There are over 150 species cynipid wasps that induce galls on California oaks. Barely the size of fruit flies, these wasps cause some of the most exotic plant galls displaying a variety of shapes (conical, spherical, stalked, disc-shaped, spiny, etc.) and colors (pink, red, yellow orange, and more). Cynipid wasp galls are species specific. Most arise on leaves and buds, with fewer species inducing galls on other plant tissues such as roots, stems or acorns.
After an adult cynipid wasp deposits an egg, a shell of plant tissue develops around the egg. Once the larva hatches and begins feeding, the chewing action of the larva releases compounds that redirect the plant cells to form gall structures. An adult wasp emerges after a period of pupation. The adult wasps live about a week during which they mate, lay eggs and begin the life cycle again. Depending on species, cynipid wasps can reproduce sexually or parthenogenetically without any male involvement. Many species exhibit alternation of generations (heterogeny) with a spring sexual generation and a summer asexual generation, each emerging from a different gall, often on a different plant organ.
Oak apple galls are induced by the cynipid wasp Trichoteras vaccinifoliae, formerly called Andricus vaccinifoliae. These spherical galls are originally fleshy and yellow-green with red mottling. Some have bumps on the surface. By fall, the gall is dry, light brown, paper thin and brittle. They occur singly, or occasionally in small groups on the older branches of canyon live oaks (Quercus chrysolepis) or huckleberry oaks (Quercus vacciniifoliae – see my previous post “Huckleberry Oak” on 10-22-18). These monothalamous galls (single chamber) have a central larval chamber suspended by numerous radiating fibers. The larva in the central chamber is sustained through nutrients transmitted through the radiating fibers to the chamber wall. Tannins and starches are absent from the nutritive tissues of this gall but do occur in the outer layers of the gall walls. These compounds are thought to provide a defense against larval “enemies”. There does not appear to be any alternation of generations with this species of cynipid wasp.
I was delighted to find these oak apple galls, a gall that I had not seen before. The huckleberry oak galls were photographed along Stony Creek Trail in the Six Rivers National Recreation Area (Del Norte County CA) while the galls on canyon live oak were on trees along Althouse Creek near Cave Junction OR.