Monarch Natural History
Monarch butterflies (Danaus plexippus L.) are large, colorful insects admired by people over much of our planet. This admiration is partly due to the great distances that they migrate in the fall to protective winter habitat followed by dispersal in the spring to widespread breeding habitat. Since fall migrant monarchs are the "great grandchildren" and/or "great-great grandchildren" of the previous years migrants, it is amazing how they find their way to relatively small and isolated traditional overwintering sites.
Milkweed and Lifecycle
At summer breeding habitat, adult monarchs nectar on many kinds of flowers, including milkweed host plants. Host plants are species on which an insect lays its eggs. In much of western North America the primary host plant is narrow-leaf milkweed (Asclepias fasicularis), which typically occurs, in isolated patches. Monarch eggs hatch after 3 to 5 days and develop during five larval growth stages called instars. The complete larval stage lasts approximately 2 weeks and during this time the larva eats milkweed and grows rapidly. The last or fifth stage larva changes into a stationary chrysalis, which completes metamorphosis during the next 10 to 14 days, finally emerging as an adult. In the leaves of milkweed are plant toxins called cardiac glycosides that cause the larvae and adults to taste bad to most birds and some mice that try to eat them. The bright and contrasting colors of both the larva and adult help these predators learn to avoid eating them in the future.
Above left: A late stage chrysalis just prior to emergence
Migration, Overwintering, and Diapause
During late summer and early fall, day length shortens, average temperature decreases, and milkweed plants age. These changes cause monarchs to develop differently from the previous summer generations. They store more fat in their abdomen and the organs that normally produce eggs or sperm do not develop until much later in their adult lives. These changes help them make their long distance journey to overwintering sites.
In eastern North America they migrate and overwinter in several mountainous areas dominated by Oyamel fir forests near Mexico City. West of the Rocky Mountains they migrate to wooded sites located along the Pacific Ocean from around San Francisco to San Diego, California.
At these sites they spend the majority of time roosting in tight clustering formations in the canopy of one or more of the dominant tree species in the habitat. They make brief flights away from the clusters on relatively warm, sunny days to drink, nectar, or reposition themselves in the canopy.
Fall migration patterns
In western North America two types of monarch roosting habitat are generally recognized - autumnal or transitional roost sites and overwintering habitat. Autumnal roost sites are occupied early in the fall and generally host relatively small populations of monarchs (tens to hundreds of butterflies). Monarchs roost at these sites for a few weeks but abandon them by early December and move to overwintering habitats. In contrast, monarchs (hundreds to thousands of butterflies) occupy overwintering habitats throughout the fall and winter, and the butterflies remain until February and March, which is when mating and spring dispersal, occur.
Above left: An overwintering habitat in the Oceano Campground in Pismo State Beach Park. Encroaching sand dunes may eventually destroy the habitat.