The eastern North American monarch population is notable for its annual southward late-summer/autumn instinctive migration from the northern and central United States and southern Canada to Florida and Mexico. During the fall migration, monarchs cover thousands of miles, with a corresponding multigenerational return north in spring. The western North American population of monarchs west of the Rocky Mountains often migrates to sites in southern California, but individuals have been found in overwintering Mexican sites, as well. Non-migratory populations are found further south in the Americas, and in parts of Europe, Oceania, and Southeast Asia.
Like all Lepidoptera, monarchs undergo complete metamorphosis; their life cycle has four phases: egg, larva, pupa, and adult. Monarchs transition from eggs to adults during warm summer temperatures in as little as 25 days, extending to as many as seven weeks during cool spring conditions. During their development, both larvae and their milkweed hosts are vulnerable to weather extremes, predators, parasites, and diseases; commonly fewer than 10% of monarch eggs and caterpillars survive.
The range of the western and eastern populations of D. p. plexippus expands and contracts depending upon the season. The range differs between breeding areas, migration routes, and winter roosts. However, no genetic differences between the western and eastern monarch populations exist; reproductive isolation has not led to subspeciation of these populations, as it has elsewhere within the species' range.
In the Americas, the monarch ranges from southern Canada through northern South America. It is also found in Bermuda, the Cook Islands, Hawaii, Cuba, and other Caribbean islands,the Solomons, New Caledonia, New Zealand, Papua New Guinea, Australia, the Azores, the Canary Islands, Madeira, continental Portugal, Gibraltar, the Philippines, and Morocco. It appears in the UK in some years as an accidental migrant.
