An alarming plunge in the number of monarch butterflies migrating from the US and Canada to Mexican winter colonies has experts worried. Although masses of sleeping butterflies still hang like clumps of dead leaves from branches in the El Rosario sanctuary in central Mexico, biologists say their population this year is the smallest ever and down three-quarters from 2004.
At El Rosario, the butterflies are threatened because of deforestation: logging has left the lower slopes bereft of their tree cover, forcing the creatures to cluster high above, thereby making them more vulnerable to the cold.
Many butterflies have also died of the unusually harsh weather en route; some have been gobbled up by birds in Mexican forests, as is evident from the hundreds of torn-off wings scattered on the forest floor. At home, in Canada and the US, pesticides sprayed on the milkweed plants (on which the butterflies lay their eggs) are killing their larvae.