Monarch butterflies (Danaus plexippus) are found across North America. During the spring and summer months, the western monarch lives in California and other states west of the Rocky Mountains. In the fall, western monarch butterflies head west to overwintering sites along the Pacific coast. The eastern monarch is found in the central and eastern US, and in southern Canada. The Eastern Monarch overwinters in central Mexico, migrating through the central US in a massive funnel in the late fall.
Meet the Monarch!
Monarch Life Cycle
The life of a monarch begins when the female butterfly lays a tiny egg on a leaf. She secretes a special kind of glue to attach the egg firmly to the leaf. She flies from one plant to another, laying hundreds of eggs over a 2-5 week period.
Just because one monarch lays hundreds of eggs, do not expect to see clusters of eggs on a milkweed plant. Typically there will be one or two on a plant, with more sometimes appearing late in the summer.
After about four days, a larva, or caterpillar emerges from the shell. It begins nibbling first on the egg shell and then on any available milkweed leaves. Milkweed is absolutely essential to monarchs. It is the only plant on which monarchs lay their eggs and it is the only plant that monarch caterpillars eat. In addition to providing basic nutrition, the milkweed leaves contain toxic chemical compounds. These compounds are ingested and sequestered within the caterpillar, and subsequently in the butterfly as well. These toxins, signaled by the monarch’s distinctive coloration, makes the monarchs themselves toxic and are distasteful to predators, who then avoid the monarchs. Learn more about milkweed in the section on loss of native milkweed.
The caterpillar consumes a lot of milkweed over the next two weeks, expanding to almost 2,000 times its original mass. During this period, the caterpillar also molts five times, each time emerging in a new skin. These five stages are referred to as “instars.”
After two weeks, the caterpillar is ready to pupate. It spins a silk pad from which it hangs upside down, shedding its familiar yellow, white, and black striped skin for the last time. Its new layer of skin hardens into a pupa, or chrysalis.
After 8-15 days, the black, orange, and white wing patterns of the butterfly become visible through the pupa covering. This is not because the pupa becomes transparent; it is because the pigmentation on the wings develops at the very end of the pupa stage.
When the butterfly emerges from the pupa, its abdomen contains mostly body fluids. Its wings are shrunken and crumpled looking. The butterfly hangs upside down and pumps fluids into its wings until they expand and stiffen. Then it flies off to feed on nectar plants. Learn more about the nectar plants that monarchs need in the section on home garden habitats.
Female monarchs have wider veins on the their wings compared to males, and often look slightly darker than males.
Males have a black spot on a vein on each hind leg. These spots are made of specialized scales which produce a chemical used during the courtship of many species of butterflies, although it does not seem to be important in the case of monarchs.
Becoming Adults!
The first several days after emerging from the chrysalis, adult monarchs remain in their natal area, making short flights to nearby sources of nectar. Females begin mating during this period.
Depending on the time of year when they are born, they may stay local or they may migrate west.
If the butterfly is in the breeding generation — typically born in the spring and summer — it lives out its life in the natal area somewhere east of the Pacific Coast. There are typically three generations of monarchs in this pattern.
In contrast, butterflies that emerge in late summer and early fall enter reproductive diapause. They feed on nectar plants for several days to two weeks and only then do they begin migrating toward the Pacific coast sometimes traveling hundreds of miles. Known as the “super generation,” they spend the winter clustering along the coast with other monarchs. In the spring they foray back inland to mate and lay eggs.
Learn more about the migration and overwintering phase in the next section!
Monarch Migration and Overwintering
Western monarchs are well known for their epic migratory voyage when they leave their overwintering sites along the California coast and head east for the spring, summer, and fall.
Successive generations during this migration period spread throughout the Western states, feeding on nectar, mating, and laying eggs.
The fourth generation heads back to the coast, where it spends the winter roosting with other monarchs, leaving the cluster only to find nectar.
The Western monarch overwintering sites are mostly located along the coast between Mendocino and San Diego County within five miles of the ocean. They look for large trees where they can shelter from wind, rain, and predators. They often select eucalyptus trees, Monterey pines, and Monterey cypresses as roost sites, as well as oak, sycamore, and redwoods.
Not until February or March do members of this generation head out, searching for milkweed and starting the cycle over again.
Between October and March, Western monarchs overwinter in large colonies to roost, primarily on eucalyptus trees, Monterey pines, and Monterey cypresses and occasionally on oaks, sycamores, and redwoods.
These trees provide wind protection and dappled sunlight, acting as a “thermal blanket and a rain umbrella.” Access to trees of varied height is also important so that monarchs can move around when microclimate conditions such as wind and temperature fluctuate.
Overwintering sites must also provide a source of fresh water as well as nectar plants. One advantage of eucalyptus trees is that they bloom during the overwintering season, offering a convenient source of nectar during a period when relatively few flowers are available.
Increasingly, it is becoming more difficult for monarchs to find adequate overwintering sites along the coast. Eucalyptus trees are potential fire hazards, and are being removed in some areas. These and other overwintering trees are also being removed to make room for housing.