Galls are abnormal growths that occur on leaves, twigs, roots, or flowers of many plants. Most galls are caused by irritation and/or stimulation of plant cells due to feeding or egg-laying by insects such as aphids, midges, wasps, or mites. Some galls are the result of infections by bacteria, fungi, or nematodes and are difficult to tell apart from insect-caused galls. Seeing the insect or its eggs may help you tell an insect gall from a gall caused by other organisms.
In general, galls provide a home for the insect, where it can feed, lay eggs, and develop. Each type of gall-producer is specific to a particular kind of plant.
Galls may appear as balls, knobs, lumps, or warts, each being characteristic of the causal organism. In addition to the unusual structure of galls, they draw attention due to their range of colors: red, green, yellow, or black. Factors such as weather, plant susceptibility, and pest populations affect the occurence of galls on plants from year to year. Oaks are one of the most susceptible, being host to over 500 different wasps, aphids, mites, and midges that cause galls on leaves and twigs.
Plant gall damage is usually an aesthetic problem and is not considered serious. Affected trees ordinarily show little injury, although foliage of young trees is sometimes completely deformed. On ornamental trees this condition can be unsightly.