Buy and use cages to protect your caterpillars. The best kind will have a fine mesh screen. Get cages that have a flexible cable framework, similar to the fold-up sunscreens that are used in front windows of cars. A cage with fine mesh screening and a flexible cable framework will take literally 3 seconds to deploy.
DON’T rig up a do-it-yourself version of a cage: you will waste a lot of time, lose a lot of caterpillars, and expend much more effort than you needed to.The configuration that works is: milkweed cuttings, placed in water-filled plastic bottles, raised in fine-mesh-flexible-frame cages. Please just accept this advice, since it is the product of many hours of trial-and-error.
Do a search on these terms: mesh cage butterfly.
Some brand names are Restcloud, Qingo, Miraclekoo, Ueetek, Trasfit, Oxel, Petforu.
A cage of 36 inches is ideal.
When you see an newly-emerged adult fluttering around: open the cage door and let the butterfly go.
When you find a tiny caterpillar, bring it to the refuge cage and place it on a milkweed cutting.
–The refuge cage protects the caterpillars against the tachnid flies. Out in the open, the caterpillars have little defense against the tachnids. Once a caterpillar is stung by a tachnid fly, it is doomed.
–The refuge cage protects caterpillars against multiple garden enemies, such as ants, lizards, praying mantii, and birds.
–The refuge cage keeps the growing caterpillars in a confined space, so that, if they fall off the milkweed cuttings they can easily get back onto the milkweed. Out in the open, if a caterpillar falls off a milkweed plant, it will wander off looking for another milkweed plant to climb onto, but oftentimes the caterpillar will be lost. On the other hand, if a caterpillar leaves a milkweed cutting in the refuge cage, the caterpillar can be easily repositioned onto the plant.
–The inside of the top of the refuge cage provides a great spot for the full-grown caterpillars to attach to when they decide to turn into a chrysalis. In the wild, the full-grown caterpillars will leave the milkweed plant and crawl up to 40 feet away, then crawl up onto a structure or plant to find a place to turn into a chrysalis. This journey makes them very vulnerable to attack by multiple adversaries.
–The refuge cage protects newly-emerged female Monarchs from overly-aggressive male suitors. The newly-emerged female Monarch needs several hours to deploy her wings. If she is disturbed during this phase, her wings will not fully expand, her wings will remain crinkled, and she will never be able to fly.
–The refuge cage can be easily protected from the summer sunlight: get a swath of sunscreen mesh, fashion it into a cap, and place it over the top of the refuge cage. Developing Monarchs, from egg thru young adult, do not tolerate temperatures over 90 degrees Fahrenheit. They become dehydrated, turn dark, and die.
REFUGE CAGES ARE FRAGILE
If the grow-cage is full of potted milkweeds, it CANNOT be dragged to a new location. The dragging causes rips in the undersurface of the cage. To move a refuge cage, first remove the stone weights and the water-filled plastic bottles, then carry it to its new location. Whenever you open a refuge cage, do it at night, when tachnid flies are not active.