What has eaten my tree alive?

I used to have a 20ft beautiful tree in my garden,it's dying right now. What I see is worm/slug looking critters,many many of them. They are grayish in color with faint black stripes across their backs and milky colored undersides. The largest I saw was 1/4" long and 1/16" wide.They hang on tight to the leaf and eat it down to nothing but the skeletal remains. The leaf remains are left with a dark brown sandy, seedy, type residue or deposit stuck in them. I need to ID these creatures fast because they are incredibly quick eaters, and I,m afraid they are going to devourer my entire garden before I even figure out what they are. So thanks everyone for your help!