The round balls commonly found on the branches, leaves, bark and acorn of oak trees are abnormal growths known as galls. They form in response to growth regulating chemicals produced by insects and mites when they lay eggs on the tree. Some early botanists thought galls were normal parts of the plant.
Over 700 species of gall wasps are known to exist in North America, along with a number of gall making flies. Each species makes its own type of gall on a specific part of a specific species of oak. The galls provide a source of protein-rich food for the larvae developing in them, and they also serve to protect the larvae from natural enemies and insecticides.
Leaf galls may be unattractive, but they don't tend to hurt oak trees. A heavy infestation might cause premature leaf drop or cause the leaves and stems to become deformed, but trees soon recover. However, heavy infestations of twig and branch galls, like horned and gouty oak galls, can kill a tree. Oak species that are susceptible to horned oak gall are pin, scrub, black, blackjack and water oaks. Oak species that are susceptible to gouty oak galls are pin, scarlet, red and black oaks.
Controlling galls isn't usually necessary because most galls don't harm trees. In addition, insecticides are ineffective against them since larvae are protected inside the galls. However, if you want to reduce the number of galls on your tree, you can collect and destroy infested leaves. Lightly infested trees can be pruned, and infested twigs and branches should be destroyed if you're dealing with gouty oak galls or horned galls. Pruning isn't practical on heavy infestations. Encourage healthy growth by fertilising trees in the fall.
Gallmaker larvae have several natural enemies, including caterpillars, parasitical wasps, birds, vertebrates and fungi. Some caterpillars kill and eat the larvae of gallmakers, while others destroy leaf galls by eating the host leaf and gall tissue. Woodpeckers, rodents and squirrels are able to open large woody galls, and smaller birds eat larvae in thin-walled galls.
Tannic acid is produced from galls and used as a flavouring agent, as a means of tanning leather, as a means of clarifying wine, in the manufacturing of inks and dyes, and as a medical treatment for cold sores, diaper rash and sore throat, and to control bleeding. In addition, the high starch and sugar content of galls has made some species a source of food for livestock.