MEXICO CITY — Monarch butterflies have made a big comeback in their wintering grounds in Mexico, after suffering serious declines, experts said Friday.

The area covered by the orange-and-black insects in the mountains west of Mexico City this season was more than three and a half times greater than last winter. The butterflies clump so densely in the pine and fir forests they are counted by the area they cover rather than by individual insects.

The number of monarchs making the 3,400-mile migration from the United States and Canada declined steadily in recent years before recovering in 2014. This winter was even better.

This December, the butterflies covered 10 acres compared to 2.8 acres in 2014 and just 1.66 acres in 2013.

While that’s positive, the butterflies covered as much as 44 acres in 1996.

“Now more than ever, Mexico, the United States, and Canada should increase their conservation efforts to protect and restore the habitat of this butterfly along its migratory route.” said Omar Vidal, director of the World Wildlife Fund in Mexico

The United States is working to reintroduce milkweed, a plant key to the butterflies’ migration, on about 1,160 square miles within five years, both by planting and by designating pesticide-free areas. Milkweed is the plant the butterflies feed and lay their eggs on, but it has been attacked by herbicide use and loss of open land.

“It is time for celebration because we see the beginning of success,” said Dan Ashe, director of the U.S. Fish and Wildlife Service. “But our task now is to continue building on that success.”

– The Associated Press