Pollinator Week!: Not So Common Milkweed

Today is the last day of National Pollinator Week, designated 13 years ago to raise awareness about the urgent issue of declining pollinators.  Most of the usual parades, speeches, and balloon drops were canceled this year, but at my house, the week was marked by the blooming of common milkweed (Asclepias syriaca).  Common milkweed has beautiful flowers, and smells as amazing as any orchid, but what it is really known for is being the most eaten milkweed of monarch butterfly caterpillars (Danaus plexippus plexippus).  (Over 90% of monarchs choose to lay their eggs on common milkweed.)

A monarch on meadow blazingstar.

The Migratory Monarch

Monarchs are part of a truly astonishing natural phenomena: a multi-generational, continental migration.  North from Mexico in March, monarchs search out milkweed and lay the year’s first generation of eggs in Texas.  The new butterflies continue migrating north, reaching throughout the Eastern United States and into Canada by early June. This is their summer breeding area, and they search out milkweed and lay the second generation of eggs.  The third generation is typically laid in July, and the fourth generation in August. These late summer butterflies, make the long journey back to Mexico, where they overwinter in a small number of colonies (this last winter there were 11 colonies), in select Oyamel fir (Abies religiosa) forests , until the spring warms up, milkweed begins to grow, and they begin their northern migration again.    

Monarch migration map, created by the U.S. Fish and Wildlife Service

Because Monarchs overwinter in colonies, it allows researchers to measure their population size.  They measure the density of monarchs on the fir trees and the hectares (1 hectare = 2.4 acres) of fir trees covered in butterflies.  They’ve been using this methodology since the mid-nineties, and are able to reach a very good estimate of how many Monarchs have returned, and how the population is faring.

Monarch overwintering area 1993 to 2018, Graph by Andre-Phillipe Drapeau Picard CC

What we have seen over the last 30 years, is that monarch butterfly populations have declined by over 80%.  From an average of 9.3 hectares (22.9 acres) in the 1990s, to 5.8 hectares (14.3 acres) in the aughts, to 2.8 hectares (6.9 acres) in the last decade.  The lowest point for the population was reached in the winter of 2013-2014, with the population occupying just .67 hectares (1.6 acres) of forest, translating to something like 33 to 43 million butterflies. This frighteningly low population drew attention. In 2014, the Fish and Wildlife Service was petitioned to list the monarch butterfly as endangered (a decision on this is now due in December of 2020), and the Obama administration set out a presidential memorandum supporting pollinator and monarch habitat restoration and the planting of milkweed.

Disappearing Milkweed

Over the last decade, it has become increasingly apparent that a main driver of the monarch’s decline is loss of milkweed due to agricultural expansion, and the Midwest’s wholesale adoption of herbicide resistant corn and soybeans. If we go back to the beginning of the century, we see that, at the same time as the monarch population was dropping, their food source, milkweed, was also in a steep decline.  Starting in the 90s, the adoption of herbicide resistant crops (Round-Up Ready corn, soy, etc.), enabled farmers to achieve much tighter control of weeds.  For example, in Iowa in 1999, common milkweed was detected in 51% of corn and soybean fields.  But over the next decade, with increasing adoption of herbicide resistant crops, one study showed that, from 1999 to 2009, agricultural milkweed was reduced by 97%, reaching 99% elimination by 2013.  Another study in Iowa, from the same time period found 100% elimination of milkweed from monitored fields, but no matter, the reality is the same: by some recent date, agricultural milkweed was eliminated throughout the Midwest.  

Habitat for more than monarchs.

The land area this represents is vast.  At this point, over 90% of corn and soy planted in the United States is herbicide resistant, and so the total land in herbicide resistant crops comes to about 66 million hectares (scientists and their metric system!;)), or more than 163 million acres.  To put this in perspective, Iowa is 35.7 million acres.  Minnesota is 51.2 million acres.  Illinois is 27 million acres.  And then there’s still 50 million more acres of corn and soy!  Land that was once monarch habitat has been reduced to a milkweed and flower desert.  And so how many milkweeds are we talking about here?  It’s estimated that this agricultural conversion translates to a loss of 850 million milkweed stems.   

The Plan

A conservation goal for the monarch population is that they reach an average overwintering population size to occupy 6 hectares of forest in Mexico.  At this level, research suggests the population would have resilience against extreme weather events, and reduce their chances of nearing extinction by 50%.  To reach this solid number of monarchs, we will need a correspondingly respectable number of milkweeds. 

graceful milkweed plant
Milkweed has such beautiful colors.

Unfortunately, by some super twisted twist of fate, it turns out that the low density, agricultural milkweed was actually the most productive milkweed, in terms of producing monarch butterflies, historically averaging 3.9 more monarch eggs than other milkweeds on the landscape. So to replace it,  we will actually need to plant an estimated 1.3 to 1.6 billion milkweeds to make up for its loss.  What?!!  What the!  GAhhh!!  Ok, don’t freak out.  We can do this, and people have been doing this. Researchers have been looking at just how this can get done. What’s life without a few big goals, right?  

“The main finding of our study is that an all-hands-on-deck approach could be essential to restoring the massive amounts of milkweeds needed to make the monarch population healthy again.”

