By Neal Lineback and Mandy Lineback Gritzner, Geography in the NewsTM and Maps.com
DECLINING MONARCHS THIS YEAR
Near the end of February each year, scientists studying monarch butterflies at their overwintering sites in Central Mexico witness signs that the butterfly colonies were “breaking up.” This separation of tens of thousands of butterflies clustered together on single trees indicate that the populations are preparing for their lengthy spring migration from Mexico to the United States and Southern Canada. This year’s colony numbers were depressed by 59 percent and scientists are worried.
Probably the best known of all North American butterflies, the monarch (Danaus plexippus) is a milkweed butterfly, laying its eggs on milkweed plants on which their larvae (caterpillars) exclusively feed.
The monarch butterfly has a wingspan of about 3.5 to 4.5 inches (8.9-11.4 cm) with a highly distinguishable color pattern. The wings are bright orange with black veins. American entomologist Samuel H. Scudder first published the common name for the butterfly in 1874. He chose monarch because “it is one of the largest of our butterflies and rules a vast domain.” Some scholars believe, however, that the name may be in honor of King William III of England.
Monarchs are also found in southern South America, Europe, Russia, Indonesia, Australia and New Zealand. They are migratory in Russia, the Azores, Sweden, Spain and Portugal, but have by far the longest routes in North America.
The female monarch lays its eggs on a milkweed plant during the spring and summer breeding times. Four days later caterpillars emerge, first eating their own egg covers. They then eat the poisonous milkweed leaves, acquiring a toxin that can later repel predators. The caterpillar stage lasts about two weeks. Since they eat milkweed plants that inhabit some farmland, they are considered beneficial.
In the pupa or chrysalis stage, the caterpillar spins a silk pad on a twig or leaf and hangs upside down from this pad by its last pair of prolegs. A change of form or metamorphosis occurs during the next two weeks finally resulting in the monarch butterfly.
Annual migration of monarchs is truly amazing and unique among butterflies, making a two-way trip like some birds. In North America, massive southward migrations of an estimated 300 million begin in August and last until the first frost. In the spring only about half that number returns north due to losses from high winds, storms, snow, fatigue, starvation, bird predation and human interference.
Two different populations of monarch butterflies inhabit the United States and Canada. When the fall migration commences, the monarch population east of the Rocky Mountains travels up to 3,100 miles (4,960 km), averaging of 12 miles (19 km) per hour and 50 miles (81 km) per day. Their destinations are specific areas in the Transvolcanic Range of the Sierra Madre in Central Mexico.
The butterflies overwinter in the same 12 mountain spots every year at elevations of 8600 to 10,200 feet (2600 to 300 m). Millions concentrate there, hibernating in large clusters in the oyamel fir trees. These sites were first discovered only in 1976 and seven of the sites are now protected as important bioreserves.
The monarch population west of the Rockies spends winters at perhaps 300 minor sites in central coastal and southern California, from Monterey south. These monarchs hibernate in eucalyptus trees.
The lifespans of monarchs vary depending on the time of year. If born in the summer, a monarch only lives 2-5 weeks, during which it lays eggs for the next generation. The last generation of the year, however, enters into a non-reproductive phase called diapause before migrating south and it can live up to nine months.
In February and March, toward the end of their wintering over, these butterflies become reproductive and begin their northward migration, laying eggs on milkweeds as they go. Though only one generation migrates south, it takes 3-4 generations each year to return north by summer.
Scientists are trying to determine exactly how monarch butterflies manage to return to the same overwintering spots over a gap of several generations. It appears the butterflies inherit the flight patterns, based on a combination of circadian rhythm, the magnetic pull of the earth and the position of the sun in the sky. Whatever the exact causes, the journey of the monarch butterfly is an incredible geographic and biologic story.
The wintering grounds in Mexico this year had perhaps 59 percent fewer monarchs than last year, likely related to a disturbing decline in habitat for the butterflies. One particularly disturbing conclusion is the effect of the decline in milkweed along the migration paths owing to the use of herbicides and perhaps some genetically engineered corn in U.S. agriculture. Milkweed is particularly susceptible to pre-emergent and defoliant herbicides.
And that is Geography in the News.
Co-authors are Neal Lineback, Appalachian State University Professor Emeritus of Geography, and Geographer Mandy Lineback Gritzner. Dr. Daniel Stillwell of Puerto Vallarta, Mexico, provided valuable research assistance. University News Director Jane Nicholson serves as technical editor.
Sources: GITN 983 Monarch Butterflies Begin Journey North, Apr. 3, 2004; http://www.fs.fed.us/monarchbutterfly/migration/index.shtml; http://www.learner.org/jnorth/monarch/; http://whyfiles.org/006migration/; Urquhart, Fred A. The Monarch Butterfly, International Traveler (University of Toronto Press, 1987); and http://www.news.cornell.edu/releases/May99/Butterflies.bpf.html
This is an abbreviated Geography in the News article revised for David Braun’s National Geographic NewsWatch blog. Nearly 900 of the 1200, full-length weekly Geography in the News articles (with Spanish translations) are available in the K-12 online education resource Maps101.com, including maps and other supporting materials and critical thinking questions.