Fallen oak branches are the work of the oak twig pruner

Yards across the Lower Peninsula are littered with small bug-infested oak branches. No management strategies are necessary for now.

Since mid-June we have received many calls from homeowners and extension educators from around the state about the strange occurrence of the severed oak branches piling up in lawns. Many described the end of the branch as being neatly cut off. We appear to be experiencing a statewide outbreak of the twig pruner, Elaphidionoides villosus, (Coleoptera: Cerambycidae). This is one of our more damaging longhorned borers, which are normally considered secondary invaders of declining trees and shrubs. The twig pruner attacks healthy twigs and small branches.

Fallen oak branches: the work of the oak twig pruner. Photo credit: Howard Russell, MSU Diagnostic Services.

The presence of a small oval-shaped plug in the cut end of the branch is a dead giveaway the culprit is the oak twig pruner. Photo credit: Howard Russell, MSU Diagnostic Services.

The twig pruner cuts through the twig from the inside, but leaves the bark intact. For a short time the injured branch remains on the tree but eventually succumbs to the wind, breaks off and falls from the tree. A small oval shaped hole in the end of the branch is a tell-tale sign of the twig pruner. Look closely for this hole because the larva usually packs the opening with a frass plug to keep out predators and other unwanted guests. Twig diameters at the point of cut usually range from about 3/8 to 3/4 inch. Common host trees are reported to include oak, hickory, pecan, walnut, basswood, redbud and hackberry.

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Left, The cut end of the branch with the plug removed. Right, An exposed oak twig pruner in his tunnel. Photo credits: Howard Russell, MSU Diagnostic Services.

Reports of the life cycle of the twig pruner vary somewhat. As we understand it, the females lay eggs in small twigs near the ends of live branches in late spring. The larva eats the inside of the twig then bores into the center of the branch and tunnels downward. When nearly fully grown, the larva severs the twig or branch by tunneling in circles from the center outward to the bark. Pruned twigs or branches soon break and fall. The larva continues to feed in the severed twig until it pupates. Winter is passed in the severed branch.

Although the sight of many severed branches on one’s lawn can be alarming, there isn’t much that can be done other than picking up the mess and getting on with our lives. Some suggest this pest can be readily controlled by gathering the pruned twigs and burning them. I have my doubts about this. Control with insecticides would require a persistent insecticide and a thorough, properly timed spray application. For this reason, spraying is not recommended, and in most cases, not needed.

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