Monarchs lay their eggs on milkweed,....but only on the very small baby plants, they eat the large plants but do not lay eggs on them
Milkweed in bloom is very sweet and attracts ants like crazy which in turn eat the monarch eggs. The monarch caterpillars eat the large flowering milkweed until they get a certain size and then find a place to hang in order to become the chrysalis. The life span of an adult monarch is about 2 weeks
The ones that migrate in the fall to Mexico are the last brood of the season and they are actually smaller then normal and thus better suited to migrate long distances. The monarchs that return in the spring are a new batch, born on their wintering grounds down south. On a pond tour last weekend, one of the homeowners was really into this and she showed us her conservatory where she raises monarchs to release in the wild.
She had them in every aspect of their life from eggs to caterpillars of all sizes to about a dozen chrysalises hanging from everywhere. In the wild monarch survival rate is about 10%. She averages 90%
Here is what I found most fascinating: She "tags the last brood of the year with a little sticker that goes on it's wing and keeps records of whether it is male or female then releases them to the wild.