Milkweed
Pollination
There
are 73 species of native milkweeds (genus Asclepias)
in the United States. Many of them are in the western US and over a dozen occur
in California.
Milkweeds
have a distinctive and complex method for pollination, involving hoods that
hold nectar, pollen sacs joined by a string, stigma slits, and a special
structure that holds everything together.
Here’s
how it works.
Milkweed
flowers have five petals that either point away or open wide from the rest of
the flower.
Above
the petals, a special gynostemium structure has fused male and female
parts. Surrounding the gynostemium are
five white hoods that fill with nectar. Many times, as in the picture below, the
hoods have curving horns sticking out that bend back to the top of the
gynostemium. The hoods and gynostemium
top are waxy and slippery, so pollinator legs tend to slip between the hoods.
Narrow Leaf Milkweed - Photo by Wilde Legard
In
addition to the exotic white hood and horns, each male stamen grows an anther that splits into two pale-purple
pollen sacs, connected by a string and a black sticky structure. This whole
unit is called a pollinium.
When
pollinators come to gather pollen from the waxy hoods, their legs often slip
between the hoods and get tangled in the pair of pollen sacs. They carry the
pollen sacs to another milkweed where their legs are likely to slip in again.
This time they leave the pollen sacs behind, right next to a slit where the
female stigma sits.
Photo by Wilde Legard (detail)
Here’s
a photo of a Carpenter bee carrying pollinia from a
different kind of milkweed flower. The petal
colors and hood shape are different but the pollination system is the same.
Photo by Beatriz Moisset, https://bugguide.net/node/view/13049
Here’s
a variation (Antelope Horn Milkweed – A.
asperula) where there are no hoods but the nectar is presented in green
stripes on the gynostemium. The honeybee
picks up and deposits pollinia that you can see on
its feet.
Photo from Wikipedia - public domain.
Here’s a
close-up of a honey bee taking nectar from a narrow leaf milkweed. Note its tongue in one of the hoods where the
nectar is produced. The bee already has
many pollinia attached to its legs.
Photo by May Chen
Each pollina has two pollen sacs attached by a thread that
tangles on the pollinator’s legs.
Pollinators
tend to be fairly large insects, attracted to the milkweed’s large nectar
stores and with long enough and strong enough legs to slip into the stigmatic
slits and pickup or leave pollina.
Photo © Chris Helzer
Here’s
a movie you can download of a honey
bee with its right back leg trapped in a milkweed flower. It struggles mightily and manages to free
itself with life and limbs intact.
What a
surprising and amazing system!
Have
fun taking a closer look the next time you see a Milkweed.
Want More?
The
USDA has written an excellent article that describes milkweed pollination in
more detail: https://www.nrcs.usda.gov/Internet/FSE_PLANTMATERIALS/publications/nvpmctn12764.pdf
Here’s
a fun one by Chris Helzer of The Nature conservancy:
https://prairieecologist.com/2021/01/26/milkweed-pollination-a-series-of-fortunate-events/
Here’s
a great article from Virginia Tech on milkweed pollinia:
https://collection.ento.vt.edu/2016/08/05/milkweed-pollinia-revisited/
Corrections/Comments: bruce@PlantID.net Copyright: https://PlantID.net/Contributors.aspx |