Milkweeds have very distinctive flowers, with unique shapes and features. I guess it shouldn’t be surprising that their pollination story is equally interesting.
Common milkweed (Asclepias syriaca) flowers. Lincoln Creek Prairie – Aurora, Nebraska.
First, milkweed flowers don’t produce thousands of of individual pollen grains that can each get carried away to other flowers by visiting insects. Instead, milkweeds have what are called “pollinia”, or waxy masses of pollen that are designed to stick to insects. You might think that a flower with a specialized pollen structure like that would have a system to make it easy, or even automatic, for any visiting pollinator to pick up and deliver that pollinia to the next flower. After all, there are countless stories of flower types that facilitate pollination by forcing visiting insects to hit the right spots as they forage for nectar and pollen.
With milkweeds, not so much.
In order for an insect to pick up one of the pollinia from a milkweed flower, its leg has to slip into a tiny slit between the anthers along the side of the flower. As the insect struggles to pull its leg back out of that tiny opening, it might emerge with a pollinia stuck to it (or it might not). However, in order to complete the pollination cycle, that SAME insect has to then visit ANOTHER milkweed flower and have the SAME leg slip into the anther opening in THAT flower! AND even if that happens, the pollinia still needs to come in contact with exactly the right spot inside the flower.
Here is a slightly zoomed in look at the milkweed flowers, with arrows indicating the tiny slits between anthers, into which an insect’s leg must slip in order to pick up the waxy bundle of pollen.
To be fair, the milkweed’s strategy does prevent it from just “giving away” pollen to any insect that happens by, and there probably is some evolutionary advantage in that. On the other hand, studies have shown that the pollinator insects – often bees – that pick up and deliver the pollinia often do so at a cost. First, the pollinia are heavy enough that they can substantially slow down the flights and movements of the bees. Second, and probably more importantly, it’s not uncommon for an insect to lose part or all of its leg trying to extricate itself from a milkweed flower. Seems like an odd strategy to attract insects to help you with a critical life function. I guess the nectar must be pretty spectacular…
Regardless, it seems to work often enough – there are lots of milkweed seeds around every fall!
If you’re interested in learning more about milkweeds, pollination, and other information, a great description – with lots of really nice photos – can be found in this excellent article by Brian Johnston in the online Micscape magazine.
If you want to delve into the scientific literature on this subject, you might start with this article in the journal Ecology.
