Meant to add on my post above--I believe the two worms--forest tent caterpillars and army worms--are related. I just got around to pulling out my wildlife book, and tent caterpillars are Malacosoma americana while the army worms are Pseudaletia unipuncta.
Dawn, you're right--the tent caterpillars are gross. We had them really bad the one year, years ago, and they were just like you describe--wall to wall, when they get on your deck or the wall of your house it looks like that deck or wall is moving. They strip the trees almost overnight, and then there are the tent webs they make...and the worms hang down on web threads, or they just plain fall out of the trees and land on you when you pass under the trees. Ick. Studies have also shown that if bred mares eat the worms there is an increased risk of abortion. There is, of course, the likelihood that during an infestation horses will eat some of the worms while grazing.
Army worms are much the same, but on the ground--again, a moving mass of worms that covers the ground--when the "army" of worms passes through a field they leave nothing behind.
