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Several kinds of insects and mites cause curious swellings on plants called galls. Some of these growths become as large as a baseball and are smooth in appearance. Others may be smaller than the head of a pin. Some have very strange shapes with spines and knobs on them. They are common on oak, willow, poplar, rose and many other plants grown in California. The native oaks support more than 90 types of galls. No part of an oak is free from infestation. Galls may be found on leaves, flowers, buds, twigs, branches, roots and even acorns.

The most common gall inducers are tiny, dark wasps called cynipid or gall wasps. Certain moths, beetles, flies and a few other insects and mites also form galls. Galls consist only of plant tissue. In most cases, normal plant cells have been stimulated to multiply at an unusually high rate by the activity of a gall inducer. To successfully form a gall (and thereby successfully reproduce) the insect or mite must deposit its eggs at a very precise moment in the plant’s growth cycle.

A gall wasp initiates the process by piercing a selected plant part with its egg-laying device and depositing an egg or eggs inside the plant tissue. Fluids deposited with the egg or produced by the larva after hatching cause the plant cell multiplication process to begin. The larva produces additional substances that maintain and control cell division that determines the size and shape of the developing gall. The larva develops within a cavity inside the gall, feeding on material produced on the cavity lining. At maturity, it transforms into a pupa, and later it becomes an adult that chews its way out of the gall. By causing the plant to form a gall, the gall inducer has made the plant provided food and shelter for its offspring.

Each gall inducer forms a gall of a particular size, shape and color; no other species forms one quite like it. (View more gall images) Gall inducers are specific about the types of plants and the plant parts they attack. Some galls contain more than a single larva of the gall inducer, but usually each lives within its own cavity.

Most insect or mite caused galls in California are not harmful to the plant. Several cause a scorching or spotting of leaves and a few result in death of twigs they infest. In nearly all cases prevention of gall formation is exceedingly difficult and is not considered practical. For many insect and mite species that cause galls, a means of prevention or control is unknown.