Oak leaf galls. Flat pink galls on the upper surface of an oak leaf. Such growths are commonly found on oak leaves, as well as on oak twigs and acorns, and are produced by the larvae of gall wasps, family Cynipidae. Gall wasps often lay eggs on oak leaves. The eggs hatch and the developing larvae produce substances which stimulate tumorous growth in the surrounding plant tissue. The masses of tissue, called galls, provide the larvae with a food supply and protection from predators. Galls of various shapes and sizes are also produced by bacteria, fungi, mites, or a combination of these agents.