The traditional wintering site for tens of millions of monarch butterflies in central Mexico is under continuing threat after conservationists failed to halt the onslaught of illegal logging in the area.

Monarch butterflies gather on a tree branch at the Monarch Butterfly Biosphere reserve in Sierra Chincua, Mexico. (Photograph: Dario Lopez-Mills/AP) The butterflies are in the middle of their annual journey of up to 2,800 miles from eastern Canada to the small area of evergreen fir forest that acts as their wintertime sanctuary. But, despite an unprecedented drive to protect it, deforestation is threatening the Monarch Biosphere Reserve and its visitors.

A report from the WWF showed deforestation of the area up nearly 10% over the last year, at 260 hectares (650 acres), reversing a downward trend established with the help of unparalleled efforts by the authorities and conservationists.

"The problem is more complicated than we had thought," said Omar Vidal, director of WWF Mexico. "It is very worrying."

Before the latest figures came out activists and government officials were hinting at victory in the battle to protect the mountainside reserve, which was formed in 1986 from land owned by 38 communities. Deforestation soared after the arrival of the logging mafias in 2001, reaching a peak of 460 hectares in 2006. The impending disaster led to unprecedented efforts to protect the reserve's 11,000 hectare core. Police and the army manning checkpoints cracked down on trucks piled high with logs leaving the reserve and local people were offered financial incentives to conserve the forest, and advice on other ways of making money, such as tourism.

"We were making such good inroads with the local people we thought it was only a matter of time before all the communities joined in," said Ernesto Enkerlin, head of the National Commission of Protected National Areas. A 48% drop in deforestation the previous year fired the optimism, boosted by the declaration of the reserve as a World Heritage Site this summer. But meanwhile the logging mafias had cemented ties to the Crescencio Morales community, which is now responsible for 92% of deforestation in the reserve.

The latest figures have led to calls for emergency measures to persuade the community to switch to conservation. But the loggers have a reputation for violence and intimidation, and the authorities worry that rewarding the transgressors would send the wrong message.

Lincoln Brower, an expert on the monarch, said the fact that the butterflies, which arrive in November, often head for the same patch of forest their great grandparents abandoned the previous spring adds to the threat. One colony, he said, arrived at a traditional site in 2006 only to be wiped out because inadequate tree cover allowed temperatures to drop too low. There was nothing obviously stopping them moving to healthy forest nearby. "The logging has got to stop. Otherwise it's a catastrophe," he said.