Maple Petiole Borers Not Uncommon This Time of Year

Photo by V. Shadrick

I don’t need to look outside to know that our trees, flowers, landscapes and gardens are up and growing; I can tell by the increased number of calls and emails to the office.  One of the most interesting emails that I got last week had to do with maple trees.  The first line of the email was “The leaves on our maple trees are falling off like it is October.”  Sure enough when I got to their home, the front yard was covered in what appeared to be healthy, green maple leaves and the culprit was Maple Petiole Borer.

According to UK Extension Entomologist Dr. Lee Townsend, a sudden drop of many sugar maple leaves in May and June can be due to injury by a small wasp larva that burrows in leaf petioles. These stems usually break at a darkened area near the leaf blade. Usually, infestations are limited to sugar maples and only about 15% to 20% 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 occurs 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. If the wasp is involved, splitting the petiole carefully near the leaf blade should reveal the larva or the tunnel.

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 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.

The borer larvae 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. There is one generation each year.

Maple petiole borer infestations are infrequent and unpredictable. Also, they do not appear to harm tree health so insecticidal control is not recommended. It may be possible to reduce future infestations somewhat by picking up and destroying infested stems, the short sections without leaves, about 7 to 10 days after the first leaves fall. This 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 fallen leaves will not reduce the population because the insects are not in that portion.

For more information on landscape pest and problems, contact the McLean County Cooperative Extension Service at (270)273-3690.  Educational programs of the Kentucky Cooperative Extension Service serve all people regardless of race, color, age, sex, religion, disability or national origin.

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