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MEXICO CITY (AP) – The stunning and little-understood annual migration of millions of Monarch butterflies to spend the winter in Mexico is in danger of disappearing, experts said Wednesday, after numbers dropped to their lowest level since record-keeping began in 1993.

Their report blamed the displacement of the milkweed the species feeds on by genetically modified crops and urban sprawl in the United States, extreme weather trends and the dramatic reduction of the butterflies’ habitat in Mexico due to illegal logging of the trees they depend on for shelter.

After steep and steady declines in the previous three years, the black-and-orange butterflies now cover only 1.65 acres (0.67 hectares) in the pine and fir forests west of Mexico City, compared to 2.93 acres (1.19 hectares) last year, said the report released by the World Wildlife Fund, Mexico’s Environment Department and the Natural Protected Areas Commission. They covered more than 44.5 acres (18 hectares) at their recorded peak in 1996.

Because the butterflies clump together by the thousands in trees, they are counted by the area they cover.

While the Monarch is not in danger of extinction, the decline in their population now marks a statistical long-term trend and can no longer be seen as a combination of yearly or seasonal events, experts said.

The announcement followed on the heels of the 20th anniversary of the North American Free Trade Agreement, which saw the United States, Mexico and Canada sign environmental accords to protect migratory species such as the Monarch. At the time, the butterfly was adopted as the symbol of trilateral cooperation.

“Twenty years after the signing of NAFTA, the Monarch migration, the symbol of the three countries’ cooperation, is at serious risk of disappearing,” said Omar Vidal, the World Wildlife Fund director in Mexico.

Lincoln Brower, a leading entomologist at Sweet Briar College in Virginia, wrote that “the migration is definitely proving to be an endangered biological phenomenon.”

“The main culprit,” he wrote in an email, is now genetically modified “herbicide-resistant corn and soybean crops and herbicides in the USA,” which “leads to the wholesale killing of the monarch’s principal food plant, common milkweed.”

While Mexico has made headway in reducing logging in the officially protected winter reserve, that alone cannot save the migration, wrote Karen Oberhauser, a professor at the University of Minnesota. She noted that studies indicate that the U.S. Midwest is where most of the butterflies migrate from.

“A large part of their reproductive habitat in that region has been lost due to changes in agricultural practices, mainly the explosive growth in the use of herbicide-tolerant crops,” Oberhauser said.