The maple petiole borer was responsible for noticeable amounts of premature leaf drop from sugar maples over parts of western Kentucky in 2013. The first activity in 2015 (Figure 1) was reported from Webster County.
Damage Due to Petiole Borer
The larval stage of the small wasp burrows into leaf petioles, which break at darkened areas near the leaf blade. Infestations usually are limited to sugar maples; up to 30% of the leaves may fall to the ground in a severe case. While spectacular, the leaf drop usually has little effect on healthy trees.
There can be other causes of leaf drop. For example, build-ups of scales or aphids, or drought stress, can cause leaf loss, but these typically occur later in the year. Leaf drop due to borers is seen earlier in the season and the leaf blades may still be green. Leaves from trees stressed by sucking insects or drought usually turn yellow before they drop. To confirm damage due to petiole borer, split the petiole carefully near the leaf blade, and look for the larva or tunnel.
Petiole Borer Development
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 the leaf stem for 20 to 30 days. The weakened stem breaks and the leaf floats to the ground.
Borer larvae generally remain 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. The mature larva ( about 1/3 inch long) leaves the stem through a hole in the side and burrows into the soil. It will change to the pupal stage and remain in the soil until the following spring when the wasp 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