Curiosity of Plant Galls:
Explaining the unusual gall growths or deformities in plants can be challenging. Different galls can be found on virtually any plant part, including leaves, flowers, twigs, bark, or buds of trees and shrubs. Galls caused by insects or mites can typically be identified by the characteristic size, shape, color, and structure that develops.
Insect/mite galls are produced by the interaction of plant hormones with strong plant-regulating chemicals released by the gall maker as they feed. The immature larvae live their entire lives within an individual gall, which provides excellent protection for the larvae as they feed and mature.
This blog contains information on horned and gouty oak galls. When the number of galls infesting an oak is few, tree damage is insignificant. However, tree injuries become more severe when gall numbers reach high levels.

Curious & colorful leaf galls created by maple bladder galls mites may cause alarm to some people but they are mostly harmless. (Photo Credit: Steven K. Rettke, Rutgers Coop. Ext.)

An extreme infestation of horned oak galls on branches of pin oak tree. (Photo Credit: Steven K. Rettke, Rutgers Coop. Ext.)