Well I'm not a great lover of dahlias but I think that earwigs are wonderful (I'd never harm any of those beautiful creatures), so perhaps I shouldn't be answering this question ;-)
If you smear vaseline petroleum jelly on the stems, it stops the climbing to eat the petals. Does not stop them from eating the leaves though. Try a flower pot on a stick stuffed with shredded paper, they will hide in here so you can tip them out the next day. Neither of these methods should cause the earwigs any harm.