Monarch butterflies at Cerro Pelon Sanctuary in Mexico (Photo: Ana Moreno)

“Every year, World Wildlife Fund (WWF)–Mexico, in collaboration with the National Commission of Protected Natural Areas in Mexico and the WWF-TELMEX Telcel Foundation Alliance, monitors the area occupied by monarch butterflies that have migrated from northern breeding grounds in the U.S. and Canada to overwintering grounds in the mountains of central Mexico (see figure). Their findings for the 2023–2024 winter season, released February 7, document a 59 percent decline from last year: 0.90 hectares (ha) compared to 2.21 ha last year (a hectare is about 2.5 acres). This value is used to estimate the size of the eastern North American monarch population from year to year. While the number of monarchs in a hectare varies from year to year and is difficult to estimate, our best estimate is that the average is about 21 million.

“Insect populations are notorious for their annual fluctuations, but this value is concerning. It is the second lowest ever reported; only the winter of 2013–2014 was lower (0.67 ha). In 31 years of measurements, only six years have declined over 55 percent from one season to the next and a drop this dramatic has never before occurred after a year as low as last year’s 2.21 ha. While monarchs did rebound after the low of 2013, numbers this low leave the population more vulnerable to catastrophic events, like a winter storm in the next month before monarchs leave their wintering grounds, or weather conditions that lead to low reproductive success in next spring or summer.”

Read the full article by retired Arboretum director Karen Oberhauser
at Monarch Butterfly Fund»