Activists from Mexico, the United States and Canada are asking the U.N. World Heritage Committee to include the Monarch butterfly wintering reserve on a list of sites considered in danger.

UNESCO designated the 139,000-acre (56,259 hectare) reserve in the mountains west of Mexico City a World Heritage site in 2008.

Monarchs from the U.S. and Canada migrate 3,400-miles (5,470-kilometers) each year to winter in the forest reserve.

But the number of butterflies wintering in Mexico has dropped steeply in recent years, leading to worries the migration might end.

Writer and activist Homero Aridjis said Monday that adding the Monarch site to the list is needed to spur governments into greater efforts to protect the butterflies' habitat.

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Much of the milkweed the butterflies depend on has been killed by herbicides used in conjunction with genetically modified crops in the United States.

Activists hope the U.S. government can create corridors of milkweed and limit pesticide use along the butterflies' migration routes and in their natural habitats.

"Mexico has made great strides in protecting the monarch reserve itself from illegal logging and other threats, but we can't save the monarch migration without more help from the United States and Canada," wrote Aridjis, a former Mexican ambassador to UNESCO who played a key role in winning the reserve's World Heritage listing.

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