Noticeable premature leaf drop of maples can sometimes occur in May. Leaves that drop are perfectly green and usually the leaf blade has absolutely nothing wrong with it. So, what's happening? It could be due to an insect called the maple petiole borer.

The larval stage of the small wasp burrows into leaf petioles, which break at darkened areas near the leaf blade (Figure 1). 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.

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.

For more information about this topic, contact the Pulaski County Extension Service office at 606-679-6361 and request ENTFACT-405 Maple Petiole Borer.

The Lake Cumberland Master Gardeners have pine straw mulch for sale at the Pulaski County Extension office. It is sold in bales for $7 per bale (over 50, $6 per bale). It can be purchased during office hours 8am to 4:30pm Monday - Friday.

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Come to the Pulaski Co Extension office on June 20 at 6pm and Stump the Extension Agent. This program will focus on weeds - bring in any that you don't know or are having a hard time controlling. This is a free program.

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