Caterpillars on Parsley

Last year I had a yellow and black caterpillar in herb garden which wiped out my parsley before I could get them under control. This year they have started early as I found a small one dining on my dill. Can you tell me what they are and how I can get rid of them once and for all?

This sounds like the caterpillars of the swallow tail butterfly are producing a next generation on your members of the parsley family. Since they fly in, you can't get rid of them once and for all-I hope!!! Why not put some netting over your plants -and this would include dills, parsleys, fennels and carrots- to prevent the adults from laying eggs on your plants. But for those of us who love butterflies -except maybe the overprolific cabbage butterflies- plant a few extra dills or parsleys (their favourites) outside the netting to maintain biodiversity!