Natural History
Gall on the branch of a tree
Photo credit: Larry Kohrnak, SFRC

Galls are abnormal growths in shrubs and trees and can be found on nearly every part of the plant -- leaves, roots, stems, trunks, buds, flowers, seeds, and fruits. Galls can be caused by a variety of organisms including bacteria, fungi, nematodes, and mites. The most common cause of galls is insects. Although these galls may be considered unsightly, they are rarely harmful to the plant and are usually helpful to the insect.

A complex relationship exists between the insects that cause the galls and the plants that bear the galls. The insect will lay eggs in a plant and at the same time release a chemical or other stimulus into the plant. The plant responds with its own release of growth hormone, leading to abnormal growth around the eggs that becomes a gall. Feeding damage and other insect secretions can cause the same result. The gall is made of protein-rich tissue and is strong in structure, providing food and protection for the developing insects. Once a gall begins to form, nothing can stop its growth.

Over 2,000 types of galls are produced by insects and the majority of these are made by wasps or tiny flies.


Identifying Characteristics

Identifying the stress: Galls vary greatly in size, color, and texture depending on the type of insect causing the gall. It is often possible to identify the gall-causing insect based on the type and appearance of the gall. Where the gall is located on the tree is also very important in determining the cause. Galls can form on any part of a tree or shrub and there may be numerous galls on a single plant and plant part. Look for any swollen abnormality.
Susceptible trees: Any tree can produce galls, although oaks are often favorite hosts.



Click on any thumbnail to see a photo. Use left and right arrows to navigate. Use "esc" to exit the lightbox.


Learn More