MEXICO CITY — The government's environmental watchdog said Monday that millions of monarch butterflies, which migrate south each winter from Canada, had died from the cold weather rather than deliberate poisoning by loggers, as some environmentalists had alleged.
Last week, Homero Aridjis, head of the environmental lobby Group of 100, said loggers were believed to have sprayed pesticides on the orange and black butterflies to try to regain about 216 square miles of forest nesting ground protected by the government.
Aridjis said the butterflies had been found with a strange luster on their wings and that there was a smell of pesticide and gasoline in two sanctuaries in the protected reserve.
But the Mexican environmental watchdog Profepa announced that a scientific analysis of 300 butterfly corpses from the Cerro San Andres sanctuary in central Michoacan state showed no traces of toxic substances from pesticides.
It concluded that the butterflies had died from the cold.
Millions of monarchs migrate about 3,000 miles each year to escape the icy winter in Canada and the United States. They tend to settle in the warmer fir forests of Michoacan, about 70 miles west of Mexico City.
The butterflies normally arrive in early November and return north at the end of March.