I don't know the answer but, you might try tossing one in a jar in your fridge for the night and then let the jar warm up for a number of hours/days and see what happens. Give it some pepper leaves to eat while enduring your experiment.

I wouldn't count on it. I had a problem in one of my high tunnels with them. I pulled off the plastic at the start of a snow storm. 4 inches of wet snow and freezing temps, 1 day later, I was inspecting the dead plants and there was a hornworm. It was very cold, but very much still alive. When I pulled him off, it started to wiggle. He met death under my foot!

Hornworms might be able to survive a blast of cold air, but they usually don't survive a blast of Bt. Misterbaby.

Oh good Lord! Squishing one of those hosses under your FOOT!? I can only imagine the resulting carnage.

I know they overwinter in the ground as pupae (I've dug them up before), so I would assume that once they feel the cold coming on, they start bedding down for the winter. I can imagine a sudden cold front might catch a few off guard, but the normal thing for them to do is to bury themselves down where they won't freeze (soil is a great insulator).

In spring they emerge as moths ready to lay eggs on your newly planted tomatoes.

They might die but their eggs will still be in the soil. If you are already rotating your crops then perhaps you could also grow a companion plant to attract predators. For example, sweet alyssum is easy to grow, easy to find and will attracts wasps. The wasps will lay their eggs on caterpillars and hornworms and the offspring will eventually kill the pest.

Walmart generally has seed packs of Sweet Alyssum for around 20cents.

If it gets cold enough to kill the hornworm it will also kill your pepper plants. Those rascals are masters with their camouflage but it's best to pick them off or spray with BT.