Goldenrod Gall Fly
Have you ever wondered what the ball-shaped growths on goldenrod stems are? These spherical formations are made by the goldenrod fly. They are about the size of house flies, but related to fruit flies, they mate during the summer.
Plant tissue of a goldenrod stem is pierced by the female’s ovipositor and an egg is deposited in the wound. Healing soon covers the small opening thereby sealing the egg inside the expanding tissue. The injury to the stem alters the plant’s chemistry at that point resulting in a globular enlargement that is referred to as a goldenrod ball gall.
Goldenrod in bloom.
The egg hatches and the larva lives within this tiny chamber lined with a layer of juicy thin-walled cells, the pith of the gall.
As the cells are eaten they are quickly replaced for as long as the tiny quarter-inch-long cream-colored larva inside requires food. The outer hardened cells of the gall serve as protection for the creature.
A truly amazing thing happens before the larva changes into its pupal or cocoon form. It consumes the plant tissue toward the outer wall and, in doing so, produces an escape tunnel right up to the paper-thin outer shell. This tunnel does not close in. Having done that, the larva squirms back to the central chamber where it undergoes the next stage in its metamorphosis, the cocoon.
Without the escape tunnel the goldenrod fly, having emerged during early summer, would have no way of gaining its freedom. Lacking chewing mouth parts, it would be imprisoned. Properly prepared, it now crawls through the escape hatch, perhaps dissolves the thin outer plant tissue with body fluids, and flies away. Tiny, very neat round holes in the old galls next summer, from last year’s plants, will indicate that the occupants have left. What a miraculous life cycle!
Some of the fascinating mahogany-colored ball galls you find now will have been visited by downy woodpeckers. A small excavation, always on the side of the gall, reveals the dark, opened, and empty interior. The larva will have been "fished" out by the woodpecker’s long barbed tongue.
Thanks to doorbell.net for the great information on the goldenrod gall fly.