Caterpillars are very susceptible to a variety of bacterial infections, including bacteria we all carry on our hands without knowing it. Be sure to always wash your hands thoroughly before handling the caterpillars.
Caterpillars are relatively fragile creatures. Handle them very gently. If you are changing their host plant, it is best to put the fresh host plant into the cage, then wait a few hours for the caterpillars to crawl onto the new host plant on their own. Then you can remove the old food.
Alternatively you can use a small paint brush to carefully move the caterpillars from one plant to another, this works for us. Remember how many caterpillars you have and count them all before you throw out old host plant so you do not accidentally throw away your caterpillars, it happens to the best of us!
Pokey Pillars
Do not pick up caterpillars with branching spines! These spines can deliver a very painful sting.
Lazy Pillars
If your caterpillars seem lethargic or have changed color, do not handle them. They are probably preparing to molt or form their pupa and are very vulnerable at this stage. Or they may be sick. If your caterpillars die, remove them from the cage immediately to help prevent infection of the other caterpillars in the cage.