Saturday, October 5, 2013

When did "Common Milkweed" Become Common?

Monarch on common milkweed. © Beatriz Moisset. 2010

The prairies of this continent used to be rich in biodiversity before they were plowed under and turned into cropland. We can only guess at the structure of those lost plant communities by studying the remaining plots of prairie. A couple dozen species of milkweed (Asclepias) prospered in the Midwest two centuries ago, each adapted to its own habitat. Certain species preferred high moisture, others, drier spots. Some showed a preference for coarse, loose, damp, or undisturbed soils; still others did well on almost any soil type. Some needed more sunlight than others. A few survived drought or fire better than others.

Among this variety of habitat preferences, common milkweed, Asclepias syriaca, a "weedy" or pioneer species, does best in disturbed areas, patches that, for a variety of reasons, have lost their established vegetation. In a few years, it is displaced by other species when the ecological succession continues. Two hundred years ago, disturbed spots must have been rather uncommon. Everything changed when the prairie became farmland and was annually plowed. Many species of these plants lost ground under the new treatment. However, plowing created ideal conditions for the common milkweed. It can grow between rows of plantings and along edges, and its deep rhizomes, underground stems, allow it to survive year after year despite plowing. Is this when common milkweed earned its name?

For many years, farming practices in the Midwest continued to disrupt the land creating the conditions that this plant prefers. Farmers dislike this plant for its tenacity. It is said that farmers rejoice at the sight of big fat yellow and black caterpillars devouring the cursed weed. By now, you probably guessed that those caterpillars are monarchs.

USDA map of corn production

Monarch butterflies feed on most types of Asclepias, but Asclepias syriaca is by far their main food source. Perhaps, when the prairie became our breadbasket, it also became the monarch's breadbasket, who took advantage of the spreading weed.

In the meantime, all along the Eastern United States, forests were being cut down. A tree-covered area is not the right habitat for any types of milkweeds. When forests were cleared, the conditions favored by common milkweed emerged. Populations of monarchs must have grown along with the expansion of this plant.

If all this is true, and much of it may be just speculation, then these butterflies must have benefited from these man-made changes. We must ask ourselves: what were the monarch's populations like before the expansion of common milkweed numbers? Were they as abundant before the early 1800s as in recent times? What were the population sizes in their overwintering sites in Mexico? We became aware of the monarch butterfly migration in the past one hundred years. The whole story of their incredible trip to Mexico became known only in recent years. Curiously, no references to monarchs in Mexico can be found until 1890. By then, the transformation of the Midwest with its expansion of common milkweed was well underway. Were their numbers in Mexico so low before this time to escape notice?

Present day discussions about monarchs and their preferred food plant, common milkweed, seem to accept the numbers reported around the 1950s as the norm. Those were the highest numbers ever recorded, and it is assumed that these had remained the same for a long time, perhaps from the days when glaciers receded, tens of thousands of years ago. However, we must consider the possibility that milkweeds and monarchs were never as abundant as in the twentieth century; that this is largely a man-made phenomenon. The numbers of both, plant and butterfly have been going down steadily in recent years. Is the new normal similar to the old normal of hundreds of years ago?

Monarch caterpillar on common milkweed. © Beatriz Moisset. 2010

List of Articles

Beginners Guide to Pollinators and Other Flower Visitor

© Beatriz Moisset. 2013


  1. A very interesting perspective! Thanks for this post.

  2. I never thought of it this way. You are right, though, that everything we do has an impact on the environment. I grow several kinds of milkweeds and lots of other plants that get lots of insects feeding on them. I try to plant a lot of things that could have possibly grown as native plants here in the past, but I do not know for sure what was here, and what I acquire is not going to replicate the past. The past is gone, and it is good we are planting what we can for the critters.

  3. What we need to keep in mind as we read this and conjecture on the past is that prairies were actually a bit more widespread in pre-settlement times than people think. Most of us imagine that they started in some grand line west of the Mississippi, but in fact recovering burns, sandy exposed areas along water bodies, oak savannas, and even pure tallgrass stretches could be found as far east as Virginia, New York, and eastern Ontario. While it is true that milkweed can be seen in abundance along roadsides and railbeds, disturbed areas are hardly a modern invention. The map up there leaves out a few key details (aside from Canada and Mexico); corn production has since taken over lands that would have been largely tallgrass prairie anyway. Even to the east, there were still once herds of buffalo that stomped out "natural" road right-of-ways! Where I am from in northern Ontario, prime monarch summer vacation land, we still have a decent abundance of grazing ungulates, fairly open sandy coniferous forests, and what I can assure readers here to be an abundance of A. Syriaca. This is in a land of few roads, utility corridors, railroads, or much of human development.

