Snowstorm Devastates Monarch Population in Mexico

An unusual weather pattern in central Mexico has resulted in the deaths of tens of millions of monarch butterflies (Danus plexipus) in the Oyamel forests west of Mexico City. The death toll at one site, El Rosario, was estimated to be as much as 81% (Monarch Watch). Together, all affected sites represent the largest monarch overwintering locations in Mexico.

The director of Monarch Watch described the event in a Monarch Watch news bulletin:

"A massive moisture-bearing weather system moved into central Mexico late on 11 January 2002 which was followed by clearing skies and overnight lows in the mid to low 20s on the mornings of 14-16 January. What followed was unprecedented mortality at the monarch overwintering sites. In the following text I will summarize what is known at this time and will try to provide a perspective for this catastrophic event." ~Dr. O. R. Taylor, Director of Monarch Watch

There are two populations of monarchs in the US. One population spends its summers in the eastern portion of the US while the second population inhabits the western resgions of the country. Both populations migrate, spending their winters in warmer climes. The eastern population overwinters in central Mexico and the western population overwinters along the California coast (Kane, 1999).

Thus, it was the eastern population that was affected by the snowstorm. This means that sadly, there will be tens of millions fewer monarchs that make their way back toward the eastern US next spring.

The monarch life cycle and migration process is complex. In February and March, the monarchs in Mexico and southern California awaken from their hibernation and begin their migration northwards. They lay eggs and then die. Those eggs hatch in March and April, forming the next generation. They develop from egg to caterpillar, then form a chrysalis and finally transform into an adult butterfly. This process is repeated throughout the summer, as two more generations reproduce and die. The fourth generation then migrates southward to Mexico and southern California where they hibernate through the winter before starting the cycle once again.

Find out more:

• Catastrophic Mortality at the Monarch Overwintering Sites in Mexico (Monarch Watch)
• Monarch Butterfly Population in Jeopardy (CNN)
• Kane, E. 1999. 'Danaus plexippus' (On-line), Animal Diversity Web. Accessed September 14, 2005.