By Golly, They're Galls In That Oak!
Durango Herald, Sep 11, 2005
Jennifer Kleffner, Community Resources Coordinator

Courtesy Allison Pease

As you admire our Gambel oak acorns turning brown in their caps this fall, take a closer look at the leaves of this plant.

You might notice a strange round reddish growth on the leaves, ranging in size from 1/8 to 2 inches across. These are apple oak galls, named for their transient resemblance to apples. If you look closely, you might notice a small hole in the side of this gall, where an insect has emerged.

Galls, as we often explain to visiting elementary school students, are simply insect homes created by a plant. An insect lays her eggs on a plant in an area where the cells are developing rapidly, that egg hatches, and through a complicated interaction in which the insect mimics the plant’s growth hormones, the plant grows a “tumor” around the insect larvae. The larvae feeds on the plant, is protected from predators and the elements by its tough home, and eventually emerges as an adult to start the process anew. Insect galls rarely do serious harm to the plant.

Each species of gall insect (certain aphids, wasps, moths, beetles and midges) has a favorite host plant and forms galls of a characteristic shape. Common galls in our area occur on rabbitbrush, sandbar willow, and the cooley spruce gall. Galls are rich in resins and tannic acid and have been used in the manufacture of permanent inks and astringent ointments, in dyeing and in tanning.

The apple oak gall is caused by a small (1/8 inch) harmless wasp with a fabulously weird two-stage life cycle called cyclical parthenogenesis. This involves alternation between sexually and asexually reproducing generations.

The cycle starts when both male and female adult wasps hatch from oak leaf galls in June and July. They mate and then drop to the ground. Female wasps burrow into the soil at the base of the tree and inject fertilized eggs into the roots. These eggs hatch, and the all-female larvae create root galls.

The root larvae mature for more than a year before emerging as wingless female wasps. These females crawl out of the soil and up the tree trunk in early spring. They inject unfertilized eggs into the center vein of newly growing oak leaves.

The male or female larvae hatch, eating and growing, and the apple galls grow with them (one gall per wasp). When the larvae are full-grown, they pupate and come out as an adult male or female wasp. The two-stage life cycle then begins again.

I have often seen small galls like strings of beads on the rib of an oak leaf. In the fall, each one showed a small hole where its occupant had emerged. Isn't nature amazing?