Do you have any pictures? Did you see any eggs of any sort on the underside of the leaves? The only caterpillars that I've caught on my plumeria were monarch butterfly caterpillars. Apparently milkweed is in the same family as plumeria, so while unusual, it isn't totally unknown for this to happen. I moved the caterpillars to milkweed shortly afterwards, and they were much happier. I'm sure there are other moths/butterflies that have caterpillars that would eat plumeria leaves, though.

I have no ideas on environmentally nuking them except maybe BTK but then the caterpillars have to ingest it to die, meaning they have to eat the leaves. I am expecting a mass invasion of the tent caterpillar this year, unless the wind blew all the moths in a different direction when they hatched :) I have stocked up on a large roll of very light frost cloth and am going to sew long tubes, slip them over the grafts and tie wrap each end. They won't be on long enough to girdle the tree. I refuse to let the evil little worms kill my efforts.

Carol, you are right about caterpillars doing better in cool weather with the wasps disappearing. I have found 2 'wild' caterpillars in the last couple of days...the first living ones I have found since early July. The chrysalis, which are mainly attached to my patio fence, hardly ever survive - they are often 'adopted' by a garden spider. That is why I move them to a safe place whenever I find them. I keep them right outside my window, attached to a repurposed jewelry tree, where I can watch them hatch. They are protected by a 'moat' to protect them from ants, spiders.

The following is simply pure peculation, but previous replies have been concerned only with controlling the ants, whereas I am looking at a different aspect of your original post. That said - Are you certain the ants actually are eating the caterpillars?? Perhaps something like the following is going on – although swallowtails are NOT the caterpillar species associated with ant activity. (See below.) [Or perhaps the question to ask is -- are you certain your caterpillars are swallowtail larvae? I'm sorry to suggest it since the larvae of Papilionidae & Lycaenidae families are very different in appearance & size!] Here is an excerpt from Wikipedia (I know, but…sometimes…) "A myrmecophile is an organism that lives in association with ants. Myrmecophiles may have various roles in their host ant colony. Many consume waste materials in the nests, such as dead ants, dead larvae, or fungi growing in the nest. Some myrmecophiles, however, feed on the stored food supplies of ants, and a few are predatory on ant eggs, larvae, or pupae. Others benefit the ants by providing a food source for them. Most associations are facultative, benefiting one or both participants but not being necessary to their survival, but many myrmecophilous relationships are obligate, meaning one or the other participant requires the relationship for survival. Myrmecophilous associations are best known in butterflies of the family Lycaenidae. Many lycaenid caterpillars produce nectar by specialized organs and communicate with the ants through sound and vibrations.[7] The association with ants is believed to reduce the parasitisation of the butterfly caterpillars.[8] " And even though this video from National Geographic concerns completely different butterfly species, it is fascinating to watch, none the less! Caterpillar-ant symbiosis Nature is endlessly and wondrously fascinating, mysterious, and obscure!