I noticed a large number of green leaves littering the ground beneath a shade-tree sized sugar maple today in southwest Ohio; not a surprise given the recent high winds and heavy rains. However, a closer look revealed the shed leaves all had very short petioles. The other part of the broken petioles remained attached to the tree and looked like toothpicks. This is the "calling card" of the Maple Petiole Borer (Caulocampus acericaulis).
This non-native sawfly was introduced into the United States from Europe. Although sugar maples are generally preferred, other maples may occasionally be infested. Fortunately, while the leaf drop may appear dramatic, the actual impact on the overall health of affected trees is minimal, so controls are not necessary.
The sawfly spends the winter in the pupal stage buried 2 - 3" in the soil beneath the affected tree. Adults emerge in the spring and 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 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 continues to be attached to the tree. Eventually, this portion of the petiole will also detach and drop to the ground where the larva crawls into the soil to pupate. There is one generation per year.
The initial symptoms of a maple petiole borer infestation are highly variable. Some leaves may become "scorched" and wilted while still attached to the tree with the petioles collapsing and turning brown just prior to leaf drop. Other infested leaves show no outward symptoms and appear perfectly healthy when they drop from the tree. However, all of the fallen leaves will retain only a very small portion of the hollowed-out petiole. Since larvae stay inside the portion of the petiole that remains attached to the tree, raking and destroying fallen leaves will not reduce the sawfly population.