Milkweeds and the Bees | Prairie Public Broadcasting

Milkweeds and the Bees

Jul 18, 2020

 

Many years ago I took a garbage bag and set out to collect some flowers of the common milkweed for my botany labs.  I found a good patch of the milkweeds and proceeded to cut off the flower heads, letting them drop in the bag.  When I had collected enough flower heads I tied up the bag and put it in the prep room freezer for safe keeping.  

 

Later, when I opened them up in lab, several dead and frozen bees dropped out from amongst the flowers.  They apparently had been working the milkweed flowers.  I quickly noticed that many of these bees had little yellow saddlebag-like structures with two pollen sacs or pollinea attached to their legs!  Some plants have evolved interesting mechanisms to help ensure pollination, and the common milkweed is a case in point. 

 

If you have ever looked closely at the flowers of a milkweed, they are different!  We will bypass a full explanation of the flower structure, but the casual observer will notice five “hoods” that look something like modified and folded petals pointing upward and outward.  These hoods surround the male and female reproductive structures which are in the center. If you look at the flower from the side, you will between the hoods is a small vertical slit with a small ridge on each side.  If there is a little dark spot (a gland) above that slit, the pollinia or little saddlebags of yellow pollen are still in place.

 

If a bee or other insect alights on the flower there is a good chance that one of their legs will slide down the side of the flower between the hoods. And there also is a good chance that one of the many hairs on the bee’s legs will slip into the slit.  The flower and slit is structured so that when the bee pulls up its leg, it will likely pull up the pollinea.  The whole structure resembles a very small wishbone, with the pollinea at the bottom of each branch.  If all goes well for the milkweed, the bee will fly to another milkweed flower, slip its leg over the side, and slip that pollen into one of those slits, breaking it off, and thus pollinate the flower.  Job done! Cross pollinated!  The next generation will soon be on its way.

 

If the common milkweeds are not flowering near you already, they will be soon. If you get the opportunity, check out those flowers.  They are quite intriguing.  Plus, you may be able to lift out those pollinea with a small pin or needle. 

 

-Chuck Lura

 

Natural Resource Conservation Service note on milkweed pollination:

https://www.nrcs.usda.gov/Internet/FSE_PLANTMATERIALS/publications/nvpmctn12764.pdf