We really don’t know why monarch butterflies are disappearing

Monarch butterflies- the most iconic of American insects- have declined to perilously low numbers this winter:

Total Area Occupied by Monarch Colonies at Overwintering Sites in Mexico. The decline is statistically significant, and numbers that were bad 20 years ago would be considered good today.

This is horrific. We’ve lost over 80% of the butterflies.

The waning of our monarchs has lead to the inevitable speculations as to the cause. Which is fair enough. But I’d like to point out the ideas are just speculation.

No one really knows why Monarchs are declining, or why 2012-13 is such a bad year. 

Lacking the proper experiments to determine how various factors affect the butterflies, and lacking solid data about their milkweed food sources, assigning causes to the phenomenon simply isn’t possible. The monarchs are disappearing. That’s all we can say with certainty.

Candidate causes abound, though.

  • -The severe heat and drought of 2012 across much of the monarch’s summer range may have reduced the monarch’s host plants and stressed the remaining animals.
  • -The extensive use of agricultural herbicides in conjunction with herbicide-resistant crops could be eliminating the monarch’s food supply, perhaps causing the gradual decline over the past 10 years.
  • -The destruction of milkweed habitat for corn ethanol production, also a gradual new phenomenon in the past 10 years, could reduce the monarch’s food.


I had always kind of assumed we’d be able to blame the Mexicans for the eventual extinction of our treasured monarchs. After all, our eastern butterflies converge each winter on a fragile, postage-stamp patch of a forest near Mexico city. It wouldn’t take more than a couple days for los bulldozeros (or, whatever) to knock out the winter refuge for the entire monarch population. But since the Mexicans appear to be upholding their end of the migration, this loss may be on us.

People who care about North America’s most famous butterfly should be treating the decline as an emergency. One more bad year could spell the end.

In particular, we need two big things. First, we need the science. This means money to ramp up monitoring & experimentation. A lot of it. And soon. It’s hard to know how to bring the monarchs back without knowing why they are disappearing. Second, we need milkweed planted. A lot of it. And soon. While we don’t know why the monarchs are declining, the most plausible explanations involve the butterfly’s food sources, so reversing milkweed loss should be an obvious step.

I was going to add a photo of a monarch to this post, but the whole thing is just too damn depressing.

34 thoughts on “We really don’t know why monarch butterflies are disappearing”

  1. I’m interested to hear more about your evidence that the wintering habitat in Mexico is being maintained at levels that are biologically relevant for the monarch. My understanding is that there’s some amount of illegal logging. Problems everywhere, attribution is hard, etc.

    1. Yep … illegal logging plus too much tourism visitation being allowed to monarch habitat areas in Mexico. We DO know at least part of why; sorry, Alex, and I can’t believe you didn’t mention either one of these.

      1. Perhaps, but the monarchs are still taking up only a very small space within the Mexican reserves. Proportionally, far more of their summer habitat has been destroyed than their winter habitat.

        1. Do you have a reference for the proportion of degraded habitat being greater in summering vs wintering habitat? I hope I don’t sound like a jerk asking, I can’t find the answer on the web and I really am interested.

  2. A shame. At least the decline is being detected and hopefully some action taken to reverse the trend. A lot of things disappear before even one has time to realize they are gone. Pelecinid wasps use to be so common around the Rancho Grande station in n Vzla. but it´s been years since we saw the last one, the same goes for Leptogenys tiobil, presumably now somewhere higher up in the mountains. Some 6-7 years ago populations of a species of Polistes species simply crashed in Maracay and the surrounding valleys. Totally gone. From being everywhere, I mean like dozens of nests on trees and farm buildings, to zero. Along with that some other wasps also checked out including a smaller Polistes that is just now making a timid comeback. The larger Polistes is still gone though.

  3. I’m currently reading ‘Four Wings and a Prayer’ by Sue Hapern, written over 10 years ago now it’s an exploration of the monarch migration and the scientists who have studied it. it was recently made into a documentary. Both book and documentary focus largely on the loss of the forests in Mexico (mostly through illegal logging) as the causes of the declining migration, perhaps when the book was written the issues you’ve mentioned for monarchs in the US weren’t yet such an issue. Imagine losing the amazing spectacle of the monarch migration, imagine losing the butterfly itself, just awful….

