What Are Those Funny Bumps on the Leaves?
Not to worry – these wart-like bumps, called “galls” are harmless and rarely affect the health or vigor of the plant. Galls are abnormal growth on plants usually caused by feeding of mites or insects. Galls can form on almost all plants, although in our area, we usually notice them most on maple, oak, ash or hickories.
As the new leaves are coming out of the buds in the spring, tiny mites or insects will feed on the leaves, causing a hypersensitive response by the plant. The plan’s cells rapidly divide, forming a wart-like gall. The mite or insect may then lay its eggs in the gall tissue, where the young develop and complete their life cycle.
Our most common galls occur on silver or red maples, and are caused by tiny mites. The galls can take a variety of forms, from perfectly round bead-shapes to long spindle-shapes. The galls may change in color as the season progresses, turning from red to green and eventually black. If there are a lot of galls on a single leaf, the leaf may curl and become deformed, and eventually drop. Once the galls form, they cannot be removed from the leaves. The mites feed and mate inside the galls during summer, then overwinter in cracks and crevices of tree bark. In spring, the mites migrate to the swelling buds to feed on newly developing leaves.
Because of the large amount of green leaf tissue on trees, galls typically do not affect the trees’s health, even if they cause some of the leaves to drop prematurely. The galls are purely cosmetic for us. Control is difficult because when the mites are inside the galls during summer, they are protected from insecticide and miticide sprays. When the mites are migrating toward the buds in early spring, they are susceptible to dormant oil sprays. However, because the galls are harmless to the plants, there is no reason to go through the time, expense and environmental risk involved with control.
So don’t worry about your trees with the galls on the leaves. Enjoy the interesting patterns and colors the galls provide and use them to teach your children about nature and interactions between people, plants and insects.