I don't have a clue as to what it might be, but I can tell you I would make up some hot pepper spray or garlic spray or even an egg solution and put on them. If you do the egg it has to sit at least 24 hours to get stinky. Maybe that will stop the destruction.
I hate that for you. I know how upsetting that can be.

I hope you try one of these and it helps,
:) Fran

Both rabbits and deer love toad lillies. I would bet your problem is rabbits. They they chew one stem down and eat a few leaves then move on to the next stem. The deer usually eat it all.

It MIGHT also be young fawns, trying things to see if they like them. I would try Fran's suggestion(s) of spraying with a deterrent, and see if that helps. Irish Spring deodorant soap will also ward off deer - maybe - hung up in an old pantyhose leg or mesh bag. A tallish fence might also help temporarily.

Monster rabbits in the mountains are multiplying faster than usual. They eat every new leaf of everything.

Here's what's working for us: urine. Yessir.

For those who aren't built with the right equipment (after dark,please), collect said valuable liquid in a pail and transfer -via- funnel to a jug.

Lightly pour some of that yellow gold around the plants or group of plants. Watch the dwatted wabbits steer clear.