For the next month, more than a million monarch butterflies will fly from inland California to roost at winter nesting grounds at hundreds of sites from Santa Cruz to Baja California.
East of the Rocky Mountains, an estimated 300 million more monarchs are thought to be passing through Texas in a trek from Canada and the U.S. Midwest to wintering sites in Mexico.
Both migrations, says a conservationist from the Sacramento office of the U.S. Fish and Wildlife Service, are "on par with the migration of the wildebeests of the Serengeti."
As the monarchs complete their odyssey, a continent-wide debate has erupted afresh over how to protect one of North America's most awe-inspiring insects.
In the most intense of three separate arguments, the monarch has become the symbol of the movement against biotechnology. The butterflies are threatened, biotech's critics say, by corn genetically engineered to emit the toxic bacterium Bacillus thuringiensis (Bt).
Intended as a control for the European corn borer, Bt corn seeds of various toxicities were introduced during the last five years and now account for one-quarter to one-third of U.S. corn production. This summer, Bt corn covered an estimated 22 million acres concentrated down the center of the Corn Belt in the Dakotas, Nebraska, Minnesota, Iowa, Kansas and Oklahoma.
Alarmed conservationists point out that milkweed growing in that territory is where an estimated half of North America's monarchs breed during summer months.
Corn pollen is wind-borne. In 1998, scientists began to question whether pollen from Bt corn was being deposited on those milkweed plants and, if so, whether it then was harming developing monarch larvae.
In May 1999, researchers from Cornell University reported in the journal Nature that monarch larvae feeding on Bt corn pollen had a 44% mortality rate and that survivors were stunted. Subsequent studies at the University of Iowa recently underscored the threat.
Six months after the Cornell report, it was disclosed at a Chicago conference that the U.S. Environmental Protection Agency had not conducted tests on the effect of Bt pollen on "non-target" butterfly larvae before the government licensed the corn.
But last month, in a preliminary risk-assessment report, the EPA decreed that the threat of Bt corn to monarchs "was not sufficient to cause undue concern."
Though lobbyists for the biotech and corn industries celebrated the report, the reaction among America's leading monarch researchers was cool.
"What we need is to understand if monarch breeding and pollen shedding of the Bt corn is co-occuring," said Orley Taylor Jr., head of the Division of Biological Sciences at the University of Kansas and founder of the monitoring group Monarch Watch.
A newly completed field survey may soon show that the two events not only occur simultaneously in the Corn Belt, but in closer proximity than even the conservationists had imagined.
In a 10-scientist, four-university study led by Michelle Prysby and Karen Oberhauser of the ecology department of the University of Minnesota, researchers monitored 20 cornfields this summer.
They also monitored nonagricultural sites within a kilometer of the fields.
Against the researchers' own expectations, they found just as many monarchs in milkweed growing within the cornfields as in milkweed in the nonagricultural sites. They also found that, contrary to EPA assumptions, monarchs used the fields at all stages of corn growth.
"Monarchs use the fields when corn is short, and when corn is tall," Oberhauser said. This increases the likelihood that the butterflies would be breeding in fields when the corn was shedding the toxic pollen. That dangerous overlap, Oberhauser said, seems to be greater, the further north you get.
After protests following the 1999 Cornell findings, use of Bt corn fell from about a third of the national crop to a quarter this year. But a spokesman for the National Assn. of Corn Growers, Stewart Reeve, said the two things were not necessarily connected. Farmers simply used less Bt corn, he said, because the previous year's planting had so successfully eradicated pests.
"Farmers need to have access to biotech crops and to selectively use them where it makes the most sense for their profitability," he said. As for abandoning the product in the event that it is shown conclusively to be harmful to monarchs, Reeve said, "Obviously, if the regulatory system comes through and says it is no longer approved, we would have no other choice in the matter."
Oberhauser will present her findings to the EPA, conservationists, biotech firms and farmers today in Washington. As they meet, the return of the butterflies to Mexico highlights a second dilemma: the whittling away of their wintering grounds in the fir forests of the Transvolcanic mountains of central Mexico.
More than half of the forests sheltering the Monarchs have been lost to illegal logging since the Mexican sites were discovered in 1975.
Both Oberhauser and Monarch Watch's Taylor are working with a 3-year-old charity, the Monarch Sanctuary Foundation, to cooperate with the Mexican government in preservation efforts.
To Oberhauser, Americans must resist pointing fingers and both the Mexican and U.S. governments must examine their own policies. "It's so much more clear cut in Mexico," she said. "You can go see where there used to be trees where there are fields, and you can see the logging tracks. It's less tangible in the U.S.: a few more cornfields, a few more housing tracts, a few wetlands being drained. But it's just as important."
Meanwhile, west of the Rockies, western monarchs, which take several generations to complete their annual migration from the Continental Divide to the coast and back, face a third distinct set of threats.
Researchers Kingston Leong and Dennis Frey of Cal Poly San Luis Obispo say trends as diverse as the felling of eucalyptus trees, the development of the state's mountainous coastal enclaves and the overzealous weeding along highways and railways conspire against the butterflies.
But ecologist Stephen Malcolm from Western Michigan University said that in his visits to California, he has seen admirable flexibility in accommodating the monarchs. "We were visiting landowners who were prepared to sign over easements to conserve over-winter Monarch habitats on their land," he said. "I think that a possible solution would be increased awareness and getting people involved in these projects with their own properties."
Frey and Leong agree, encouraging Californians to also plant milkweed to support monarch larvae in summer months.
Meanwhile, Californians have been welcoming the return of monarchs to the coast in style. Last year, Pacific Grove attracted 60,000 butterflies. Two weeks ago, the townspeople hosted a parade, replete with children dressed in extravagant butterfly costumes. " Said organizer Moe Ammar, "We take our monarchs seriously here."
More on Monarchs
* For planting tips: Butterfly Gardening in Southern California, by Brian V. Brown and Julian P. Donahue; $4 from the Natural History Museum of Los Angeles County, 900 Exposition Blvd., Los Angeles, CA 90007. (213) 763-DINO. Donahue was curator of moths and butterflies at the museum for 23 years.
* For information about protection of Mexican wintering sites, contact the Monarch Butterfly Sanctuary Foundation online at www.mbsf.org or write to 2078 Skillman Ave. West, Roseville, MN 55113.
* For information on school projects, conservation and research, contact Monarch Watch c/o Orley Taylor, Dept. of Entomology, Haworth Hall, University of Kansas, Lawrence, KS 66045. (888) TAGGING (toll-free) or (785) 864-4441 or online at www.MonarchWatch.org.
* On Dec. 9, the Natural History Museum of Los Angeles County will offer a monarch tagging field trip to Santa Barbara. It will be suitable for schoolchildren in grades five and older. Members: $20, nonmembers: $25. Contact Mary Baerg, Youth and Family Programs Coordinator, (213) 763-3535 and e-mail: firstname.lastname@example.org or go to http://www.nhm.org.
Emily Green can be reached at email@example.com