Almost everyone knows the monarch butterfly (Danaus plexippus). The adults are large with wings colored a deep orange with black borders containing white spots; the caterpillars are pale green and ringed with black and yellow. It is the one butterfly that almost everybody recognizes by sight, and many people have watched it develop from egg to adult in school science classes. Given ample milkweed to feed on, the caterpillars develop from egg to pupa in about three weeks and emerge as adults about ten days after that.

In the summer, monarchs occur throughout the United States and southern Canada. Adults from the wide-ranging population east of the Rocky Mountains spend the winter in fir forests in a small part of central Mexico; in early spring they migrate to the southern states to lay their eggs on milkweeds and produce a second generation. This second generation migrates northward, following the green-up of the milkweeds, and two or three other generations may be produced that move farther and farther north before the autumn return migration to Mexico. Thus, unlike birds, the monarch's annual migration cycle involves several generations rather than the same individual.

West of the Sierra Nevada the population winters in groves of eucalyptus or Monterey pines along the California coast and has a similar multi-generation cycle that reaches as far north as southern Canada. However, the population in the intermountain west, including Nevada, is something of an enigma. It was once thought that these monarchs are all part of the population that overwinters in California and spills over the Sierra Nevada and Cascades. However, naturalist Robert Pyle followed migrating monarchs from Washington state to northern Arizona and concluded that a few of the butterflies were headed toward Mexico instead of California. Does this mean that some of Nevada's monarchs are part of the eastern population that overwinters in central Mexico? Or is there yet another, presently undiscovered, wintering area in the southwestern United States or northern Mexico from which part of our monarch population comes every spring?

Because the monarch is in trouble, this question may never be answered. Farming practices that eliminate milkweeds from field edges, broadcast spraying of pesticides, planting of genetically modified crops that contain a bacterial gene for synthesizing a compound specifically toxic to moths and butterflies (to combat moths that are crop pests), the onset of climate change, elimination of nectar sources needed by migrating individuals, and the destruction of overwintering areas all are threats to monarchs. While the species as a whole is unlikely to become extinct (there are non-migratory populations in Florida and elsewhere in the world), the multi-generational migration is now an endangered phenomenon. The monarch's life history strategy requires huge numbers of individuals to guarantee that enough will survive to produce the next generation of migrants; one or two bad years on the overwintering or breeding grounds easily could drive the migratory populations below some threshold number that results in rapid extirpation.

Once one of the most common butterflies in towns and agricultural areas in Nevada, monarchs are now getting very rare here. University of Nevada Reno biologist Dennis Murphy, a butterfly specialist, saw none during the summers of 2005 and 2006 and just two in 2007 even though he did extensive field work throughout Nevada. Other biologists report similar observations. Whether this represents a general decline or is just a low point in a population cycle is unknown. However, the loss of the monarch from the state would mean that Nevadans of the future will never experience a beautiful and once common species that delighted and informed past generations.