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What Causes Oak Galls?
What Causes Oak Galls?
I recently received some pictures of some weird looking growths in an Oak tree. They looked like a growth called a gall, which looks like a tumor growing on a branch or leaf. They are usually the result of the feeding or egg laying of certain insects that cause the plant's cells to multiply at a dizzying rate. Immature stages of the insect or the larva feed on the inside of the gall and often use the gall as protection from predators.
The gall in this tree is a Horned Oak Gall. They grow to the size of a golf ball and have an interesting life cycle. The adults emerge from the gall in early summer and lay eggs on the leaves of the tree. The resulting larvae cause oblong blister-like galls to develop on the leaves.
About three months later, new adults emerge from those galls, they mate and the females lay eggs in the twigs of the tree. Small marble sized galls appear and grow together to form the larger gall. The cycle continues on and on, from leaf to twig, and this can lead to deformity of the tree if populations become too large.
Treatment is possible, but it requires specialized equipment that will inject insect control materials into the tree. This requires a tree care professional who is trained to properly inject these materials to do the job correctly. Sometimes, if the tree is not too large, the twigs or branches that have galls on them can be pruned out as soon as the galls appear during normal tree maintenance to reduce the ongoing life cycle.