Galls are abnormal growths or deformities found on plant leaves, stems and flowers caused by the presence of a tiny insect or mite. Plant galls come in a wide variety of forms, textures and colors; bumps, warts, spheres, spines, fuzzy, hairy, smooth, etc.
The tiny insects and mites that cause galls to form are called gallmakers. Galls occur on young, developing plant tissue because of an intricate interaction between the gallmakers and the plant. There are thousands of species of insects and mites that produce galls on hundreds of species of plants. In the presence of a gallmaker the plant abruptly changes the course of normal growth and modifies growing tissue into a special swelling that surrounds the tiny insects and mites. Each gallmaker attacks specific tissues on a specific plant producing a unique and distinctive gall.
Most insect galls do not seriously affect the vigor of healthy plants. Leaf galls are aesthetically displeasing and may cause some premature leaf drop but they do not directly kill the plant. Occasionally, a heavy or prolonged gall infestation on small branches or roots may weaken or kill portions of a tree or shrub.
Galls cannot be "cured" after they have formed. That is, spraying or treating does not make them go away. Preventive treatments applied before the galls form may be effective but are not usually practical. Control with insecticides is not recommended.
For more information on the 24 most common galls in Iowa please see handout IC-417 "Insect Galls on Trees and Shrubs" from Iowa State University Extension.