Do you see maple leaves unexplainably dropping in late May or June? Although this may be the result of drought, squirrels, or aphid infestations, there is a good chance this damage is caused by the Maple Petiole Borer (Caulocampus acericaulis).

Adult sawflies (non-stinging wasps) lay their eggs in the leaf petiole (stem) of the maple leaves. They most frequently target Sugar (Acer saccharum) and Norway (Acer platanoides) maples. After hatching, the 1/3"-long, cream-colored larva, tunnel through the petiole for 20-30 days. They disrupt connective tissues, which causes the leaf to suddenly drop. Shriveled, dark-colored petioles connected to the green leaf are easily identifiable.

Don't worry. Maple Petiole Borers normally damage about 10% of the leaves. They rarely affect more than 25% of the canopy. Healthy trees can easily recover from an infestation of this magnitude. Feeding damage subsides in a few weeks.

Don't spray. Using insecticides is not recommended due to the high cost. It is also difficult to cover the entire canopy, and necessary equipment isn't available to the average property owner.

Don't bother. Raking up the leaves for aesthetic reason is an option; however, raking will not reduce infestations (the borers are in the petiole still attached to the tree). Removing petioles after they fall (approximately 10 days after leaf fall) is tedious and not guaranteed to be effective. Adult sawflies may fly in from adjacent areas to lay eggs next spring.

Keep the tree healthy. Preventative care is the best choice. Keep the tree properly pruned, watered, and protected from physical damage. Fertilize if nutrient deficiencies are detected in a soil test.

For more information contact the Department of Forest Resources Extension and Outreach office at 612-624-3020.

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