Monarch Migration...

with clarifications by Steve Malcolm

The monarch butterfly (Danaus plexippus) is one of the most well-known butterfly species in North America. They are fascinating not only for their beauty, but also because they are one of the few migratory butterfly species of the United States. Unfortunately, due to extrinsic factors, monarch migrations are being threatened.

There are two U.S. populations of monarchs: the western population, which exists west of the Rockies along the Pacific coast from Marin county to Baja California, and the eastern population which stretches from Southern Canada south to Mexico. The western population breeds in the Rockies and migrates to the coast to overwinter in blue gum trees, an exotic species from Australia. Unfortunately, these trees are not being protected from development. Therefore, monarch populations of the west may begin to be seriously threatened.

The eastern population of monarchs undergoes an incredible migration. Their critical overwintering site is in Mexico, where they roost in high altitude forests of Abies religiousa, a type of fir tree. The entire Mexican population exists only at ten different sites which are each about 5 hectares and all together only make up about 800 square kilometers (18 x 18 miles). Each site contains approximately 10,000,000 dormant monarchs. These butterflies start to arrive along the Gulf coast of the USA at the end of March and through April when they mate, lay eggs on southern milkweeds, and die. Their offspring continue the migrational pattern by flying further north to repeat the same process, arriving in the Great Lakes states in early June. These butterflies then lay eggs in the north and initiate three generations feeding on the abundant milkweed Asclepias syriaca (the common milkweed). It is this accumulation of butterflies in the northern USA and southern Canada that then migrates south in the Autumn, flying all the way back to Mexico through October, where they will overwinter and reproduce the young that will begin another migrational journey north the following spring. All together, there are 3-4 generations of monarch butterfly each year.

A very small number of monarchs overwinter in pine trees at a few sites along the Gulf coast of Florida, near Tampa and possibly west of Tallahassee.

Several factors threaten the overwintering sites in Mexico. Many forests are being destroyed by loggers, an industry that threatens so many species worldwide. Also, due to the ever-increasing human population, expansion of villages into the overwintering sites is creating problems. Pesticides pose yet another threat to the monarch population. Ecotourism, which sometimes can create money to support species' conservation, is negatively affecting the monarch population because human presence is disruptive to the monarch's fragile ecosystem. Another problem is that several lepidopteran species act as "pests" and invade the roost trees of the monarch. Due to the number of threats to the monarch populations, only time will tell if yet another species will be destroyed by the unfortunate practices of an uncaring, exponentially-growing human population.

Stephen B. Malcolm, Department of Biological Sciences, Western Michigan University, Kalamazoo, MI 49008, USA


S.B. Malcolm and M.P. Zalucki (editors), Biology and Converation of the Monarch Butterfly. Natural History Museum of Los Angeles County, Science Series 38, 425 pp.

Primack, R. B. 1993. Essentials of Conservation Biology. Sinauer Associates, Sunderland, MA.

Sayre, J. K. personal conversation: May 1996.

Schneck, M. 1990. Butterflies: How to Identify and Attract Them to Your Garden. Rodale Press, Emmaus, PA.

Tiebout, H. T. III. personal conversation: January 1995.West Chester University, West Chester, PA.

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