This "Spittlebug Condo" was on a plant in a field along the Cumberland Trail. There were several of the nymphs sharing this bubble home. The nymphs are green and black, two are easily visible on this stem. They suck the juice from the plant stem, when the slippery "leftover juices" are excreted, the nymphs whip them up into a foam. Not many predators will try to go through the foam to eat them! I've heard kids call this "snake spit".
Hemiptera Nov. 29, 2007 Boxelder bugs are not harmful, but they can be pesky if they invade homes in the winter. During the summer they lay their eggs on the leaves, branches, and trunks of female boxelder trees. I have not read that they are poisonous, but I would suspect they are distasteful due to their red and black coloration. They look similar to the Milkweed bug.