  4. There is some really interesting evidence emerging that pesticide usage is a much better correlate for global grassland bird declines than agricultural intensification. I wonder if this is the case for Monarchs and many other insects that may frequently inhabit many of the same open and edge habitats as these birds. This is a sad state of affairs for our wildlife.

  5. There was an irruption of Monarch butterflies in Saskatchewan last summer. I don’t know how or why they got here, only that the odds of successful reproduction would be extremely low since milkweed is not a common species in most of the province. I do know that a few successfully reproduced in the garden of an extended care home where I volunteer. They have a “butterfly” garden which included milkweed, though Monarchs normally don’t come here.

  6. Two comments:
    — I was concerned last year about the severe and extensive drought conditions over the whole lower Midwest, Texas and northern Mexico for a variety of reasons, but particularly for the monarchs’ sake, because of the effects on milkweeds I noticed. Around here, almost all the local populations had gone dormant well before the migration, thus reducing the production off the migrating generation. Further, the flowers of migration season nectar sources such as “Aster” & Liatris withered on the vine, so to speak. Could it be that whatever other problems monarchs are having, the weather was the primary cause of the decline?
    — The native milkweeds of your area may be difficult to acquire, and once acquired, are at least initially rather slow-growing. I recommend planting tropical milkweed Asclepias curassavica, as much as you have space for. This milkweed is highly attractive (seemingly more than some temperate zone natives) to ovipositing monarchs, fast-growing even after defoliation by the caterpillars, is very heat tolerant with minimal watering, and has ample cardenolide content that can be sequestered by the larvae, rendering them, and the adults they mature into, quite distatsteful to predators.

      1. Milkweed has virtually disappeared in most of the U.S. & Dow Chemical Co. latest milkweed killing pesticide was approved by the EPA a year ago but that approval has been retracted in 2015. However there are simply too many interacting other pesticides having saturated the U.S. from coast to coast affecting milkweed & monarchs, not to mention other butterflies,bats, birds & bees & every other living thing, that planting a few milkweed seeds can remedy but good luck.

  7. In 2012, at least in the eastern third of Texas, conditions were much improved from the devastating drought in 2011. Milkweed was relatively abundant along roadsides (especially NE corner of state) and highway right-of-ways. Nectar sources from northeastern Texas into the hill country were in also relatively good shape. Now, further south and west in Texas I can’t say.

    George Cates with Native American Seed in Junction, Texas posted the info. below to the Native Plant Society of Texas Facebook page. Native American Seed is working with the Xerces Society to develop large-scale propagation of native milkweed seed.

    “Native milkweeds are easy to grow!

    I would like everyone to know that the picture painted by Monica is not that bleak. She left off the other half of my quote in the title of her article. It is not clear at this point if the mass production will work or not because no one has ever tried it before. We are on our third year with the project. We are enjoying great success and are doubling our plot sizes this year. It takes these two milkweed species at least three years to reach full maturity and so we will be able to better know the potential soon.

    On another cheery note, these two species (as well as many other asclepias sp.) are EASY to germinate and establish from seed. If anyone wants to see photographs please inquire with NAS Facebook page.

    The truth remains there is no substitution for effort, and no matter what anyone says Tropical milkweed is in no way shape or form a reasonable substitute for our native milkweeds. If we buy into that slippery slope we can just start planting KR bluestem because its so much easier to grow.

    Even Monica herself says that tropical milkweed is akin to junk food and in the absence of anything else it’s all the caterpillars have to eat…. Does anyone else see the problem here? As long as you support the tropical milkweed production that’s all you will ever get. As long as you believe you can’t grow native milkweeds you can’t.

    Native American Seed is committed to providing a reliable and steady supply of local native milkweed seeds, raising awareness, and imparting knowledge about these species for the benefit of both the imperiled Monarch butterfly and the responsible people who care greatly about their survival.”

  8. Ironically, Alberta had a flush of Monarchs last year, an unprecedented sightings as far north as Edmonton, where I live. Many people who had milkweeds in their gardens were fortunate enough to see a new generation.

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  10. This is indeed depressing. I was concerned about Monarch populations 20 yrs. ago, and now to see these reports is very sad for me.

    Your readers might be interested to know that I found swamp milkweed, Asclepias incarnata, to be a very favorable food plant for monarch larvae. It is easy to grow in a garden setting and in towns where common milkweed might not be allowed. (Despite its name, it does not need swampy soil). It’s a lovely plant and seeds can be ordered through Prairie Nursery in Westfield, Wisconsin.

