I found this caterpillar in our garden on Friday July 8. On Sunday I moved it to another Narrow Leaf milkweed plant that is larger than the one on which it hatched, because the original plant was too small and running out of leaves to eat.
The Monarch female typically lays only one egg per plant and milkweed is the only plant she will choose. Milkweed contains a toxin that protects the larva and the adult from predators (there is some question whether that holds true for all milkweed plants, so researching appropriate native host milkweeds is a good idea). In our area, southern California, one of the preferred host milkweeds is Narrow Leaf milkweed aka Asclepias fascicularis. I think I saw a female checking out suitable plants in out yard about a month ago.
Our resident caterpillar is an eating machine. I think it has molted a couple of times since Friday. I estimate that it is in the fourth “instar,” a term that refers to the intervals between each molt. I noticed white dots on its “legs” which are a sign of the fourth instar. From here it will eat lots more milkweed, molt once more and then hang out as a chrysalis for eight to 15 days.
I think that this caterpillar, once it becomes a butterfly, will live some five or six weeks. I read that many western Monarch migrate from their overwintering spots in California to breed in various parts of the southwest. Those long distance travelers began their migration in late winter or early spring. The strictly “local” set do not migrate and live shorter lives.