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Measuring Monarchs and Milkweed

May 15, 2017


  • Population Trends
  • Recent Research

It is common knowledge that the North American monarch population is declining. From a high in 1996 to their record low in 2013, the eastern monarch population size fell by over 90%. These population size estimates allow us to assess the status of the population, and set goals for restoring it to a sustainable level. Monarchs’ declining numbers have gained a lot of attention, but how is the monarch population measured anyway?

Each fall, monarchs emerging east of the Rocky Mountains begin an epic journey of up to 2500 miles to forested mountains in central Mexican to survive the winter together. After they have arrived at their overwintering sites in Mexico, and are clustered in oyamel fir trees, the population size is measured. There are too many monarchs to count individually (unlike the western population in California), so the area of forest the monarchs occupy is used as a substitute for the size of their population. This is why monarch population targets are often in units of hectares (1 hectare is about 2.5 acres) instead of number of butterflies.

Monarch scientists have determined that bringing the eastern monarch population size up to 6 hectares of overwintering monarchs by 2020, and sustaining it, would significantly reduce its risk of extinction. Without this increase, the eastern North American monarch has a more than 20% chance of reaching numbers so low that recovery would be unlikely (Semmens et. al 2016), so it is critical that we work together to restore enough habitat to bring the monarchs back to at least 6 hectares.

To estimate how much milkweed (as part of diverse habitat that also includes nectar plants) we need to plant to meet this ambitious goal, we need an accurate estimate of how many butterflies are in a hectare of occupied overwintering habitat. There have been several different approaches to measuring overwintering monarch density. A new study by the Monarch Conservation Science Partnership combined the six most widely used measures of monarch density into one more comprehensive estimate. Their work suggests there are about 21.1 million butterflies per hectare (Thogmartin et. al 2017). Therefore, the goal of 6 hectares of monarchs by 2020 translates to 127 million butterflies.

Combined with previous research, this figure allows us to understand how much milkweed we need to plant to support 127 million butterflies. Nail et. al, 2015 used citizen science data to determine how many milkweed plants it takes to produce one migratory monarch. They found that for every 29 milkweed plants on the landscape, one fall migratory adult monarch is produced. So, to reach 127 million butterflies, a total of about 3.6 billion milkweed stems are needed across the monarch breeding range. Using current estimates of how much milkweed remains on the landscape (Pleasants 2017), these calculations suggest that we have about 1.8 billion milkweed plants to go (Thogmartin et. al 2017).  

“Monarchs in eastern North America are a beloved insect, but they’re in jeopardy, partly due to the loss of milkweeds in cropland,” said Wayne Thogmartin, a USGS scientist and the lead author of the report in a USGS press release. “Our study helps specify the conservation needs of this charismatic species.”

This seems like a daunting number to restore across the landscape, but the good news is, with everyone on board, we can make a difference and support a sustainable monarch migration. The agricultural Midwest is an important priority area for restoring milkweed, but Flockhart et. al 2017 found that overwintering monarchs in Mexico come from all across the eastern U.S, and habitat is needed everywhere to provide ample resources for monarchs, wherever they are. 

With over 1 billion more milkweed plants needed across the summer breeding range, we need everyone to help, today! It’s a great time of year to start monarch conservation activities, and there are ways for everyone to participate. Get involved by planting milkweed and nectar plants, spreading the word to your community, supporting monarch habitat conservation, and monitoring any milkweed habitat you already have!



Flockhart DTT, Brower LP, Ramirez MI, Hobson KA, Wassenaar LI, Altizer S, Norris DR. (2017). Regional climate on the breeding grounds predicts variation in the natal origin of monarch butterflies overwintering in Mexico over 38 years. Global Change Biology doi: 10.1111/gcb.13589

Nail KR, Stenoien C, Oberhauser K. (2015). Immature Monarch Survival: Effects of Site Characteristics, Density, and Time. Ann Entomol Soc Am 108 (5): 680-690. doi: 10.1093/aesa/sav047

Pleasants, J. (2017). Milkweed restoration in the Midwest for monarch butterfly recovery: estimates of milkweeds lost, milkweeds remaining and milkweeds that must be added to increase the monarch population. Insect Conserv Divers 10 (1): 42-53. doi: 10.1111/icad.12198                         

Semmens BX, DJ Semmens, WE Thogmartin, R Wiederholt, L López-Hoffman, JE Diffendorfer, JM Pleasants, KS Oberhauser, OR Taylor. (2016). Quasi-extinction risk and population targets for the Eastern, migratory population of monarch butterflies (Danaus plexippus). Sci. Rep. 6, 23265; doi: 10.1038/srep23265.

Thogmartin WE, Diffendorfer JE, López-Hoffman L, Oberhauser K, Pleasants J, Semmens BX, Semmens D, Taylor OR, Wiederholt R. (2017). Density estimates of monarch butterflies overwintering in central Mexico. PeerJ 5:e3221


The Monarch Joint Venture is a national partnership of federal and state agencies, non-governmental organizations, and academic programs working together to conserve the monarch butterfly migration. The content in this article does not necessarily reflect the positions of all Monarch Joint Venture partners. Header photo by Wendy Caldwell.