Gall makers, a group of small insects, cause oak galls on leaves, barks or roots of a tree. These insects release a chemical that causes a reaction with the tree's hormones to form a gall. This gall provides the insects with the perfect place to reproduce since it contain a protein food source and protection for the larvae, which are the insects' offspring.
Twig and Stem Galls
If you see a solid, woody growth on the branch of an oak tree, you are looking at a twig and stem gall caused by wasps. These types of galls can exceed more than 2 inches in diameter and their weight can cause the branch to droop. Gouty oak gall and horned oak gall are two types of twig and stem galls. Horned oak galls infect pin, scrub, black, black jack and water oak trees, while gouty oak galls only form on scarlet, red, pin or black oak trees. If the gall you see is smooth, it's a gouty oak gall, and if it looks like it has horns protruding from it, it's a horned oak gall. When your oak tree becomes infected with twig and stem galls, prune the affected branches. Insecticides generally do not work against these types of galls. If the tree if heavily infected, consult an arborist.
Oak Apple Galls
Oak apple galls appear on the petioles and midribs of oak tree leaves and grow to be 1 to 2 inches in diameter. Although these round masses may feel spongy, a wasp larva lives in the hard center of these growths. When the galls dry and turn brown, the walls become thin allowing the mature wasp to escape. Although apple galls appear to be rather large, they cause little to no harm to your oak trees, but if you wish to diminish the infestation, you can remove and destroy the galls before they dry out and brown.
Vein Pocket and Leaf Pocket Galls
Maggots cause vein pocket and leaf pocket galls. These galls form along the mid and lateral veins of the leaf. Small flies called midges lay their eggs on the leaf just as it begins to flatten. When the eggs hatch into maggots, they feed on the veins of the leaf causing a gall to form. After the maggots mature, they drop to the ground. Vein pocket galls form on the leaves of scrub and pin oak trees. Leaf pocket galls form only on red or pin oak trees and are caused by a different species of midge than the midges that form the vein pocket galls.
Jumping Oak Galls
Affecting valley oak trees and California white oak trees, jumping oak galls are caused by wasps. The females lay eggs on the leaves during the summer months causing circular, seed-like galls to form on the leaves. One wasp lives within each gall. When the wasp matures, the galls will fall off the leaves. Once on the ground, the wasp inside the gall may cause the gall jump off the ground giving the jumping oak galls their name. The wasps don't emerge from their galls until the following spring.