Danaus plexippus is indeed perplexing down here in the tropics. Monarch butterflies are generally migratory, but it seems, as happens with many tourists, that once they get down here, they have a Jimmy Buffett moment and can't be bothered to leave. Too much sun, too many mojitos. It happens.
I decided, after Irma's wrath leveled my garden, to plant more butterfly-friendly things. Of course milkweed was purchased and planted up.