Latin America Correspondent
Between 20 and 30 million of the world's most beautiful butterflies, of the North American Monarch species, may have been wiped out by snow and freezing weather this week in the Mexican mountain forest where they spend the winter.
Mexican ecologists who visited the Oyamel fir forest in the state of Michoacan reported that at least a third, possibly half, of the estimated 60 million hibernating Monarchs had frozen to death or been buried under a foot of snow.
The picturesque orange-and-black butterflies are renowned for their 3,000- mile odyssey from the northern United States and Canada to the same Mexican forest every winter, taking up to three months at 30 miles a day and an altitude of one mile.
"The situation is devastating," said Homero Aridjis, a Mexican poet and leader of the "Group of 100" lobby of environmentalists. "Two of our biologists visited the butterflies' winter reserves and estimated that at least one- third had been wiped out, blown from their perches on the fir trees and buried in the snow. It's possible up to half of them have died."
Despite the catastrophe, the Monarchs, known to science as Danaus plexippus, are not endangered as a species by their latest losses. They are more threatened by increased logging of their reserves, which is removing their protection from the elements, Mr Aridjis said. They will probably restore their numbers quickly if breeding conditions are good on their way back to the north, he added.
An unusually bad snowstorm which occurred in 1992 wiped out at least 70 per cent of the hibernating Monarchs in Michoacan state, but the butterflies are already close to their previous levels.
After flying down in October from as far north as Ontario, the Monarchs settle beneath the normal snow line over several square miles of forest west of Mexico City on mountains that reach more than 10,000ft.
Visitors who trek to see them in normal weather are stunned by a solid landscape of orange and black as the butterflies cover the ground and trees. If disturbed, they almost black out the sky and sound like a heavy drizzle as they flutter in their millions.
It is impossible to visit the area without trampling hundreds, if not thousands of Monarchs underfoot, since they also cover all pathways, a dilemma which has led Mr Aridjis's group to call on the Mexican government to restrict access to the reserves.
Picnickers who play loud music and leave litter also endanger the butterflies, but their biggest problem of recent years has been the gradual deforestation of the area by loggers.
The local Oyamel fir trees usually act as an umbrella for the butterflies against heavy rains and rarer snow storms, such as the one last weekend.
On their flight from the north-eastern US and Canada the butterflies stick to the US east coast and invariably make a "pit stop" around St Marks in Florida to gorge themselves on milkweed, which will give them enough body fat to survive the winter.
While milkweed's poisonous sap is avoided by most other creatures, it is absorbed by the Monarchs and gives them a natural defence against predators.
Monarchs often live up to 10 months, mostly surviving the epic journey south but dying while laying their eggs along the Gulf coast of the US - in Louisiana, Mississippi, Alabama or Florida - on their more westerly return trip.