Welcome Clownboy. There's really nothing you can do other than pick them off or get rid of them by some other means. They can eat your passie to the soil. I have the opposite problem, I've been trying to attract Gulf Fritillary butterflies with no luck. At least my passion vines look good. lol Do you have multiple vines growing? Maybe you can put in some passies just for the cats. I do that for the hornworms that go after my brugs and daturas. I grow some host plants just for them. I pluck them off my "good" plants and put them in their little food patch. Good luck.
Karyn

As matter of fact .. I have soo many Gulfs dive bombing round it looks cool ... And soo darn many cats too boot off my vines also .. I have 2 burghandy passion vines , 1 blue which is like a scraggler vine , and 1 purple vine which Im to beleive is supposed to be such the large vine .. I got it so these cats can eat n the vine would grow to be big n strong like popeye Ha .. Im a welder , so I built this huge trellis for it to grow on .. Ohhh and I also have a Giant Dutchmans vine too with HUGE cats on them ... BIG like thumbs !!!

You get pipevine swallowtails too! I have loads of aristolochias and no pipevine swallowtails either. All i have are the tiger and black swallowtails that like fennel, parsley and dill. I also have loads of monarchs on my asclepias but I really want some gulf frits and pipevine swallow tails. tell some of your butterflies to fly up my way. I have plenty of food for their babies. lol
Karyn

Ok , I sent the troops yer way .. Heck , I peel dozens off each day to keep my vine in good shape