Blech! – makeupanid
Yuck! I hate these things! – craftermom
The nests are created by Eastern Tent Caterpillars (Malacosoma americanum). The eggs overwintered and hatched in spring. The larva are social and live in this shelter they build together, emerging to eat tree leaves when the weather is fine. Most often, the tents are built in the crotches of trees, but I have also seen them suspended from smaller branches, as pictured below:
Because they can defoliate trees in years when their populations are high, they are considered a pest. If you “google” them, you will find several sites that not only tell about their life cycle and natural history, but they will also give tips about controlling them.
Learn more: The following websites have some neat pictures of the eggs, and adults.
Apparently, the populations fluctuate – there are lots one year, and fewer the next. I’ve been seeing quite a few around this spring. How about you? Are you seeing lots around your place this spring?
UPDATE (5-9-2008): Read this blog post from The Marvelous in Nature for some really, really good pictures and information on these guys!!!! tenting-it-with-the-family