Many plants growing throughout Illinois are susceptible to attack from gall-forming organisms. A number of organisms cause plant galls, including viruses, bacteria, fungi, nematodes, mistletoe, mites, and insects. Insects, without a doubt, produce the most galls. Gall-forming insects include beetles, wasps, moths, flies, midges, sawflies, thrips, scales, adelgids, aphids, psyllids, and twig borers. A wide range of plants are hosts of insect gall-formers; however, the most susceptible are oaks, cottonwood, poplar, and willow. Oaks (Quercus spp.) are susceptible to a wide diversity of gall-forming insects. A gall is an abnormal swelling caused by a gall-forming insect, which lives part of its life in the gall, feeding inside the gall on the surrounding contents of plant cells.
The insect feeds on plant cells that are abundant in carbohydrates, protein, and fats. As the insect feeds, it injects growth-inducing chemicals into the plant tissues. These injected chemicals cause plant cells to discontinue their normal growth pattern, which results in the creation of enlarged cells that divide until an abundance of reorganized tissue surrounds the insect. Gall-forming insects may modify plant development in different ways, including directly disrupting the plant’s hormonal balance or altering the cells’ DNA. In addition, insect feeding or egg-laying by females may result in the formation of galls.
The primary groups of insects that form galls on oaks are cynipid wasps. These wasps are responsible for 80% of the oak galls, which are typically located on leaves and branches. Cynipid wasp adults are 1 to 6 mm in length, antlike, and deep black in color. Galls formed by cynipid wasps can range in size from 1 to more than 50 mm in diameter, and they are often round or irregular in shape. Many oak galls are large and very apparent. Female cynipid wasps lay eggs into actively growing meristematic tissue. Feeding by the wasp larvae causes a growth reaction in oak leaves, which results in the formation of galls. The wasp larva feeds on gall tissue and pupates within the gall, and then the adult chews an exit hole, which allows for emergence. 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 plant parts. In fact, a single cynipid wasp species may contain members that cause two distinctly different types of galls, which has led to confusion in determining the many types of oak galls.
In general, oak galls are not considered a problem as the galls cause minimal, if any, apparent reduction in plant vigor and growth. It is my personal opinion (although biased) that oaks look more attractive and “aesthetically pleasing” when galls are present. However, two galls that may damage oaks are the horned oak gall, Callirhytis cornigera, and the gouty oak gall, Callirhytis quercuspunctata. Cynipid wasps are responsible for both galls. These galls can girdle plant stems, causing branch dieback by cutting off the flow of water and nutrients to the portion above the gall. The best—and really only—way to deal with oak galls (or galls in general) is by simply pruning them out, because once the gall is formed, then options are limited.