Remnants of summer’s tall goldenrod don’t look like much in November, yet they harbor unseen lives. Globular swellings partway up the stems suggest a strange departure from the normal goldenrod plan. What are they? If you slice one open, you’ll see a plump larva in a central cavity surrounded by spongy plant cells.
The spherical bulge is a plant gall that starts off in early summer after a tiny fruit fly (Eurosta solidaginis) lays solitary eggs upon leaves surrounding the growing tips of young goldenrod stems. Each hatchling larva crawls around before boring into the plant stem, where it seeks undifferentiated cambium cells, for it is only during early growth that embryonic plant tissues can be induced to change. Under strict chemical control by the larva’s secretions, the stem is forced to stop its own normal cellular development and produce abnormal tissue wrapped in a tough covering. The enclosed gall insect then feeds upon the cells of this contrived goldenrod tumor.
When the plant ages in the fall, the larva is stimulated to hibernate. Tolerating sub-zero temperatures by means of an intra-cellular “antifreeze,” glycerol, and a slowed, super-cooled metabolism, the immature insect becomes dormant, but not before preparing an escape tunnel leading almost to the outside in the upper half of the gall. Quiescent all winter, the larva pupates in early spring. After metamorphosing for a month or two inside the gall, the new adult crawls to the previously prepared entrance and rams through the last layer using a swollen portion of its head. Wings dried, it flies away, never to eat again during a two-week lifespan. An adult goldenrod gall fly’s only role is to mate and start the life cycle over again.
The first gall I remember seeing as a boy grew on an oak. It was a beautiful rose-and-yellow sphere about the size of a golf ball, with great curving horns projecting all around. The tree appeared to have decorated itself with a living ornament, and it took me awhile to learn that a wasp had brought it about. Over 300 species of wasps and flies are responsible for oak galls of every description.
Two months ago outside my study window, I noticed little scarlet spindles arising from the upper side of green maple leaves. They too were galls, caused by a diminutive mite, evidence that galls are formed by more than insects.
Galls come in all shapes and sizes. Besides flies, wasps, and mites, parasitic gall-makers include bacteria and viruses, fungi, roundworms, and hundreds of insects. Each is a specialist concentrating upon one kind of plant.
Galls occur on lichens, mosses, ferns, many seed plants, and even seaweed. Fossils that date before the history of life on land show galls growing on seaweed in the ocean, hundreds of millions of years ago, when organisms started evolving into the part-time parasites that create galls.
Most scientists who study gall organisms and their effects upon plants are unfamiliar to the general reader. The exception is Dr. Alfred Kinsey, the same person who later published statistical reports on human sexual behavior. Originally, Kinsey was a specialist who studied the life histories of gall-producing wasps known as cynipids. If you notice an oak tree bearing dozens of smooth brown spheres about two inches in diameter, you’re looking at one kind of cynipid gall.
To form a gall, a plant must produce quantities of oversized cells with enlarged nuclei. The stimulus for this abnormal growth comes either from secretions of the animal that lays eggs on a plant or from reactions to the behavior and chemistry of a parasite. The parasite’s secretions probably imitate plant hormones by diverting the host’s growth into a distinctive, highly specific gall. These alien chemicals control plant tissue development so precisely that some insects cause remarkably similar galls in dissimilar plants.
The one-sided association between gall-maker and host benefits the parasite, while the plant suffers altered growth patterns and a reduction of essential functions, such as the transportation of sap. Forced to develop a gall, a plant provides food, shelter, and protection for a growing larva that is equally specialized during its youthful imprisonment.
In basic terms, a plant gall is a cancer-like abnormality that is initiated and precisely regulated by another organism. Considered in this light, the idea of galls could have encouraged science fiction like The Invasion of The Body Snatchers. The difference is that gall-making organisms can’t afford to destroy the host plants upon which they depend. In effect, they say to the plant, “Build me a safe house according to my exact instructions, and feed me well, even if it is an expensive affair for you. If you do the job properly, I won’t bother you further.”