Urban species #261: Oak apple gall wasp Amphibolips confluenta
Acorns aren't the only round objects falling from oak trees this time of year. Spheres, about the size of golf balls or slightly smaller, green and mottled with red, mysteriously appear on the sidewalk. Later, many more will be found--larger, light brown like dead leaves, sometimes entire, more often with a single hole in the thin crust. These mysterious objects are not pods from outer space, or oak tumors; they are galls: the protective nurseries for young insects, usually given the quaint name "oak apples." The mother insect enlists the help of oak trees, symbolic of strength and live itself, to shelter her growing offspring. Known, somewhat anticlimactically, as the oak gall wasp, the tiny non-stinging insect deposits her egg in the flesh of an oak leaf. Along with the egg is a hormone that induces the oak tree to form a gall around the wasp egg. The larva develops within a shield of plant material, a capsule suspended in the center of the orb.
At least 200 species of organisms use oak trees to form galls of one kind or another. Other plants that often play host to gall wasps and gall mites and other creatures include goldenrods, maples, and roses. Galls cause very little damage to the host plant, and gall wasps are not considered to be serious pests.
The gall is not an impregnable fortress, and many predators recognize wasp galls as sources of food. Birds, notably woodpeckers and chickadees, poke holes in galls when they encounter them aloft. Fallen galls are fair game for squirrels and other mammals, and enterprising predatory wasps may gnaw their way into galls in trees and on the ground. If the gall wasp larva escapes all these predators and survives to adulthood, it chews its way out, leaving a small neat hole in the papery gall, which falls with the leaves of autumn.
Sliced open to see the larva inside.