Steven E. F. Brown

Yesterday, Kieran and I explored another of the Bay Area’s great mountains — Mt. Diablo. We were looking for bones, like we found last week on Mt. Hamilton. But although we did find a few bones — a deer’s shed antler and leg bone, plus another cow shank — we found a wonder of nature far more queer and marvelous.

We found three different types of oak galls, all of them caused by tiny cynipid wasps that are poorly understood by human beings.

The first galls were bright pink “crystalline” galls of Andricus crystallinus. Each tiny, cone-shaped gall is fuzzy, like a little pink Christmas tree, and sometimes a whole forest of them covers the underside of a single oak leaf. These galls are from the “parthenogenic” generation of the wasps — when females lay eggs without mating, producing clones of themselves. There’s a second generation of these wasps in early spring, when males and females mate.

Then we found many enormous “oak apples” — galls of the wasp Andricus quercuscalifornicus, which is even more poorly understood than the Crystalline Gall Wasps. No males have ever been identified of A. quercuscalifornicus. It’s possible that the males are so tiny and different that if they’ve been caught, they’ve been erroneously identified as a separate species. But for now, no one’s sure they even exist.

On one of the trees, I also found a spectacular, alien-looking “urchin” gall of the wasp Antron quercusechinus. Again, like the Crystalline Gall Wasps, these have two generations a year. The spiky, urchin galls hold only females from eggs laid in the parthenogenic cycle. Galls that hold males and females from the other generation are merely small green bumps on the tree that you’d never notice.

These galls are just three of the estimated 95 different types of oak galls in California — there’s an entire, wonderful world all around us that we hardly notice.

Tiny crystalline galls on a fallen oak leaf. We found thousands of these galls.

A close view of a leaf with the A. crystallinus galls.

Some leaves were completely covered with a mini forest of fuzzy galls.

Another view of A. crystallinus galls.

Andricus crystallinus galls on leaves still attached to an oak tree.

A double “oak apple” gall.

Oak apple galls grow from the branches, rather than from the leaves.

An urchin gall made by Antron quercusechinus.

Not a gall, but rather an enormous fungus growing from the trunk of an oak tree!

The steep slopes of Mt. Diablo are covered with oak trees of many species.