The butterflies are on their way to Mexico. They come from as far north as Winnipeg, Manitoba, and as far east as the islands of Maine. Many take a well-flapped route down the Eastern Seaboard before veering across the Gulf Coast. If they can make it through the gantlet of Texas, they will cross the Rio Grande and converge on a few acres of forest in mountains about 60 miles west of Mexico City. There, they spend the winter roosting, thick as quilts, on the branches of oyamel fir trees. In spring, they’ll head back north.
But it’s not clear how many will make it this year to their Mexican retreat, or what kind of condition they’ll be in when they get there.
They need water. They need flowers. They need nectar. The monarch butterfly is a hardy and vigorous insect, but whatever compels it to migrate south does not tolerate much flexibility in the itinerary. Going through Texas on the way to Mexico is what they’re hard-wired to do. And Texas is scorched.
“They’re going to be encountering a thousand miles of hell as they go through a nearly waterless, flowerless, nectarless landscape,” said Chip Taylor, an insect ecologist at the University of Kansas and the director of the nonprofit organization Monarch Watch.
The migrating monarchs must overcome a host of challenges to their way of life, and their numbers have dropped in recent years. A critical problem is the widespread adoption throughout North America of herbicide-tolerant corn and soybeans, said Lincoln Brower, a professor of biology at Sweet Brier College who has been studying monarchs for decades.
These genetically modified crops enable farmers to spray herbicides on their fields and wipe out weeds without hurting the corn or soybeans. But the milkweeds that are eradicated are crucial to the life cycle of the butterflies. Scorned by farmers, milkweeds are a diverse genus of plants, with more than 120 species identified, that co-evolved over the millennia with the butterflies.
“We’re talking about hundreds of thousands of acres of land that are virtually sterilized except for human food crops,” Brower said.
Conservation groups have encouraged people to plant milkweeds to help the monarchs. And in Mexico, where illegal logging has damaged some of the butterfly’s winter habitat, the Washington-based conservation organization American Forests has teamed with other groups to plant more than 900,000 trees in the past five years, said American Forests spokeswoman Lea Sloan.
But nothing is constant in any ecosystem. Climate change is driving wetter winters that leave soggy butterflies exposed to cold snaps, Taylor said. The butterfly population that migrated north this year was smaller than normal, he said, as is the number heading south. As they fly through southern Kansas, Oklahoma, Texas and into northern Mexico, they are encountering areas hit by what the U.S. government rates as “exceptional drought.”
They’ll improvise. They’ll visit gardens and the flowers along drainage ditches. Texas residents typically see thousands of monarchs in clusters. The Journey North Web site reports that in Fort Stockton, Tex., trees are “dripping with monarchs.” From Midland, Tex., a resident reports, “Every flower patch has hordes hovering.”
This year’s migration through Texas seems to have shifted west by up to 300 miles, Taylor said, and many monarchs are heading toward a border crossing near Big Bend National Park. Beyond is a baked desert.
Taylor predicted a record-low number of monarchs roosting in Mexico. From 1994 to 2003, the butterflies covered an average of 23 acres of forest, but since then the average has dropped to less than 11 acres. The lowest number came in the winter of 2009-10, when the insects covered less than 5 acres.
The butterflies typically arrive fat and happy, having gorged on nectar for thousands of miles. If they arrive thin and bedraggled, they could be more vulnerable to winter storms and below-freezing temperatures.
“By the time they get to Mexico, they’re butterballs. They use that fat to get them through the winter and back to Texas,” Brower said. But this year might be different, he said. “We’re really concerned about how much energy the butterflies have to sustain them through the course of the winter.”
The monarch butterfly — Danaus plexippus — has been making this trek in eastern North America for thousands of years, at least since the North American ice sheets retreated at the end of the Pleistocene era (a separate population west of the Rockies migrates to coastal California). Only in recent years has the migratory adventure of the monarch been carefully studied and mapped. The overwintering site in Mexico was not discovered by researchers until 1975.
Monarch butterflies have a life cycle that beggars belief. The butterflies that roost in Mexico fly north in the spring, mate in Texas or thereabouts, lay eggs on the leaves of milkweeds and die by the end of April. The larvae that emerge from the eggs are tiny. The caterpillars molt a number of times, growing dramatically, then enter a pupa stage. Inside the shell of the chrysalis, the butterfly forms. It emerges, lingers for a few days and then starts flying — north, in many cases, with butterflies following the milkweeds up to the Great Lakes and far into Canada.
Depending on the latitude, these butterflies can spawn two or three generations. Come early August, emerging butterflies will begin the great migration south. The waves from Canada will overlap with butterflies emerging farther south. The speed of the migration picks up steadily, and by this time of the year the creatures are motoring toward Mexico as if turbo-charged.
They are guided by navigational clues — celestial, magnetic — that scientists haven’t decoded. No single butterfly makes the entire round-trip journey. How a butterfly finds the same set of mountains in Mexico visited by a grandparent or great-grandparent is a mystery.
The butterfly isn’t endangered, but this migratory pattern could be, Brower said.
“The migratory biology of the monarch is a phenomenon. It’s an endangered biological phenomenon,” Brower said.