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11 February 2002

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, Monarch Watch

Catastrophic Mortality at the Monarch Overwintering Sites in Mexico

A report (Brower, et al., in prep; released for publication on 12 February 2002) of the deaths of tens of millions of monarchs at the two largest monarch overwintering sites in Mexico once again confirms the long held view that it is the overwintering sites that hold the key to the continuation of the monarch migration in eastern North America. Each fall hundreds of millions of monarchs migrate from Canada and the eastern United States to overwinter in Oyamel fir forests high in the Transvolcanic Mountains west of Mexico City. The Oyamel forests provide a cool and moist environment which shelters the monarchs from extreme temperatures allowing the butterflies to pass the winter in a relatively inactive state. The monarchs form dense clusters on the trees in the forest with densities of 10 million or more butterflies per hectare. The butterflies take flight on warm days visiting flowers for nectar and damp areas for moisture but for the most part they are inactive and sustain themselves through the winter by metabolizing fat stored in their abdomen. The forests provide substantial protection for the monarchs and the temperature and moisture changes within the forest are modest compared to the changes which occur in cleared areas adjacent to the colonies. Degradation of the forests at the colony sites is likely to break down this protection leading to higher mortality and greater vulnerability of the clustered butterflies to the occasional snow or freezing rain. It is clear that monarchs require relatively intact forests to successfully overwinter. The challenge is to maintain the integrity of these forests when the economic conditions are such that the local landowners (ejidatarios) view the trees as a source of income.

Catastrophic mortality at the overwintering sites occurs from time to time. Historically most of the weather extremes leading to the deaths of masses of monarchs have been limited to one or a few colonies. Last year, a late season snow and ice storm (2-3 March 2001) killed modest numbers of monarchs at the two main colonies, El Rosario and Sierra Chincua but appeared to devastate some of the smaller colonies in the mountains to the east. A cold snap earlier in the year killed hundreds of thousands of monarchs at the San Andreas colony, a site with a badly degraded forest. Initially, the monarch mortality at San Andreas was attributed to intentional spraying by loggers, a claim that created a furor in the press and conservation community. A subsequent investigation showed that these monarchs died as a result of cold weather.

The Recent Snowstorm and Freezing Weather

The most recent monarch die-off was due to a weather pattern of unusual scope and severity. On January 11th a massive moisture-bearing weather system moved into central Mexico. This was unusual for January which is typically a dry month in central Mexico.

Rain began falling in Ocampo and Angangueo, the towns nearest the El Rosario and Chincua colonies, around sunup on the 12th . By Sunday morning 10 cm of rain was recorded in Angangueo. The rains continued and turned to snow on Sunday afternoon in the colonies. About 10 cm of snow fell at El Rosario and 5 cm at Chincua. Following the snowfall, the skies cleared and the overnight lows for the next three nights were in the 20s. Large numbers of butterflies were on the ground by Monday morning in piles variously described as 10-15 cm deep. By Wednesday the 16th, half of the El Rosario colony was on the ground - with the majority of the butterflies appearing to be dead or dying - and many still falling from the trees. Chincua seemed to fare better and by the 16th many butterflies were climbing back up on the vegetation and were flying. The colony at El Rosario, being at the edge of a large deforested area, may have had greater exposure to freezing temperatures than the colony at Chincua which was located in a tall stand of Oyamel fir trees on a more protected slope. Snowfall per se seems to be less of a factor in killing monarchs than the effect of wetting the butterflies combined with temperatures below 26 degrees F.

The Living and the Dead

It will take some time to fully assess the mortality and long term impact of the January freeze. Brower and his colleagues have provided preliminary information on the numbers of monarchs killed and still alive at El Rosario and Chincua. The monarch mortality at Sierra Chincua was estimated to be 74% and that at El Rosario approximately 81%. At this writing (6 Feb) it appears that the area occupied by living butterflies at both sites combined is approximately 1.29 hectares. This is the lowest number of butterflies ever recorded at these sites for January. These low numbers and the rates of mortality are unprecedented in the known history of the monarch overwintering sites. Although 60-70% of the total monarch population typically overwinters on these two mountains each year, it is critical to assess the 30-40% of the population overwintering at the remaining colonies. Some colonies may have fared better and others worse than El Rosario and Chincua. The weather pattern that prevailed from 11-16 January was so pervasive that it is likely that the butterflies in all colonies were exposed to sub freezing conditions. If these colonies were also decimated by the extreme cold, the number of returning butterflies could be precariously low.

