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2017-2018 Monarch Numbers Down: A Battle on Two Fronts


This year’s overwintering monarch butterfly population experienced a 14.77% decrease from the previous winter, following the trend of steady decline observed over the past two decades. These overwintering monarchs represent the individuals that survived the long-distance migration from as far north as Canada all the way down to Mexico.


Causes of Decline

There are several potential explanations for the disappointing overwintering numbers this year. Higher temperatures during the fall in the Midwest and Northeast caused a lag in the migration, which makes for a more risky journey. The high number and intensity of tropical storms and hurricanes during the migration also likely caused higher than normal mortality along the way. Beyond this, monarchs still face a reduction in habitat as a result of agriculture and urban/suburban development.


Overwintering Report from Mexico

In early March 2018, the World Wildlife Fund-Mexico released their much-anticipated report on the population size of monarchs overwintering in the mountains of central Mexico (find the full report here). Overall there were 9 colonies of monarchs that collectively covered 2.48 hectares (6.13 acres). It is estimated that every hectare of forest cloaked in butterflies represents 21.1 million individuals. This would put the current overwintering population at just over 52 million monarchs, which may sound like a substantial number. To put it in perspective, the recommended total population size to ensure the preservation of the monarch migration is 6 hectares. The population has not exceeded this size since 2006.

Overwintering Report from California

While the migration down to Mexico receives most of the attention when it comes to monarch conservation, there is actually a second, smaller migration down the California coast. This migration is less studied but these monarchs are known to come from western states reaching as far as Washington, Utah, and Idaho. To estimate the population size of the western overwintering colonies, volunteers conduct counts during the Xerces Society’s Western Monarchs Thanksgiving Count. Although volunteers in 2017 visited more sites than any previous year, they observed fewer than 200,000 monarchs (full report here). Following a similar trend to Mexico, two decades ago volunteers reported over a million monarchs even though they only visited half the sites.


What can you do?

As people become more and more invested in the monarch cause (planting milkweed and pollinator gardens, restoring prairies, reducing pesticides), it may seem disheartening that there has not been an apparent boost in the monarch population. As the monarchs are challenged to overcome a changing climate and evolving landscape, we can help by providing habitat that will give them their best possible shot. We can all pitch in to help reach the goal of 1.3 billion milkweed plants on the landscape in order feed enough monarchs to fill 6 hectares next winter.

To learn more about the overwintering monarch population and how the population is estimated click here and to learn more about their milkweed host plant click here.


Monarch butterfly nectaring on a Common milkweed plant (Asclepias syriaca). Photo credit: Laura Lukens

Photo credit: Cover photo and all other photos were taken by the author unless otherwise noted.

About the author:

image00 Hayley Schroeder is an undergraduate at UGA studying Ecology and Entomology. She is an enthusiastic foodie, advocating for food that nourishes not just the people eating it, but communities, farmers and the earth as well. She is also a friend to all insects and can be easily spotted on campus by her butterfly net. Contact her at hayleyadair37@uga.edu. More from Hayley Schroeder.

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