Help! My dahlias are coming up but being killed by some sort of leaf chopper. They don't look like they're going to make it. The ones I've planted in pots are doing fine, but those in the ground are being eaten alive. Not by snails; I've put snail repellent down already.

I've grown them in the same place for years. This leaf chopper seems to be a new pest. How to get rid of it?