Millions of monarch butterflies in the eastern United States and Canada migrate to El Rosario sanctuary in Michoacán state each fall to overwinter, clustered together in high-elevation oyamel fir forests. They gather in about 12 colonies in the sanctuary, which have numbered as high as 380 million butterflies in 1997 and as low as 14 million in 2014.
According to sanctuary officials, upwards of 130 million butterflies have reached El Rosario sanctuary in Michoacán state this year which is about 30% higher compared to recent years. Marino Argueta, a park official said:
Millions of butterflies arrived this year and the climate … up until now has been very favorable, they’ve even come down to the sanctuary entrance.
This year, an increase in the number of overwintering monarch butterflies has also been reported in central California’s Pacific coast, where the butterflies migrate from inland areas of the Western United States.
Monarch butterflies are threatened by pesticides, global climate change, loss of breeding habitat, and illegal logging of the forests where they migrate for the winter. Eradication of milkweed plants both in agricultural areas as well as in urban and suburban landscapes is also cited as a major reason for their population decline. The population of eastern monarch butterflies which migrate to Mexico has declined by more than 80% over the past two decades. The decline is much steeper for western monarchs, with more than a 99% reduction in population in recent years.
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