The number of monarch butterflies that completed an annual migration to their winter home in a Mexican forest sank this year to its lowest level in at least two decades, due mostly to extreme weather and changed farming practices in North America, the Mexican government and a conservation alliance reported on 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 Zitácuaro, 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.

“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.