These older larvae will likely pupate by the following week's monitoring, and thus will not affect our population counts. It is very important that you collect only 4th and 5th instars, and that you release the resulting healthy adults back into your monitoring site. The container should be easy to open, since you need to clean it every day. It should have a screen covering or holes for air flow and should allow you to see the larva inside. Unless you plan to move the pupae, the cage should be large enough for the adult to expand its wings when it emerges.
High temperatures can kill the larvae.
Cages must be cleaned daily. Simply empty out the caterpillar frass and old milkweed. Larvae must be provided with fresh milkweed daily. You can pick several days' worth of caterpillar food, and keep it in a plastic bag in a refrigerator. It stays fresher in the cage if you keep the end moist by wrapping it in a wet paper towel and covering the towel with aluminum foil, or use the plastic containers in which single cut flowers are sold by florists.