Experts believe that the monarch butterflies that arrive in swarms of millions from northern United States and Canada to spend winter in Mexico, are disappearing.

Environmentalists warn that the number of monarch butterflies overwintering in central Mexico has reached a historic low since the World Wide Fund for Nature (WWF) started to monitor the size of the butterflies' winter colonies in 1993. In the 20 years of keeping records of the monarch's winter habitat, the largest area covered by the butterflies reached 44.5 acres (18 hectares) but this season, the migratory butterflies covered only 1.65 acres (0.67 hectares).

"There have always been ups and downs in the populations, but over the last 10 years we have seen a clear downward tendency," said Omar Vidal, director general of WWF-Mexico.

Lincoln Brower, a professor of biology at Sweet Briar College who has studied the overwintering, migration and conservation biology of the monarch butterfly, sees three major reasons why the population of the monarch is dwindling. In the study "Decline of monarch butterflies overwintering in Mexico: is the migratory phenomenon at risk?", Brower and his team of researchers cited deforestation in Mexico, severe weather and industrialized agriculture as main culprits to the monarch's declining population.

"Severe weather was working against the butterflies for the last two years. Another is the progressive deterioration of the overwintering habitat in Mexico due to illegal deforestation. But the third and probably the most egregious problem is the result of industrialized agriculture in the Midwest," Brower said, explaining that the use of herbicides as well as the explosion of herbicide-resistant genetically modified soybean and corn crops are destroying the milkweed flora, a crucial larval food source for the monarch butterflies.

"The Monarch migration, the symbol of co-operation between the three countries, is at serious risk of disappearing," Vidal said, adding that North American leaders should agree on a plan to protect the monarch butterflies because their annual migration is the symbol of unity between Mexico, Canada and the United States.

Nonetheless, he pointed out that the butterfly isn't in danger of disappearing altogether. "The butterfly as a species isn't in danger of extinction," Vidal said. "What is in danger of disappearing . . . is the migration of the monarch from Canada through the United States to Mexico."

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