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Each winter researchers calculate the size of the eastern monarch butterfly population in Mexico by measuring how much space they cover in the forests where they winter. 

This winter the butterflies cover less than one hectare, the smallest population in a decade and the second smallest in 30 years of monitoring. 

Drought is thought to be the primary reason for the population decline which increases the risk of further losses during the spring migration.  

“Severely concerning with regard to continuing to ramp up the protection efforts and the conservation efforts that we have for the species,” said Wendy Caldwell, executive director of the St. Paul based 

The coalition of government agencies, nonprofit organizations, businesses and academic programs works to protect monarch migration.

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The butterflies will start their northward migration in late February. They typically arrive in Minnesota by mid-May.   

Monarchs depend on timely access to milkweed plants along their migration path, so spring weather will be critical, said Caldwell. 

“If it’s a warm spring monarchs tend to sometimes move faster than the milkweed emerges or they might get too far north and then experience a late season frost,” she said. “So we’re hoping for normal spring conditions.”

While the winter population this year covers less than a hectare of land, scientists have determined the population should cover a minimum of six hectares to sustain a resilient monarch population. 

Caldwell said there has been progress in expanding habitat for monarchs across the Midwest and it’s disheartening to see the population decline this winter.

“Monarchs are resilient, we’ve seen them lower before and we know that they can recover,” said Caldwell. “So I feel hopeful that we can still move them in the right direction.”

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Dan Gunderson

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The overwintering population of monarch butterflies in Mexico is at the lowest point in a decade, according to the results of an annual population survey. The butterflies overwinter in Mexico before migrating north, arriving in Minnesota by May. 

Wintering monarch butterfly population hit hard by drought

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