I've ruined many a garage-sale-hosting homeowner's day by dragging them out in their yard and explaining to them that bagworms are killing their trees. I'm not an entomologist or even an etymologist (but I might be an Entenmannsologist) but I can tell you what you need to know about this devastating grossness.
Bagworms like many kinds of trees, but where their camouflage is most effective is on evergreens. Their cocoons look wildly like pine cones and I completely understand why this slides right past many homeowners. The cocoons are held on by silk that is so strong that it actually strangles the branch, but that's not where most of the damage is done. In May and June (and sometimes later) when the little fellers poke their heads out of the cocoons to eat, they will consume EVERYTHING in the reach of their cocoon. EVERYTHING.
I have seen cases of infestation that were so BAD that upon walking up the the trees with the Bagworms in them... I could HEAR them eating, from many feet away. I swear that sound will haunt me for years to come. Ew.
So, how to get rid of them? The easiest and most effective is the most time consuming: pick them off the tree by hand. Get out the ladder, bribe a local kid that thinks that gross is cool. Whatever. Just do it. Remember the silk that's attaching it to the branch? See if you can get that off too. Once they are picked they need to be smashed to bits, burned or drowned in soapy water. Personally I would do all 3- smash, burn, drown.
Or you can spray them. It can be done but you need repeated treatments over 2 or even 3 months because you have to actually come in contact with the worm, not just spray the impermeable cocoon. You can useBT (organic) or Sevin (not organic). I'm afraid this is also a total pain, so just do me a favor and hand pick them, OK?
I am including these handy You Have Bagworm door hangers (full size in the forum!) that you can print and cut out and leave anonymously on "the infested's" front door. This way, you can do your part to keep trees alive while not bringing the garage sale to a screeching halt.
Have more questions? I have answers, believe it or not. I'm in the forum.