Climate Change

Canary in the Cornfield: Why the Fuss about Monarchs?

February 26, 2014

Follow Lisa
Monarch feeding on orange glory, Asclepias tuberosa, a variety of milkweed. Photo courtesy of Lewis Feldkamp.
Monarch feeding on orange glory, Asclepias tuberosa, a variety of milkweed. Photo courtesy of Lewis Feldkamp.

Everyone is talking about the record low count of monarchs at their overwintering site in Mexico, but what does the science say is happening to them and why does it matter?

Monarch butterflies have a special place in the North American imagination. They are beautiful, plentiful, and have a legendary predator-repelling capacity.

They are the state insect or butterfly of seven US states and an important ecotourism resource in Mexico, where millions of monarchs overwinter on oyamel fir trees.

With a range that covers the United States, watching monarchs was once a shared cultural experience, but that is changing fast.

No scientists are arguing that monarchs will go extinct, but numbers are dwindling at an alarming rate.

The most recent overwintering population in Mexico covered about 0.67 hectares of forest – a record low. That’s down approximately 44% from the 2012-2013 overwintering population, which was already very low (Rendon-Salinas 2014).

The record high was in 1996-1997, when the monarchs covered about 18.19 hectares of forest (Rendon-Salinas 2014).

Why are the numbers so low?

The short answer: Monarchs need two things, milkweed and very specific overwintering conditions. Both of these are at risk.

Overwintering and Oyamel Fir Trees

There are two large North American populations of monarch butterflies. The eastern population is the largest. These butterflies come from Canada and various US states, but they all (or very nearly all) gather in Mexico to overwinter, primarily in the Monarch Butterfly Biosphere Reserve (MBBR).

Overwintering butterflies go into a state called diapause – it’s not exactly hibernation but they’re not fully active either.

During diapause they can preserve calories without eating, but they can’t easily fly and they require a stable temperature range to stay in this state.

Monarchs overwintering in the Monarch Butterfly Biosphere Reserve. Photo by Flickr user Pablo Leautaud through a Creative Commons license.
Monarchs overwintering in the Monarch Butterfly Biosphere Reserve. Photo by Flickr user Pablo Leautaud through a Creative Commons license.

If it gets too cold, these butterflies freeze, but if it gets too hot, they become active and have to search for nectar. The most devastating recorded freezing event in 2002 killed off 75% of the butterflies in the observed area and up to 90% estimated in other areas (Barve 2012, Brower 2004).

It turns out that dense forests of oyamel fir trees provide the most stable temperature for monarchs to overwinter.

Deforestation in the oyamel forests where monarchs overwinter can be a major threat to the species (Barve 2012, Brower 2012, Lopez-Garcia 2012, Vidal 2013).

Take out even a single tree and the monarchs near the open area are more likely to freeze (Brower 2011).

Increased protections and monitoring by the Mexican government and conservation organizations has all but stopped illegal logging in the MBBR (Vidal 2012), making deforestation less of a threat.

Unfortunately, the overwintering sites are still threatened by extreme weather events and climate change.

The places with the highest densities of butterflies now are predicted to have more extreme cold/wet events by 2050, making them unsuitable for the butterflies (Barve 2012, Brower 2012).

In addition, the trees that monarchs seem to favor and perhaps rely on when overwintering, the oyamel firs, are threatened by predicted warming trends in the region (Saenz-Romero 2012).

Starting from the prediction that there will be a 1.5C rise in temp by 2030 (a prediction questioned by Chip Taylor of Monarch Watch on Yale E360) a study by Saenz-Romero suggests that suitable habitat for these trees will quickly disappear in its current range until there is no suitable habitat left in 2090.

A surprising threat to monarchs from climate change has to do with the way that they navigate.

Monarchs’ decisions about which way to fly are affected by temperature. Exposure to colder than usual temperature in the fall causes monarchs that were headed south to turn around and fly north (Guerra 2013, Kyriacou 2013).

That’s right: if they hit colder temperatures, they might fly north for the winter!

It is also possible that if temperatures in their overwintering grounds were not cold enough, they would continue to fly south until they found a colder area (Guerra 2013, Kyriacou 2013).

Milkweed and Changes in Agriculture

Despite the improvements in protecting the MBBR, monarch numbers have continued to plummet.

This turned the attention of scientists to another phase of monarch life. In the spring and summer, monarchs of the eastern population spend their time in Canada and the United States, breeding and laying eggs on milkweed.

Monarch caterpillars are uniquely adapted to eating milkweed. It is likely that they need milkweed (any species of milkweed) to survive (Luna 2013).

Monarch caterpillar on common milkweed, Asclepias syriaca. Photo by Flickr user Benimoto through a Creative Commons license.
Monarch caterpillar on common milkweed, Asclepias syriaca. Photo by Flickr user Benimoto through a Creative Commons license.

