This thing was sitting on the ground beneath my red oak tree. It’s the only one I’ve seen on my whole tree. Do you know what it could be?
This is called “oak apple gall” and is created by a cynipid wasp. There are many wasps, mites and flies that cause this type of growth to form on leaves and branches of oak trees. The galls come in many different shapes and sizes but are formed as a result of a chemical the insect injects into the plant. The gall is formed around the larvae which then feed on the interior of the gall and later emerge as adults.
Though galls typically are not harmful to the tree, they can be considered unsightly. There can be severe infestations of galls which can cause twig dieback. In general, the best approach is to ignore the presence of the galls or remove them if they bother you. Chemical control is not effective nor practical for these insects.