For the monarch butterflies, life is complicated enough even in a good year. This fall, though, they had to deal with Texas.
The monarchs in recent weeks have been beating their way south and west across eastern North America, riding winds a thousand feet above the ground, covering 25 miles or more every day. Early in October they reached a vast area in Texas stricken by drought and charred by wildfires.
The first butterflies have been seen crossing into northern Mexico, still heading south. They come from as far north as Winnipeg 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 make it through the gantlet of Texas they cross the Rio Grande and eventually converge on a few acres of forest in mountains about 60 miles west of Mexico City. There they’ll roost over the winter, thick as quilts on 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 animal, but whatever compels it to migrate to Mexico 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.
Experts expect to see the smallest overwintering population ever recorded. From 1994 to 2003 the monarchs covered an average of 23 acres of forest, but since then the average has dropped to fewer than 11 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,” said Lincoln Brower, a professor of biology at Sweet Briar College who has been studying the monarchs for decades. But this year may 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 (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.
A critical problem for the butterflies is the adoption by farmers of herbicide-tolerant corn and soybeans, Mr. Brower said. 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,” Mr. Brower said.
The 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.
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 at first. The caterpillars molt a number of times, growing dramatically, then enter a pupa stage, turning into a chrysalis. 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, October, the creatures are motoring toward Mexico as if turbo-charged.
They are guided by navigational clues — celestial, magnetic — that scientists haven’t yet 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 scientific mystery.
The butterfly isn’t endangered, but this amazing migratory pattern could be, Mr. Brower said: “The migratory biology of the monarch is a phenomenon. It’s an endangered biological phenomenon.”