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Twig Borers and Girdlers on Oaks — Enchanted Gardens


A different long-horned beetle, Oncideres cingulata, girdles rather than cuts off the leafy twigs and it has a similar life cycle as the Borer. The Twig Girdler is about ¾” long but it is grey-brown with a lighter stripe across its wing covers (its elytra) and its antennae are about the same length as its body, not longer. Adult beetles emerge in late summer when the females girdle a twig and lay a single egg in the cut portion so their eggs can feed on the dying wood; these larvae don’t eat live wood, unlike the Twig Borer. The girdled terminal quickly falls to the ground that first summer. The Girdler Beetle can be identified by looking at the cut end with its smooth V-shaped outer cut and ragged torn inner wood, quite distinct from the Twig Borer’s cut appearance with the visible tunnel inside. The Twig Girdler larvae bore further into the fallen twig, spending the winter and the next spring inside until they pupate and emerge as adults to start the cycle again.

During late summer, the cut-off twigs, with their green leaves attached, litter the ground under infested trees or worse, they stay snared in unreachable branches, very noticeable and often long-lasting, and to the tidy landscaper, frustrating. Their remarkably persistent green leaves eventually brown but don’t drop off since no abscission cells form as they normally would each autumn for leaf drop. You can diagnose the source of these branches by their ends, a rather smoothly cut surface, very unlike any branches ripped off by the wind.

I have to confess I have never noticed nor identified either beetle; I only saw their littered handiwork. And in a cursory examination of the fallen leafy terminals in my yard, they all seem to be from the Twig Girdler, not the Twig Borer, so each clump of leaves lacks a central eaten-out tube and a frass plug. But I will look more carefully in the future. These beetles do not do much damage to healthy trees, only to the gardener’s peace of mind. The natural world continues to amaze and mystify, but at least I have finally solved another minor mystery.  

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