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
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.
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