MEXICO CITY — The stunning and little-understood migration of millions of Monarch butterflies to winter in Mexico is in danger of disappearing, specialists said Wednesday, after numbers dropped to their lowest level since record-keeping began in 1993.
The situation was blamed on factors including displacement of the milkweed (which the butterflies feed on) by genetically modified crops and urban sprawl in the United States. It cited extreme weather trends and dramatic reduction of the butterflies’ habitat in Mexico due to illegal logging of trees they depend on for shelter.
After steep and steady declines in the previous three years, the black-and-orange butterflies cover only 1.65 acres in the pine and fir forests west of Mexico City, compared with 2.93 acres 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 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 marks a statistical long-term trend and can no longer be seen as a combination of yearly or seasonal events, specialists 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 like the Monarch. The butterfly was adopted as a 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 e-mail, is genetically modified ‘‘herbicide-resistant corn and soybean crops and herbicides in the USA,’’ which ‘‘lead 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 most of the butterflies migrate from the US Midwest.
‘‘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.
Extreme weather — severe cold snaps, unusually heavy rains, or droughts in all three countries — have also apparently played a role in the decline.
But the milkweed issue now places the spotlight on the United States and President Obama, who will visit Mexico on Feb. 19, with events scheduled for Toluca, a city a few dozen miles from the butterfly reserve.
‘‘President Obama should take some step to support the survival of the Monarch butterflies,’’ said writer and environmentalist Homero Aridjis.
It is unclear what would happen to the Monarchs if they no longer made the annual trek to Mexico, the world’s biggest migration of Monarch butterflies and the second-largest insect migration, after a species of dragonfly in Africa.
There are Monarchs in many parts of the world, so they would not go extinct. The butterflies can apparently survive year-round in warm climates, but populations in the northern United States and Canada would have to find some place to spend the winter.
The migration is an inherited trait. No butterfly lives to make the full round-trip, and it is unclear how they remember the route back to the same patch of forest each year, a journey of thousands of miles. Some think the huge masses of migrating butterflies release chemicals that mark the path and that if their numbers fall low enough, not enough chemical traces would remain .