As we wrap up autumn, there has been a bounty of wonderful things Fall-ing all around us. In addition to some brightly colored leaves, you may have also noticed something underneath our Oak trees. In addition to the acorns scattered about you may also spy some bizarre and colorful little shapes littering the ground…galls! They come in a wide variety of shapes, colors, textures, and sizes. Little kisses, witch’s hats, flying saucers, puff balls, urchins, etc.

Each of those tiny vessels is a larval nursery of a tiny cynipid wasp. Early last spring, the mother wasp laid her eggs on a blue oak in the western foothills of Mount Diablo and flew away, never to return. The oak took over from there. It built this gall for the larva of the wasp, supplying it, free of charge, with food, water, shelter, and protection until it matures and flies away. The tree, minus the mother’s DNA, is the parent of the wasp larva inside. The gall itself is made of oak.

Stranger still, blue oaks make unique nurseries for more than 40 species of these tiny native wasps. They obtain their architectural plans from the wasp mom, who tells them the specifications of her species either through her egg laying or the hatching and munching of her larva. The oak then makes up the order from scratch at its own expense.

How wasp galls manage this free ride isn't yet explained in full by researchers, but it’s fair to say that the oak is reprogrammed by the wasp. Maybe the wasp triggers an autoimmune disorder, making the tree unable to tell the difference between wasp cells and its own. Maybe the egg or the larvae irritate the oak like a grain of sand in an oyster, and the gall structure is the oak’s defense: a gall as a pearl. Highly unlikely, I know. It seems certain, however, that galls do no damage to trees in most cases, though long-term droughts could change that.