Milkweed used to be abundant in the United States, especially in agricultural fields. That changed with the introduction of glyphosate-resistant crops. Milkweed all but disappeared from agricultural lands (Luna 2013, Pleasants 2013).

At the same time, because of increased demand for corn and soybeans in biofuels, more land has been planted with these glyphosate-resistant crops, leading to an estimated 58% decline in milkweed habitat in the Midwest (Pleasants 2013).

There is strong evidence to show that the Midwest is an important area for monarch production. Even though monarchs are typically only counted at their overwintering grounds in Mexico, stable isotope measurements can be used to tell where butterflies were hatched. The data accumulated thus far shows that most of them are from the Midwest (Flockhart 2013, Miller 2012, Pleasants 2013).

Furthermore, a study by Pleasants that estimated monarch production based on availability of milkweed (taking into account loss of agricultural habitat) revealed a positive correlation between estimated monarch production in the Midwest and the subsequent size of the overwintering population in Mexico (Pleasants 2013).

That is to say – as milkweed friendly habitat in the Midwest disappeared, the number of monarchs overwintering in Mexico dropped.

Going forward, climate change may also play a role in the suitability of monarch habitat in the US, but there are many variables that make it difficult to determine what effects it will have (Zipkin 2012).

See Richard Conniff’s excellent interview of Chip Taylor on Yale E360 for more detail on the impact of agricultural changes on butterflies.

Where are the butterflies going?

The drop in butterflies counted in the overwintering grounds is disturbing, but it does not necessarily match up to the drop in monarch butterfly numbers overall.

The butterflies could be going somewhere else.

As mentioned above, there are two major populations of monarchs in North America. They are called the eastern and western populations, but they are not genetically distinct, indicating that they continue to interbreed (Lyons 2012).

In recent years, the western population has grown.

Furthermore, some North American monarchs stay in Florida year-round or travel to the Caribbean (Flockhart 2013). In Mexico, there are some small clusters of butterflies farther east than the main overwintering population (Barve 2012).

It could be that as conditions in the MBBR have become increasingly unsuitable for overwintering, more monarchs have chosen these alternative locations.

Beyond the North American populations, there are also introduced populations of monarch butterflies in Hawaii, New Zealand, and Australia. These colonies have shown that monarch migration is highly adaptable.

Why is the overwintering phenomenon important?

Even if it is possible for butterflies to survive elsewhere, it is important to try to save the overwintering phenomenon.

Not only is it an amazing sight that we should want to preserve for its own sake and for future generations, it contributes to monarch health, to the economy, and to the preservation of pristine forests and biodiversity.

Lincoln Brower told Science Now that when butterflies stay in the same place for many generations, as they might if the overwintering phenomenon ends, they are more susceptible to the deadly Ophryocystis elektroscirrha parasite.

Ecotourism is a significant source of income for people living in and around the MBBR (Lopez-Garcia 2012, Lopez-Hoffman 2010, Vidal 2013).

ser Aquiles Carattino through a Creative Commons license.
Tourists enjoy the butterflies in the Monarch Butterfly Biosphere Reserve and provide a benefit to the local economy. Photo by Flickr user Aquiles Carattino through a Creative Commons license.

Money from ecotourism is an important incentive for people to protect the forests. Without this many people in the area would have little choice but to turn to logging as a source of income (Vidal 2013).

In the United States, butterflies may be a “canary in the cornfield,” as Brower calls them in the aforementioned Science Now article. The loss of monarchs could be the first sign that the widespread planting of glyphosate-resistant crops is irrevocably disrupting food webs in the Midwest.

What can we do?

See Matt Miller’s previous Cool Green Science post for some of the most important ways that we can help keep monarch butterflies going.

Other things you can do to protect monarchs include:

  • Carefully planned, controlled burning of milkweed patches can help make sure that have new growth at the times when monarchs are coming through and laying eggs (Baum 2012).
  • Be careful what kind of milkweed you plant. Be sure that it is a species that grows locally. Karen Oberhauser warns Science Now that some tropical species grow year round, which encourages butterflies to stay in one place instead of migrating.
  • Continue to protect the MBBR by tightening logging restrictions (Brower 2011, Navarrete 2011) and paying people there more for ecosystem services (Vidal 2013).
  • Relocate oyamel firs to habitats that will be more suitable as the climate changes (Saenz-Romero).


Lisa Feldkamp

Lisa loves all things citizen science and enjoys learning about everything that goes on four legs, two wings or fins - she even finds six and eight-legged critters fascinating at a safe distance. She has a PhD in Classical Literature and Languages from the University of Wisconsin - Madison and enjoys reading Greek and Roman literature or talking about mythology in her spare time. More from Lisa

Follow Lisa

Join the Discussion

Please note that all comments are moderated and may take some time to appear.


  1. Like George Washington Carver, “I love to think of nature as an unlimited broadcasting station, through which God speaks to us every hour, if we will only tune in.”

