The Gall of the Oak

by Dr. Carlos de la Rosa,

Conservancy Chief Conservation and Education Officer

What are those colorful, whimsical shapes we find occasionally on the leaves and branches of oaks, willows and coyote bush on Catalina Island? You'd know them if you saw them. They show up as red “bumps” on the leaves, as little groups of growths, or as swellings on the stems. Some of them look like flowers or fruits, but they are placed in the wrong places on the tree, like on the middle of a branch or on a leaf.

These bumps, growths and swellings are called “galls” and they are triggered by some insects, particularly some species of wasps and moths, by mites and even by viruses and bacteria. The galls can be imagined as tumors that the plant grows in response to chemicals produced by insects.

There are several hundred species of gall-triggering organisms in California, and a number of them have made it to Catalina. Some of the most spectacular ones appear on oaks. One of my favorite galls, and one that is among the most common, is activated by tiny wasps of the family Cynipidae. These diminutive insects, barely a few millimeters in length, force the oak tree to grow round-shaped capsules that provide the growing wasp larvae with food and shelter. The little wasps live their lives inside these capsules and chew their way out as adults, to mate and lay their eggs somewhere else on the oak. The abandoned capsule often serves as shelter to other small insects, including a rare wasp that uses the holes left by the cynipid to lay her own eggs.

Next time you are out hiking on Catalina, look for these growths on the oaks and on other plants. If you look very, very closely, you may see one of the cynipids walking about in its tiny realm. In the mean time, come along and explore the shapes and colors of Catalina’s galls and the insects that stimulate their growth.

Learn More:

For an amazing poster of gall wasps in California, go to this website: