MAPLE PETIOLE BORER
(Hymenoptera: Tenthredinidae: Nematinae)

The maple petiole borer, Caulocampus acericaulis (MacGillivray) is a small sawfly wasp of European origin that causes leaves to fall from trees in May and June. This leaf drop causes considerable alarm to the homeowner. However, damage is rarely severe enough to harm maple trees. The only host attacked in Canada is the sugar maple, Acer saccharum.

Only one generation of this sawfly occurs each year. The adults appear in May and eggs are laid in a puncture near the base of the petiole (leaf stalk). The larvae bore into the petiole and tunnel towards the blade while feeding, leaving only the external petiole wall. The larva cuts the petiole off a short distance below the leaf blade but remains in the stub of the petiole attached to the twig. Approximately tens days later, the larvae drop to the ground and burrow into the soil. A chamber is constructed for pupation in the fall. The insect overwinters in the pupal stage and emerges as an adult the following spring.

In spring, damage is characterized by the presence of green leaves with 1-2 cm. long sections of petioles, on the ground. Dangling leaves in the crown of the tree may also occur, as well as petioles remaining on twigs but without the leaf blade. A developing larva can be found if the petiole is cut open.

There are no recommended control measures. Collecting leaves on the ground is of no use since the larvae remain in the petiole on the tree for a few days. Spraying with an insecticide is unlikely to provide appreciable control.

W.A. Attwater

PDCF-023 (Revised 2002/01/14)