Galls are irregular plant growths which can be stimulated by the reaction between plant hormones and powerful growth regulating chemicals produced by some insects, mites, nematodes and fungi. Galls may occur on leaves, twigs, flowers, buds or roots. Many plants serve as gall hosts, but certain plant groups are more attractive to gall producers than others. The Oak family is by far the most popular (with 805 species of gall makers; the next largest family being the Daisy family, with less than 200 gall makers). Galls on oaks are most often caused by small wasps or midges.
Each gall-making species of insect produces a uniquely shaped and colored gall. Thus, it is possible to identify the insect within a gall just by noting the appearance of the gall itself as well as what plant it is on.The growth of the galls takes place in the spring. Gall-making insects lay eggs on the host plant, and the insect larva resides inside the gall that the plant forms. The galls provide the insects within them with both shelter and food. Because many oak leaves persist well into the winter, there is still the opportunity to find galls, though some may be lacking residents at this stage, as many insects emerge as adults in the fall after pupating within the galls.