In late August a couple of years ago, I spotted a tiny, bright orange wasp on an oak gall. As I took a series of photographs, I witnessed an astonishing behavior.
Before spilling the crazy details, here’s a little background. If you look closely at white oak trees at Mount Pisgah Arboretum this time of year, you’ll see small, round balls under some of the leaves. The orbs are yellow-orange with darker orange spots. These are galls. When a speckled gall wasp cuts into the tissues near the budding leaves of the tree and lays an egg, the tree makes a gall to scar over the area. The gall in turn protects the wasp egg.
Usually, the gall wasp grows happily, feeding on the spongy parts of the gall, pupates, and emerges as an adult. But sometimes, another wasp steps in to use the cozy, protected gall and the larvae inside for its young. The Torymus genus wasp is a parasitoid of the speckled gall wasp, and lays eggs inside the gall, where the Torymus larvae use the gall wasp larvae as food.
In the series of images, you can see the red-eyed, orange wasp probing for the best place on the gall for her to insert her eggs. Then, she slides the ovipositor out from its sheath. Yes, that tiny but long wand is just the outer covering for a needle-sharp, yet hollow, implement the wasp uses to lay her eggs. (Side note: These wasps do not sting.)
Next, she stretches her abdomen skyward, positions her legs solidly underneath her like a platform, and begins to drill her ovipositor into the gall. In the final photo, the Torymus has plunged the ovipositor down as far as it will go, and she is likely laying eggs in the center of the gall, next to the gall wasp egg or larva.
Keep your eyes on the galls this fall.
Stay curious!
See more of Karen’s work here.
Sources:
Simple explanation of oak galls: https://mirrorofnature.bigwhisper.com/oak-gall/
Accessed 8/11/24.
Details of the ovipositor mechanism, including the three valves that help keep the long, slender organ from folding or buckling: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7495304/. Accessed 8/11/24.