During the summer months round galls are a common sight on goldenrod. One cause of galls is insect larvae, which produce chemicals to induce the abnormal growth. The hard exterior of the gall provides a safe haven and the inside becomes a source of food.
Many kinds of insects (as well as mites and fungi) cause galls, often unique to each species on preferred host plants (more on this subject at BugGuide). Here's the culprit responsible for the round goldenrod galls: a member of the Fruit Fly family, the Goldenrod Gall Fly (Eurosta solidaginis). There's no sign of an ovipositor on this insect, which tells us that it's a male.
This fly is roughly 8 mm long (the measurement is by "rule of thumb", metaphorically speaking).
Adult flies are emerging in greater numbers every day so it was just a matter of time before I stumbled across a female (note the ovipositor) ...
... and a mating pair.
Images of females ovipositing, starting the cycle anew.
Until I took photos of this fly a few days ago and researched its life cycle on the Internet I had no idea as to the cause of the goldenrod galls. And I had always assumed that winter and early spring (when no self-respecting insect would be active) galls with holes in them were the result of predation by Downy Woodpeckers. Perhaps not: it seems that the fly larva prepares its emergence tunnel in the autumn before pupating for the winter. You learn something new every day ...
There were actually two larvae ensconced within the gall in the following photo. When I opened it I fortuitously split one of the exit tunnels (circled and labelled in red) right down its length. I don't have a picture of the other larva that was living in the center of the gall (its passage, which goes into the gall at a right angle to the plane of the cut surface, is annotated in blue) – it wasn't so lucky and fell victim to my jackknife.
