How do I prevent them come back? ==>>> you dont ... and they may never come back ... you treat.. for a CURRENT and EXISTING problem ... and then.. ONLY .. if they are a threat to teh tree.. and a little cosmetic damage.. is usually not a threat to a tree ... and never treat on a future potential problem.. that would be akin to polluting.. since there is nothing there that argues in favor of hard core chems ... and without even seeing the tree ... let me guess.. the worms did no apparent current damage.. so they didnt need to be treated last year either ... its just not needed.. and a waste of money ... keep in mid.. its a tree.. not a child.. it doesnt need inoculations.. education.. college etc... nor will it ever need to be fed ... keep that clear in your mind... its not a child ... its a tree .. [unless a soil test indicates something is missing from the soil itself ...] ken

These are tomato hornworms, pretty common and it's good to just hand pick them. I've read that they glow in the dark if you shine a blacklight on them at night. That would make it much easier to find them if true. Inspect the whole plant, also looking on the undersides of the leaves and check for smaller ones and the eggs which are small green globes. The black deposits are 'frass' (bug poo) from the caterpillars. As they get bigger sometimes the frass looks like pellets and can be greenish. If you happen to see one that has some white cocoons on it, let that one remain because it is an incubator for parasitic wasps that help keep these under control. Google natural control for tomato hornworm for other suggestions.

Sounds like it could be an owlet moth larva, also called a looper caterpillar. There are many kinds of loopers, though, and a photo would help.

You have black swallowtail butterfly caterpillars. The website I have linked will help you out. On the right side of the page, click on "Black Swallowtail." Here is a link that might be useful: Raising butterflies