Most of the wasp species in the world cannot sting. Stingers are believed to have evolved from ovipositors, a sort of drill that insects use to place their eggs deep into plant tissue, or into other insects. These tiny wasps are specialist parasites; they seek out a specific insect or plant in which to lay their eggs. The larvae can exert a remarkable amount of control over their host, altering their bodies and behaviors.
Mathematical models suggest there are more species of these wasps than any other order of life. There’s a wasp for almost every insect and plant, and even other wasps! Since they are so small and live hidden inside other organisms, these insects are poorly understood; it is a frontier of science. It’s much easier and more appealing to catch and catalog beetles or butterflies. However, we can observe the bizarre structures some of these wasps induce in their host plants: galls.
Walking in the woods, you have likely noticed a variety of lumps and bumps on leaves, stems, and tree trunks. The biggest of these, the burls, are from trees walling off a diseased section with wood and bark. Galls are much smaller, only a few inches across. They are temporary structures, and usually on softer tissue. Unlike burls, the plant has little say in its creation. Gall-formers emit chemicals that alter the plant’s hormonal system. These hormones trigger the plant to grow tissue where it normally wouldn’t, tricking the host into growing a bed and breakfast for the uninvited guest.
Ironically, one of the main predators of gall inhabitants is other parasitic wasps! Galls often include very high concentrations of phytochemicals to deter interlopers, and these likely account for their bizarre coloration. Oaks are a common host, as they naturally have a high amount of tannin, a bitter, astringent compound that deters herbivores. These compounds have been sought after by humanity for millennia as medicine, tanning agents, lamp fuel, and ink. The Declaration of Independence and the Constitution were written with oak-gall ink.
Wasps are the most famous of the gall-formers, but they aren’t the only ones. This ability has evolved independently in all corners of the tree of life: mites, aphids, beetles, flies, nematodes, fungi, bacteria, viruses, and even other plants, such as parasitic mistletoe.
Galls from the black cherry leaf mite, a food source for caterpillars of the cherry gall azure butterfly. (Photo by: Justin Cifello)
This diversity accounts for the wild array of forms and colors galls can take. If you find an unusual shape on a plant, it’s most likely a gall. Galls usually only occur in small amounts and are generally considered harmless to their hosts, though there are some exceptions.
Some are even beneficial, like the root nodules formed by the rhizobium bacteria that give the legume family the unique ability to breathe atmospheric nitrogen. Without their ability to grow their fertilizer, agriculture would not exist as we know it. Some gall-forming bacteria are capable of horizontal gene transfer and provide novel mechanisms for biotechnology. Galls are an important winter protein source for birds, and the galls used provide microhabitats for other organisms.
Visible in every season, galls are a fascinating reminder of nature’s unending complexity.
Written by Justin Cifello, Wareham Land Trust Member and Volunteer