae are about 1/3 inch long at maturity, cream-colored with brown heads. The process of tunneling into the leaf petioles disrupts the connective tissue, causing leaves to fall, often very suddenly. The larvae remain in the portions of the petioles that are still attached to the twigs. About 10 days later, that part of the petiole also drops to the ground. The larvae leave the petioles to burrow into the soil to pupate. There is one generation per year.
Typically, infestations occur at low levels. During rare occurrences, up to a quarter of the leaves may drop but normally no more than 10% of the leaves will drop. In some years, maple petiole borers may not cause any problems.
Management is difficult and unnecessary. Although seeing many leaves on the ground in late spring is disconcerting, the actual number of leaves involved is relatively small. Trees easily tolerate some defoliation. Maple petiole borers will not compromise tree health or detract from its appearance.
Since the problem is minor, applying an insecticide is unnecessary. Most homeowners do not have equipment that could provide adequate coverage of all the leaves in a large tree. Even with proper equipment, an effective spray application is difficult to time.
Raking up leaves to prevent the larvae from overwintering in the soil is not effective as the maple petiole borers are not in the petioles attached to the leaves. Waiting and then raking up the petioles that contain larvae is difficult and not very practical. Even if some maple petiole borers are removed from your yard, the adults can fly in from adjacent areas to lay eggs on maple leaves the next spring.