Milkweed Pollination: A Series of Fortunate Events

Most of us know a friend or relative who isn’t content to follow the standard path in life. Why do things the simple easy way when there’s a more complicated option available? Maybe you’re even that person yourself. If so, you’ll appreciate the pollination strategy of milkweed plants.

A painted lady butterfly feeds on nectar from a butterfly milkweed flower.

Most flowering plants that attract insects as pollinators have adopted a fairly simple strategy for getting their embryos fertilized. They use color, patterns, and/or scent to draw bees or other insects to themselves and then reward them with nutritious pollen, nectar, or both. As a tradeoff, when those hungry insects come in to feed, they rub against anthers and pick up tiny pollen grains that lodge in their hair. Upon visiting another flower, some of those pollen grains rub off the insect onto strategically placed stigma and there you have it – fertilization.

That basic process is tried and true, and responsible for seed production in thousands upon thousands of species.

“Yawn,” say milkweed plants.

Or maybe they say, “Hold my beer.”

Milkweed flowers make lots of nectar, and most also emit an attractive scent. They produce pollen too, but instead of loose pollen grains, milkweed pollen grains are glommed together in a kind of waxy sack. Botanists call that sack a pollinium, and milkweed flowers create them in pairs (pollinia), connected by two arms stretching out from a small dark structure called a corpusculum. The entire combination looks like a pair of yellow saddlebags.

As a quick aside, the creation of these waxy pollinia isn’t unique to milkweed plants. Orchids create them too. If they could, I bet orchids and milkweeds would very much enjoy swapping tales about how they trick insects into hauling those saddlebags around for them. You can find lots of stories about orchids and their tricks on the internet – they have no shortage of fans who like to talk about how beautiful and special orchids are. No argument here, I just think those orchid fans might consider throwing a little love toward milkweed too.

If you’ve ever really inspected a milkweed flower, you’ll have noticed that it looks pretty different from how a kindergartner would draw a flower. It has its own unique structures, perfectly suited to carry out its crazy pollination strategy. As part of that strategy, the milkweed flower doesn’t put those pollinia (saddlebags) on display where any old insect could pick them up and carry them off. Instead, they hide them behind a vertical slit on the side of the stigma – the central ‘hub’ of the flower.

Sorry, another quick aside here. The botanical term ‘pollinarium’ refers to the entire package of two pollinia, arms, and corpusculum. I mean, that’s fine. But wouldn’t ‘pollinarium’ be the perfect name for the compartment on the side of the stigma in which the milkweed plant stores the pollinia? “The pollinia can be found in the pollinarium.” That just sounds right. But no. That compartment is called a ‘stigmatic chamber’. Such a waste…

Here is the slit that leads to the stigmatic chamber in a common milkweed flower, where the pollinia are stored.
On this showy milkweed flower, you can see five slits located around the edge of the stigma in the center of the flower.

The milkweed’s nectar – the whole reason insect visitors come in the first place – is located in each of five sections of the colorful star-shaped ‘corona’ that surrounds the central stigma where the pollinia are hidden. Visiting insects walk around the flower, sticking their tongues down into the openings where the nectar is stored. For most flowers, this is the part of the story when insects get pollen all over themselves as they search for nectar. In the case of milkweed flowers, those pollinia are hidden away within the ‘stigmatic chamber’ (sigh) far away from the fuzzy faces and bodies of those insects. Seems like a dumb design, right?

However, as insects crawl around on milkweed flowers, moving from one to another nectar bucket, there are only so many places to step. The stigma is one of those few places, and now and then, a leg inadvertently slips into the slit where the pollinia are stored. It’s a tight fit, and insects have to struggle a little to yank their leg back out. As they do, they often catch their foot on the corpusculum between the saddlebags and when that happens, the whole package pops out and dangles from the leg.

“Aha!” says the milkweed, “told you it would work.” (I like to imagine that milkweed flowers say this every single time it happens.)

Those little yellowish globs stuck to this paper wasp’s feet are pollinia, extracted from swamp milkweed flowers. Clumsy wasp or clever flower? You be the judge.
Pollinia are attached to two of this large milkweed bug’s feet too.

I want to be absolutely clear that an insect’s leg sliding into that slit is a complete accident. Insects are not going this on purpose. In fact, it’s not unusual for an insect’s leg to get so securely stuck in the slit that it has to pull its leg off to escape – or die while trying.

Ok, the milkweed has scored its first points, but the game is only at halftime.

In order to complete the pollination process, pollen from one flower has to enter the stigma of another flower and fertilize the embryo. In the case of milkweed, that means the same insect that accidentally stepped into a stigmatic chamber (ugh) on one flower has to make the same mistake again on another flower. With the same leg.

