I'm kicking myself for not taking a picture of it after pulling it off the tree; I'm hoping to not find another one but I'm snapping a picture if I do! I've only got this one citrus tree in a barrel, I'd like to put a dwarf lemon in the barrel next to it; everything else in my yard is non-citrus and I haven't seen these caterpillars on anything else before.

I doubt it is a big or problem so go ahead and plant your second citrus. Enjoy!

Almost surely an Orange Dog, the larva of the Giant Swallowtail butterfly. To my knowledge it is the only butterfly that lays its eggs on citrus. You can spray for them as do the citrus growers (big pest in California); or in the case of garden citrus, you can just watch for the eggs/young caterpillars and pluck them off.

After a quick google search, I figured that's what it was too. But when I looked at pictures of the orange dog, the one on my tree looked nothing like it. I guess I'll just have to keep a close eye out for any more to take a picture.

Do they typically only eat the foliage? I found a few holes in some leaves here and there, but I have several hollowed out immature oranges.

If you found them in your fruits I don't think they're orange dogs aka giant swallow tail caterpillars, they only eat leaves, as far as I know. At least the ones I find on my trees only eat leaves. I'd suspect hawk moths or some other type of larvae that consume fruits instead of foliage.

Evan

Is there some kind of non-toxic anti-parasitic that I can put on the plant? Or would I need to know the specific caterpillar in order for it to be effective?

This is what I use on my citrus trees. I don't think it will kill the existing caterpillars, but should help control future issues. It works well and is organic.

Thanks ev, I'll give that a try.