Milkweed (members of the genus Asclepias) flowers are unique and fascinating structures that attract a myriad of insect pollinators. The insects are rewarded with sweet nectar that milkweed flowers produce in copious amounts. Yet, insects sometimes cannot pull free from the nectar trough and perish in the grips of the milkweed flower.
My garden currently has two species of milkweed that are native to the area, butterfly weed (Asclepias tuberosa) and swamp milkweed (Asclepias incarnata). Having these plants just outside my door allows me ample opportunities to observe and photograph their flowers and their insect visitors.
Anatomy of a milkweed flower
Milkweed’s flower anatomy is complicated but I’ll try to explain it using as little botanical jargon as possible.
About 20 to 70 individual milkweed flowers are arranged in an umbrella-shaped structure called an umbel. The individual flowers about ¼ to ½ inch across, are radially symmetrical, and have 5 reflexed petals. Atop the petals is a corona consisting of a circle of 5 hoods and 5 horns. The hoods hold the nectar that attracts insects. Enclosed within the corona is a structure that I’ll call the reproductive chamber. The reproductive chamber has 5 vertical slits around its perimeter which allow access to the female ovaries and male pollen that lie within it. Unlike most flowering plants, though, the pollen of milkweeds is not transported as individual grains. Instead, hundreds of pollen grains are packaged into golden, teardrop-shaped sacs called pollinia which are strung together by strong filaments. The pollinia are joined in pairs by a small, black, grooved structure called a corpusculum. The corpusculum is visible at the top of the slit in the reproductive chamber if the pollinia are still within.
Enter stage left – pollinating insects
A variety of insects are attracted to the plentiful nectar present in the hoods of milkweed flowers. As the insect climbs over and around the flowers, its leg may slip into and become stuck in one of those vertical slits in the reproductive chamber. The insect then twitches and tugs to free its leg. In the process, the insect may also catch onto the groove of the corpusculum. When the insect pulls its leg free, it may pull with it the corpusculum and pair of pollinia and fly off with them attached. The pollinia stay attached to the insect as it continues to feed. At a subsequent flower, the insect may again slip a leg into the slit in the reproductive chamber and pull out more pollinia. Large bees can carry up to 10 pairs of pollinia which look like golden chains dangling from their legs.
But how do the pollinia carried by insects make their way to a receiving flower? Basically, by the same process, just reversed. If an insect’s pollinia-laden leg slips into a reproductive chamber that has already been emptied of its pollinia, pollinia might be pulled free from the leg during the struggle and become lodged in the chamber. If the inserted pollinia came from a different milkweed plant, the insect has performed the valuable service of cross-pollination, a mandatory step for milkweed seed production.
A fatal tradeoff
Most of the time, insects are able to free themselves from the slit of the milkweed reproductive chamber within a few seconds. Occasionally, though, the grip of the milkweed may be too strong, and the insect perishes on the flower with its leg caught in the slit. If you look closely at several umbels of milkweed flowers, you can usually find a few dead insects, usually smaller bees and ants, that enjoyed milkweed nectar as their final meal.