In Mexico, migrating monarch butterflies gather on trees. The number of migrating butterflies has shrunk as trees and food have disappeared. (Kirsten Luce/Associated Press)

The stunning and little-understood migration of millions of monarch butterflies to spend the winter in Mexico is in danger of ending, experts said last week, after numbers dropped to their lowest level since recordkeeping began in 1993.

In a report, they blamed the disappearance of the milkweed plant the species feeds on, extreme weather trends and the illegal cutting down of trees the butterflies depend on for shelter.

After steady declines in the past three years, the black-and-orange butterflies now cover only 1.6 acres in the forests west of Mexico City, compared with 2.9 acres last year, said the report, released by the World Wildlife Fund and by Mexico’s Environment Department and Commission of Protected Natural Areas. The butterflies 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.

The decline in the monarch population is a long-term trend, experts say.

A monarch butterfly perches on a tree at the Sierra Chincua Sanctuary in the mountains west of Mexico City. (Marco Ugarte/Associated Press)

Lincoln Brower, an entomologist (insect scientist) at Virginia’s Sweet Briar College, called the migration “an endangered biological phenomenon.”

Karen Oberhauser, a University of Minnesota professor, said farmers in the Midwest are making more use of weedkillers because some crops aren’t harmed by the chemicals. But the chemicals kill milkweed, the monarchs’ food source.

Severe cold snaps, unusually heavy rains and droughts also seem to have played a role in the decline.

It’s unclear what would happen to the monarchs if they no longer made the trek to Mexico. There are monarchs in many parts of the world, so they would not become extinct. The butterflies apparently can survive year-round in warmer climates, but populations in the northern United States and Canada would have to find someplace to spend the bitter winters.

The migration is an inherited trait. No butterfly lives to make the full round trip, and it is unclear how they know the route to the same patch of forest each year, a journey of thousands of miles to central Mexico. Some scientists think that the butterflies release chemicals that mark the migratory path and that if their numbers fall low enough, not enough chemical traces would remain, and the route-marking might no longer work.

Butterfly guide Emilio Velazquez Moreno, 39, and other farmers in Macheros, a village inside an officially protected wintering ground, have planted small plots of milkweed, hoping to provide food for the monarchs if they stay in Mexico year-round. Velazquez Moreno, who has been visiting the butterflies since he was a boy, said: “We have to protect this. This comes first; this is our heritage.”

— Associated Press

For a look at the monarch butterfly migration, check out the movie “Flight of the Butterflies 3D” at the Smithsonian’s National Museum of Natural History, 10th Street and Constitution Avenue NW, Washington. For showtimes, a parent can visit www.si.edu/Imax/Movie/71.