Citizen Mark Holly recently stopped by Camp Salmen with a small bough from a Live Oak that had several odd, round galls growing on its twigs. They were a little larger than an acorn, spherical and smooth, and each had a tiny hole about the size of a pencil point. Mr. Holly was curious about them so we cut one open. We were gall-darned if it wasn’t full of tiny, black, bulbous-looking ants living in the chambers within; and the ants appeared to be looking after tiny white babies. It appeared that we had busted into someone’s scene of domestic tranquility.
Furthermore, Mr. Holly had observed wasps chewing on some of the leaves near the galls and were apparently getting drunk and falling to the ground. “What was that all about?” we wondered.
Galls are induced irritations to a tree’s tissues causing outgrowths that usually do no harm to the host. They’re like a plant’s version of a wart. They can be caused by fungi, bacteria, abrasion and insects, and happen to different parts of a plant – flowers, buds, roots, twigs and leaves.
Insects that make galls do so by introducing chemicals to a tree’s young, formative tissues in new leaves and green twigs. These can produce warty-looking bumps and blisters on leaves like what tiny psyllids do to Hackberry tree leaves or when gall-making wasps create tan, pom-pom like fluff on the underside of oak leaves. These structures allow the young of these species to hide away the live-long summer and leisurely suck up the sugary juices made by the plant’s photosynthesis.
On the internet Mr. Holly’s gall looked most like that of an English Oak Marble, though these are caused by wasps, not the ants we found. Dr. Bob Thomas of Loyola University, and headman of the Louisiana Master Naturalist Program, helped us clear up the confusion. He sent photographs of the ants to Raymond A. Mendez, an Entomologist in Arizona who tentatively identified the ants as possibly of the genus Crematogaster that specialize in keeping small colonies in tiny spaces like hollowed-out acorns and galls or Acorn Ants of the genus Temnothorax who do battle with others of their kind to determine who gets to hollow out acorns or galls and live in them. We’re preserving the ants in alcohol to send to Mr. Mendez for further scrutiny.
He also thinks the ant babies found in the gall may be someone else’s. They could have been the larva of the gall fly, or wasp who originally made the gall to be their home, intending to raise a family in it. They may have also been aphid babies, kidnapped from somewhere else and held, not for ransom but for a meal.
As for the drunken wasps, Mendez related this to Dr. Bob: “I am sure you have seen trees sweat in the tropics. It also happens all over (the world) and is usually caused by a virus that can produce alcoholic exude. Beetles, butterflies, flies and other critters love the stuff and just either hang out drunk or fall to the ground. Another possible reason, trees sprayed with insecticide to control mosquitoes, tent caterpillars or other critters just sits on the older leaves and kills non-target insects, it’s like friendly fire in warfare.”
All in all, it’s astonishing that either through a long evolutionary process of trial and error, or divine intervention, an insect happens to have a chemical in its bag of tricks to force another organism like a tree to yield it a home, in this case a pretty round sphere and one that would be contested between tiny bugs.