Many plants throughout Illinois are subject to attack by gall-forming organisms. Organisms that can cause galls on plants include viruses, bacteria, fungi, nematodes, mistletoe, mites, and insects. Most galls are produced by insects. Gall-forming insects include beetles, wasps, moths, flies, midges, sawflies, thrips, scales, adelgids, aphids, psyllids, and twig borers. Many plants are hosts of insect gall-formers, but particularly cottonwood and other poplars, willow, eucalyptus, ficus, and oak. Oaks (Quercus spp.) are susceptible to a wide diversity of gall-forming insects. A gall is an abnormal plant swelling caused by the gall-forming insect, which lives part of its life in the galls, feeding inside the gall on the surrounding contents of plant cells.

The insect feeds on plant cells that are rich in carbohydrates, protein, and fats. As the insect feeds, it injects growth-inducing chemicals into the plant tissues. The injected chemicals cause plant cells to abandon their normal growth pattern. This gives rise to enlarged cells that divide until an abundance of reorganized tissue envelops the insect. Insect gallers may control plant development in different ways, including directly disrupting the plant’s hormonal balance or altering the cells’ DNA. Galls may be formed by insect feeding or egg laying.

The primary group of insects that form galls on oaks are cynipid wasps, being responsible for approximately 80% of oak galls. These galls are generally found on leaves and branches. Cynipid wasp adults are 1 to 6 millimeters long, antlike, and jet black in color. The galls they form can range from 1 to more than 50 millimeters in diameter and are round or irregular shaped. Many oak galls are large and very noticeable. Female cynipid wasps lay eggs into actively growing meristematic tissue. The feeding of the wasp larvae causes a growth reaction in oak leaves that results in the formation of galls. The wasp larva feeds on the gall tissue, pupates within the gall, and then the adult chews an exit hole to emerge. The life cycle of gall-forming wasps may be complex, involving alternations between generations of sexual and asexual individuals. Galls of these generations may differ in appearance and may be found on different parts of plants. Thus, a single species of wasp can contain members that cause two distinctly different types of galls. This has caused confusion in determining the many types of oak galls.

Oak galls are generally not considered a problem as the galls cause little, if any, apparent reduction in plant vigor. In fact, they sometimes increase their aesthetic quality. However, two galls that may damage oaks are the horned oak gall and the gouty oak gall. Both galls are caused by cynipid wasps. These galls can girdle plant stems and may cause significant branch dieback. The best way to manage oak galls is simply by pruning them out, because once the gall is formed, options are limited.