Random Natural History: Valley Oaks and their Galls.

Ok, time for a short bit of natural history. I live in the Sacramento Valley in northern California. The dominant tree species (outside of urban areas) seems to be the Valley Oak (Quercus lobata). Now, there aren’t a whole lot of trees in the valley, so it’s pretty lucky that Valley Oaks are fairly spectacular.

Valley oak (Quercus lobata) on Joseph D Grant County Park Canada de Pala Trail

They are also little ecosystems unto themselves. The first thing most people notice about them are oak apple galls, so called because they bear a disturbing resemblance to (rotting) apples.

"Oak apple" galls of California Gall Wasps (Andricus quercuscalifornicus, Cynipidae, Hymenoptera) on Valley Oak (Quercus lobata, Fagaceae)

Trees can often be so laden with them that they actually look like cultivated apple trees. The galls are woody, though, not squishy like actual apples. What is a gall, you ask? Good question. A gall is essentially a plant tumor. In many cases (as here) galls are caused by insect parasites. An adult insect lays eggs in the tissue of a plant, and those eggs release hormones that induce the plant to form the gall. Galls can provide food and shelter for their hosts until they are ready to mate and lay new eggs. Galls can be quite complicated structures, the result of parasites evolving very refined control over their hosts over time. As a result, galling insects can frequently be identified by their galls alone. Oak apple galls are caused by a wasp, Andricus quercuscalifornicus, but are exploited by a constellation of at least 20 other arthropods that feed on the galls, A. quercuscalifornicus, and each other.

These aren’t the only galls associated with the Valley Oaks. There are at least two more. One of which is fairly bizarre and the original inspiration for this post: the California Jumping Gall. This gall is also caused by a wasp, Neuroterus saltatorius. In contrast to the oak apple galls, these galls are tiny, only about a millimeter across. What they lack in size, however, they make up for in quantity. These galls form on the undersides of oak leaves by the hundreds of thousands. When they mature, they drop off the leaves, wasp larva still inside. Once on the ground, they start “jumping”. The larvae violently fling themselves around inside the gall, presumably to try to move it into a sheltered spot where they can finish out their life cycle and emerge the following spring to lay new eggs.

The galls are dropping now in my neighborhood, and the result is that sidewalks and gutters under valley oaks appear to be full of jumping grains of sand. It’s a pretty weird sight:

Here’s a link to another video:

ARKive video - California jumping gall wasp - overview

Well, that’s all I’ve got for now. I’ll end on another photo of an amazing Valley Oak.

Quercus lobata VALLEY OAK/ROBLE