These small spheres have started to appear on the goldenrod. I now know they are created by a the Goldenrod Gall Fly, thanks to the Audubon Guides! The fly’s larva will winter within this lump (known as a gall, of course) and emerge in spring.
I have always been a bit fascinated by the galls. I had no idea what purpose they served and, since I typically noticed them only in winter, had no idea what plant they were on. What I have noticed is that they are exceptionally hard. But in deep winter, you will see that many of them have been opened, with a hole drilled into the side. Now, understanding what is at the center of this gall, it makes sense that birds are opening them. I had supposed that there must be seeds, but larva and glucose would be far more tasty!