Nature: Sonoma County is bursting with galls

One foolproof hiking strategy to keep young spirits up and prevent the “are we there yet?” syndrome is to turn your walk into a treasure hunt. And few treasures are so perfectly huntable in Sonoma County’s wild places as oak galls in the fall.

These colorful, flamboyant, often candy-like structures are found on a wide variety of plants, but it is the oak woodland that yields the best treasure trove. Galls on oak leaves are just invisible enough to make searching for them fun, but are frequent and findable enough to reward even the most jaded hiker.

What is a gall and who makes them?

Technically, a gall is nothing more than a plant’s response to an invading organism, but that’s like saying a rainbow is nothing more than bent light. The vast array of colorful galls found just on Sonoma County’s oak trees speaks to one of those magical processes in nature that is almost too mind-blowing to grasp. On the leaves of oak trees from late spring until fall are galls that look like Hershey’s kisses, sea urchins, candy-cane volcanoes, dunce caps, strawberries, pumpkins, flying saucers, clubs, goblets, stars and fuzzy slippers.

Galls are formed when an organism - the larva of a wasp, for example - releases a chemical signal that causes the host plant to produce swellings of a size and shape unique to the invader, not the plant. Think about it. A plant builds a house to the specifications dictated by a blueprint provided by a tiny invader. The invader not only gets a protective nursery in which to grow up, but it gets all of its food for free.

While all parts of the oak can be galled, including stems and roots, most galls are found on the leaves because their rapid growth insures a steady food supply for the baby growing inside.

Most galls on oak trees are made by tiny cynipid wasps, also known as gall wasps or simply gallflies, most of which are smaller than a grain of rice. The galls themselves range in size from the minuscule to macro, the largest being the ubiquitous California oak galls, often called “oak apples.”

And no, oak apples do not taste good (too high in tannins), although similar types of galls have been used by humans for centuries to make inks and dyes. The Founding Fathers wrote the Declaration of Independence using a kind of iron gall ink.

This year is particularly bountiful for the beautifully bizarre crystalline galls that look like clusters of red beaked cones engulfed in cotton candy or tiny shards of glass. Found on the underside of the leaves of blue oaks, the crystalline galls tend to clump together, more like a wasp condominium than a house.

I have seen blue oaks so laden with the reddish clumps of crystalline galls that they look like fruit trees. And now that the leaves are starting to fall, I am noticing trails in the Sonoma County Regional Parks that are dusted with crystalline hairs beneath oak trees.

One reason oak galls are so prevalent in Sonoma County is because of our diversity of oak species. No native tree or shrub supports more types of gall wasps than oaks. Furthermore, among the county’s 10 oak species, none supports more gall wasp species than blue oak (Quercus douglassii).

Gall guru Ron Russo, who has spent a lifetime hunting and studying galls, reports that blue oaks alone support at least 50 species of cynipid wasp.

If you are uncertain which oak trees are blue oaks, look for bluish, slightly lobed leaves and acorns with caps that look like beanie hats. Blue oaks lose their leaves when it starts getting too dry rather than waiting for the chill of winter. Because of California’s drought, they can be distinguished from other oaks right now by their crown of yellow and brown leaves. Or just look for the trees ornamented with clumps of red fuzzy slippers.

The really mind-blowing question when looking at oak galls is “Why go through all that trouble?” Surely a wasp larva can make do with a tiny efficiency apartment. What’s the point of building a castle? And why does one species create a striped volcano and another a strawberry hut? It’s not to attract mates. It’s not camouflage.

Could it be that nature has created this beautiful phenomenon simply because she can?

Jeanne Wirka is an interpretive naturalist and resident biologist at the Bouverie Preserve near Glen Ellen.