Q. Our big, old sugar maple is dropping an awful lot of leaves recently. I have also noticed a lot of dead leaves scattered throughout the crown of the tree. Is it just the hot, dry weather we have had recently, or is it something more serious?
A. There is an insect called the maple petiole borer (Caulocampus acericaulis) that causes the kind of damage you describe. These introduced sawflies have been very active this spring, and a number of homeowners have called our office understandably concerned about their maple trees shedding so many leaves. I recently purchased a house, and have noticed their activity on two sugar maples in my own yard. The good news is that these pests rarely cause enough damage to warrant control measures. They only produce one generation a year, so once the larvae pupate, they will cause no further harm to your tree.
Adults emerge in May and lay their eggs in the petioles, or leaf stems, near the base of the leaves. The larvae hatch out and tunnel through the petioles, consuming much of the inner tissue. Once their feeding destroys that tissue, it cuts off the supply of water and nutrients to leaves and they dry up on the tree. Withered leaves often fall from the tree from their own weight or in the wind. The petiole simply snaps where the larva has tunneled through.
Larvae remain in the part of the petiole still attached to the tree. They mature in about a month and drop to the ground. Then they burrow into the soil where they pupate until the following spring.
There are no insecticides specifically labeled to control maple petiole borer since they generally do not cause enough damage to require spraying. Since the larvae burrow into the soil, raking leaves in the fall does nothing to reduce their population. An otherwise healthy tree can withstand the damage with no problem.