The monarch butterflies (Danaus plexippus) of North America are renowned for their long-distance seasonal migration and their spectacular winter gatherings in Mexico and California. Monarchs face many threats, and population monitoring at overwintering sites has documented significant declines in the number of monarchs returning to those sites.
In 2008, the Commission for Environmental Cooperation (CEC), a tri-national organization covering the United States, Canada, and Mexico established by the North American Free Trade Agreement, published the North American Monarch Conservation Plan. The Plan identifies several factors that have contributed to the steady decline of monarchs across their native range:
• loss of overwintering sites in Mexico due to deforestation;
• degradation of overwintering habitat in Mexico due to forest fires, diversion of water for human use, and poorly-regulated tourist activity;
• loss of overwintering sites in California due to development;
• degradation of overwintering habitat in California due to aging trees;
• loss of breeding habitat due to the ongoing decline of native milkweeds (Asclepias spp.), their larval host plants; and
• disease, parasitism, and predation.
Because of their migratory lifecycle, effective conservation strategies for monarch butterflies must protect and restore habitat across their entire range. The Xerces Society, with support from the Monarch Joint Venture and the USDA Natural Resources Conservation Service, has initiated a monarch protection campaign that focuses on two major issues: conservation of overwintering sites in California and the restoration of breeding habitat in key regions of the United States.
Read about how Xerces is working to protect and manage monarch overwintering sites in California.
Read about how Xerces is working to restore monarch breeding habitat by increasing the availability of native milkweed seed.
Neither the United States nor Canada has protected any habitat along the major migration corridors for monarchs. Mexico has protected only tiny fragments of the habitat to which the monarchs migrate. Because so little protection is afforded these butterflies, the IUCN recognizes the annual migration of the monarch in North America as an endangered biological phenomenon, although this designation does not afford the species any actual protection.
In the western U.S., overwintering populations of monarchs along the California coast have declined from over 1 million individuals counted at 101 sites in 1997 to less than 60,000 individuals counted at 74 sites in 2009. Most scientists believe that this decline is due to the loss of milkweed from a prolonged California drought and the extensive use of pesticides.
western monarch migration
Two main populations of monarchs exist in North America: one population is east of the Rocky Mountains, and the other is west of the Rocky Mountains. There has been recent evidence that suggests that these populations may intermix. The western population of monarchs makes a migration similar to its eastern counterpart, but overwinters instead at more than 200 coastal sites along the California coast, from north of San Francisco south to the Mexican border. Tens of thousands of individuals may be present at an individual monarch grove.
The monarchs that migrate away from these overwintering sites in the spring move up into California’s Central Valley and Sierra Nevada foothills, and north to Oregon and Washington. By the end of the summer, after three generations have passed, some offspring can reach as far north as British Columbia. Although only one to two million monarchs migrate back to California’s coastal groves in the fall (as opposed to 100 million to Mexico), the western monarchs are still important.
eastern monarch migration
The monarch butterfly migration east of the Rocky Mountains is one of the world’s magnificent natural events. Adult monarchs that emerge from their chrysalis in early fall will fly from the United States and Canada to overwintering grounds in Mexico, some traveling as far as three thousand miles. These are the only butterflies in the world to make such a long, two-way migration each year.
In the fall, monarch butterflies from east of the Rocky Mountains return to high-altitude oyamel fir forests in central Mexico. It is here that they spend the winter in dense aggregations. These monarchs begin migrating north in March and early April to the Gulf Coast of the southeastern United States, where the females lay their eggs on milkweed plants. One to two generations of monarchs are produced in the Gulf Coast States in the spring. These offspring of the wintering generation continue northward without their “mothers”, where they recolonize the northern breeding range. Because southern milkweed plants die in June, the migration must continue north in order to utilize the milkweed resources of central and northeastern North America. This allows monarchs to produce up to three additional summer generations. The cycle begins again in the fall when this new generation of monarchs heads south.
The monarch, indigenous to the New World, occurs throughout North and South America. It also has established breeding populations in Australia, a number of Atlantic and Pacific Islands including Hawaii and Bermuda, and the eastern coast of Spain.
Monarchs lay their eggs on milkweed plants (genus Asclepias). The caterpillars that hatch from the eggs feed on the milkweed leaves, incorporating the plant’s toxic compounds (cardiac glycosides) into their bodies. As the caterpillars grow, they become more and more distasteful to potential predators. They usually pupate near the plants on which they feed. When the monarchs emerge as adults, they either continue migrating to the north and east or turn around and start the journey back to their overwintering grounds, depending upon the time of year.
Xerces Society Conservation and Management Guidelines (1993)
North American Monarch Conservation PlanWestern Monarchs at Risk: The Plight of Monarch Butterflies Along the West Coast
Protecting Monarch Groves
California Pollinator Plants: Native Milkweeds
The Legal Status of Monarchs in California
Xerces Society Policy on Butterfly Releases
SE Monarchs, Milkweeds, & Hostplants Brochure