These tent caterpillars drive me crazy. I have had them in my ornamental flowering crab, and last year, they nearly ruined it. It was loaded again this time. Now I'll admit that over my lifetime, my first tendency when approaching a pest is to think,”What can I put on that to kill it?” Meaning, basically, which chemical. I fired first, and I did not ask too many questions.
Don't get me wrong. I still believe in the accurate and judicious use of pesticides. But my above remarks really highlight why their use can become problematic, and they should not be applied in haste, but thoughtfully, as a last resort when no other choice is available. Not just because you want to take care of the problem and more or less, get it over with.
So I've been suffering with these things. There really isn't a good solution anyway, chemically, that I have found, once they hatch and form those tents. They were up high in this tree. I manually took down the ones I could reach. That's nasty and disgusting, by the way. Plastic gloves, armour, whatever you can find, would be good to put on first. Last year we resorted to cutting the limbs off where they were. Not really a good option either.
Well. I was reading University of Missouri-Extension Newsletter yesterday and had a HUGE aha! moment. You can remove them without chemicals! A strong stream of water melts those tents and sends the worms plunging to the ground-although sometimes they hit the limb below and you have to, uh, spray them again with high pressure….and the little frontier person in me felt a rather strange satisfaction in seeing these vile little invaders go DOWN. 
After about thirty minutes and becoming soaked (totally worth it) I have not one of those unsightly encampments left in that flowering crab, which happens to sit in front of my door. It was just such a “duh”! I couldn't figure out how to reach them, and this was embarrassingly simple!
They are not supposed to be able to make it back to the tree…….I'm a Missouri Show-Me lady, so the jury's still out on that one, but we'll see. At least I feel better. ~JB was here.
Update: 2013! My tree made it, not sure what those little indentations were, but it's fine. The tent caterpillars barely got a start this year before "water woman" was on the go again. My tree has beautiful foliage now and I can report that the "water works method" really works well!