Sunday I went to the winter meeting of the Iowa Prairie Network, and attended a workshop on milkweeds by Tom Rosburg and Deb Lewis. There was a lot of good information about how milkweeds are pollinated. I knew a little about it as a consequence of the time I have spent photographing insects, especially butterflies, and especially on milkweeds. There was also some information that was new to me and that I had not really thought about.
First of all, Iowa has 16 species of milkweed. This one is called prairie milkweed, Asclepias hirtella.
Milkweeds have an unusual type of pollination. Pollen grains in milkweed flowers are combined in the flower into a structure called a pollinium–in milkweeds this is a saddlebag-shaped structure. The structure has a large number of pollen grains covered with a waxy coating. Orchids have a similar arrangement but in most flowers the pollen grains are separate.
If you look closely at the hind leg of this wasp, you can see a milkweed pollinium dangling from it. The flower is swamp milkweed.
A good discussion of milkweed pollination can be found here.
Common milkweed is very attractive to a wide variety of insects, especially hairstreaks. These banded hairstreaks both have pollenia on their legs, so are likely pollinators of the plant.
Just because an insect is attracted to milkweed does not mean that it is a pollinator. This beefly is attracted to the flower but is not likely to collect the pollenia on its legs like butterflies do.
Pollination of milkweeds is mostly accomplished by large insects–butterflies, large wasps and bees (although the bees do not collect the pollen like they do for other flowers), flies, and beetles.
One of the consequences of this type of pollination is that statistically it is not likely to happen. Flowers are on umbels of several dozen flowers. We have all seen milkweed pods. The botanical term for those pods is “follicles”
These are follicles for common milkweed. Each milkweed plan can have a few follicles–maybe as few as two, maybe as many as a half dozen. Compare that with the total number of flowers produced–many dozens of flowers. Five percent or less of the flowers produced actually got pollinated.
One pollinium produces all of the pollen for each follicle. Therefore all of the seeds in each follicle have the same mother and the same father. Male and female parts are found on each milkweed flower, and the milkweeds are not able to self pollinate.
There is a plant found here in Iowa that is in a group close to milkweeds. That plant, Indian hemp or dogbane, Apocynum cannabinum, has a milky sap similar to milkweeds. Like milkweeds, it is very attractive to butterflies. It does not produce pollenia, but has pollen that sticks together in groups of four.
There is a discussion of pollination in dogbane here. It also touches on why pollenia might have evolved. The hypothesis discussed suggests that this arrangement uses less pollen than the flowers would use otherwise, and therefore saves energy for the flowers. I am not sure I buy that. It seems to me that it might have more to do with competition for the resources used by the plant to set seed.
Apocynum cannabinum produces a follicle similar to milkweeds, but each seed would not necessarily have the same male contribution like in milkweeds.