Monarch larvae can be reared in many different types of containers, such as aquariams, glass jars, plastic food containers, etc.
Rearing containers should be at least three inches deep and should have ventilated lids (use either mesh lids or poke holes in the lid). Larvae will survive best and attain a large size if you keep the food fresh, the container clean, and the humidity and crowding low.
Controlling humidity is very important. If the humidity is too low your leaves will dry out and will not be a good food source; if the humidity is so high that condensation forms in the container, mold may develop on the frass (feces) and/or the leaves. These conditions seem to favor the development of diseases that could spread rapidly from larva to larva.
Your larvae will do best if only a small number are reared in each container. To minimize the possibility of cross-contaminating containers, DO NOT transfer larvae from one container to another. Should a larva die of an apparent disease, for example lose its shape and color, transfer the healthy larvae to a clean container with new leaves and clean the container with the dead larva with hot soapy water or bleach. You can find examples of rearing containers and cages here.
To rear monarchs outdoors on living plants outdoors you will need to protect the larvae from numerous parasites and predators. Protection can be provided by using mesh “sleeves” with draw-strings on either end. The sleeve is placed over a plant, the larvae placed inside, and the drawstrings tied tightly. This method works well and little care is involved.
Please click here for instructions for making several types of homemade monarch cages.