A friend's wife has a section in her back yard garden with milkweed plants. Every year monarchs come in and lay their eggs on the leaves and she brings them into the house and raises them in an insect cage I built for her. I think last year she raised over 50 caterpillars to maturity and released them.
I don't know how they find her plants each year because it's a small backyard surrounded tucked into a wooded area. If I didn't know better, I would say the ones that arrive there are ones that were released the prior fall.
Anyway, it's pretty cool and she usually gives me several and some for my neighbor for our insect cages.