Strange cocoon

It's been on my weeping willow for weeks now. I haven't disturbed it because it's managed to hang on through a couple pretty big thunderstorms, so I figure it deserves to be there. This photo is from July 15. I just went down to check on it, and it's still there, and a little bigger. It appears to be a cocoon of some sort. What's odd is that once I saw it actually eating part of a leaf. The wormy thing inside was munching along the spine of the leaf, clearing one side but leaving the other intact. It doesn't seem to be eating anymore, just hanging there, although it does move around.

Just for kicks I thought I'd try investigating on the web by typing in "stick cocoon" and sure enough, there it is. It's called a bagworm. They say it's a bad thing and you should kill it. Actually, they say I should have killed it a long time ago, and by now it's probably had babies. Or maybe it was one of the babies that hatched back in June. I don't know if I can bring myself to go down and rip it off. The tree seems to be fine. In fact, it's got lots of new growth, which is a very good sign, because for the first few months it seemed very weak. But this article uses the word infestation, and I would hate for my now healthy tree to suffer, so I guess I should go cut the stick-cocoon-thingie off the branch. Okay, here I go...

...Well, that wasn't so bad. It didn't scream or anything, so that's good. And now my tree will grow to be big and strong. :)