I noticed a great flurry of activity on one goldenrod stalk and soon found the reason. These spiky little critters are Publilia treehopper nymphs, the adults of which appear in late spring to early summer. The ants tend their "herd" for the sweet honeydew the treehoppers exude, and in return they protect them from harm. When I stuck my camera too close, the ants jumped on my lens and began running all over the camera, quite prepared to do battle :-)