A few weeks ago I noticed these tiny red aphid nymphs (Hemiptera: Aphididae) feeding on a goldenrod (Solidago sp., Asteraceae). Scattered among them were a few larger adults. These insects use their piercing-sucking mouthparts to siphon sugary juices from their host plants.
There are well over a thousand aphid species in the United States, so it can seem daunting to try identifying these tiny insects beyond the family level. Like many phytophagous (plant-eating) insects, however, aphids can be pretty particular about what they choose to eat. Based on this host specificity, I suspect these aphids are in the genus Uroleucon and may be U. nigrotuberculatum.