To conserve a migratory phenomenon it is necessary to conserve all of its components.

Causes of the diminution

It is evident that in order to conserve a migratory phenomenon, it is necessary to conserve all of its components. In the migration of the Monarch, these components are:

  1. The reproductive populations (in Canada and the United States),
  2. The migratory route (in Canada, the United States and Mexico) and
  3. The hibernation (in the United States and México).

The pressures and threats differ on each component of the migratory phenomenon. The reproductive populations include all of the phases of the life cycle: eggs, larvae, chrysalises and adults. The larvae are herbivores and feed mainly on milkweeds (Asclepias spp.). The adults are nectarivores and feed on a wide variety of nectar producing flowers. The reproductive populations develop in open zones such as pastures and agricultural areas where the milkweeds, while the populations in the hibernation sites require forests to give them protection.

The concentrations of animals in restricted sites are very susceptible to impacts, since, under these circumstances, even a small perturbation can affect a large population.  For this reason, much of the conservation of the Monarch butterfly has concentrated on the hibernation sites. It is also possible to estimate populations in these sites.

What causes the drastic diminution of the Monarch butterflies?

Three causes have been identified as being responsible for the diminution:

1. Diminution of milkweeds and habitat reduction in the United States (reproductive sites and migratory route)

The majority of the caterpillars that will become the migratory generation, known as "Methuselah", feed on a species of milkweed (Asclepias syriaca) that grows in the fields of soybean and maize of the Midwestern United States. Milkweeds are considered weeds that impact upon agricultural production and, in addition, are toxic to livestock. For this reason, they have been combated and eliminated from agricultural fields since the 1970s using increasingly powerful herbicides. Currently, the main herbicide in use of wide spectrum and extreme toxicity is glyphosate, better known as "Roundup" from Monsanto. The use of these "super" herbicides is possible due to the adoption of varieties of transgenic crops that are highly resistant to these compounds. However, elimination of the milkweed causes the loss of the main food source for the Monarch butterflies in their caterpillar stage.

Thanks to economic incentives for the production of ethanol from maize, the planted area of maize in the United States has expanded from 31.5 million hectares in 2006 to 39.3 million hectares in 2013.  If that wasn´t enough, intensified agriculture applies large quantities of insecticides.

In addition, intensification of cultivation has resulted in the reduction of habitat. Fields in fallow, alternate crops in rows and drains along the highways were milkweeds used to grow, as well as land that was previously conserved as part of the Program of Conservation Reserves of the United States, have been converted into monocultures. It is known that these practices have caused a 58% decrease in the abundance of milkweeds (Asclepias), for which reason Monarch abundance reduced by 81% between 1999 and 2010, as well as that of other species of pollinator insects critical for the good health of natural ecosystems.

Furthermore, some agricultural zones have been transformed into urban zones, roads or commercial centers, destroying the Monarch butterfly habitat.

2. Degradation of the forests of hibernation in México

Degradation of the forests affects the survival of the Monarchs in the hibernation sites in Mexico. In addition, changes in the composition and structure of the forest also affect the water capture on which thousands of inhabitants depend.

Despite the fact that the butterflies are extremely resistant, extreme drops in temperature, together with winter storms and snow, substantially increase their mortality. Monarchs have some substances in the hemolymph (the blood of the insects) that allow them to lower the freezing point of their tissues and, in this way, can tolerate down to -14˚ C if there is no excess moisture on their bodies. However, if the butterflies get wet during the winter storms, the extremely low temperatures kill a large part of the population.

Events of massive mortality due to low temperatures and rain have occurred in past decades (2001-2002, 2003-2004). While these events form part of natural mortality, the removal of trees due to deforestation has reduced the capacity of the forest to maintain a microclimate that is suitable for the protection of the butterflies.

An analysis of the plant cover in 2006 highlights that only 55% of the area of the Reserve is comprised by dense forests, the other 25% is very disturbed forests and 20% are open areas with no trees. From 1986 to 2006 more than 10,500 hectares of forest have been lost in the Reserve.

Core zone

Deforestation by “large scale illegal logging” by organized groups in the region increased during the decade 2000 to 2010, affecting 2,179 hectares in the forests of the core zone of the Monarch butterfly Biosphere Reserve. The first evidence of deforestation in the core zone was reported in 2003. The logged area duplicated up to 2007 and decreased from 2007 to 2009. Large-scale illegal logging has been controlled thanks to the joint participation of governmental authorities and civil society; however, the small-scale logging continues to gradually modify the structure and composition of the forests.

Unplanned tourism

During migration, the butterflies feed on nectar, increasing their fat reserves in order to weather the season of hibernation. During some sunny days in winter, the butterflies detach from their congregations and fly before returning to rest on the firs. During this activity, they use the energy that they accumulated during the journey, and in this activity increases, they can use up their reserves in Mexico and not have enough fat to carry out the return flight.

The hibernation sites are visited by more than 100,000 tourists in the season from November to March. Unplanned tourism can cause too many perturbations to the butterflies that cause and increase of flights during the winter and overuse of their energy reserves.

3. Extreme climatic conditions

The life cycle of the Monarch depends on the climatic conditions of their surroundings. Temperatures above 35˚C are lethal for the larvae and the levels of eclosion reduce in excessive conditions of heat and dryness. Extreme climatic fluctuations in spring and summer affect the survival and fecundity of the adults, such that the migratory generation of butterflies in autumn is reduced along with the numbers that arrive in Mexico.

It is very probable that the lower density of butterflies found in the hibernation sites in 2012-2013 was due to the fact that the climatic conditions of that summer were the hottest and driest in the last 117 years, which could have caused the next annual cycle and hibernation of 2013-2014 to be of low density.

Main actions

Six themes have been identified as being of priority for the protection of the species, their migratory phenomenon, and their habitat in Mexico (High Level Working Group for the Conservation of the Monarch butterfly Migratory Phenomenon in Mexico: GANMM, by its Spanish acronym), with their corresponding activities.

1. Economy of Conservation

Develop and consolidate productive programs, projects and activities that provide sufficient income for the wellbeing and comprehensive rural development of the communities that live in the Monarch butterfly Biosphere Reserve and its zone of influence.

Foster and strengthen the different existing productive initiatives in the Monarch butterfly region for a productive diversification that creates "green" jobs.

2. Restoration and conservation

Maintain the health of the Monarch butterfly hibernation habitat and its zone of influence and of the migratory route.


3. Research and monitoring

Foster scientific research on priority themes that support decision making for the conservation of the Monarch butterfly migratory phenomenon.


4. Inspection and Surveillance

Promote inspection and surveillance that contributes to the reduction of environmentally unlawful actions in the MBBR.


5. Social participation, environmental education and conservation

Design a strategy of education, communication and dissemination for the conservation of the Monarch butterfly at different scales.


6. Coordination and Funding

Establish a national working group for the conservation of the Monarch butterfly.