Your BSTs shouldn't get to be more than 2" (5 cm), and mine often don't get bigger than about 1 3/4" (4.4 cm). As for buying parsley from the grocery store, you probably could do it, but be careful! You will want to get organic / pesticide free parsley.
You also said that you still had some cats that were about 1 cm long. They are probably in their 3rd or 4th instar rather than their 1st instar.
Hope this helps,
-Jmcat

Thank you! Hopefully they'll calm down soon. Do you see all the stalks they've left? I ended up putting two of the pots I bought yesterday morning on top of the previously chomped up plants.

Sorry I got the instar thing wrong. This is the first time I'm paying attention to caterpillars, since I kept silkworms as a kid. The +/- 1cm guys are still black with the white dot in the middle of their backs, which is why I thought 1st instar.

I have to say, I think they're absolutely exquisite once they're big and stripy - they look like someone has painted them with waterpaints.

Thank you again,
Nik

I agree - the cats are absolutely beautiful! As for the smaller cats, they actually keep their black with a white stripe coloring until their 3rd or 4th instar (it varies between individuals).
-Jmcat

You can also try carrot tops if you can find them. Rue is another option although, BST often don't like to change their food type, at least that has been my experience. Dill and fennel are also host plants.

I've bought Parsley from the grocery store but wash well with a mild soap and rinse well. They seem to like curly best, and this is a last ditch effort. Good luck.

I ended up buying Italian parsley from Whole Foods. It's organic. :)

I'm now down to 10 cats, from the 17 I had over the weekend. I'm not sure where those 7 have crawled off to, but at least I've managed to keep them supplied with food.

Thank you for your help!
Nik

I'm glad you had success. Whole Foods is better choice, but they still have some chemicals on them, so wash good if you can.