We faced the same dilemma, and solved it by realizing we can buy parsley at the grocery store, but we've never seen swallowtails for sale.
This year we planted three times the parsley we thought we'd need, a big patch of dill, and some fennel. We have lots of butterflies and adequate dill and parsley. They ate the fennel!
I see you have the flat-leafed Italian parsley. Delicious, isn't it? Hope you get a little bit, anyway! And bless your heart for agonizing over the decision. Too many folks would just get rid of the "worms", never trying to find out what they are, or will become.

You simply have to decide for yourself. I will tell you that these butterflies, though beautiful, are not rare or endangered. Their habitats are, however, which explains why everyone ends up with a batch when they plant parsley, dill, fennel, carrots, and other members of this plant family (Apiaceae/Umbelliferae).

Another reason why we often end up with so darned many of them is that birds won't eat them, and I doubt that other predators will either. They taste awful, apparently. Parasitoids are about the only control.

I believe that you have the right to enjoy your parsley. Personally, I would probably remove them from some of my plants while letting them have others. Next year, consider planting for the whole crowd. Or exclude them from your plants entirely by placing a cage around your plants (like a tomato cage), and covering it with tulle.

Enjoy the butterflies!! They are beautiful. I had a swallowtail caterpillar on my fennel. It didn't eat the whole plant. Besides, fennel like parsley will regrow!

Here is a picture of the caterpillar that was visiting me 2 weeks ago...

FataMorgana

Try contacting a butterfly enthusiast via a local club. Somebody there may be interested in rescuing the caterpillars. I know this may sound strange, but I have a lady near me who has done this for me. I can't afford to have my potted plants for customers get damaged. In the garden, they are welcome to eat away. They plants will grow back.

F. DeBaggio

To me it is amusing that some people plant dills, parsley, fennels just to attract caterpillars(cats).Not me, I like to eat my parsley myself. If I find a cat. nearby I just pick it up and take it somewhere far away from my herb garden.
As for butterflies, I like them but they have an ugly past (grin)

Use some lightweight floating row cover over the herb patch/pots from day one, so that the butterflies cant' get in there to lay eggs in the first place -- a little less attractive in the yard, but prevents the moral dilema of "what do I do now?"

yep, decided we'd rather let the kids see caterpillars become cocoons, become black swallowtails. First one flew away yesterday. The kids really thought it was great. We had a little butterfly habitat and the youngest was the one that noticed a butterfly flitting around inside. I've been picking parsley for the caterpillars to eat every day.

Excellent!

It will be an experience your kids will remember always - and you can't buy those types of memories!

FataMorgana