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Mass Mortality of Monarchs in Mexico
This page contains archived content and is no longer being updated. At the time of publication, it represented the best available science.
Like migratory birds and many retired couples, monarch butterflies head south
for warmer weather in the wintertime. During the summer hundreds of millions of
monarchs live and breed throughout the eastern United States and southern
Canada. As the fall equinox approaches and temperatures begin to drop, they fly
southwest through Texas and into central Mexico. Here they congregate by the
millions at high altitude in fir forests on 12 separate mountain ranges until
the spring Equinox. They then reverse the course of their migration and travel
This winter, however, the monarch migration ended in disaster. On January 12,
2002, a winter storm blew in over central Mexico. The cold temperatures and rain
wiped out up to 250 million (80 percent) of the monarch butterflies nesting in
Mexico. These false color images taken by the Moderate-resolution Imaging
Spectroradiometer (MODIS) aboard NASAs Terra satellite show the average land
surface temperatures across Mexico from January 11-20 for 2001 and 2002. As is
indicated by the increase in the green and dark green pixels, much more of the lands
surface was freezing in the monarchs habitat this year than in the last.
The freezing temperatures may have not been the sole reason for the monarchs
demise. Intense logging in the mountains of Mexico have degraded the fir forest
by 44 percent over the past 28 years. The monarchs cluster in the branches of
mature trees and shelter themselves from the elements. With the forests
substantially thinned and reduced, the monarchs were more exposed to the harsh
winds and rains of the winter storm that hit on January 12th. Were the forests
left intact, a higher percentage of the monarchs may have lived. But given the
millions of monarchs that did survive, scientists predict that the insects
should pull through despite the storm and return again to Mexico next year.
References: Anderson, J. B., and L. P. Brower. 1996. Freeze-protection of overwintering
monarch butterflies in Mexico: critical role of the forest as a blanket and
an umbrella. Ecological Entomology 21:107-116.
Calvert, W. H., W. Zuchowski, and L. P. Brower. 1983. The effect of rain,
snow, and freezing temperatures on overwintering monarch butterflies in
Mexico. Biotropica 15:42-47.
Brower, L. P., G. Castilleja, A. Peralta, J. Lopez-Garcia, L.
Bojorquez-Tapia, S. Diaz, D. Melgarejo, and M. Missrie. 2002. Quantitative
changes in forest quality in a principal overwintering area of the monarch
butterfly in Mexico: 1971 to 1999. Conservation Biology 16:1-15.