Sunday, December 7, 2014

Oak Galls

One of the Oak Apple Galls (Amphibolips spp), a type of Cynapid wasp

     Galls are growths caused by an organism (usually an insect or mite) in an another organism (normally plants). There are over 2,000 different types in North America. Each type is very uniform in shape, host plant specific, and generally only occur on specific parts of their hosts. How the gall maker induces the plant to grow in such a uniform manner is not completely understood. The gall inducer lives inside the gall, which provides protection, nourishment, and water. It is for all practical purposes, its edible home.

Oak Bullet Gall (perhaps Disholcospis globulus?), caused by a Cynapid wasp.

     I am unaware of any plant that hosts more gall species than the oaks (Quercus spp.). Over 800 kinds of galls need oaks in order to survive. The vast majority of these galls are produced by the larvae of tiny wasps in the Cynapid family. There is not much known about the life cycle itself of most gall producers, and oak gall makers are no different. Here a few that I've encountered.

Lobed Oak Gall (Cynips strobiliana) on Swamp White Oak. This Cynapid wasp is usually restricted to Bur and Swamp White Oaks.

Gouty Oak Galls (Callirhytis quercuspunctata?) caused by Cynapid wasps.

Wooly Oak Leaf Galls (Callirhytis lanata) formed by Cynapid wasps.

Oak Wool Gall, another type of Cynapid wasp.

Horned Oak Gall (perhaps Callirhytis spp?), formed by another Cynapid wasp.

     If you were to look inside any active gall, you would find the gall maker inside feeding. If you see holes on the outside of the gall itself however, there is a good chance that gall maker(s) has already exited through the opening. Galls may contain one or multiple occupants within the overall structure depending on the species. Below you can see the larvae that was living inside a pin oak gall (click to enlarge as always).

Larvae inside of an oak gall

     Although the majority of oak galls are produced by Cynapid wasps, a few are caused by some others organisms, especially various midges. A few of these are presented below.

Oak Pill Gall (Cincticornia spp?) caused by one of the gall midges.

Oak Leaf Galls (Polystephe pilulae?) caused by midges.

Vein Pocket Galls (Macrodiplosis qoruca?) caused by midges.

     This is just a small sample of the many kinds of oak galls, and I have done my best to identify them, but please realize that there is so much still left to discover about galls and disagreement about others. It is amazing what little we know of even some of the organisms that live right outside many of our homes. It just goes to show the vastness of the natural world, even that found near the nation's capitol.


  1. I enjoyed reading your post on Oak Galls. I was surprised you didn't mention that they were used to make ink in the past. I wondered if there are so many types of oak gall if they all contain the chemicals needed to make oak gall inks? I found an interesting video on making the ink with iron sulfate that interacts with the tannic acid.

  2. This is a GREAT article! Thank you. Always wondered what those were exactly.

  3. I was told this is what is causing our oak trees to die.

    1. Not at all! It would be a poor evolutionary strategy to kill the only think that keeps you alive. They would have killed them all off a long time ago if that was the case.

  4. Very interesting! I have seen many of them, knew they were from some kind of insect, but didn't realize they were all pretty much related in the same way. :)

  5. That was fascinating! I didn't know anything about these except my parents called them galls. Question: Are the Latin names below each of the pictures for the species of oak, the insect, or the gall? I wouldn't think the gall would get its own name but-- Thanks!

  6. Thanks folks. I did not include anything on their use as inks, partially because I do not think they all can equally be used, especially the differences between those produced by Cynapid wasp as opposed to midges. Galls in general do not cause their hosts to die, they are not parasatoids. Having evolved with specific hosts, it would make poor evolutionary sense to kill off the only thing you can feed on, at least through a part of your life. They may result in some aesthetic damage, but rarely really harm a healthy tree. The names I provided are the best I could do for the gall maker, the animal that lives in the gall for at least a part of its life. Thank you.