I've not actually seen any webworms yet...some insects (like fireflies) thrive on all this wet weather we've been having, and some don't. Anyway, it's well past the time when, if webworms were going to be seen in Gate City, Virginia, we'd be seeing them.
Old books used to identify Spring and Fall Webworms as two separate species, but more recent online information about these insects describes them as two races or sub-species of Hyphantria cunea. The southern type have two generations per year and can thus be described as Spring Webworms, but in places where they're common they're also seen in autumn. As discussed at Wikipedia, the types can be distinguished by colors; southern-type moths have more dapples on their wings.
Moths emerge during warm weather and lay small clumps of eggs on leaves. About a week later, these eggs hatch into tiny, fuzzy caterpillars. They will eventually grow up to about an inch long. The color can be yellow, greenish, or blackish drab; the ones I've seen all looked yellow. Magnified photos show markings on the undercoat below the long guard hairs:
The caterpillars spin thin, flimsy webs around the leaves they're eating. Webs grow as the caterpillars grow, but I've not seen a web expand beyond one branch of a bush, as some online sources apparently have.
After four to six weeks, these caterpillars shrink down to small drab pupae that can be mistaken for dirt in the leaf litter at the base of their host plants. The plants are usually unharmed, although a webworm family may defoliate one or two branches. The damage these little animals do is mostly cosmetic. They may pupate for most of the year, or for most of the summer, then reappear in autumn.
The moths are white, cream, or beige with or without brown or black spots on the wings. Going by the wing shape and patterning, entomologists classify Hyphantria cunea as a tiger moth, which makes webworms more closely "related" to woolly bears than to either tent caterpillars or walnut caterpillars...although webworms can look like miniature tent caterpillars:
Webworms become a problem when, where, and as pesticides kill the creatures that naturally keep their populations in balance:
Several kinds of wasps really know how to love Hyphantria cunea. The blogger known as Prof Josh mistakes webworms for silkworms (duh...silkworms are domestic animals) but does show the strong of stomach exactly how wasps love webworms...
Not sure where this was taken, but I can tell the person who seemed surprised that the wasp looked so "black" that Polistes fuscatus, the species I welcome at the Cat Sanctuary, can look black even when they're not backlighted. Fuscatus means dark, right? While there are small wasps that specialize in parasitizing various caterpillar species, Polistes fuscatus chews up lots of different insects, including several that are toxic to other predators (tent caterpillars, tobacco worms), to make the paper of which it builds its nests. Humans do not usually have to watch this process.
If you don't have a Vicious Spray Cycle going on, there is seldom a need to do anything about webworms. Understand that one webworm family is not going to kill a bush or tree; unless there are dozens of webs, the best thing for your shrubbery is probably to leave the caterpillars alone. If there are dozens of webs, then you probably have a Vicious Spray Cycle going on and may have a hard time getting the predator species established, but persevere. Webworms are a native species and are naturally well balanced.
While trying to establish predator populations, a cheap and simple way to reduce webworm populations would be to poke a long stick through a web, shake the branch, and whack worms as they land on the ground...I say this theoretically because I've never bothered. Webworms haven't been a serious pest in any place where I've ever seen them.
Webworms don't bite or sting, noticeably, although their hair can be itchy and may raise a mild rash if you get enough of it down your shirt on a hot day. As this web site has noted before, that's also true of human hair.
How to tell webworms apart from tent caterpillars, etc.: Most obviously, they only ever grow to about half the size of tent caterpillars, and in real life they usually look plain yellow or green. Even if they look black there's no dramatic white stripe down the back. Webs are spun at the ends of branches and cover mature leaves.
Walnut caterpillars also spin their webs at the ends of branches and incorporate mature leaves, but they're bigger than webworms, have a different shape and color, and, in my part of the world, appear later in the year.