Question
I have what I think is a silver Maple about 23-30 years old. Very recently the leaves have started to fall off. The veins on the leaves, nearest the branch, appear to be turning brown to black and then the leaves fall off. The far ends of the veins near the tips of the leaves are still normal looking. The leaf itself is still mainly green and in good shape, once it falls off, except for the veins being discolored. I live 30 miles west of Chicago. High temp summer with lots of recent rain. Any ideas on what might be happening? My local Arboretum could not help via email, so anything you might be able to add would be appreciated!

Answer
It sounds like Maple Petiole Borer Damage.

A sudden drop of many sugar maple leaves may be due to injury by a small wasp larva that burrows in leaf petioles (stems). The stems usually break at a darkened area near the leaf blade.

Usually infestations are limited to sugar maples and only about 25% to 30% of the leaves fall to the ground. While spectacular, the leaf drop has little effect on tree health. Buildups 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 have turned yellow before they drop. Splitting the petiole carefully near the leaf blade should reveal the larva or the tunnel. The main symptom is the fallen leaves. These usually have a short part of the petiole still attached although it is usually black and shriveled.


There is one generation each year. Infestations begin as the adults, small wasps about 1/6" long, appear in May and lay their eggs in petioles near the 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.

The 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" 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.

Maple petiole borer infestations are infrequent and unpredictable. Also they do not appear to harm tree health so insecticidal control is not recommended. In addition, probably preventive treatments, applied well before leaf drop, would be necessary. 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 and must include all infested trees in the vicinity to be most successful. Raking and disposing of the leaves will not reduce the population because the insects are not in that portion.