Monarch butterfly population surges in Mexico

The monarch butterfly population has soared in its Mexican winter sanctuary this season, marking a recovery for the threatened species that migrates across North America, officials said Friday.

The orange and black butterfly covered 4.01 hectares (9.9 acres) of forest in the 2015-2016 season, more than tripling last year's figure of 1.13 hectares (2.8 acres), according to the Mexican government and the World Wildlife Fund.

While researchers measure the population by the area it covers, it estimates that there were 140 million butterflies this year in the mountains of central Mexico.

"The area occupied by the monarchs in the Mexican sanctuaries has increased in the last two seasons, which suggests the start of a recovery of this butterfly," said Omar Vidal, WWF's Mexico office director.

The rebound comes after the population hit an all-time low of 0.67 hectares in 2013-2014.

Alejandro del Mazo, the head of Mexico's office for protected areas, gave credit to the joint actions taken by the Mexican, US and Canadian governments to reverse the decline.

The goal, which follows a mandate given at a 2014 North American summit, is to increase the area to six hectares by 2020. This compares to a high of nearly 19 hectares in 1996-1997.

The decline has been blamed on illegal logging in their Mexican wintering grounds and the drop in milkweed on which they feed due to the use of pesticides in the United States and Canada.

The butterflies travel some 4,000 kilometers (2,500 miles) from Canada to spend the winter in a mountain reserve straddling the states of Mexico and Michoacan.

They usually arrive at their nesting ground between late October and early November and head back north in March.