Vol. 17 Issue 1, Spring 2012
By Kerry Bzdyk
Photo by Liam McGranahan
Those of us who make a habit of observing the natural world in great detail have all had the experience of seeing something on a plant or tree that looks alien and at the same time looks like it belongs there. These strange formations on plants and trees are very likely galls. Galls are abnormal plant growths caused by fungi, insects, mites, and sometimes viruses and bacteria. Often these growths appear to be part of the normally developing plant and resemble fruit or seeds. Others are large, unusual bulges in twigs or leaves. Each type of gall found is specific to a plant and, if insect-caused, an insect species. Oak trees are a favorite host for several types of galls.
Most galls on oak trees are caused by insects; most commonly, very small wasps. The abnormal plant growth is stimulated by chemicals or other stimuli from the developing wasp egg or larvae. These chemicals actually interfere with the normal plant growth to form a protective and nourishing habitat for the growing wasp. Each type of gall found is specific to an insect species and plant. Galls do not cause any permanent damage or harm to the host tree and control is not needed.
Many types of galls occur on oak trees including leaf galls, roly-poly galls, oak apple galls, and the fascinating wool sower galls which are specific to White Oak trees. They are caused by a tiny wasp, Callirhytis seminator. Secretions from the grubs of this wasp cause the formation of this wooly and fibrous growth in the spring and early summer. These galls are about an inch or more in diameter and resemble a cotton ball or white pom-pom. They form on the twigs and smaller branches of the White Oak. While they look soft, they are actually somewhat hard. Inside the white wooly part of the gall are small structures that resemble seeds in which the tiny wasp is developing. The gall protects the growing insect from predators, and the plant tissue involved feeds it as well. It’s a one-sided relationship that only benefits the wasp, but does no real harm to the tree.
Galls are intriguing, sometimes beautiful, and harmless. Keep an eye out for these unusual growths when taking your spring hikes this year.