The leaf-shedding handiwork of the Maple Petiole Borer (Caulocampus acericaulis, family Tenthredinidae) is becoming evident in southwest Ohio. This non-native sawfly was introduced into the United States from Europe. Although the sawfly prefers sugar maples (Acer saccharum), other maples may occasionally be infested.
Leaves that drop may appear perfectly healthy or the leaves wilt and turn brown while still attached to the tree. However, all of the fallen leaves will retain only a very small portion of a hollowed-out petiole.
Look closely at the fallen leaves for abnormally short petioles and examine the tree canopy for broken petioles that have remained attached. These are tell-tale symptoms of a petiole borer infestation as opposed to leaf loss from hail, high winds, or certain leaf diseases such as maple anthracnose.
The sawfly spends the winter in the pupal stage buried 2 - 3" in the soil beneath the affected tree. Adults emerge in the spring. After mating, the females use their saw-like ovipositors to insert a single egg into the petiole near the leaf blade. The resulting grub-like sawfly larva feeds by boring down the center of the petiole.
The larval feeding damage causes the petiole to break near the base. However, the larva remains in the portion of the petiole that remains attached to the tree. Thus, raking and destroying fallen leaves will not reduce the sawfly population.
Eventually, the portion of the petiole that remains attached to the tree stem will also detach and drop to the ground where the larva crawls into the soil to pupate. There is one generation per year.
The leaf loss from a maple petiole borer infestation may appear significant; however, our perception may not reflect reality. It’s rare for the leaf loss to rise above 5% of the total canopy. This level of leaf loss has no significant impact on the overall health of affected trees.
