Field Guide to Plant Galls of California and Other Western States Galls are something you can be forgiven for overlooking, but they’re an important and surprisingly common feature of the natural world. In fact, gall expert Ron Russo has found as many as 30 species of galls on a single blue oak, and even casual observers may run across giant ones nicknamed “oak apples” that are formed by the California Gall Wasp (Andricus quercuscalifornicus).

But what are galls exactly? The short answer is that they are any kind of abnormal swelling of plant tissues caused by a wide variety of insects, bacteria, fungi, and mistletoes. Either accidentally through irritation, or intentionally through the release of chemicals, these invaders cause plant cells to proliferate or grow abnormally large, creating a tumor-like growth.

Galls typically arise when insects, such as cynipid wasps or tephritid fruit flies, insert their eggs into plant tissues, and the plant swells until it forms a protective growth around the developing larvae. It’s a case of insects co-opting plant defense systems for their own ends.

Galls come in an astonishing variety of shapes and sizes and can be found on nearly any type of plant. They may be as small as the period at the end of this sentence or as large as four feet across; they may be smooth, warty, spiny, or hairy; and they may be shaped like cups, saucers, donuts, sea urchins, or caterpillars.

Even more amazing is that out of the thousands of species of insects and other organisms that cause galls, each one creates its own distinctive and unique type of gall. This allows biologists called “cecidologists” to identify and study these galls with a high degree of specificity.

Spring is the best time to observe galls because insects prefer to lay their eggs in rapidly developing plant tissues like buds and shoots which are being supplied with enormous amounts of sugars and carbohydrates. The developing insect larvae then co-opt these nutrients for their own growth.

California has a particularly rich diversity of galls associated with its oak trees, so an oak grove is an excellent place for a spring walk in search of galls. Try finding some “oak apples” which start out smooth red or green before aging to dark brown, and look like out-of-place potatoes dangling among the oak branches. Examine a few leaves closely and you might discover the bubblegum-pink sea urchin-shaped galls of the crystalline tube gall wasp (Trichoteras tubifaciens). Or the measles-spotted ping pong balls of the speckled gall wasp (Besbicus mirabilis).

Not only are galls fascinating in their variety and ecology, but very little is known about the insects that cause them. Many species remain to be discovered and named, and virtually nothing is known about the identification and biology of even some of the commonly observed galls.

If you’ve ever wanted to try your hand at discovering new species, cecidology might be the field for you. If nothing else, the fact that so little is known about galls might add a little mystery to your next walk this spring, so keep your eyes open and see how many types you can find.

David Lukas, co-author of Sierra Nevada Natural History