Don’t come knocking, cause there’s nobody home in a brown oak apple gall.

Oak apple galls are those fragile looking, paper-like round things on the ground or attached to branches. These ping-pong ball sized orbs are readily seen in the woods now that the leaves are coming down. They were home base for an insect, but by this late in the season, these domiciles are empty. Their summer residents left even before Memorial Day!

In their no vacancy season, they were inhabited by one single wasp, so the spongy, fiber-filled oak apple galls were hardly overcrowded. These dwellings are now unoccupied because their former resident literally flew the coop. And it is about time that these insects could fly, since gall wasps are able to become airborne in only one part of their life cycle.

Gall wasps exhibit “alternation of generations,” meaning that they have life stages that don’t necessarily resemble each other. In the case of the oak apple gall wasp, one stage starts in the springtime when a wingless gall wasp injects her egg into the base or midrib of a newly forming leaf. The wasp egg causes the oak leaf to swell and create the familiar gall, which in the summer is green like the leaf. The gall is part of the tree, not a part of the insect.

Naturalist Edwin Way Teale described life in a gall this way, “Imagine yourself living in a globelike room with greenish walls bulging outward and upward and then arching in to meet above your head. Imagine such a room constructed of succulent, edible material, forming a house that at once provides food and shelter, plenty and protection. That is what you would find if you traded places with one of those gall insects that now live in the globular swelling . . . .”

The protected egg enveloped within the gall grows and matures through the spring. By summer, it has metamorphosed into a flying adult wasp that will leave the protection of its surroundings in order to mate. On every gall you will see the hole which was the escape hatch for the wasp.

The flying emergent wasps find each other and mate, then the female moves on to her next stage. She burrows in the ground and lay eggs on the roots of oak trees. Root galls form to house the next generation, and the developing larva will remain in the root for over a year before a wingless female wasp emerges. She will climb up the oak tree and begin the cycle anew by laying eggs in the newly developing leaf buds. Return to paragraph four and start over, if you want to see what happens next.

The plant host is simply a victim, whose mutated flesh accommodates the wasp. In this vein Henry David Thoreau asked, “Is not disease the rule of existence? There is not a lily pad floating on the river but has been riddled by insects. Almost every shrub and tree has its gall, oftentimes esteemed its chief ornament and hardly to be distinguished from the fruit. If misery loves company, misery has company enough.”

It’s striking how depressing Thoreau sounds here, but perhaps you would feel the same if you were an oak tree and your flesh had been invaded and deformed. It is a bold insect that would be so aggressive — or maybe just wasps that have a lot of gall.

Suzan Bellincampi is director of the Felix Neck Wildlife Sanctuary in Edgartown.