A cousin to the eastern tent caterpillar, the forest tent caterpillar (Malacosoma disstria) doesn’t actually make tents; instead, it constructs silken mats along tree trunks and branches. Like its cousin, however, it also congregates in large numbers and the mats can be just as unsightly and large as the tents. The caterpillars are harmless to people, and usually only visually annoying or slightly damaging in trees, although in large groups they can defoliate or even kill trees. They grow to become mid-size brown moths which do not damage plants.
They live all over the US, but are more common east of the Mississippi River. There is one generation per year, with populations being cyclical, and varying widely from year to year.
This photo shows the dark color phase of the larva; they also come in a lighter blue flavor, but both have the distinctive white spots along the midline.