  2. Regarding that: “Starting from the prediction that there will be a 1.5C rise in temp by 2030 (a prediction questioned by Chip Taylor of Monarch Watch on Yale E360)… “, seems to me that there is a minor misunderstanding. On the Yale E360 interview, Chip Taylor mentioned that we projected a temperature increase of about “two degrees centigrade in that 17-year period”. That is not exact. We estimated an increase of 1.5 oC for the decade centered in year 2030 in comparison with the average temperature for the 1961-1990 period, and not in comparison to 2012 or so. Because at present it has occurred a warming of approximately 0.8 or 1.0 oC, we can expect that from today (2014) to 2030, it will happen an increase of approximately 0.5 or 0.7 oC, which is quite realistic. In any case, Chip Taylor is absolutely right when it says that “if the planet has temperature rises that are that fast, we’re not going to be talking about monarch butterflies. We’re going to be talking about survival of a lot of things”. Yes, by the end of the century, the ecological disaster will be beyond imagination. See our paper in:

    1. Thank you Dr. Saenz-Romero for the clarification on the temperature shift predicted and the length of time that it is likely to take.

  3. It’s not just Monarchs that are declining, but because of our incredible increases in mono-culture ,herbicide, glyphosate, use over the last decade.Not only the toxicity of these chemicals, but that they destroy diversity, of plant life, milkweed, and any other plant but the intended crop, multiply that by hundreds of millions of acres being treated, and we wonder why nature is in decline.The Prairie is our most impacted eco-system , most prairie birds are declining. All our native Prairie grouse are. The Atwater’s prairie chicken less then a hundred left in the wild. The lessor Prairie chicken will probably be listed, the Sage grouse also, the greater prairie chicken range and numbers greatly diminished to the flint hills and sand hills.

  4. I agree the Monarch is a ‘canary in the coal mine’, in that the annihilation of insect populations throughout the Midwest is largely unseen and unstudied, except for the Monarch. I am not as sanguine as you about their future; I believe we’ve already passed the point of no return for them and for many other insect species, which leads to poor prospects for the birds that need them, etc., etc., etc.

  5. The invasive species black swallow-wort certainly doesn’t help either. I learned recently that the mortality rate of monarch larvae is 100 percent when they feed on this species, which is also from the milkweed family. It’s an early detection species we are certainly keeping an eye out for here in Indiana.

    1. Thank you Susan – I hadn’t seen anything about the black swallow-wort in my reading. Do you have the names of or links to any articles that talk about it. I would be interested in finding out more.

  6. I mow around all patches of milkweed on my farmstead and other grassy areas. I have done this for years, because I am aware that Monarchs eat them. I live near Westhope, ND

  7. The Basic Theme

    to rescue life on Earth
    gently reduce our population,
    safely recycle all our waste
    and carefully grow the wilderness
    to return the balance of Nature
    for an evergreen village civilization
    so wise and great at least by 2028
    women are free and men cooperate.


    1. An overwhelming majority of our environmental problems could be solved by reducing our total population.

  8. Ive seen this wonderful sight of these clusters of butterflies in the coastal Calif area,and it’s a magnificent experience.We must change our use of chemicals for their survival

  9. […] February 26, 2014. Canary in the Cornfield: Why the Fuss about Monarchs? Monarch populations face a multitude of threats and stresses. Not only are monarchs being impacted by a sharp decline in available habitat (which is milkweed), but also the disastrous effect of temperature and precipitation shifts in their overwintering grounds. Midwest milkweed, the plant that the majority of North American monarchs depend on to begin their life cycle before hibernating in Mexico, is estimated to have been reduced by 58% since the introduction of glyphosate resistant crops to agricultural fields–hence making them a ‘canary in the cornfield’. But once the butterflies arrive in Mexico to end the season, primarily in the Michoacán’s reserve, changes in climate disrupt the stability required to remain in a hibernation-like state of diapause. Warmer temperature will waken monarchs to search for nectar, and butterflies rarely survive cold events like the record 2002 freeze that killed off 75% of monarchs wintering in the area. Monarch populations may be relocating to warmer territories year round, raising vulnerability to the deadly Ophryocystis elektroscirrha parasite by skipping seasonal migration. Read more. […]

  10. What milkweed is the best plant for the Texas Hill country

    and where can one buy plants or seeds. I have 5 acres that I would love to raise some milkweeds for the Monarchs.

  11. How did we get to be so compliant? We should plant backyard milkweed and consider relocating fir forests, rather than addressing the real causes of this problem? Why aren’t we hopping mad that GM crops and our run-away greenhouse gas emissions are causing this issue?

    Our children are truly going to shake their heads.

    1. I totally agree. Nobody is doing anything to help, and people are just complaining. If they really cared about these wonderful creatures they would do something. Farmers, scientist, all thoses people who are studying these almost extinct creatures should find away or fix or recreate the forest that had a fire.