Oaks Have a Lot of Gall

By Anne Schuster, conservation specialist at Wolf Haven International

Honeypot galls caused by a cynipid wasp. Photo by Anne Schuster If you ever find yourself standing amongst oaks, look up to the branches or down at the fallen oak leaves. You will probably see some funny looking knobby round things stuck to the branches and leaves. These oddities are called galls. Galls are abnormal structures that grow on plants as a result of invading viruses, bacteria, fungi, mistletoe, nematodes, mites, and most often, insects (especially wasps).  In addition to wasps, there are species of fly, moth, beetle and true bugs that also make plant galls. The good news is that the types of wasps that cause galls do not sting.

Apple oak gall caused by the California oak gall wasp (Andricus quercuscalifornicus). Photo by Anne Schuster. To make a gall, a female insect pierces the plant with her egg-laying structure (called an ovipositor) and puts at least one egg in the plant. When the egg hatches, the gall begins to form, induced by chemicals delivered with the egg from the mother or by chemicals released by the larva. The larva then grows in the gall, feeding off of the plant, pupates in the gall, and then chews its way out as an adult. Every step of gall formation is very species specific. Oak trees are the plant group most prone to getting galls, and there are over 900 known species of wasps that make galls on oaks, and many more non-wasp insects. Each species of wasp will probably only lay eggs in one small part of one species of oak, and the galls will look completely different from any other species of galls.

The showiest galls on the oaks at Wolf Haven are caused by wasps. The Speckled Gall Wasp (Besbicus mirabilis) makes the round, mostly hollow, spotted galls on the underside of oak leaves. The California Gall Wasp (Andricus quercuscalifornicus) causes apple oak galls; these stem galls are very large, often growing close together in clumps, and can be used to make ink. Fun fact: the United States Constitution was written in ink from an east coast species of apple oak gall.

Spotted galls caused by the speckled gall wasp (Besbicus mirabilis). Photo by Anne Schuster.

Why should we care about galls? In addition to its contribution to the backbone of our modern democracy (as ink used by the Founding Fathers), the huge diversity of gall-inducing insects adds to the healthy biodiversity of our planet.  These insects are also involved with forest trophic cascades, and most of them are great pollinators, despite their small size.  They also offer a great opportunity for a fun outdoor family field trip –who can be the first one to find a gall?