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.
One familiar plant gall is the maple bladder-gall often seen as bright red bead-like growths on upper leaf surfaces of silver and red maple. The causal agent is an eriophyid mite. The mites feed inside these galls. The galls are green at first. Later they turn a reddish color and by the end of summer they may be almost black. By fall, the mites have left the foliage to overwinter on the twigs, usually at the bases of the buds.
Oak apple gall, caused by several species of gall wasp, consists of large, dry galls attached to the midrib or petiole of a leaf. As the galls mature they become papery. The single larva in each “apple” is inside a small and very hard seedlike cell.
Horned oak gall appears on red and pin oaks and is also caused by a wasp (Callirhytis cornigera). Small, blister-like, oblong leaf galls appear along veins on undersides of leaves. They turn into dark brown, spherical twig galls. The horns develop the second or third year after the wasp’s eggs are laid and the larvae inside are nearing their full size.
Crown gall is one of the most studied plant diseases. The disease is caused by a soil-inhabiting bacterium (Agrobacterium tumefaciens). More than 600 plant species in over 90 plant families are susceptible to this disease, although relatively few species sustain significant damage. Galls form on roots and stems, especially at the root collar – the junction of roots and stem. Young plants with large or numerous galls tend to be stunted and predisposed to drought damage or winter injury. Galls continue to enlarge as plants grow and can disfigure woody stems.
Fortunately, galls, while unusual and sometimes even alarming in appearance, cause little permanent injury and seldom result in the death of the plant. For this reason chemical sprays are rarely necessary or recommended to treat gall infestations.
By the time the galls become noticeable, the insect or mite causing the injury is protected from chemical sprays. If only a few galls are present, the affected part of the plant may be removed.