Silkworms naturally are inclined to only eat fresh mulberry leaves, and it's hard to convince them to eat anything else after they've started, so they're usually raised and sold with a special green "chow" (I'm assuming is made with some sort of dried mulberry leaves, but I have no idea)
Hornworms come with a different chow, I think there are tobacco hormworms or tomato hornworms, either way I think both those leaves make for toxic feeders, hence the chow instead. But same idea, special hormworm chow.
The butterworms, I don't think there's anything, I think they're just imported the way they are, I'm not sure anyone here "raises" them, I don't even know if you can, maybe the adult bug form is a pest threat here?? Just a guess.
Anyways, seems to me I've seen mentioned on the forum before maybe shredded carrots or lettuce of some kind as a way to tide a silkworm over?? but that could be wrong.
I think the standard is to just gutload them with their appropriate chow. Since their whole lives are just eat & poop, they're pretty gutLOADED anyways, if food has been available to them.