As many as 200 million Monarch butterflies may migrate to Mexico this year — a nearly tenfold increase over 2004, when unfavorable weather, pollution and deforestation caused a drastic decline in the population, environmental officials said Tuesday.

Last year, fewer than 23 million butterflies survived long enough to leave habitats in the United States and Canada for sanctuaries in the state of Mexico, which borders Mexico City, and neighboring Michoacan state.

That was at least 75 percent lower than expected, but should usher in a Monarch resurgence this year, officials said.

"In the past, very low numbers have recuperated and produced surprisingly high populations," Jose Bernal, director of inspection for Mexico's environmental protection agency, Profepa, said after a news conference to kick off Mexico's Monarch butterfly season.

After especially cold weather reduced the butterfly population to 28 million during the 2000-2001 migration period, the number of Monarchs swelled to 93 million the following year, he said.

No one knows for sure how many butterflies make the annual migration south, but Bernal said 2005 estimates will be aided by a new counting system that better determines the tens of thousands of butterflies on each acre of territory.

The Monarchs' annual 3,400-mile journey from the forests of eastern Canada and parts of the United States to the central Mexican mountains is an aesthetic and scientific wonder.

The butterflies began reaching Mexico last week and usually continue to stream south until early December. The spectacle of millions of orange and black butterflies carpeting fir trees attracts 200,000 visitors a year. Authorities plan to open four major sanctuaries to tourists on Nov. 19.

Hector Gonzalez, Profepa's deputy prosecutor for natural resources, said officials have significantly reduced the rate of deforestation, which has for decades devastated the areas where butterflies winter.

He said the number of people arrested for illegal logging is on the decline, as is the amount of timber seized from butterfly habitats. Satellite imagery of the sanctuaries also confirms the drop in deforestation, he said, though officials failed to provide concrete data to support the claim.

"This by no means puts us in the position of being calm," he said. "Reduction and complete eradication of deforestation remains a permanent goal."

Police officers and federal agents have for years patrolled Monarch wintering grounds in an effort to stop illegal logging and authorities have set up checkpoints along nearby highways to seize timber as it leaves the area.

This year, a new 15-officer police force will patrol butterfly areas. In the past, armed logging gangs have responded to anti-deforestation efforts with violence.

Profepa's chief prosecutor, Ignacio Loyola, said bands of thugs often control illegal logging in Monarch areas, but that impoverished residents also cut down trees for firewood and to make room for subsistence farming.

He said federal officials have begun a number of programs to stimulate economies in butterfly areas, creating jobs in tourism and construction so as to discourage illegal logging.

"It's not about simply saying, 'Nobody can touch anything here' and forgetting about the people who live there," Loyola said.