The fascinating life of oak gall wasps

The fascinating life of oak gall wasps

Earlier this month my colleague, LyAnn Graff, brought in an interesting looking fuzz ball surrounding a thin leafy branch of a burr oak tree. It was about the size of a golf ball, but with red-tipped small points on the wooly surface. I figured it must be an oak gall of some kind and the burr oak leaf attached was my first clue. I’ve seen oak galls plenty of times, but this one was different.

I told LyAnn that there was probably an egg or larva from a tiny insect inside the fuzz ball she had brought into the office. While she was fascinated, she also though it a good idea for me to take it home, preferably before hatching!

In my bookcase are two excellent books about oak trees and I pulled them down to see what I could find about this fuzzball. “The Nature of Oaks,” by Douglas W. Tallamy is an excellent source of information on oaks and the countless species of insects and other creatures relying on this mighty tree of the forest. “Oak: The Frame of Civilization,” by William Bryant Logan is also an excellent source of information on the role oaks played across the globe for thousands of years. Both volumes had lots of information about oak galls and here’s what I found out.

Tallamy describes oak galls being make from a cynipid gall wasp, but not what we typically think of as a wasp, “cynipids are tiny and cannot sting you and most people would dismiss them incorrectly as midges or gnats – that is if they noticed them at all.” They are gall wasps due to their amazing ability to “chemically manipulate the plant tissues to form galls, plant growths that provide both protection and food for cynipid larvae. There are nearly 800 species of gall wasps in North America, and most of these have specialized relationships only with oaks.”

According to Logan, cynipid wasps have been using oak trees for laying eggs and raising larva for 30 million years. “The galls are extraordinary, many-layered and labyrinthine. They exist not simply to feed growing larva but also to defend it against weather and especially against the numerous parasitoids that would either eat the larva or take over the house for their own broods.”

Logan’s book had illustrations of 23 different oak galls and included two that identified LyAnn’s as from the wool sower gall wasp. The two pictures showed one as it would appear brand new, and one in late autumn.

I did a bit more searching about the wood sower gall wasp and found an interesting blog “Sea Shore to Forest Floor,” by naturalist Elise Leduc, where she describes the life cycle of the wool sower gall wasp, Callirhytis seminator.

“These pink-spotted, white-cottony wool sower galls are about the size of a ping-pong ball and are produced by a tiny species of Cynipid wasp, Callirhytis seminator, and are most commonly found on white oaks. C. seminator eggs, which are laid in winter, hatch just as new leaves are appearing on the oaks in the spring. The newly emerged larvae secrete chemicals that stimulate the plant to develop the gall tissue. Interestingly, each puffy gall is a conglomeration of small hairy galls, each produced by an individual grub, joined at a common spot on a twig. Although I chose to let these galls remain undisturbed, if you pull one of these galls apart you will see small seed-like structures, each containing a developing wasp.”

In my rambles I do find oak galls, usually in the fall when the larva has matured and left the gall. It seems the most common around these parts are what are called oak apple galls, or at least those are the ones I find. You too have probably seen these crisp brown balls, very light and hollow with a papery outside and light pithy center. You might have noticed the small holes where the wasp has exited its summer abode. The wasp of the apple gall is Amphibolips confuenta. I have also found them when they are bright green with a hard outer shell and still with the wasp larvae inside. They get their name because the inside resembles the inside of an apple.

There is much to learn about the unique relationship between oak trees and the many species of tiny oak gall wasps. Our ancestors also found use for the galls for various purposes. One of the more fascinating bits of history is that oak galls were used for making iron gall ink. In “Oak: The Frame of Civilization,” Logan describes gall ink as “far superior to soot ink – called India ink. It flowed onto the page better, making a cleaner sharper line and, above all, once set was virtually indelible. Where clarity and permanence were wanted, oak ink was unsurpassable. Governments used it for their official documents. Among the first modern government contracts were those for the ink supply. The U.S. Constitution and the Declaration of Independence were both drafted with it.”

Our natural world is full of surprise and wonder. Thanks to LyAnn, I was introduced to another fascinating connection between the plant and insect world. I’ll be on the lookout for more oak galls and hope to find additional varieties created by the fascinating oak gall wasp. The wool sower oak gall she found is now on my back porch. I put it in a cup with the oak stem in a bit of water and plan on checking it regularly to see if I can catch a glimpse of the tiny cynipid wasp.

We live in beautiful place called The Last Green Valley. It’s like having a National Park right in our own backyard. I hope you’ll join me to care for it, enjoy it, and pass it on.

Bill Reid is the Chief Ranger of The Last Green Valley National Heritage Corridor. He has lived in and explored the region for over 40 years and can be reached at



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