I had no idea what I was looking at.
The plant was the wild rose (Rosa woodsii), but what were those spiky red balls on its leaves? Adjacent clues — spider’s threads, spittlebug’s froth — turned out to be irrelevant.
Rather, these are galls provoked by the rose-leaf gall wasp (Diplolepis polita). The wasp lays eggs on the leaf and the leaf responds by encasing them in a gall, inside of which grow one or more wasp larvae. The gall provides the larvae with both protection and food. What chemical stimulus would prompt the wild rose to respond in this way seems to be a mystery.
These spiky red galls are the plant’s response to the eggs of the rose-leaf gall wasp.