By Linda Williams, DNR Forest Health Specialist, Woodruff; Linda.Williams@wisconsin.gov or 920-360-0665

A green maple leaf found on the ground with a broken petiole (leafstalk) due to damage by the maple petiole borer. / Photo Credit: Wisconsin DNR.

Some sugar maple trees in the northern half of Wisconsin experienced leaves dropping to the ground this spring.

These leaves were green and had no apparent areas of damage, but they covered the ground under some trees. A closer look showed these leaves had short petioles (leafstalks) that had been broken off when they fell, which indicates a tiny sawfly larva called maple petiole borer was to blame.

The sawfly larvae bore into the petiole of maples and feed within the petiole, creating a weak spot that can break, causing the leaf to drop to the ground. Although the amount of leaves lying on the ground can look alarming, the damage is rarely noticeable when looking up into the tree’s crown.

The last multi-year outbreak of this insect occurred in northeastern Wisconsin from 2005 to 2010. Damage is light, even during outbreaks, and the health of trees is not impacted, so no control is necessary.