If cultural remedies really aren't working, you could try a B. thuringiensis solution, either spray or dust. If I remember correctly, there is no required waiting period before harvest, but check the label to be sure and it will need a few days or so for it to kick in anyway.

Have you tried shaking the plant or spraying it with the hose? I find that usually knocks a lot of them off.

If you can see the caterpillars and it isn't a large area, hand pick them off.

This isn't specifically directed at you, krazycoupons, as we've seen this exact scenario many times over the years here. But I'm always mystified when people talk about using some pesticide or another application to deal with caterpillars on a very limited number of plants. I would definitely try spraying off with water and/or hand-picking the insects off before resorting to any other means. That is always safe for food purposes and only targets insects that are a problem

Of note, parsley, carrots, and other Apiaceae can be host plants to some large and beautiful butterfly caterpillars. (http://gardeningwithconfidence.com/blog/2012/07/19/parsley/) I would willing sacrifice some parsley harvests as the plants will regrow to have more butterflies. ;)

FataMorgana

I've tried hand picking them off for a while, and give the plant a good squirt every day. It only became a bigger problem for me when I noticed several baby caterpillars all over the plants. I have four plants in a rather large wooden planter box. I'm not sure if I'll try the dust yet because I don't want to harm them, just deter them from my plants. Rain is the forecast for the next three days, so hopefully that will help wash some away!

Personally, I would just plant extra for the cats!

i used to have a lot of black swallowtail cats on my parsley you're certain these aren't going to be butterflies??

I'd gladly sacrifice the parsley for the butterflies.