The Number and Condition of the Remaining Butterflies

At this point, the number and condition of the remaining butterflies is the most important issue. It is this remnant of the population that must survive the rest of the winter, and the return migration, to recolonize the summer breeding habitat in the United States and Canada. Beginning in late February, the survivors will move north with the females laying eggs on newly emerging milkweed plants as they migrate through the southern states and into the Midwest. The number of egglaying females is important. If the population is too low, the production of first generation monarchs may not be sufficient to recolonize the northern breeding areas in May and June resulting in a low population of fall migrants in September. Monarchs have demonstrated a remarkable capacity to rebound. Last year the number of returning females was at an all time low (4-5 million). Yet, due to excellent spring conditions in Texas and favorable conditions in the summer breeding area, the fall population in 2001 exceeded 200 million butterflies. Unfortunately, it appears that the conditions (i.e., soil moisture, predators, etc.) in Texas and the Midwest will not be as favorable this coming breeding season. It is important to monitor the numbers and survival of the remaining butterflies. If the population is as low or lower than that of last spring, it could take the monarchs more than one season to rebound from the freeze of 2002.

Evolutionary Perspective

The January freeze prompts the question: How important is catastrophic mortality to the overall dynamics of the monarch population? Historically, extremes in the weather are part of the environmental background in which monarchs evolved. In general, species which occasionally experience high death rates due to catastrophic mortality have high birth rates and therefore the capacity to recover their numbers when conditions return to normal. The monarch fits this pattern. Female monarchs have the ability to produce 400 or more eggs in their lifetime and when conditions are favorable a sufficient number of caterpillars survive to the adult stage to replace and even increase the population.

If this is the case, do we really need to worry about catastrophic mortality in monarchs? (Biologists call this "density independent mortality" since the mortality is due to extrinsic events that are unrelated to the density of the population.) In stable environments catastrophic mortality could depress a population for a short time, perhaps a few years, but we would expect the population to recover. However, habitat degradation which contributes to the mortality by increasing exposure to the weather extremes could have a longer lasting impact on the population. This is our concern with monarchs. If the forest in the overwintering areas is degraded and this degradation leads to higher than normal attrition of the overwintering monarchs and even higher mortality in snow, ice or freezing rain storms, then the time needed for the population to recover after catastrophic mortality will increase. Furthermore, if the number of overwintering colonies is reduced due to deforestation, the vulnerability of the population to catastrophic mortality is likely to increase. Biologically this will be the equivalent of putting all the eggs in one basket and sooner or later catastrophic mortality at the remaining colonies could result in a substantial reduction of the population. Measures of the mortality of monarchs in forests of differing qualities are needed to determine the relationship of mortality to the integrity of the forest and the most favorable conditions for overwintering monarchs.

The monarch mortality in this particular instance can not be directly attributed to the condition of the forests. The weather pattern from 11- 16 January was massive and it was the rainfall that wetted the butterflies combined with the extreme low temperatures that killed the monarchs. Conversion of landscapes over large areas, such as changing forests to agricultural lands, increases the albedo (reflection of incident radiation) which can result in lower rainfall and greater temperature extremes over broad regions. This may have been a factor. Clearly, the severity of this weather event was unusual and was more extreme than any winter weather recorded since the monarch overwintering sites became known to scientists.

In retrospect, it appears to be fortunate that the January freeze occurred this year, a year in which the overwintering population was robust (about 100 million butterflies). Even though estimated mortality due to the January freeze is extremely high (>80%) the number of surviving butterflies may be sufficient to recolonize the breeding areas without a long-term depression of the population. Had this storm occurred last season when the overwintering population was at an all time low (28.3 million), it is likely that it would have taken the population many years to return to normal levels of 60-120 million overwintering butterflies.


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. [10pp 900K PDF - download*]

Brower, L. P., D. R. Kust, E. Rendon-Salinas, J. Miller, C. Fernandez del Rey, K. R. Kust, K. Pape. 2002. Cold front causes catastrophic mortality of monarch butterflies overwintering in Mexico in January 2002. [In preparation.]

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(1): 42-47. [6pp 500K PDF - download*]

Calvert, W. H., M. B. Hyatt and N. P. Mendoza Villaseñor. 1986. The effects of understory vegetation on the survival of overwintering monarch butterflies, (Danaus plexippus L.) in Mexico. ACTA ZOOL. (ns) 18:1-18. [9pp 600K PDF - download*]

Links to other online articles

February 12, 2002
Storm in Mexico Devastates Monarch Butterfly Colonies

February 12, 2002 07:34 PM ET
Mexico Butterfly Deaths May Hit Future Migrations
By Pav Jordan

Wednesday, 13 February, 2002, 12:24 GMT
Mass butterfly deaths after storm

(Filed: 13/02/2002)
Millions of butterflies killed by freak storm
By Ronald Buchanan in Mexico City wbutt13.xml&sSheet=/news/2002/02/13/ixworld.html

February 13, 2002
Monarch Butterflies Dying in Mexico

February 14, 2002
Mass Monarch Butterfly Die-Off Likely the Result of Deforestation and Lack of Tree Cover, WWF Says
World Wildlife Fund

* PDF files are downloaded as compressed zip files. To view these documents they must first be decompressed with a zip utility and then opened with Adobe's FREE Acrobat Reader software. Necessary software is likely to already be installed on your computer, but you may also download it from and/or

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