From what I read, the elm sawfly is one of the biggest in North America! The larvae get up to 2" long so yep, it's a doozy! They have a lot of hosts other than elm ("hosts include elm (Ulmus), maple (Acer), birch (Betula), willow (Salix), and basswood (Tilia); adults girdle bark on twigs"). My first thought when I saw your plant was that it was a little willow sapling, is that possible? I'm terrible at trees, lol! Sounds like they're really generalist though so they could be eating almost anything... funny that you mentioned that they wouldn't uncurl: "While feeding, the larvae usually coil their posterior around a leaf or twig. At rest the larvae roll into a characteristic tight coil." Good observations!
Solomon - black DSH - king of kitchen raids (11)
Gracie - Mutterooski X - scary smart (9)
Jaida - GSD - tripod trainwreck and gentle soul (4)
Heidi - mugsly Boston Terrier X - she is in BIG trouble!!! (3)
Audrey - torbie - sweet as pie (11 months)
Patrick - blue - a little turd (but we like him anyways) (6 months)
Boo, our Matriarch (August 1 1992 - March 29 2011)