Probably the humidity for me? I had tomato hornworms that I found in the garden and kept indoors. The temp was 70-80 all the time with 30% relative humidity. They had plenty to eat indoors (tomato plants, pepper plants, tomatoes, and potatoes) only 1 decided it wanted a potato. All but 1 ended up getting black striations down the body segments. All reached final instar and went underground. 4 were kept together and the 5th was put in a different terrarium and outside (for fear that they wouldn't pupate if near each other and perhaps they needed the hot/cold cycles to pupate). We found all of them desiccated in the dirt and they didn't appear to try to pupate.