The areas occupied by monarch butterflies during the 2012-2013 season at the reserve in the western Mexican state of Michoacán have been reduced by 59 percent, the National Protected Natural Areas Commission, or Conanp, said.
An inspection by Conanp officials and representatives of the WWF-Telcel alliance "found that the forest surface occupied by the butterfly colonies in December 2012 was the lowest in two decades," the environmental agency said.
Nine hibernating colonies were found in the second half of December, covering 1.19 hectares (2.9 acres), the agency said in a report.
"This figure represents a decrease of 59 percent with respect to the 2.89 (hectares) occupied in December 2011," the environmental agency said.
"Extreme climate fluctuations in the spring and summer in the United States and Canada affect the survival and reproductive ability of the adults," WWF Mexico director Omar Vidal said.
The World Wildlife Fund and Telcel, the Mexican unit of wireless giant America Movil, teamed up in 2003 to promote the protection of Mexico's wilderness areas and protect 17 species, including the Monarch butterfly, jaguar, whales, dolphins, sharks and turtles.