    1. I’ve got a young swamp milkweed in my prairie garden- it flowered for the first time last summer! Here’s hoping it becomes monarch food this year.

      1. I would suggest growing more than one plant. In my experience, it is not a long-lived plant. Aphids can weaken it, too, plus there is some little fuzzy caterpillar that can bother the plant, too. I’m sure you will identify it if you see it. I once had a plant with 11 stems. It only lived three years. Most of mine only get one or two stems.

        Interesting factoids:
        Orioles will love the fibers of overwintered dead stems so leave them up. The fibers were once used for fabric weaving/spinning, or there was a scheme to do so… fascinating history. Also, there was a huge use for milkweed silks as a better form of kapok. I cannot remember the full story on that but am sure you could find it online.

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  13. Christopher McLaughlin

    Well, what happened between the ’96-’97 and ’97-’98 years when the hectares covered went from 18.19 to 5.77 and again in the ’03-’04 to ’04-’05 years when they dropped from 11.12 to 2.19 hectares? Aren’t either of those declines drastic or am I misinterpreting the data?

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  15. I was wondering if wasps could also be a factor. We have so many wasps in our area and see them going after all sorts of caterpillars. The caterpillars can’t seem to get big enough before the wasps find them. I have to keep my monarch caterpillars in a butterfly enclosure in order for them to survive. I have so many milkweed plants but it doesn’t seem to matter because the caterpillars are sitting ducks.

  16. Leslie Penning

    In Missouri, I have also seen a decline. Not just Monarchs, either. Usually by this time I have numerous species in my yard, and adjacent fields. I am unsure if the neighboring farmed fields have been treated, and the milkweed and plants nearby have been victims of the over spray, or if was the very wet spring we had. It’s very discouraging. I have seen more mourning cloaks than I remember, but the tiger swallowtails, fritillaries (sp!) , and even the milkweed beetles, and tiger moth caterpillars, are non-existent so far this summer. Usually by now the milkweed have the moth caterpillars and beetles. I have nothing. We need to figure this out.

    1. And if u figure it out…? It can only be spraying if u have nothing. Why don’t you find out if the neighboring farmed fields have been treated & with what, just for your curiosity and ours. Nothing you can do if the insects have died off already from some insecticide except learn what the product is.

  17. 7/31/2013An Observation from the Western Adirondack Park in New York State: I have seen NO monarch butterflies this year and find this especially disturbing. We have been visiting this area since 1981 and have lived here for since 2007. Every summer, we have been graced with these butterflies until this year. They typically arrive in June and leave in late August or early September. We had a long winter and cold, wet spring this year, waking up with snow on the ground until the end of April. The first 10 days of May were warm & sunny & then we had a freak snow storm the day after Mother’s Day & a late frost at the end of May. June was soggy and we have had extreme temperature swings in July ranging from the 90’s to the low 40’s combined with high humidity and more rain. I suspect our changing climate has played a roll in this. There are plenty of milk weed plants here. I fear that we will not see any this summer as they are well over a month late now.

  18. I have not seen one monarch butterfly here in Hamilton, Ma. this season. I reserve 1/4 acre in milkweed which is very healthy and tall (Asclepias syriaca). Of course, Lincoln Brower has predicted for years, that the migration, not necessarily Danaus plexippus, would go extinct.

  19. I live in the northeast of U.S. & 25-30 years ago saw milkweed growing everywhere. I mean it is a weed, meaning ‘grows on it’s own,’ right? My question was how can a land mass as large as the United States make a weed disappear from the whole country, poof! Just like that. Cant find any answer on Google which automatically goes to dozens of Monarch butterfly sites the second it sees u begin typing the word, milkweed. We used to cook the leaves like spinach, after soaking the milk out of them. And was nothing more beautiful than the mid-summer large, dark pink, beaded flower, that eventually became a large green pod, that eventually turned to a brown pod & eventually burst open w/ delicate flat shaped seeds attached to masses of white silk ‘angel hair’ and the more the brown pod split apart the more the white silk fluffed in every direction hanging out of the splitting pod & was eventually caught in a breeze & floated away carrying the little brown flat seed with it. Never thought this much about them back then, like they say, u don’t appreciate something till it’s gone. Miss the Monarchs but think I actually miss the milkweed more.

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