Cold snap wipes out millions of monarchs

The deaths of unusually high numbers of monarch butterflies in their winter nesting grounds has deepened the mystery surrounding how many of them exist. Researchers have suggested the death toll is twice as high as the previous estimate of their entire population.

Lincoln Brower, a zoologist and monarch expert, said: "This data is telling us we have to go back to square one" in estimating how many monarchs make the 3,000-mile migration from North America to Mexico.

He estimated that 150 to 250 million butterflies may have frozen to death this winter, the worst on record and up to twice the 110 million believed to have wintered in Mexico. Roberto Solis, the manager of the government butterfly reserve, estimated the death toll to be nearer 30 million, which is still well above the usual mortality rate of 10 to 15 per cent.

Mr Brower, who saw piles of dead monarchs a foot deep, said the deaths in the states of Mexico and Michoacan had resulted from freezing weather. Deforestation might have contributed, he said, by reducing the forest canopy, which regulates temperature.

But he said the monarchs were also threatened in the United States, where herbicides killed the plants the monarchs used for food. (AP)