It's due mostly to extreme weather and changed farming practices in North America, the Mexican government and a conservation alliance reported Wednesday.
The area of forest occupied by the butterflies, once as high at 50 acres, dwindled to 2.94 acres in the annual census conducted in December, Mexico's National Commission of Natural Protected Areas disclosed at a news conference in Zitacuaro, Mexico.
That was a 59 percent decline from the 7.14 acres of butterflies measured in December 2011.
Because the insects cannot be counted, the combined size of the butterfly colonies is used as a proxy in the census, which is conducted by the commission and a partnership between the World Wildlife Fund and the Mexican cellphone company Telcel Alliance.
"We are seeing now a trend which more or less started in the last seven to eight years," Omar Vidal, the head of the wildlife group's Mexico operations, said in an interview. Although insect populations can fluctuate greatly even in normal conditions, the steady downward drift in the butterfly's numbers is worrisome, he said.
The latest decline was hastened by drought and record-breaking heat in North America when the monarchs arrived last spring to reproduce. Warmer than usual conditions led the insects to arrive early and to nest farther north than is typical, Chip Taylor, director of the conservation group Monarch Watch at the University of Kansas, said in an interview.
The early arrival disrupted the monarchs' breeding cycle, he said, and the hot weather dried insect eggs and lowered the nectar content of the milkweed on which they feed. That in turn weakened the butterflies and lowered the number of eggs laid.
But an equally alarming source of the decline, both Taylor and Vidal said, is the explosive increase in American farmland planted in soybean and corn genetically modified to tolerate herbicides.
The American Midwest's corn belt is a critical feeding ground for monarchs, which once found a ready source of milkweed growing between the rows of millions of acres of soybean and corn. But the ubiquitous use of herbicide-tolerant crops has enabled farmers to wipe out the milkweed, and with it much of the butterflies' food supply.
"That habitat is virtually gone. We've lost well over 120 million acres, and probably closer to 150 million acres," Taylor said.