Galls on the Valley Oaks of Effie Yeaw

October 11, 2018

Written by Mary Lou Flint, Docent, Effie Yeaw Nature Center

Have you noticed odd, tumorlike growths on our oak trees as you walk the woodland trails? These growths are oak galls, and fall is the perfect time to observe them.

Most oak galls are made by tiny cynipid wasps that lay their eggs in stems or leaves.  Gall growth by cynipids is induced when hatching gall wasp larvae start to feed and begin to release an auxinlike plant growth hormone. The hormone causes rapid expansion of the treeís tissues into a gall, which provides food, protection and a stable microenvironment for development. By fall the galls and the wasps that induced them have reached their maximum size, matured, changed color and, in some cases, started to drop to the ground.  

Although many species of trees and shrubs host insects that cause galling, oaks have by far the most gall-producing insects associated with them. Over 200 species of insects are known to cause oak galls, each inducing a unique shape. Specific gall makers are generally associated only with one of the three major oak groups (black, white, intermediate). Although there are several gall makers that attack black oaks such as interior live oak (Quercus wislizensii), the most common and visible galls seen at Effie Yeaw occur on valley oak (Quercus lobata), which is a white oak.


The California Gall Wasp

The California gall wasp, Andricus quercuscalifornicus, produces the large, round oak apple galls frequently observed on valley oak. These are the biggest insect galls found in the western United States. The life cycle of this wasp and the galls it produces are illustrated below (Figures 1 and 2). Only females are known for this species, and they reproduce parthenogenetically, laying eggs without mating.

Females of the California gall wasp emerge in fall and deposit a dozen or more eggs directly into stem tissue. However, galls do not begin to form until spring when trees begin to flush out. At about that time, gall wasp eggs hatch and larvae start to feed and release the plant growth hormones that stimulate gall formation. Galls rupture through the stem tissue, grow rapidly and reach full size in about two months. Galls are at first smooth and shiny and reddish or green but turn light brown by late summer and darken as they age.   

The wasp larvae are shiny white and legless and develop in chamber within the gall.  More than one larvae may be in a single gall. When full grown, larvae pupate within the gall and emerged adults may rest just inside the walls of the gall until conditions are favorable for egg laying. Once out of the gall, the adult wasps donít live for much more than a week. While some wasps emerge in fall, others may not pupate and mature for another year. Galls may stay on trees for several years well after all wasps have left the gall. Sometimes a black sooty mold grows over old galls on trees. Although some people may think the galls are unsightly, they donít injure the oak tree.

Oak apple galls provide food for a community much larger than the California gall wasp itself. Over 20 species of other insects have been associated with the galls including other tiny wasps, flies and caterpillars that feed on gall tissue (inquilines), wasp species that parasitize the gall wasp (parasitoids) and other wasps that parasitize the parasitoids (hyperparasites). Sapsuckers and other birds may dig into galls to feed on the insects.  Galls on the ground provide food for other animals.


Figure 1.  Life cycle of the California oak gall wasp. The female wasp (about 5mm long) lays eggs in the stems of the oak tree in fall. In spring, larvae hatch, begin to feed and produce a plant growth hormone that induces gall formation. The cross section of the gall shows two larvae inside. Galls are at first green but later turn darker. Larvae pupate and emerge as new adults in fall. (Molly Keller illustration).












Figure 2.  Old (brown) and new  (green) apple oak galls produced by the California oak gall wasp. 














Other Common Gall Wasps on Valley Oak

I have observed several other types of galls on valley oak at Effie Yeaw over the past year. These include red cone galls, spined turban galls, jumping galls, yellow wig galls, and flat-topped honeydew galls. Examples of these are pictured below with captions. I know there are a number of other species likely to be present. Keep your eyes out for them. A good reference for California oak gall photos is Joyce Grossís web page

Figure 3.  Spined turban galls (left) caused by the wasp Antron douglasii and red cone galls (right) of Andricus kingi on underside of valley oak leaves. Both of these galls drop to the ground when leaves fall in autumn but adult females donít emerge until February to lay eggs within the tissue of newly emerging buds in spring. 



Figure 4.  Flat-topped honeydew galls produced by Disholcaspis spp. wasps on stems of valley oak. These galls produce honeydew that attracts ants and yellow jackets. 










Figure 5.  The jumping gall waspNeuroterus saltatorius, causes seedlike galls on the undersides of valley oak leaves. These galls drop to the ground in fall and the larvae inside wiggle or jump to work themselves beneath the leaf litter to hide from predators. Larvae pupate on the ground in the late fall-winter and emerge as adult wasps in March or April.    











Figure 6.  The wasp Andricus fullawayi produces yellow wig galls, which have a dense mat of long hairs on their surface. These galls occur on the midrib of leaves.  











All photos:  Jack Kelly Clark, used with permission from the University of California Statewide IPM Program.

Text:   Mary Louise Flint, Docent, Effie Yeaw Nature Center