Wayne Thogmartin, U.S. Geologic Survey researcher

The Deckhands

  • Conservation Reserve Program land (CRP)
  • marginal agricultural land converted to the Conservation Reserve Program
  • governmentally protected grasslands
  • roadside rights of way
  • powerline and rail lines
  • urban and suburban areas

A great paper from 2017, “Restoring monarch butterfly habitat in the Midwestern US: ‘all hands on deck’ ” by Wayne Thogmartin et al., demonstrates that the “all hands on deck” approach would be able to meet the milkweed needed to support a 6 hectare monarch population, but with a big caveat. It would mean looking to the agricultural sector to move 1/2 of the current marginal crop land into the Conservation Reserve Program, or a similar set-aside status. Given current government subsidy programs, crop insurance, and ethanol mandates, planting marginal land in row crops is still economically incentivized, and so marginal land entering a conservation-like status is not a certainty. But the 2018 farm bill did provide for an additional 3 million acres to be moved into the CRP program (equivalent to a bit less than a quarter of marginal crop land), so although it remains a large task, it looks like things can still move in the right direction.

Ask not what milkweed can do for you. Ask what you can do for milkweed.

But what are they asking of us, the urban/suburban, gardener/gardening club types, native plant enthusiast/maniacs? Well, they separate different levels of urban intensity with different expectations of stems of milkweed per acre, but on average for an urban/suburban area, in their highest level participation scenario, they are anticipating 3 milkweed stems per acre. My first reaction was to think they were certainly asking too little, but then I did the math for my town. My little 25,000 person town covers 10,580 acres, so that’s going to be about 32,000 milkweed stems. That sounds totally daunting, but lucky for us, with milkweed, one plant, left to it’s own devices, will likely become at least 10 stems in a couple years (though it would be great if a botanist/horticulturalist could verify this line of thinking!), which would knock down the number we truly need to plant to about 3,200. And we could round up to 4,000, to account for any milkweed crop losses, no problem. That’s totally do-able, right? It’s a lot. It will take a big effort, but I think it’s totally do-able.

Milkweed plays a big role attracting monarchs at the Public Library’s pollinator garden.

So, there is a large task ahead, especially for us in the Midwest, because the Midwest is where the huge losses occurred, and the Midwest is considered the monarch’s core breeding range.  In fact, a stable isotope analysis has shown that, historically, the majority of overwintering monarchs came from the Midwestern corn belt.


Urban/suburban Gardeners! Let us pick up our trowels!  In a world full of complicated questions, here is a simple question: Can you plant more milkweed?  And more importantly, can you think of two neighbors and three or more friends that you can convince to plant common milkweed?  Milkweed is needed not just in native plant enthusiasts’ yards, but in many yards, across the city, all the parks and empty lots, grocery store parking lots, along our city roadsides, in the medians, traffic circles, churches, libraries, every single school. Oh, the list goes On.  Widespread, chemical free milkweed (and nectar flowers), in town. This is our job.

But we don’t have to plant milkweed alone. There are many organizations working to save monarchs and pollinators. And importantly, there is a wonderful organization helping to create partnerships and provide coordination for this huge, multifaceted conservation effort, the Monarch Joint Venture. If you feel like getting to work, finding a local group, planting some milkweed, planting some nectar plants for migration (can’t forget those!), their website is a great place to find resources or find a group to work with.


Restoring monarch butterfly habitat in the Midwestern US: ‘all hands on deck.’ Wayne E Thogmartin et al 2017 Environ. Res. Lett. 12 074005

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. Pleasants, J. (2017), Insect Conserv Divers, 10: 42-53.

Monarchs in decline: a collateral landscape‐level effect of modern agriculture. Stenoien, C., Nail, K.R., Zalucki, J.M., Parry, H., Oberhauser, K.S. and Zalucki, M.P. (2018), Insect Science, 25: 528-541.

Neonicotinoid-contaminated pollinator strips adjacent to cropland reduce honey bee nutritional status. Mogren, C., Lundgren, J. Sci Rep 6, 29608 (2016).

A 10,000 Foot View of Monarch Conservation, Monarch Joint Venture

Monarch Conservation Implementation Plan 2019, Monarch Joint Venture

Pollinator Week: Pollinators, Plants, People, Planet, Pollinator Partnership

Monarch Butterflies, The Journey North

Interpreting surveys to estimate the size of the monarch butterfly population: Pitfalls and prospects. Pleasants, J. M., Zalucki, M. P., Oberhauser, K. S., Brower, L. P., Taylor, O. R., & Thogmartin, W. E. (2017). PloS one, 12(7), e0181245.

Massive Milkweed Restoration Could Help Save the Monarch Butterfly, by John Daley, Sierra Club

Pollinators and 2018 Farm Bill, by Scott Hoffman Black, Xerces Society

5 replies on “Pollinator Week!: Not So Common Milkweed”

This is such good information. I now have 5 different species of milkweed and have never seen a monarch caterpillar. Now I know why. I am looking for common milkweed ASAP!


That’s great! It was news to me, too! In past years we’ve had a good number of caterpillars on swamp milkweed, but I haven’t seen them on any of the other species so far, either.


Ah ha! Sara and I were just looking at milkweed in a national park by the beach last week. The caretakers had wisely roped off an area of milkweed that borders a soccer field.

Also, impressed my friend today when a swallowtail of some sort landed in a field of chamomile we were tossing a frisbee in today. Putting your posts to work!


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