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Most insects get swatted, squashed or sprayed without a second thought. But not the monarch butterfly. Despite its kinship to such dreaded pests as gypsy moths and European corn borers, the black and orange butterfly has sailed straight into the hearts of a continent.

Every year, millions of monarch butterflies migrate south to wintering grounds in Mexico

Every year, millions of monarch butterflies migrate south to wintering grounds in Mexico.

Eye in the Sky

Admiration goes deeper than sun-kissed wing markings and a jazzy polka-dot torso. People are inspired by the monarch’s annual migrations of continental proportions. Each fall, monarchs leave milkweed fields all over eastern Canada and the United States to fly south in an unrelenting stream through Texas and into Mexico. By December, millions of monarchs have arrived at remote over-wintering sites in a transverse volcanic mountain range south of the Tropic of Cancer.

Fascinated by this extraordinary phenomenon, tens of thousands of adults and children track monarchs during their fall passage. How, they wonder, does an insect that weighs no more than a penny and has a wingspan no larger than a child’s outspread hand make a journey of up to 2,200 miles (3,500 kilometers) or more?

How, indeed? After hundreds of monarch studies, no one really knows.

Still, to be intrigued by this issue is to be plagued by another: The migration could be in serious trouble. Environmental disruptions throughout the monarch’s vast range have turned the wondrous passage into “an endangered biological phenomenon,” warns Lincoln Brower, research professor of biology at Virginia’s Sweet Briar College and the continent’s foremost authority on monarch migration.

Achilles Heel in Mexico

The immediate threat, Brower says, is fragmentation of the rare forests where eastern monarchs find the particular conditions they need to survive winter. Mexico’s high-altitude Oyamel fir stands offer temperatures warm enough to prevent the butterflies from freezing, yet cold enough to keep their reproductive systems dormant until spring. In addition, the density of the firs, on which the butterflies thickly festoon themselves, protects them from rain and snow while retaining enough humidity to prevent their bodies from drying out. The Oyamels may also act like giant hot-water bottles, keeping the butterflies somewhat warmer than they might be otherwise.

These conditions, extremely rare if not unique, may explain the great effort monarchs make to get to them. Each fall, virtually the entire eastern population retreats to these few mountain tops to wait out winter.

But the safety of the retreats is now in doubt. According to a recently released study, logging has dangerously reduced both the size and density of the Oyamel stands. Aerial photographs of the region 30 years ago show a forest of nearly 2,000 square miles (500,000 hectares). Today, only a tenth of it remains. The largest tract today is 20 square miles (5,800 hectares), five times smaller than the largest tract 15 years ago.

“We’re losing more than 2 percent of the forest every year,” says Brower, one of the authors of the study with colleagues at the National Autonomous University of Mexico and the World Wildlife Fund. “At that rate, by 2050 it will be gone.”

Limiting logging in the over-wintering areas would seem an obvious solution, but so far that has proved easier said than done. Fifteen years ago Mexico decreed a reserve for the monarch of approximately 62 square miles (16,000 hectares) that consisted of no-logging zones at five known over-wintering sites. Local residents have largely ignored the restrictions, saying they are too poor to care about the monarchs and angry they were never compensated for the loss of the land.

With new concerns increasing about the future of the Oyamel forests, on November 9 Mexico tripled the size of the protected area. The enlarged reserve of 216 square miles (56,000 hectares) links known over-wintering sites and expands the no-logging zones around them.

The critical difference, proponents says, is a new trust fund that will compensate approximately 60,000 local people for the loss of their logging rights and help create alternative economies. The fund, established by an anonymous donation of U.S. $5 million that was contingent on expanding the reserve, will be administered by the World Wildlife Fund and the Mexican Fund for Natural Conservation.

“Hopefully this is seed money for the U.S. $30 million that will be needed to do the job,” says Brower. “If the trees are left alone, they grow back pretty fast. In 20 years the forests could be in pretty good shape.”

Troubles Up North

Logging in Mexico is not the monarch’s only problem. Increasing use of herbicides throughout the butterfly’s northern breeding range is an even larger threat in the long run, says Brower.

Herbicides don’t kill butterflies directly, but they annihilate milkweeds along with other plants. Milkweeds are critical to the species because they are the only plants on which monarch caterpillars can feed. All over the United States herbicides are replacing costly mowing as the primary method of controlling weeds along roadsides, power line right-of-ways, and agricultural fields—areas that together comprise much of the monarch’s breeding grounds.

“Herbicide use at that level,” says Brower, “is catastrophic for monarchs.”

Monarchs are also threatened by insecticides and other methods used to control insect pests. Sprays that kill gypsy moth larvae also kill monarch caterpillars. And the caterpillar’s similarity to the European corn borer makes monarchs susceptible to blowing pollen from corn that has been biologically engineered to kill the corn pests. Sprays that kill mosquitoes can also kill monarchs.

There is considerable disagreement about the degree to which any one factor affects the overall monarch population. “Monarchs are not showing a profound tailspin,” says Ann Swengel, co-editor of the July 4th Butterfly Count, an annual survey of breeding butterfly populations, sponsored by the North American Butterfly Association.

Even so, says Brower, the cumulative effect of environmental onslaughts on the butterfly may not become obvious until it’s too late. “The combination of killing monarchs and killing the plants they need,” he says, “is a double whammy.”

Eye in the Sky is a weekly series that brings you the story behind the headlines using satellite imagery, remote sensing, aerial photography, and maps. This feature is developed by National Geographic News with the sponsorship of the National Imagery and Mapping Agency (NIMA) and Earth-Info. Check out maps and imagery at

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More Information
•  Each fall monarch butterflies from east of the Rocky Mountains migrate up 2,200 miles (3,500 kilometers) or more to over-wintering grounds in a volcanic mountain chain south of Mexico City.
•  Like some birds, monarchs take advantage of thermal updrafts and tailwinds to accomplish their long journeys.
•  Monarchs are able to orient on the sun and respond to the earth’s magnetic fields, but no one knows exactly how they find their way each fall to rare Oyamel fir forests in the Michoacán highlands south of the Tropic of Cancer.
•  In a normal year about 250 million monarch butterflies festoon the firs, waiting out winter in a state of reproductive dormancy.
•  Spring reawakens the butterflies’ reproductive organs and they mate. Females head north again to lay their eggs and begin the cycle anew.

More Information
Passage of the Heart

Few events in the natural world excite people more than the fall migration of the monarch butterfly. Between August and November, tens of thousands of adults and children herald the butterflies as they stream through gardens and parks on a grand passage from northern breeding grounds east of the Rocky Mountains to over-wintering sites south of Mexico City.

The largest organized monitoring effort is Monarch Watch, a program launched in 1991 as a research project by University of Kansas. It has since blossomed into multi-faceted educational outreach program involving people all over Canada, the United States and Mexico.

“At least 100,000 children and adults participate in Monarch Watch in some capacity each year,” says Orley R.“Chip” Taylor, a professor of entomology at University of Kansas who directs the program.

Many participants serve as volunteer taggers, who gently net migrating monarchs, affix tiny numbered tags to their wings, then release the butterflies to continue their journeys. Recaptured taggees help researchers answer questions about the routes monarchs take to Mexico and their mortality rates along the way. During the fall 1999 migration season, Monarch Watch volunteers tagged 64,000 to 74,000 butterflies, says Taylor. About 700 of those were recaptured in Mexico.

Monarch Watch volunteers also track the fall migration through the program’s Internet discussion group. Their messages mention how many monarchs they have seen, when and where. Sometimes the message is, “Help! Anyone have extra tags?”