    1. Indeed, the prairies were much wider than the area occupied by corn fields at present, which is what this map represents. I have seen maps of the historical range of tall and short grass prairies and agree with you; but couldn't find one for this blog. If you are interested on maps of monarch generations' distributions, you may want to look at this article by Flockhart on multi-generational colonization by monarchs., available as a PDF document. It has been established that the bulk of monarchs reproduce pretty much in the same area where most corn and soy beans fields are.
      I am not surprised that you have plenty of milkweeds in Ontario. I can say the same about the Eastern US. It may not be a scientific statistically proven fact but an accumulation of personal observations, including mine. The trouble is that, if the monarchs don't find enough milkweeds in the Midwest, the next generations that migrate to Canada or Eastern US are severely reduced. How did the monarchs fare in the area with abundant milkweed mentioned by you?

  4. This is a very interesting point of view, however you fail to consider the other factors in play that are affecting the current monarch and milkweed population such as:

    a) The accepted use of Roundup in both commercial and residential settings that is killing all types of beneficial weeds and host plants, not just milkweed.
    b) Global warming which has been responsible for the extreme temperature changes that have kill tens of millions of monarchs due to the unexpected and sever temperature drop in their wintering areas.

    What you say about the past number of monarchs may very well be true but that should not be allowed to overshadow our current issues of the drastic decline in monarch and milkweed population.

    1. I haven't covered the topics you mention because I planned to do so in a future post. I was simply posing some questions because nobody else seems to be doing it. Moreover, the size of historical populations may not be a guarantee of survival. Take the passenger pigeon; their numbers were immense when Europeans arrived. It is possible that they became even more numerous when Native Americans were devastated by European diseases. None of that prevented their extinction a couple of centuries later.

      I am just as concerned about monarchs as you are, or I should say, I am primarily concerned about entire ecosystems and I notice that sometimes people lose perspective by narrowing their concerns to just some iconic species. You may want to read the comment I left recently in another blog:

      "According to some recent studies, most of the monarchs in Canada and the East Coast (fourth and fifth generations) are descended from the ones born in the Corn Belt (second and third generations). It seems that the weakest link in the chain is the Midwest where herbicide resistant crops plus herbicides are decimating the common milkweed. Trying to strengthen the other links may be futile.
      Our milkweeds and nectar plants, here in the East, are bereft of monarch butterflies. The same thing applies to the Oyamel forests of Mexico where overwintering monarchs used only a small fraction of the available habitat last year.
      If we want to save the monarchs, we need, most of all, to stop using herbicides in our breadbasket, which until recently was also the monarch's breadbasket."

      If you want to know what I will be discussing next, you may want to take a look at some of the references I intend to use:
      Flockhart. Multigenerational colonization.
      Pleasan, Oberhauser. Milkweed loss in agricultural fields because of herbicide use: effect on the monarch butterfly population
      Impact of GM crops on pesticide use in USA.

  5. Thank you and I appreciate your detailed reply. I was unaware of your previous posts as I followed a Facebook link to this post. I look forward to reading more of our blog posts.

  6. Your post ignores known disturbances that caused frequent turnover to less competitive, non-climax species in the native prairies. To quote a paper from the Wyoming Game and Fish Department which talks about the original state of the prairie:
    "Buffalo, prairie dogs, and fire were crucial factors in maintaining grassland habitat. Historically, the Great Plains grasslands consisted of a complex pattern of distinct patches of grasses and forbs. These patches were created by events, often referred to as “disturbances”, like intense grazing or vigorous wallowing by large herds of migratory buffalo, steady cropping of grass and digging by prairie dogs, wildfires, or water collection in low spots. Each type of disturbance created a subtly different habitat patch, and each patch changed slowly as it recovered from the disturbance. The result was a “shifting mosaic” of habitat patches across the prairie landscape."

    So, while Asclepias syriaca may have increased in numbers in some disturbed habitats, we must consider the possibility that before the prairies were commandeered for wheat, corn, soy, and pasturage, they supported even larger areas of disturbed soils.

    1. Thanks for the additional information. I am glad to learn more. If the "grasslands consisted of a complex pattern of distinct patches of grasses and forbs" caused by disturbances, that diversity might have benefited other species of milkweeds, not just A. syriaca -something else worth pondering about. This material is hard to come by; this is why I posed this subject as a question. We just don't know enough. Sometimes one gets the impression that nothing happened before the 1950s. Monarch Watch has provided statistics from that time on. I have contacted them without success. Perhaps, they are not interested in going further back in time. Of course any estimates would have to be rough ones as there were no censuses then.

      Another thing that intrigues me is the lack of references to monarch butterflies in pre Columbian and Colonial Mexico. If they were as numerous then as in the 1950s, somebody would have recorded this phenomenon. I keep trying to find out more in the subject. I didn't include this in the article because as you know: lack of evidence is no evidence of lack. But, I keep wondering.