More specifically, for all this to work, a milkweed flower needs an insect to:

  • accidentally slide one of its legs into a slit on the side of a milkweed flower’s stigma
  • successfully extract the leg again (don’t die!) and snag the pollinia on the way
  • leave the pollinia on its leg (insects often attempt to pull them off but they seem to stick pretty well)
  • visit a different flower
  • accidentally slide THE SAME LEG into a slit on that flower
  • successfully extract the leg again, this time dislodging the pollinia and leaving it inside

There are, of course, structural features of the flower that ‘encourage’ insect legs to slide into the correct slits. Similarly, there are structures that help ensure the leg will hook the pollinia as it is pulled back out (or leave a pollinia behind the second time). In other words, pollination is not all due to random chance. But still. It’s a pretty crazy process.

Depending upon your perspective, the alternate strategy employed by milkweed flowers is either risky and overly complicated or quirky and fun. Milkweed plants probably don’t care which terms any of us choose to describe their approach. The countless milkweed seeds floating around each autumn is testament to their success.

Well done milkweed; here’s your beer back.

Evidence of success – rows of fertilized seeds, ready to be launched into the world.
This entry was posted in Uncategorized by Chris Helzer. Bookmark the permalink.

About Chris Helzer

Chris Helzer is the Director of Science for The Nature Conservancy in Nebraska. His main role is to evaluate and capture lessons from the Conservancy’s land management and restoration work and then share those lessons with other landowners – both private and public. In addition, Chris works to raise awareness about the importance of prairies and their conservation through his writing, photography, and presentations to various groups. Chris is also the author of "The Ecology and Management of Prairies in the Central United States", published by the University of Iowa Press. He lives in Aurora, Nebraska with his wife Kim and their children.

31 thoughts on “Milkweed Pollination: A Series of Fortunate Events

  1. I have been doing milkweed and monarch monitoring for two years now and never knew any of this! After reading your blog I went back and looked through some of my milkweed photos and found several e smokes if this that I never even noticed. Let me know if you think I’m right.

    And thank you so much for your weekly blogs. I enjoy each and every one of them and look forward to hearing from you at the upcoming TPE conference.

    Amy Chamberlin TPE Volunteer- Moely Remnant Prairie

    On Tue, Jan 26, 2021 at 8:48 AM The Prairie Ecologist wrote:

    > Chris Helzer posted: ” Most of us know a friend or relative who isn’t > content to follow the standard path in life. Why do things the simple easy > way when there’s a more complicated option available? Maybe you’re even > that person yourself. If so, you’ll appreciate the pollin” >

  2. Sorry. I hit send before attaching this other photo.

    Thanks again,

    Amy

    On Tue, Jan 26, 2021 at 8:48 AM The Prairie Ecologist wrote:

    > Chris Helzer posted: ” Most of us know a friend or relative who isn’t > content to follow the standard path in life. Why do things the simple easy > way when there’s a more complicated option available? Maybe you’re even > that person yourself. If so, you’ll appreciate the pollin” >

  3. Having recently journaled this whole thing, after extensive research to make sure I had a thorough understanding of the process, I totally LOVE your rendition of it!

  4. Nothing in life is as straightforward as it initially seems! Thank you so much for capturing the beauty of ‘ordinary’ things and sharing your insights with us.

  5. Thank you, once again, for such a fun and enlightening post!! I feel so lucky to have found your blog; the photos AND the fun, informative writing are delightful. AND – once again, I am in awe at the strange ways Mother Nature has encouraged her critters to propagate!

  6. I have many images of bees on milkweed with upwards of 10-20 pollinia attached to its feet and legs. So much fun to see how nature works! Thanks for the detailed explanation.

  7. You are such a fantastic writer! I’d heard about this before either during a presentation you gave, or perhaps when you were a part of a landscape tour somewhere, but you STILL had me with your first sentence. Thank you so much!

  8. Hi Chris,

    I thought you might be interested in this slide from my power point presentation to beekeepers.

    I can send the link for the rest of it if you are interested.

    Finding the pollinia on sticky boards under screen bottom boards in beehives was a mystery for over a year until I discovered the connection.

  9. Hi Chris, Thanks for such an enjoyable and informative post. You have a delightful style of writing that makes it easy to take in. And your images are spectacular. The milkweed’s mantra has to be, “accidents do happen!”

  10. Wow, thanks for an entertaining telling of this fascinating pollination story. I can’t wait for my various milkweeds to bloom again this summer so I can examine them more closely than I ever have before.

  11. Well, this made my day! Thank You! I didn’t know any of this and can’t wait to closely inspect the next Milkweed I see.

  12. A few years ago I brought milkweed pods into my house to clean the seeds out of them and inadvertently introduced milkweed aphids to my hoya (wax plants and rope plants) and orchids. It took 4 years to eradicate them form my house plants. So milkweeds and orchids have other similarities other than their pollen packs. Also you shouldn’t give beer to milkweeds. That is contributing to the delinquency of plants :) !

  13. Maybe it doesn’t work as g god as planned. Example, the Purple Milkweed.
    I have read various texts about the poor seed production of Purple Milkweeds.
    Doubtful to me that a rare specialized insect is the only bug to pollinate them as they are very similar to Common Milkweeds.

    Any wisdom the you have regarding Purple Milkweeds?

  14. Pingback: news round-up: spring edition – Ecological Relationships

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