Milkweed Mystery

I came across this munched on milkweed in the garden... I'd never seen anything like it!

I've never seen milkweed damaged like this, inside or outside the garden, so I'd assumed deer don't eat it. The damage was too high up the plant for rabbits or groundhogs, and seemed too much for bugs. 

Have you seen anything like this, and do you know who might be the culprit?