I have found small branches from my mature oak tree littered about the lawn and am very worried. Is there something I should do to protect the tree?
-- Sandy Morris, Lake Bluff
The small cut branches you are finding in the yard are likely nothing to be worried about. Both squirrels and some kinds of insects regularly trim twigs from trees with little long-term effect. I know I have them at work in my oak tree because I find cut twigs on the ground in both late spring and late summer. The tree is doing fine.
Early in the year, squirrels often cut young branches. In late summer, similar damage can be caused by insects. Two different beetles, the twig pruner beetle and the twig girdler beetle, cause twigs to fall as part of their life cycles.
An adult twig pruner beetle lays its eggs at the tip of a twig in spring as the leaves are beginning to form. As the twig grows, the eggs hatch and the larvae move to the center of the branch and begin to feed. In late summer, they move out to the sapwood just under the bark, weakening the stem so it falls. The larvae remain in the fallen twigs over the winter and emerge as adults the following spring.
Branch tips with fading foliage that turns brown are the first sign of twig pruner infestation. Then small branches may fall. If you pick up a twig and find that the inside wood is smoothly cut but the bark is ragged at the break, twig pruner larvae likely were responsible.
Another insect, the twig girdler beetle, causes a slightly different kind of damage. These beetles chew a groove around a twig and lay their eggs in the part outside the groove. The weakened twig falls to the ground, where the eggs hatch and the larvae develop and overwinter in the wood. The adults emerge in late summer to begin the cycle again. If you pick up a twig and find the break is rough in the center but the bark is cleanly cut– the opposite of twig pruner damage – the culprit is probably the twig girdler beetle.
Other trees that can be hosts for both these insects include elm, honey locust, hackberry, linden, redbud and hickory.
Neither insect typically will kill trees or damage them severely, although heavily infested trees can appear ragged and unattractive, while young trees can be deformed by repeated attacks. The best management practice is to collect and destroy the fallen branches to reduce the insect population by destroying the eggs and larvae that would overwinter in the wood. Prune out wilted and damaged branches if you can reach them safely.