A stand of goldenrod, its rugged stalks standing tall in a wintry field, is a good place to look for galls, one of nature’s small wonders. Many of the stalks sport hard, round swellings about an inch in diameter. Cradled in each, awaiting spring, is the larva of a gall-making insect. Its life cycle, like those of many other gall-makers, involves a remarkable relationship between an animal and its particular plant host.
A gall is an abnormal growth on a plant caused by another organism, most commonly an insect or a mite but also by nematodes, viruses, fungi, or bacteria. The gall-makers induce the gall to form in a specific plant, its host species, while that part of the plant is growing. For an insect or mite, the swelling becomes its larval home, providing both food and shelter as it develops. Of the more than two thousand different kinds of galls in North America, most are caused by wasps, gnats, or mites. Each has a particular host plant which it needs to complete its life cycle. Each forms a gall with a characteristic size, shape, and location on the plant – in some cases so distinctive that the plant can be recognized by the gall.
Galls are particularly common in some families of plants. Oaks have by far the most galls, with over eight hundred different species, almost all caused by tiny, harmless wasps. The willow, rose, and aster families are also hosts for gall-makers, as are spruces, pines, and firs. Some galls contain a single insect, while others contain many individuals. Gall names are descriptive and can sometimes be confusing. Willow pinecone galls look like fuzzy gray cones – growing on the tips of willow twigs. Produced by a tiny gnat, they are often invaded by many other minute insects making use of the convenient shelter. Oak apple galls are large, spherical, and can be reddish-colored. Formed by a tiny wasp, they grow on oak leaves and can be spongy or hollow inside, depending on the species. Raspberry knot galls are bulgy stem galls that house numerous small wasps, the empty galls riddled with their many tiny exit holes. Spruce pineapple galls on twigs bear some resemblance to pineapples, while blackberry seed galls, which form on stems, have a seedy look. Cherry black knot and cedar apple galls are caused by fungi and not by insects. Easier names refer to location on the plant, such as locust twig galls, poplar petiole galls, and birch bud galls. Burls, the large woody swellings on tree roots or trunks, are not true galls and are instead caused by injury, fungus, or virus.
A gall-making insect or mite lays its egg in a particular part of the plant – twig, bud, leaf, stem, or root. This stimulates the plant to grow differently in the affected region, forming a swelling. Substances secreted by the egg-laying female or the larva, or possibly the irritation caused by the chewing larvae, produce a proliferation of cells that make the gall. The tissues of the gall provide food for the developing larva, as well as protection from predators and from the elements.
Each gall-maker has a particular life cycle and spends only part its life inside the gall, which may be weeks or months. In the case of the goldenrod gallfly, Eurosta solidaginis, the female fly lays her eggs in the emerging tip of a new shoot of either tall goldenrod (Solidago altissima) or late goldenrod (S. gigantea). She lays about a hundred eggs, one per stem. The larva hatches in a few days and burrows deeper into the stem. The stem swells by growing more cells in the region surrounding the larva, and the larva feeds on the tender, nutritious tissues inside the stem. Surprisingly, galls rarely harm the plant, and goldenrods go on to produce flowers and seeds, though slightly fewer than on stems without galls. Throughout the summer, as the larva eats and grows, the goldenrod replenishes the tissues within the gall. Then in the fall, after the goldenrod stem has died, the larva prepares an exit route to use in the spring. It chews out a smooth-walled tunnel to the tough outer covering of the stem. There it stops, leaving the translucent outer layer of the stem intact, a window to the outdoors. It retreats inside its chamber and enters a dormant state to rest through the winter, protected inside its gall. Antifreeze-like substances in the larva’s body allow it to survive extremely cold temperatures. Towards the end of March, triggered by the change in day length, it turns into a pupa. In May, the adult fly emerges from its pupa case, a tiny brown-winged fly with a funny orange balloon-like structure on its head. It inflates the balloon with air and uses it like a battering ram to break through the wall of the gall. It reabsorbs the balloon, spreads its wings, and flies off to find a mate. All told, the larva spends fifty weeks inside its gall and only two weeks as a winged adult fly.
Other gall-forming insects have different life cycles. Looking around the goldenrod stand, you may notice the goldenrod elliptical gall. This one is caused by a moth that lays its eggs not in the goldenrod stem but in the leaf litter at its base. The larvae lie dormant through the winter, and in the spring they climb up young goldenrod shoots and burrow into them. The goldenrod forms an oval gall in which the larva lives for the summer, pupating and emerging as an adult in the fall to lay its eggs in the leaf litter.
Breaking open a gall is exciting as you never know what you’ll find inside. A gall might contain the original gallfly larva or its pupa. It might contain only some frass (insect droppings) or mold, if something affected the development of the fly. Or you might find the slender C-shaped larva of a parasitic wasp in place of the gallfly. Other possible guests are larvae of the tumbling flower beetle. Sometimes gallfly and beetle larvae coexist in the gall, but often the beetle larvae get the better of the gallfly and feast on it in addition to the starchy plant tissue.
Even without looking inside a gall, it is often possible to tell whether the insect has emerged. The presence of exit holes shows that the gall-maker has left and gives us clues about the timing of its life cycle. Goldenrod elliptical galls found in winter will have exit holes since the moth emerges in the fall, whereas ball galls will still have a larva inside. A hole made from the outside tells us that something may have found and eaten the larva! Chickadees know where to look for an energy-rich snack and will chip away at the galls, messily pecking bits off until they reach the larva. Downy woodpeckers use their sharply pointed bills to chisel out a neat, cone-shaped hole and their barbed tongues to extract the tasty nugget.
A stand of goldenrod is a good place to start a hunt for galls, but don’t stop there. As our eyes learn to recognize different galls, we get better at seeking them out. We’ll notice ash flower galls silhouetted against the sky, discover beaked galls on willows, pick out stem galls on blueberries. We’ll wonder at the complex link between gall-makers and plants. Don’t miss a chance to investigate every lump and bump, for it just might be a marvelous gall.
Suggested Reading
Eiseman, Charley and Noah Charney. Tracks and Sign of Insects and Other Invertebrates. Mechanicsburg, PA: Stackpole Books, 2010.
Holland, Mary. Naturally Curious. North Pomfret, Vermont: Trafalgar Square Books, 2010.
Stokes, Donald W. A Guide to Nature in Winter. Stokes Nature Guides. Boston: Little Brown and Company, 1976.
Nearby Nature 