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.
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