I have some significant specimen oak trees in my front yard, and I have been finding the lawn littered with the ends of branches. What is causing this?
— Allison Rosenberg, Lincolnwood
My driveway has been covered with the clipped ends of oak branches with green leaves over the last couple of weeks, somewhat like what you have been observing in your garden. I have two beautiful large oaks flanking my driveway. I’ve noticed the ends of branches in many home gardens, and at the Garden, that have been dropping over the past weeks due to cicada damage from the egg-laying process. These branch ends will typically have brown leaves attached if any are remaining. You can also see the slits in the bark from cicada egg laying on the undersides of the fallen branch tips. There is no damage from cicadas on the twigs at my house, and the leaves are bright green, so the damage is fresh.
It is likely that a squirrel or squirrels are responsible for the clipped ends falling from your oak trees, especially if the leaves are bright green. Look at the ends of the twigs as smooth cuts at an angle that look somewhat like they were done with a pair of pruners indicating squirrel damage. Neither the center nor the bark will have any jagged edges. Though the amount of debris falling off of a tree can be alarming, squirrels usually cause little damage to a large tree. I am not worried about my trees at home but just slightly annoyed at the extra clean up required.
There are two insects that create dying branches for their larvae to live in. The twig girdler female lays an egg near the end of a branch, crawls up the branch a short way and chews a groove all the way around the branch, which girdles it and causes it to die. The branch end or twig will eventually break off and the larva will continue to develop. The end of the branch will have a smooth external cut, but the center of the stem will be jagged. The twig pruner lays an egg at the base of a leaf. The hatching larva tunnels into the center of the branch and moves outward and stops at the bark. The branch will break off at this point, with the larva continuing to develop in the
fallen twig. The end of the branch will be smooth, but the bark will be jagged.
There is nothing for you to be concerned with or any action to take regarding your oak trees. The primary way to control twig pruner and twig girdler is to rake and destroy the fallen twigs should any control be needed. A small number of these insects on a large oak tree would not concern me.
For more plant advice, contact the Plant Information Service at the Chicago Botanic Garden at plantinfo@chicagobotanic.org. Tim Johnson is senior director of horticulture at the Chicago Botanic Garden.