Galls

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.

Gall
Photo credit: Larry Kohrnak

A complex relationship exists between the insects that cause galls and the plant with 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 secretion can cause the same result. The gall is made of protein rich tissue and is strong in structure, providing food and protection for the invading 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 symptom:	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.