A sudden drop of many sugar maple leaves may be the work of small wasp larvae that burrow into petioles (Figure 1). The weakened stems usually break at a darkened area near the leaf blade (Figure 2). Usually, infestations are limited to sugar maples and only about 15% to 20% of the leaves fall to the ground.
Leaf drop from petiole borers occurs early in the season and the leaves are usually green. On the other hand, leaves lost from stress caused by sap-feeding insects or drought usually turn yellow before dropping. To determine if the petiole borer is involved, split the petiole carefully near the leaf blade and look for a larva or tunnel. While spectacular, the leaf drop has little effect on healthy, established trees. Buildups of scales or aphids, or drought stress can cause leaf loss but these typically occur later in the year.
Insect Description & Life Cycle
There is one generation of petiole borer each year. Infestations begin as adults (small wasps about 1/6 inch long) appear in May and lay their eggs in petioles near leaf blades. Legless, white grubs with distinct light brown heads hatch from the eggs and tunnel inside leaf stems for 20 to 30 days. The weakened stems break and leaves float to the ground.
Petiole borer larvae generally remain behind in the portion of the stem left on the tree. About 10 days after leaf drop, the rest of the stem falls to the ground. Mature larvae, about 1/3 inch long, exit stems and burrow into the soil. Larvae will change to the pupal stage and remain in the soil until the following spring when wasps emerges.
Maple petiole borer infestations are infrequent and unpredictable. Insecticidal control is not recommended. It may be possible to reduce future infestations by picking up and destroying infested stems (the short sections without leaves) about 7 to 10 days after the first leaves fall. This sanitation program needs to be continued throughout the leaf drop period. Raking and disposing of the leaves will not reduce the population because the insects are not in that portion.
By Lee Townsend, Extension Entomologist