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What Are Galls?


The oak rough bulletgall wasp, a cynipid wasp, produces galls on the twigs of oak trees.

The oak rough bulletgall wasp, a cynipid wasp, produces galls on the twigs of oak trees.

Whitney Cranshaw, Colorado State University, Bugwood.org
Question: What Are Galls?

Have you ever noticed unusual lumps, spheres, or masses on trees or other plants? These strange formations are called galls. Galls come in many sizes and shapes. Some galls look and feel like pompoms, while others are hard as rocks. Galls may occur on every part of plants, from the leaves to the roots. But what are galls, exactly?


Galls are abnormal growths of plant tissue trigger in response to an injury to or an irritation of the plant, usually (but not always) caused by some living organism. Nematodes, bacteria, fungi, and viruses can all cause the formation of galls on trees, shrubs, and other plants. Most galls, however, result from insect or mite activity.

Gallmaking insects or mites initiate gall formation by feeding on a plant, or by laying eggs on plant tissues. The insects or mites interact with the plant during a period of rapid growth, such as when leaves are opening. Scientists believe that gallmakers secrete chemicals that regulate or stimulate plant growth. These secretions cause rapid cell multiplication in the affected area of the meristematic tissue. Galls can only form on growing tissue. Most gallmaking activity occurs in the spring or early summer.

Galls serve several important purposes for the gallmaker. The developing insect or mite resides within the gall, where it is sheltered from the weather and from predators. The young insect or mite also feeds on the gall. Eventually, the mature insect or mite emerges from the gall.

After the gallmaking insect or mite leaves, the gall remains behind on the host plant. Other insects, such as beetles or caterpillars, may move into the gall for shelter or to feed.

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