Tent caterpillars are a nuisance. Now, while the weather is still cool and the trees are not leafed out, you can find the egg masses and destroy them. They look like brown, shiny or crusty tobacco spittle (yeah, gross) that has solidified in a blob on the limb of the tree. If you have a few trees that you really don’t want the worms in (such as your apple trees or young decorative trees), you can examine the branches and remove them, just scrape into a jar of soapy water or something.
That’s obviously a fairly time-consuming process. :P
Bt (bacillus thuringiensis) will kill the caterpillars. This is available in a powdered form or a liquid form, from suppliers who sell organic gardening supplies. Bt is a naturally-occurring bacteria which you apply (spray or dust) to your plants. It gives caterpillars severe stomach cramps; they stop eating immediately and soon die. It does not affect beneficial insects because they do not eat the foliage. (It WILL, however, also kill other worm-type larvae that you may not want to kill, such as butterfly caterpillars, so keep that in mind.)