Mystery of the Tiny Bouncing Spheres

What’s shaped like a ball and white, less than a millimeter in diameter, and bounces like a Mexican jumping bean?

This odd question occurred to my wife, Karen, last week as she locked her bicycle to the rack near her office on the University of Illinois campus in Urbana. She’s not the sort of person who conjures up such questions hypothetically. It occurred to her because the concrete at her feet was alive with such spheres—thousands upon thousands of them—and she had never seen anything of the sort.

Initially, she speculated the bouncing spheres were associated with the many ants that were also present—eggs, maybe, or larvae, and that their movement was caused by the adult ants shifting them from one place to another. Needing to get on with her day, she snapped a photo to share later with others who might be able to shed light on the question.

As it turned out, however, the photo didn’t contain enough evidence to produce a good answer, so she and I returned to the scene for further investigation together a few days later at lunch. And what a scene it was—thousands of white balls bouncing around on the concrete, tiny enough to go unnoticed by most people walking by, but large enough to be seen with the naked eye by anyone curious enough to stop for a look.

We noticed almost immediately that the spheres moved without any help from ants. And by shielding them with our hands, we ascertained they weren’t being propelled by breezes, or just bouncing up from the ground after falling. But they did originate from above. The leaves of the massive bur oak tree standing over the bike lot were covered in them, and they popped off the leaves at a touch.

Thanks to internet access, the solution to our mystery was just a few keystrokes away. We searched “oak tiny white Mexican jumping beans” and that took us to—drumroll—jumping oak galls!

Jumping oak galls form around the larvae of tiny, stingless wasps as they feed on the leaves of certain species of trees in the white oak group. As the larvae mature, the galls fall to the ground, where they overwinter and then emerge as adults the following spring.

Although jumping oak galls have been observed in the U.S. since at least the 1870s, the questions of why and how they jump have yet to be investigated very fully. In a paper published just last summer, researchers from the University of California at Santa Cruz suggest that the jumping behavior protects the gall-wasp larvae from getting too hot and drying out, which can kill them. By jumping repeatedly, they settle down into the leaf litter, where conditions are cooler and moister than above. That’s the “why.”

As for “how,” the researchers observed that the larvae are packed into their galls so tightly it’s impossible for them to cause movement by thrashing around. As an alternative, they propose the larvae move by means of snap-like abdominal contractions, which transfer momentum to the gall shell through the fluid that surrounds them. As far as the researchers know, this mechanism for producing motion is unique to jumping oak gall wasps.

Whatever conclusions scientists eventually come to about how jumping oak galls jump, I am delighted to have found something so new to me and so weird in a spot that’s been part of my everyday experience for nearly 30 years (thanks to my observant spouse, of course). Who knows what we have yet to learn about the 1300 or so other species of gall wasps that occur around the world?