By Joe Eaton
Thursday February 23, 2012 - 05:28:00 PM
Cynipid wasp gall on blue oak.
Franco Folini (via Wikimedia Commons)
Cynipid wasp gall on blue oak.

Ron has begun noticing plant galls. This is fine, since we have a field guide: Ron Russo’s Plant Galls of California and Other Western States, in the estimable UC Press California Natural History Guide series. Galls are the kinds of things you pay attention to when nature is not otherwise cooperative. 

We stopped at the Leo Cronin Salmon Viewing Area off Sir Francis Drake Boulevard last week, on the way back from Tomales Bay. Lagunitas Creek was clear, and devoid of salmon (for which it was a little late in the season anyway.) The birds were quiet, except for the barely audible squeaky calls of something high in the redwoods, maybe a brown creeper or golden-crowned kinglet. It wasn’t a complete waste: the fetid adder’s-tongues, AKA slinkpods, were blooming along the creekside trail, and were duly admired. I crouched down to sniff one; it wasn’t all that fetid, although I’m told they can be noticeable en masse. Then we found the galls. 

They were on the brown, brittle stems of a chest-high plant along the creek. Ron identified it as thimbleberry from one remnant leaf. Most of the stems were surrounded at some point by potato-shaped lumps, often at a bend in the stem. A couple had tiny holes where something had exited. 

Later, with Russo’s book in hand, we were able to identify the galls as the erstwhile homes of thimbleberry stem gall wasps (Diastrophus kincaidii.) It wasn’t a difficult call: the guide is organized by host plant, and thimbleberries have only one gall-making associate species. The wasp larvae spend the winter in the galls. In spring, adult males emerge first and hang around waiting for the females to show up. Then, in typical insect fashion, they mate and die. 

A few weeks before, at the Consumnes River Preserve, we found spindle-shaped galls on the twigs of willows, the work of sawflies of the genus Euura. Since all leafless willows look alike, it might be possible to identify the species by the shape of the galls. 

Galls, to back up a bit, are growths on plant tissue—leaves, flowers, and fruit as well as stems—produced in response to invading fungi, mites, and insects as the host’s growth hormones go into overdrive. Russo calls them “tumorlike.” They come in a myriad of shapes, sizes, and colors.  

Some harbor complex natural communities: in addition to the original gall-inducing species for which the growth is a nursery, there may be inquiline insects that feed on the gall tissues, parasites that attack the gall-inducer’s larvae, and hyperparasites that parasitize the parasites. The thimbleberry stem galls, for instance, may house nine species of wasps that parasitize the cynipid larvae and an inquiline weevil.  

Some of the most spectacular galls occur on native oaks. The wasp Antron quercusechinus induces galls that resemble tiny purple sea urchins. Other oak galls are pearshaped, discshaped, or globose (“oak apples.”) Over 110 species of gallmaking cynipid wasps have been found on Pacific Coast oaks, plus a handful of midges and mites and a fungus. Like the willow sawflies, all oak gall wasps are specialists, using only trees from one of the three categories of oaks: black, white, or intermediate. 

The variety of shapes is a clue that galls are something different from what tree people call “reaction wood,” the growth of new tissue around a cut. “Something in the chemicals provided by gall organisms directs the expression of normal plant genes in the development and expansion of the host plant’s tissues,” writes Russo. “Scientists have been looking for a long time for this ‘blueprint’ that seems to control gall characteristics.” It’s tempting to see galls as an example of the “extended phenotype,” as Richard Dawkins defines it: part of the external environment that evolves along with an organism. Are galls subject to the pressures of natural selection? How does a gallmaker’s reproductive fitness benefit from growing up in something that looks like a sea urchin, as opposed to a Frisbee or a spindle? I hope the scientists are working on this angle as well. 

Russo says gall larvae provide food for a variety of birds, including jays, sapsuckers, evening grosbeaks, chickadees, and goldfinches. Chipmunks, ground squirrels, and western gray squirrels also take their toll.  

Do galls harm the host plant? That depends on the species involved. Some wasp-induced galls can kill oak leaves, and a psyllid—an aphid relative—causes unsightly galls on the Australian brush-cherry. Otherwise, gallmakers and their hosts seem to have reached an evolutionary truce.