Monarch Butterfly Sanctuary

Monarch Butterfly Sanctuary

  • Monarch Sanctuary Sign
  • Monarch Butteflies

Every Year Thousands of Monarchs Overwinter in Pacific Grove

Arriving in October, the Monarch Butterflies cluster together on the pines and eucalyptus trees of the Sanctuary. Their migration to Pacific Grove is so unique that Pacific Grove is nicknamed "Butterfly Town, U.S.A." The community has always welcomed the butterflies and advocated for the butterflies’ protection. Citizens of Pacific Grove voted to create an additional tax to create the Monarch Grove Sanctuary which is cared for by dedicated volunteers. The Pacific Grove Police Department enforces strict regulations that prohibit the "molestation of butterflies." The fine? $1,000. For more information on monarch viewing and to access weekly monarch counts, please go to the Pacific Grove Museum of Natural History.

The Migration Phenomenon

Where do they come from? The Monarch Butterfly (Danaus plexippus) 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 flying too much. 

Monarchs east of the Rocky Mountains spend the winters in high mountains in central Mexico. Monarchs west of the Rockies migrate to locations on the central California coast. En route, they may travel as far as 2,000 miles, covering one hundred miles per day, and flying as high as 10,000 feet. A mighty achievement for such a seemingly fragile insect!

Why is this migration so unique? In many migrating species, such as birds and whales, the same individuals travel the migration route year after year. In contrast, migrating Monarchs have never been to their destination before. In fact, several generations of Monarchs have lived and died since last year's butterflies departed.


Historically, most of the western population of Monarchs settled in the fog-shrouded Monterey pine forest of Pacific Grove. These trees provide the microclimate that they need: proper humidity, light, shade, temperature, and protection from wind.

Pacific Grove's George Washington Park is part of the original overwintering habitat. However, urbanization, aging of the trees, and environmental stresses have caused a decline of this habitat. The City is attempting to reverse this decline with an aggressive campaign of tree planting, mulching, and trail improvement. 

The proliferation of Australian eucalyptus trees, first introduced into this country in the 1850's, has also affected traditional overwintering patterns by providing a tree that is not only well-suited to sheltering Monarch clusters, but also provides the butterflies with a convenient nectar source since it blooms in winter. Monarchs cluster in the eucalyptus trees at the Monarch Grove Sanctuary, as well as in eucalyptus groves at Natural Bridges State Park in Santa Cruz, and at the North Beach Campground in Pismo Beach near San Luis Obispo.

The Monarch Life Cycle

During her lifespan, a female Monarch may lay hundreds of eggs. She deposits these pin-head sized eggs on the underside of milkweed leaves, where they will hatch in 4-5 days, depending on the temperature. The newly hatched larva feeds voraciously on the milkweed, accumulating bitter chemicals from the host plant which help protect the Monarch from predation by birds. Over the next few weeks, the caterpillar grows from 1/16th of an inch to about 2 inches in length, increasing its weight by a factor of 2,700. To accommodate this rapid growth, the caterpillar must shed its distinctively striped skin several times before it is ready for the next stage of its development.

The mature caterpillar usually leaves the milkweed to seek out a bare branch or similar sturdy surface. It attaches itself by spinning a silk button from which it hangs upside down in the shape of the letter "j". After settling down, the caterpillar sheds its skin a final time, revealing a beautiful green chrysalis decorated with delicate gold spots. Inside the chrysalis, the caterpillar's body is undergoing metamorphosis, the process by which its tissues and organs rearrange into the startling different body of a Monarch butterfly. After two weeks, the chrysalis becomes transparent, signaling that the black and orange butterfly within is ready to emerge.

The chrysalis splits open along several joints, and the Monarch butterfly carefully emerges. Its wings are still folded and crumpled from confinement in the chrysalis, so the butterfly must pump fluid from its body into the wings, expanding them quickly to full size. The butterfly then rests quietly for a few hours to allow the wings to dry and harden before embarking on its maiden flight. A female Monarch has approximately 6 weeks to seek out nectar, mate, and lay eggs before she dies.


In late summer and early fall, a special generation of Monarchs is born. These Monarchs live much longer, up to eight months. Triggered by the decreasing daylight and angle of the sun, these butterflies delay sexual maturity and begin flying south toward the overwintering grounds, up to 2,000 miles away. The Monarchs feed on flower nectar during the journey, attempting to build up fat reserves which will enable them to survive the winter months. At night they may cluster together in small groups, but as winter approaches, they move on to more permanent overwintering sites.

After arriving at their destination, the monarchs cluster in large masses to conserve heat. Their flight muscles do not function well unless the temperature is above 55 degrees. They rest quietly on the trees, resembling dead leaves, until sunlight warms them enough to fly. On warm days, the butterflies will leave the trees entirely, seeking out nectar sources with which to replenish their energy reserves, but always returning well before evening to once again cluster in the trees.

The overwintering Monarchs do not mate until the increasing temperatures and daylight hours in February trigger the development of their sexual organs. They can then be seen performing spiral mating flights, after which the coupled pair will rest overnight. The male passes a nutrient rich sperm packet to the female during mating. This extra energy will allow the female to travel far in search of milkweed on which to lay her eggs. By March, most of the butterflies have departed on the spring migration 

Spring and Summer

With the coming of spring, the Monarchs join the western migration, spreading out through the Central Valley, into the Sierras, and northeast to the Rocky Mountains, laying eggs as they go. Monarchs born in the spring and summer move rapidly through their life cycle, flying further north and east with each succeeding generation. As many as 5 generations of Monarchs may continue northward, until the shortening daylight once again reverses the direction of the migration.

For More Information about Monarchs visit the Pacific Grove Museum of Natural History website.


250 Ridge Road
Pacific Grove, CA 93950