Some 8,000 oyamel fir trees - the butterlies' unique mountain habitat each winter - were cut down in July in a bid to remove beetles that threaten the Monarch's ages-old migration.

But now another small beetle has since taken to devouring the savory tree trunks, further endangering the butterflies' winter colonies.

"We are working to determine how many trees have been affected," said Homero Gomez, president of El Rosario Sanctuary, a premier migrating spot for the Monarch in the western Mexican state of Michoacan.

Local residents, who help manage and maintain the sanctuary, have asked the authorities to fight the new intruder by using natural substances and without felling trees.

Millions of the orange and black butterflies migrate each year when the weather grows cold in Canada to make habitat in Michoacan's oyamel firs, in an annual ritual that has yet to be scientifically explained.

The Monarchs blaze a trail of some 4,500km - to the tune of 80km per day - arriving in early November in the high mountain massifs of Mexico's transvolcanic belt, where they hibernate until February in huge colonies, completely masking tree trunks.

Thousands of tourists come to observe their majestic aerial dances in the El Rosario Sanctuary, home to five million trees of various species.