Monarch butterflies that once covered 50 square acres of forest during their summer layover in central Mexico now occupy fewer than three acres, according to the latest census.
The numbers of the orange-and-black butterflies have crashed in the two decades since scientists began making a rough count of them, according to the Mexico's National Commission of Natural Protected Areas.
At a news conference on Wednesday, the commission said the count was a down 59 percent from December 2011 levels, when the insects filled 7.14 acres of fir trees in central Mexico.
"We are seeing now a trend which more or less started in the last seven to eight years," Omar Vidal, the head of the World Wildlife Fund's Mexico operations, said in an interview with The New York Times. Although insect populations can fluctuate greatly even in normal conditions, the steady downward drift in the butterfly's numbers is worrisome, he said.
The Associated Press quotes experts as saying that the decline in the Monarch population now marks a statistical long-term trend and are no longer seen as a combination of yearly or seasonal events.
Why is it happening?
Vidal points north, to the monarchs' summer home in the U.S. Midwest. The farmland there once provided plenty of milkweed sprouting between rows of corn and soybean. But the heavy use of genetically modified, herbicide-tolerant, crops have allowed farmers to wipe out milkweed and eliminate the monarchs' main food source.
In an interview with the Times, Chip Taylor, director of the conservation group Monarch Watch at the University of Kansas, said the Midwest milkweed habitat "is virtually gone. We've lost well over 120 million acres, and probably closer to 150 million acres."
Taylor says the record-breaking heat in North America last summer was also a factor.