You’ll probably first notice the silhouettes. Naked against the sky, the branches of a certain tree may look bumpy, as if drawn with a leaky pen. Or a bush whose leaves and fruits dropped long ago still seems festooned, as if hung with Christmas ornaments.
For many plants, summer’s leaves, like clothing, covered a multitude of imperfections, now laid bare. “At close range, nature is deformed,” naturalists Ed Duensing and A. B. Millmoss observe in their nature guidebook, The Backyard and Beyond. If you look closer at these bumps and lumps, you’ll see what they mean: you’ll find cylindrical swellings covered with red hair, succulent-looking orbs hanging like fruit, seedlike bumps on leaves, oozing tumorous masses.
Certain trees may even take on alter egos. Willows seem to sprout pinecones. Oaks may seem laden with apples. Of course, oaks don’t have apples. Willows don’t have pinecones. But they may have galls.
Galls are created by invaders who chemically co-opt their host plants into growing these deformities in place of a normal bud, branch, flower, leaf, or root. If you cut open the gall, you might see the culprit behind the mischief: usually it’s the pale, legless, innocent-looking larva of a tiny fly or a stingless wasp.
From the plant’s point of view, a gall is a defensive barrier. Most galls don’t hurt the plant, and usually fall off after a year or so. But from the point of view of the invader, the gall offers both food and shelter in luxuriant abundance.
Galls may occur on well over two thousand different species of plants; oaks seem particularly prone. Some appear on stems, some on bark, others on leaves, some on flowers, even roots. Although galls are “abnormal” structures, they are surprisingly common and often alarmingly numerous. Some plants are so beset with galls that the weight of the structures can bow their branches. One large oak outside Albany, New York, covered with the giant, beadlike swellings of the gouty oak gall, was estimated to host some five hundred thousand baby wasps in its galls.
Michael Quinn, a Vermont teacher, photographer and gall enthusiast, almost lost a friend in an argument over the pineappleshaped structures she spotted on the branches of a particular willow. “She simply wouldn’t believe it could be a gall—she insisted that something that abundant had to be a fruit,” he said. In the spring, at the Garden in the Woods in Framingham, Massachusetts, it takes three people several days to pluck all the bright green, golfball-sized galls off the azaleas. (These look like juicy fruits, and in fact, the Appalachian Cherokee used to harvest them as sweet-sour treats.)
Garden staff pluck galls for cosmetic reasons. Some people find them unsettling. Even the word sounds like bad news: Biblical authors used “Wormwood and gall” to evoke the bitterness and sorrow of the destruction of Jerusalem. At the Arnold Arboretum in Boston, Director of Living Collections Peter Del Tredici prefers to call galls “swellings,” and when they’re on wood, “burls.” Burls are nice—woodworkers love them. Burls on black cherry, sugar maple, redwood, and black walnut are highly prized; sawn and polished, a burl yields beautiful swirling, figured patterns in the wood. But call the same structure a gall, Del Tredici says, and plant pathologists “become terribly excited and upset.”
Some galls—the 5 percent or so caused by viruses, bacteria, or fungi—can kill the plant. Fissured, tumorlike galls on olive trees impart a rancid, bitter taste to the fruit. The presence of these olive galls is among the oldest of recorded plant diseases; Socrates’ successor, Theophrastus, complained about them two thousand years ago. (Scientists now know these galls are caused by a bacterium.) Crown gall, one of the most-studied plant diseases, afflicts fruit, nut, and ornamental trees and is also caused by a bacterium.
But most galls are caused by insects, representing a fascinating, ancient evolutionary link between plant and animal. Millions of years ago, the progenitors of today’s gall-making insects probably just munched on their host plants, and that was that. But over time, certain flies, wasps, mites, and wormlike, soil-dwelling nematodes evolved chemicals to mimic their host plants’ own growth hormones. Once these chemicals are released—by the action of the larva feeding, or sometimes by the egg in which the larva is encased—the plant responds in two major ways. Cells proliferate abnormally around the site, and the plant shunts extra sugars there.
When Jorie Hunken, a naturalist in Woodstock, Connecticut, tells children about galls, she asks the kids to imagine they’re each a baby gall wasp: “When you’re hungry, all you do is roll over and take a bite of your bed—a sweet, gooey, spongy, edible mattress.”
Some galls ooze with sugars. In his 1918 book Key to American Insect Galls, Ephriam Porter Felt, former New York State Entomologist (and virtual dean of American galls), recorded a large pin oak near Newton, New Jersey, which people called “the roaring tree” because it seemed to hum or roar. The sound was coming from the hosts of bees and flies attracted by the sweet fluids oozing from its numerous galls.
Some galls also yield commercially valuable dyes. Certain oak galls contain a high-quality pigment, a tannin, that the U.S. Treasury and the Bank of England use in preparing the inks to print currency.
Cut open a gall and you might find its architect inside. (Remember, this won’t hurt the plant, but damaging the gall dooms any inhabitant.) The pale, legless larva of a gnatlike fly overwinters in the willow cone gall, emerging in early spring as an adult. A ball-shaped swelling on the dried stem of goldenrod shelters the larva of a different fly. But you might find a beetle larva in there instead—this species parasitizes the fly larva, and then takes over the nursery as its own. Some galls this time of year are empty. If you slice open an oak apple gall in the summer, you’ll find a seedlike capsule inside, where the young of a stingless wasp is maturing. By fall, the gall has dried to a mass of fibers radiating from the center of the capsule to the now-papery skin, and the wasp has hatched and flown. Likewise, elliptical galls on goldenrods are now empty. Look near the top of the gall for the exit hole. Here an adult moth emerged from its reddish brown pupal case in September, chewing its way out.
But an exit hole doesn’t necessarily mean no one’s home. Often after the original occupants have left, spiders and mites move in, glad for the winter shelter.