These galls are made by the Goldenrod Gall Fly (Eurosta). In spring they lay their eggs on the stem of the newly growing goldenrod. The eggs hatch into a larva and the stem grows out around the larva, creating a protective shelter. However, squirrels, mice, and some birds have long since discovered that inside these hard round balls are the larvae, a much relished food source. The galls have no detrimental impact on the plants.