It's a deadly virus that is doing it. Caterpillars are just so susceptible to diseases as well as predators. Unfortunately, this "black death" also kills a huge amount of our precious butterfly larvae as well. In 2 weeks, I collected 6 tobacco worms of various sizes. Sadly, every last one succumbed to tachinid flies. It just took me this long to even find a single hornworm. Other tomato growers in the east and especially northeast, are reporting good numbers of them. I'm happy to know the sphinx is doing well somewhere. Overall, our native Lepidoptera, including garden hornworms, is declining.