Gall Wasps

Have you ever seen those misshapen lumps on the twigs of oak trees? Those peculiar growths are called galls, and they're almost always caused by gall wasps. Although they're quite common, gall wasps (family Cynipidae) often go unnoticed because of their diminutive size.

What Do Gall Wasps Look Like?

Cynipid wasps are quite small, with few species measuring over 5 millimeters in length, and usually drab in color, which makes them rather inconspicuous. It's often easier to identify gall wasps from the galls themselves. Tracks and Sign of Insects and Other Invertebrates is an excellent reference for identifying North American gall-makers from the galls they leave behind.

Cynipids infest plants in the rose, willow, aster, and oak families. Cynipid galls vary greatly in size, shape, and appearance, depending on the host plant and the gall wasp species involved. Gall wasps aren't the only organisms that trigger gall development in plants, but they are probably the most prolific gall-makers, especially in oak trees. About 80% of gall wasps target oaks specifically. In North America, well over 700 gall wasp species create galls in oaks.

Gall wasps look like tiny hunchbacks. When viewed from above, the abdomen may appear to have just two segments, but the rest are simply compressed beneath, in telescoping fashion. Gall wasps have minimal wing venation and filiform antennae (usually consisting of 13 segments in females, and 14-15 segments in males).

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What Are Galls?

By Debbie Hadley

You're unlikely to see gall wasp larvae unless you're in the habit of dissecting galls. Each tiny, white larva lives within its own chamber, feeding constantly. They lack legs and have chewing mouthparts.

What Do Gall Wasps Eat?

Gall wasp larvae derive nutrition from the galls in which they live. Adult gall wasps are short-lived and do not feed.

Surprisingly for an insect that eats so much, the larvae don't poop. Gall wasp larvae don't have anuses, so there is simply no way for them to expel their waste. They wait until the pupal stage to rid their bodies of fecal matter.

The Life Cycle of Gall Wasps

The cynipid life cycle can be quite complex. In some species, male and female gall wasps mate and the female oviposits in the host plant. Some gall wasps are parthenogenetic, and produce males rarely, if ever. Still others alternate sexual and asexual generations, and these distinct generations may use different host plants.

In very general terms, the gall wasp life cycle involves complete metamorphosis, with four life stages: egg, larva, pupa, and adult. The female deposits an egg into the meristematic tissue of the host plant. When the egg hatches and the larva begins to feed, it triggers a reaction in the host plant, causing the formation of the gall. The larva feeds within the gall and eventually pupates. The adult gall wasp usually chews an exit hole to escape the gall.

Special Behaviors of Gall Wasps

Some gall wasps don't produce galls in their host plants but are instead inquilines of other species' galls. The female wasp oviposits into an existing gall, and her offspring hatch and feed on it. The inquiline larvae may indirectly kill the larvae that induced the gall to form, simply by outcompeting them for food.

Where Do Gall Wasps Live?

Scientists have described 1,400 species of gall wasps worldwide, but many estimate that the family Cynipidae may actually include as many as 6,000 species. Over 750 species inhabit North America.

Resources and Further Reading 

• Capinera, John L., editor. Encyclopedia of Entomology. 2^nd ed., Springer, 2008.
• Frogge, Mary Jane. “Most Leaf Galls Don't Hurt Trees (Galls).” Institute of Agriculture and Natural Resources: The Nebline, University of Nebraska-Lincoln in Lancaster County, May 2012.
• Johnson, Norman F., and Charles A. Triplehorn. Borror and DeLong's Introduction to the Study of Insects. 7^th ed., Cengage Learning, 2004.
• Leung, Richard, et al. “Family Cynipidae - Gall Wasps.” BugGuide.Net, Iowa State University, 13 Apr. 2005.