Wasp galls on oaks

Every summer we get photographs of oaks carrying things that should not be there: papery brown balls hanging from the leaves, hard marbles along the twigs, a white fuzzy lump flecked with pink, or knobby swellings on the twigs. These are galls, and on an oak nearly every one is the work of a gall wasp. They look like a disease, but they are closer to an arrangement the tree never agreed to, and in almost every case the oak will be fine.

What a gall is

A gall is the tree’s own tissue, grown into a shape the tree would never make on its own. A female gall wasp, a cynipid a few millimeters long and easy to mistake for a gnat, lays an egg in a bud, leaf, twig or root. Chemical cues from the egg and then the larva redirect the plant’s growth, so the oak builds a chamber around the larva, lines it with nutritious tissue and wraps it in a wall that may be spongy, woolly, papery or woody. The larva eats the lining, pupates and chews its way out. The tree supplies the material and the energy; the wasp supplies the instructions.

Each species of wasp makes its own gall on its own group of oaks, red oaks or white oaks, rarely both, and the gall is usually more distinctive than the insect.

The ones you are most likely to see

Oaks host more kinds of gall wasps than any other plant in North America, several hundred at least. A few are familiar:

• Oak apples. Round galls up to two inches across, green in early summer and papery brown by fall, on the leaves of red oaks. A single larva sits in a chamber at the center, held by spongy fibers.
• Bullet galls. Hard, round, woody galls the size of a marble, clustered on the twigs of white oaks and bur oaks. Some kinds secrete a sweet honeydew that draws ants and yellowjackets, which end up guarding the galls from the larva’s parasites.
• Wool sower galls. A white, woolly ball flecked with pink, the size of a ping-pong ball, on white oak twigs in late spring, made of seed-like chambers, each with its own larva.
• Jumping oak galls. Tiny round galls like poppy seeds under white oak leaves. They drop in summer, and the larva flexing inside makes them hop like jumping beans.
• Horned oak galls and gouty oak galls. Woody, knobby twig swellings, marble-sized to two inches, on pin oak most of all and on black, scarlet, red and willow oaks. Horned oak galls sprout small horns as they mature; gouty oak galls stay smooth. These can do real harm.

Two generations, two galls

Many gall wasps alternate between generations that look nothing alike. A spring generation of males and females develops in small, soft galls on leaves or catkins. Their daughters lay eggs in twigs or buds, and the next generation, all females that reproduce without mating, grows in a different gall, sometimes taking two or three years to mature. For the horned oak gall, the spring blisters along the leaf veins are easy to miss; the woody twig gall that follows is what people notice.

This is also why identifying a gall to species is hard. Many gall wasps are known from a single generation, some galls have never been matched to an adult, and the adults are minute and look alike. An extension entomologist or a gall enthusiast can usually name a common gall from a good photograph and the identity of the oak. For many of the rest, “a gall wasp on a white oak” is as far as anyone gets, and far enough.

When galls matter

Leaf galls and most twig galls are cosmetic. The tree gives up a little leaf surface, a galled leaf may drop early, and that is the whole cost; a tree loaded with oak apples loses less than it would to a mild caterpillar year.

The exception is the woody twig galls, horned and gouty oak gall, on pin oaks and their close relatives. Each gall strangles the twig it sits on, so the branchlet beyond it dies. In a heavy year that adds up to a crown full of dead tips, and several heavy years in a row can thin a small tree badly. Even then, galls kill only a tree already struggling.

Pesticides are close to useless. Once a gall has formed, the larva is sealed inside living tissue where no spray reaches it. A treatment timed to the few days when adults emerge is hard to time, kills the gall maker’s own parasites, and does nothing about galls already on the tree.

What to do

• On a big tree, nothing. The galls weather, fall and are replaced by the next crop, in numbers that rise and crash on the wasp’s own cycle.
• On a small pin oak or red oak with woody twig galls, prune out the heavily galled twigs in winter, before adults emerge in spring, cutting back to a healthy side branch, and bag or burn the prunings. This reduces next year’s galls on that tree.
• Keep the tree vigorous, with mulch, water in a drought and no vehicles or fill on the roots; a vigorous oak outgrows its galls.
• Then enjoy them. Chickadees, titmice and woodpeckers tear galls open for the larvae in winter, and a single gall may house a dozen other insects, from squatters to parasitic wasps hunting the gall maker. The tannin in oak galls made the iron gall ink Europe wrote with for a thousand years. A galled oak is a busier tree than a clean one, not a sick one.

Wasps crowded on a trunk in late summer, feeding at rows of small weeping holes, are a bird’s doing, not a gall wasp’s.