Post by Jen Berlinghof

Fall is the time of harvest here in the Midwest. Golden stacks of hay bales, farmer’s markets teeming with end-of-season produce, and above all, apples. But step away from the orchard and into an oak woodland and you’ll find a different kind of autumn apple: an oak apple gall. What looks like a small, lime-green, spotted apple dangling from an oak leaf is not a fruit at all, but rather a secret abode for a tiny wasp.

There are more than 50 species of oak apple gall wasps in North America. Each one creates a unique fruit-like structure that protects and feeds its eggs and larvae as they develop. Lately, I’ve been finding many dried, spent brown husks created by the larger empty oak apple wasp (Amphibolips quercusinanis) around the Lake County Forest Preserves in northern Illinois.

The larger empty oak apple wasp (Amphibolips quercusinanis) is one of 50-plus species of oak apple gall wasps. Photo © Charley Eiseman.

Gall formation is complex and not totally understood by science. What we do know is that female gall-making wasps produce chemicals that turn plant genes on and off, allowing for gall growth. In the case of the larger empty oak apple gall wasp, a female wasp from the family Cynipidae injects these gene-altering chemicals along with one egg into the leaf buds of red oaks (Quercus rubra) and scarlet oaks (Quercus coccinea).

The egg and the larva that hatches out of it ooze said chemicals throughout their development. White fibers form, radiating from the capsule that encases the egg in the center. The skin of the gall keeps it all contained. This fibrous flesh provides all the nourishment the larva needs to transform into an adult wasp, all the while safely hidden away from the elements and would-be predators.

When occupied, this kind of gall is typically plump and green, but dries out and turns brown once the adult gall wasp emerges, leaving behind a perfectly round exit hole. Thankfully, oak apple galls generally do not damage their tree hosts.

During these early fall days, when the oak leaves themselves are also beginning to trade their vibrant green luster for tan tones, take some time to visit a preserve and try to spot oak apple galls as well as the many other galls we see in Illinois.

Want to plant an oak around your home (and maybe reap a harvest of oak apple galls next year)? Join us at our upcoming OAKtober Celebration and Native Tree & Shrub Sale, Sunday, October 6, 10 am–4 pm at Ryerson Conservation Area in Riverwoods. Select the perfect native tree or shrub for your yard at our sale (10 am–3 pm). Then stick around for workshops, nature-themed activities, and guided tours of the autumnal woodland (1–4 pm). FREE. No registration required. All ages welcome. Rain or shine.