About Monarch Butterflies
The monarch butterfly (Danaus plexippus) is a native North American insect that has evolved an adaptive survival strategy that coincides closely with the milkweed seasonal history. Milkweed is the primary food source for the monarch butterfly caterpillars. This insect takes advantage of the abundant milkweeds that grow across the United States and in parts of southern Canada during the spring and summer months. They escape the harshness of winter conditions and the lack of milkweeds by migrating to winter sites located in the high mountains of Mexico and near the beaches of coastal California.
Their survival story begins with the overwintering butterflies dispersing from their winter sites in spring to seek emerging milkweed plants on which to lay their eggs before dying. Upon hatching, the young will feed voraciously on the milkweed leaves before forming their chrysalides within 3-4 weeks, depending on temperatures. Feeding on milkweeds not only provides the necessary nutrients for the caterpillars to develop into adult butterflies, but also provides a natural protection to both monarch caterpillars and butterflies. The cardenolides, or noxious chemicals, in milkweeds are incorporated into the body tissues of the feeding caterpillars which make them, and the adult butterflies they become, unappealing to some predatory birds.
When the butterflies emerge from their chrysalides, the adults will fatten themselves on nectar from flowering plants, find mates and begin to lay their eggs on milkweeds for the next three weeks before they expire. The cycle of eggs to summer adults is repeated four or five times (generations) before the butterflies will occupy their entire distributional range that includes the United States and parts of the lower Canadian border.
During late summer or early fall, developing caterpillars are exposed to shorter day length and cooler temperatures. Under these conditions, they undergo physiological changes that result in overwintering adults. These adults differ from the summer adults in that they are long-lived (5-6 months vs. 1 month), their reproductive organs are immature (diapause), they have large amounts of body fat (which allows them to survive the winter), and they have the urge to migrate.
The overwintering butterflies begin their migration in the fall before the milkweeds die back to their rhizomes (underground stems) and before the onset of unfavorable winter conditions. The grand-offspring of the original butterflies migrate instinctively, often several thousand of miles to winter sites located in Mexico and California. These sites are important to their winter survival, allowing them to escape the harsh winter conditions of central and northern states and the lack of larval food, the milkweed. The butterflies will form winter aggregations on foliage ranging from a few hundred (California) to several million individuals (Mexico). They ingest water and survive mainly on their fat reserves. California monarchs also have a nectar source from winter flowering plants, including the eucalyptus trees in which they often roost.
Toward the end of the winter season and with increasing day-length and ambient temperatures, the reproductive organs within the overwintering butterflies awaken from their slumber (diapause) to resume development. The butterflies mate before they begin their spring dispersal from the winter sites. When the female butterflies deposit their eggs on the leaves of re-emerging milkweeds, the seasonal cycle of the monarchs in North America is completed.
Metamorphosis: Life stages of the monarch butterfly
Stage 1 - EggThe female monarch may lay hundreds of eggs during her short life. She normally deposits these yellowish pinhead-sized eggs on the underside of milkweed leaves, where they will hatch in 3 to 6 days, depending on the temperature.
Stage 2 - Larva or Caterpillar
The newly hatched larva feeds voraciously on its milkweed host plant, accumulating chemicals called cardenolides which scientists believe protect adult monarchs from bird predation and also enable individual monarchs to be genetically traced to the species of milkweed they fed on as larvae. During the next 15-20 days, the 1/16-inch caterpillar grows to approximately two inches and increases in weight by a factor of 2,700. To accommodate this rapid growth, the caterpillar must shed its yellow, white and black striped skin four times before it is ready for the next stage of its development.
Stage 3 - Pupa or Chrysalis
The mature caterpillar attaches itself to a sturdy stem or leaf of the milkweed by spinning a silk button from which it hangs upside down in a “J” shaped position. After 15-20 hours, the caterpillar sheds its skin, transforming into a beautiful chrysalis, or pupa, decorated with delicate gold spots. After approximately 10 days, the chrysalis becomes transparent, revealing the orange and black pattern of the butterfly within. Once this change takes place, the butterfly is ready to emerge from its chrysalis.
Stage 4 - Adult
As the chrysalis splits open, the monarch begins to pump fluid into its crumpled wings and quickly expands into a full-size adult. The butterfly rests for at least three hours until its wings dry and harden. Then it’s ready to join in the ongoing cycle of the monarch migration.
The Migration Phenomenon
The monarch butterfly is a widespread tropical insect that ranges as far north as Canada. It cannot withstand freezing winter temperatures. To survive, the monarch migrates to safe overwintering sites that are neither cold enough to kill it, nor so warm that it wastes precious energy necessary to survive the winter. Monarchs east of the Rocky Mountains spend the winters in high mountains in central Mexico. Monarchs west of the Rockies migrate to coastal California. En route, they may travel as far as 200 miles a day and fly as high as 10,000 feet.
In many species, such as birds and whales, the same individuals travel the same routes year after year; however, the monarchs that migrate to Nipomo have never been here before. In fact, they are the grand-offspring (4-6 generations removed) of those that overwintered here last winter season. Some scientists believe monarchs rely on Earth’s magnetic field, the position of the sun, and the polarization of the sun’s rays to find their way.
In early October, as decreasing daylight signals the approach of winter, the emerging autumn butterflies head south to their overwintering grounds. Migrants of the fall generation live 5-6 months and may travel nearly 2,000 miles to reach their ancestral overwintering sites. The monarchs feed on flower nectar during the journey to build up fat reserves which enable them to survive the winter months.
Monarchs have trouble flying in temperatures below fifty-five degrees. The butterflies hang in clusters in the trees, resembling dried leaves, until the sunlight warms them. Then they spread their wings and leave their clusters to forage for nectar and water or to find better spots to warm in the sun. On cold winter days when the temperature does not exceed fifty-five degrees, the monarchs may remain in their clusters all day.
Last updated 24 May 2013.