If I had not chosen to study flower-visiting insects in graduate school I almost certainly would have become a cecidologist, one who studies plant galls. Invaders such as fungi, viruses, bacteria, mites, and even mistletoe cause these abnormal and often strange-looking growths on plants, but most galls are induced by insects, our principal focus here. The word “gall” comes from the Latin galla, meaning oak apple, a conspicuously large gall caused by a tiny and rarely noticed wasp in the Cynipidae family. Oak apples are a common sight on valley oak trees throughout the West. Oaks in California’s Central Valley may also host jumping galls induced by another species of cynipid wasp. About the size of a flea, these tiny galls detach from the tree and drop to the ground by the thousands where the larva encased in each gall cause it to bounce about in the manner of Mexican jumping beans.
Hundreds of other species of cynipid wasps induce galls and, like other gall-inducing insects, are generally host specific, seeking out particular species of plants. When cynipid wasps deposit their eggs in plant tissues, the plant walls off the invader with a swelling or tumor-like growth that is specific to the causal species. The resulting gall provides both food and a safe environment for the larva within. Oaks host more galls than any other plants in the western United States, but other plant families, including willows and wild roses, can host a great diversity of galls. In addition to cynipid wasps other species of wasps, flies, aphids, psyllids, and thrips also can induce galls.
My studies in biology, with an emphasis in entomology and botany and therefore chemistry, sparked my latent curiosity and interest in the extraordinary world of galls. How, I wondered, do the hundreds of species of cynipid wasps induce so many different forms of galls, many of them highly ornamented? It turns out that chemical signals excreted by ovipositing or by larval saliva cue the plant to produce a gall of a particular size, shape, and color depending on the species of insect. And what fantastical forms they take! Ron Russo’s wonderful book, Field Guide to Plant Galls of California and Other Western States is a boon to anyone west of the Rockies interested in galls.
Oak galls generally cause little damage to oaks. Others like canker gall, which can damage orchard trees, and fir tree galls caused by balsam wooly adelgid, cause substantial harm to the plants. Sometimes, a secondary agent causes damage.
Grape phylloxera, a tiny sap-sucking insect that causes root deformities and introduces fungal infections, has devastated the wine industry. However, one type of gall beneficial to any gardener are the root nodules caused by nitrogen-fixing bacteria. Rhizobium bacteria species induce detachable nodules on the roots of most legumes, while Frankia species are found on the roots of ceanothus, mountain mahogany, alder, wax myrtle, buffaloberry, and bayberry, among others.
Galls serve other useful functions. Many galls play a vital role in the food web by providing food to birds that seek out the larvae within. Some galls are used as dye and for tanning. Noted for its long-lasting quality, ink made from galls was used by monks in Europe, in the signing of many historic treaties, and by the United States Treasury for printing currency. Galls have been used for medicinal purposes for centuries for everything from eyewash to treatments for cuts and burns, and even as remedies for hair loss. Gall wasps pollinate figs. Each species of fig is pollinated by a specific species of wasp; figs from Smyrna, Turkey, were first brought to California in 1880, but not a single fruit was produced until the right species of fig wasps were introduced.
Galls form most quickly in spring and early summer when plant growth is rapid. Place a few galls into a jar and you can expect to see the insect that caused it to eventually emerge. Surprisingly, you might find multiple species of insects emerging if a gall wasp’s larva has been parasitized by another species of wasp, and sometimes the parasite will in turn be parasitized. Insects termed inquilines, resident lodgers that cause no harm to the original occupant but are merely sharing space in a cozy home, often occupy mature oak apples.
Other plant phenomena produced by a gall-inducing agent include fan-shaped fasciation and witches’ brooms, dense bundles of shoots that arise from a common point on woody plants. With so many intriguing stories, galls can provide a lifetime of study for the amateur naturalist or professional cecidologist. Aren’t we lucky that the Western United States
offers us such a rich diversity of galls to engage us in exploring nature?