Nature Note: Oak Apple Galls

While Glenn Evans and his 22 Honors Biology students walked the Preserve in preparation for their fall field research, someone found a round green ball with purple spots, about one inch in diameter, on the ground (picture above). He held it up for all to see and asked for ideas of what it might be. One student guessed it was some kind of seed. “That’s a good guess,” Glenn noted, “but it’s not a seed.”

Another student was whispering with a friend but wouldn’t divulge her guess. But when Glenn revealed that it was an oak apple gall, a protective structure housing the larval stage of a wasp, she said to her friend, “See, I thought it was that!” Lesson: trust your instincts!

Note the tiny larva in the middle chamber.

Glenn proceeded to cut the gall in half, revealing a tiny white grub in a small chamber at the center of the gall. The tissues around the inner chamber provide a highly nutritious food for the larva until it pupates and leaves via a tiny exit hole.

Chickadees, downy woodpeckers, and other predators have discovered that galls can be a welcome source of protein during the lean winter months. If you find a gall punctured with a large hole, it’s a good sign that the gall served as a picnic table for some sharp-eyed (and sharp-beaked) bird. In nature, just about everything is food for something else.

You can find many other galls in nature, all providing food and shelter to various insects. Plants form galls in reaction to an insect laying eggs on or in it, usually in the spring when plants are actively growing. While the insect-plant interaction is not yet completely understood, the gall is believed to be a defensive reaction designed to contain the invader. (Fun fact: Most of the ink used in Europe from the Middle Ages until the 19th century came from oak galls, which are high in the tannins needed to make ink. Source: Dicks, L. “Small wonders” Science, Volume 365, Issue 6451, 26 July 2019, p. 329)

It’s common to find the dried remnants of oak apple galls, still attached to branches or on the ground. On your next walk, see if YOU can find these fragile brown spheres when you pass by an oak tree. Note whether it has a tiny exit hole or whether the birds got to it first!

Glenn was a great teacher in this moment. First, he prompted students to think about what it might be. He credited thoughtful guesses. And he didn’t just answer, “It’s an oak apple gall” and move on, he dissected it for all to see and provided enough context to educate and engage students’ curiosity.

The next time you find something new in nature, dig a little deeper, beyond just naming the thing. It’s a wondrous world out there, elegantly designed through millions of years of evolution. Observe, magnify, photograph, research – embrace the natural historian that lurks in all of us!

We highly recommend Mary Holland’s book Naturally Curious, which has wonderful information about what’s happening in nature, organized seasonally. It’s a wonderful resource, which supplied some of the details in this post about galls.