Galls are abnormal growths that occur on twigs, branches, and leaves. Some are small lumps; others are large, more complicated structures. They can be green, plain brown, or brightly colored. Although they look disconcerting, for the most part they are not harmful to the plant.

Most galls are caused by feeding or egg laying insects such as aphids, midges, wasps, or mites. However, some are the result of infections of bacteria, fungi, or nematodes. It is impossible to tell which is causing the gall by simply looking. Galls result from a chemical interaction between the pest and the plant’s hormones. The host plant reacts to the stimuli and provides food and shelter to the larva within the gall. The structure of the gall is strong, rich in protein, and a very safe environment for the insect to mature.

Galls have fascinated botanists for centuries, albeit not always scientifically correct. They were mentioned in a work by philosopher Theophrastus (371-287 B.C.). Because they were believed to be “supernatural growths” they were thought to foretell the future. Their contents were examined and were thought to predict everything from war to famine.

The shape and color of the gall is determined by the offending organism. They may be lumps, warts, balls, or knobs. They come in a wide array of colors and may be red, green, yellow, or black or may change color as the pest completes its life cycle. A specific gall can be identified by the shape, color, and structure. In addition, many are plant specific, making identification easier. Most have only one generation per year, making chemical control after symptoms appear of little value.

For most of their lives, the gall-making insects live within the gall, sheltered from any insecticide. In addition, the galls can be in the top of tall trees, making it nearly impossible to reach with pesticide applications. The best management is to allow natural predators like birds, mice, squirrels, and other insects do their work. Since most galls are cosmetic only and not a danger to the plant, tolerating their presence is recommended.

Most homeowners have seen galls on their deciduous trees or shrubs at some point. One of the most common is Maple Bladder Gall. The leaves develop small wart-like bumps on the leaves, predominately on soft or sugar maples. The culprit is the maple gladder gall mite. The mites overwinter in the cracks and crevices of the bark and once the buds open, they feed on the new leaves. The leaves develop hollow galls where the mites live feed and mate. The galls start out as red, turn green and eventually black. Other than being visually unappealing, there is no damage to the tree.

Oak galls are another common problem, caused by two wasps. Oak galls are horned or hornless and are up to two inches long around oak stems. In late spring female wasps emerge from stem gall and lay eggs on oak leaves. The larva feeds on the leaves and the tree produces leaf galls. By mid-summer, the adults move to the twigs where they lay eggs (the larva can remain there for two or three years) and stem galls are produced. These galls can girdle the stems and cause branch dieback, so it is best to remove all twig galls the first winter they are seen.

Because of the gall’s hard shell, chemical controls have proven to be largely ineffective. With larger galls, to prevent completion of the insect or mite life cycle, prune out twig and stem galls. Small holes in the gall indicate that the pests already have moved on repeating the life cycle.

Carol Shirk

Certified Master Gardener

Photo: Wisconsin Horticulture - https://hort.extension.wisc.edu/articles/deciduous-tree-galls/

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