Abnormal growths caused by insects
This young pin oak tree in Hopkinsville, KY, is sadly afflicted with galls. Galls are spots of abnormal growth caused by an insect. A gall can develop in any part of a plant that an insect can burrow into, and in oak twig galls, the insect is usually a small wasp.
The various university extensions are a great source of information about tree problems like this. Here's a summary of the life-stages of gall wasps from North Carolina State University Extension.
Many gall wasps develop for 2 or 3 years in woody galls on the twigs of oaks. Adults then emerge from the twig galls during the winter. They lay eggs in the buds and die. When these eggs hatch, and new growth resumes on the oak, salivary secretions of the gall wasp grub act as powerful plant growth regulators and force the tree to form the gall. Gall wasp galls typically have an outer wall, a spongy fiber layer and a hard, seedlike structure inside of which the gall wasp grub develops. Although gall wasp grubs have chewing mouthparts, they do not seem to chew plant tissue. Evidently the gall secretes nutrients which the grubs lap up.
Source: Galls on Oaks, by James R. Baker and S. B. Bambara, Extension Entomologists at North Carolina State University.
Branches with galls should be removed and destroyed before the tree's infestation becomes serious. Spraying with insecticides is rarely effective because the wasp spends most of its life as a larva, protcted inside the gall. There's a small window of opportunity when it emerges to lay eggs.
A University of Kentucky publication about oak galls states that the weight of the galls can cause stress on infected branches, and the deformation can girdle (thus killing, I assume?) a branch.
I hated to read the following quote about oak trees because I didn't realize they were so susceptible to galls.
Oaks can have numerous types of galls. Out of the over 800 species of gall making wasps in North America, 731 of them attack oaks. Oak deformities are of various sizes, shapes, and colors on leaves, twigs, flowers, acorns and buds. Galls are so commonly found on oaks that many people think the galls are typical parts of the plants.
Source: "Trees With a Lot of Gall (growths called galls on trees)" by Sandra Mason of the University of Illinois Extension.
I have only seen one other oak around here as badly deformed with galls as the little tree in the photo is. The other tree is a mature pin oak, and it is a sad sight indeed. I'm going to try to watch our oaks here at the house carefully. Hopefully we can "nip the problem in the bud" if it occurs.