Maple petiole borer: The worst is over

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.    

County Extension offices across the state have been swamped with calls from alarmed homeowners who found large numbers of fallen leaves beneath their sugar maples. Many who called Diagnostic Services were convinced their trees were dying. Not to worry – just another outbreak of the maple petiole borer, Caulocampus acericaulis (Hymenoptera: Tenthridinidae). The last major outbreak of this pest we experienced was in 2001. The worst is over for this current outbreak. I have not seen any new fallen leaves under my sugar maples for over a week now.

The maple petiole borer, a sawfly in the adult stage, was introduced into the United States from Europe. (view photos) The tiny larvae tunnel in the petioles, which eventually causes the petiole to break a short distance from the leaf blade. The damaged leaves fall from the tree in May and June. Splitting the petiole (the short green stem that connects the leaf to twig) down the middle lengthwise would reveal tiny brown pellets of frass. The larva can be found in the portion of the petiole that remains attached to the tree. When that part of petiole eventually falls, the larva crawls out and burrows into the soil where it pupates and spends the winter.

Although the appearance of many fallen leaves in May can be troubling, there is rarely sufficient leaf drop to cause any harm to the tree, and by the time the fallen leaves are noticed, there is nothing that can be done to control the damage. And there is little we can do to prevent the pest from having its way with our trees. Raking and destroying the fallen leaves will not control the population since the larvae are in the part that remains in the tree. Attempts to prevent the damage in the spring with insecticides require exact timing when the females are laying eggs and most likely will not be effective. The good news is that in most years natural enemies control the pest. There is a single generation of maple petiole borer each year in Michigan, so there is no chance of the problem continuing throughout the summer.

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