From time to time people call to ask about curiosly shaped, often brightly colored swellings that occur on oak tree leaves and twigs. The growths, called "galls," are usually the result of tiny wasps known as cynipids or gallwasps. Trees such as willows and poplars may also have galls, but our native oaks - especially the Valley oak (Quercus lobata) - support far more different types of galls than any other plants. Look closely at a Valley oak tree and you'll see that no part of the tree, from leaves to branches, is free from galls

Each gallwasp forms a gall of a particular size, shape and color, and entirely different from the galls of other species. Most people are familiar with the large "oak apples," induced by the gallwasp Andricus californicus on Valley oaks. The pinhead-sized "jumping oak gall", formed by the gallwasp Neuroteras saltatorius, attracts a good deal of attention because it falls to the ground and jumps around for several days. This must be a good year for jumping oak galls, because I've had numerous calls from people fascinated by the seedlike things hopping around under oak trees. Other galls look like pink stars, reddish cones or tan wheels. One particular twig gall resembles a tiny loaf of bread.

The tiny gallwasp begins the gallmaking process in early spring by piercing a selected tree part with its egg laying device and depositing an egg inside the plant tissue. Fluids deposited with the egg cause the plant cell multiplication process to begin. The larva that hatches from the egg produces additional substances that maintain and control cell devision. The larva develops within a cavity inside the gall, feeding on material produced in the cavity lining. At maturity, the larva transforms into a pupa, and later becomes an adult that chews its way out of the gall. By causing the tree to form a gall, the gallwasp has provided food and shelter for its offspring.

A variety of other insects inhabit or invade galls during or after the residency of the gallwasp. Some are parasites of the gallwasps. Others use the galls as a food source, or scavenge on materials left behind by the gallwasps or other inhabitants. If you place several galls in a glass jar, you're likely to see one or more species of tiny wasps and other insects emerge from the galls over a period of time.

A good deal remains to be learned about galls and gallwasps, because this area of biology has received little study. Most gallwasps are not harmful to oak trees, although several may cause scorching or spotting of leaves, and a few may result in the death of the twigs they infest. A healthy tree can support numerous galls without being seriously harmed.

If you are interested in reading more about galls on plants, an excellent book is available entitled Plant Galls of the California Region, by Ronald Russo, published by The Boxwood Press, 183 Ocean View Boulevard, Pacific Grove, CA 95950.