PIEDRA HERRADA, Mexico — The number of monarch butterflies reaching their wintering grounds in central Mexico this year may be three or four times higher than the previous season, authorities said Thursday.

Speaking during a visit to a monarch reserve by U.S. Interior Secretary Sally Jewell, Mexican Environment Secretary Rafael Pacchiano said initial reports suggest the butterfly population is rebounding.

Monarchs reaching their wintering grounds in Piedra Herrada, Mexico, could cover almost 10 acres this year, down from a high of 44 acres but higher than last year’s 2.79 acres. The Associated Press

“We estimate that the butterfly population that arrives at the reserve is as much as three and could reach four times the surface area it occupied last season,” Pacchiano said.

He did not explain how the government made the calculation, but authorities conduct informal tracking of monarch butterflies as they enter Mexico from the United States.

The population of orange-and-black butterflies making the 3,400-mile migration from the United States and Canada declined in recent years before recovering slightly in 2014, when the insects covered about 2.79 acres in the mountains west of Mexico City.

The monarchs cluster so closely in trees that their numbers are measured by the area they cover. They once blanketed as much as 44 acres.

Pacchiano said the butterfly colonies could cover 7.8 to 9.9 acres this year, and officials hope to reach 14.8 acres in the reserves by 2020.

“The United States is very committed to protecting the monarch butterfly, but we need the help of Mexico and Canada,” Jewell said before hiking an hour into the mountains to see the trees where the monarchs roost.

She said 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 designating pesticide-free areas.

“Our agricultural practices must be adapted. … We have to look at our use of pesticides,” Jewell said. “We have the goal of 225 million monarch butterflies returning right here, to Mexico, every year. We believe we can get there by working together.”

Milkweed is the plant the butterflies feed and lay their eggs on, but it has been attacked by herbicide use in the United States.

And Mexico too still has problems.

Illegal logging more than tripled in the monarch butterflies’ wintering grounds last year, reversing several years of steady improvements.

Pacchiano said the reserve’s buffer area suffered the loss of more than 20 acres due to illegal logging this year, but the tree-cutting was detected and a number of arrests were made.

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