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Wisconsin Natural Resources magazine

Wisconsin Natural Resources magazine

A dun skipper butterfly is an effective milkweed pollinator. © Gregory K. Scott
A dun skipper butterfly is an effective milkweed pollinator.

© Gregory K. Scott

August 2007

Milkweed love

Pollination is tricky, but it works!

Anita Carpenter


On a humid, late summer day common milkweed's sweet, lilac fragrance hangs heavy in the still, moist air as insects of all kinds converge on its lavender blossoms. The hungry visitors crawl over the droopy blooms, pausing often to sip a high-energy nectar meal. The meal is not without cost. Unknowingly, while drinking, the insects pick up milkweed's precious pollen and deposit it on the next nectar-rich blossom.

Milkweed pollination seems simple: offer nectar and let insects do the work. Although milkweed is a very popular dining site for hungry insects, why are so few pods produced compared to the number of flowers? With milkweed nothing is simple. The pollination process is complex and so chancy that it's a wonder it occurs at all.

For a better understanding, let's first examine milkweed's unique flower. As each of the approximately 50 individual blossoms in a flower cluster opens, the five floral leaves of the corolla split and fold back revealing a five-part waxy cup. Each tiny nectar-holding receptacle is called a hood and within each hood, a pointed horn curves toward the flower's center. The horn's function is unknown but if you observe nectaring bumblebees, the horns seem to physically direct the bees to the nectar.

Hidden deep within the flower are two ovaries, each with its own style but sharing one stigma. Ten pollen-laden sacs lie outside and around the ovaries but are also hidden within the flower. Access to the pollen sacs is through a minute vertical slit located between each hood.

Each waxy-yellow pollen sac, called a pollinium, is flat and less than 2 mm long. Two pollen sacs or pollinia are joined at one end by two tiny filaments centered with a black speck called the translator. The complete structure resembles two tiny saddle bags.

When an insect crawls around and over a flower, its leg may slip into one of the slits. The leg hooks onto the translator. As the insect flies off, the leg pulls free from the slit extracting the sticky pollinia which wrap around the insect's leg. (You can duplicate this behavior by carefully inserting a needle into the slit. Hook the visible black speck, and gently lift up and pull. Out slides the sticky pollinia which adhere to the needle.) The insect flies to another milkweed flower and as it crawls around and hangs onto the bloom, its leg may slide into another slit. The opening closes around the leg and as the insect extricates itself, the pollinia may be scraped off and left behind. The intact pollen sacs must pass through the stigma, into the style and fertilize the ovary. Often only one ovary per flower matures into a follicle we commonly call a pod.

Milkweed pollination depends on several chance happenings. Will an insect's leg slide into a slit? Will the leg hook the translator? Is the insect strong enough to pull out the pollinia? Will the insect visit another milkweed? Will the pollen-carrying leg slip into the right slit? Is the insect able to shove the pollen in deep enough to be beneficial? Will the pollinia be scraped off?

Although many insects visit milkweed, increasing the chances of pollination, are all these insects effective pollen carriers? Are the feathery-light butterflies and moths, whose legs may not slip into the slits, as efficient pollinators as the heavier, bulkier bumblebees? Probably not.

Other insects succumb to the hazards of visiting milkweed, further reducing the probability of successful pollination. Besides the hidden dangers of lurking ambush bugs and flower spiders waiting to pounce upon unsuspecting diners, some insects become entangled in the sticky blossoms. An insect frantically beating its wings usually has one leg stuck in a slit. Death is sure to follow.

This complex pollination might lead one to believe that milkweed would not reproduce very successfully. Yet common milkweed, Asclepias syriaca, is a familiar sight along roadsides and in meadows with its delicate flowers giving way to bursting seed pods later in the fall. The reproductive strategy may be chancy, but it seems to work.

Anita Carpenter enjoys the fragrance and anatomizes flowers on walks near her Oshkosh home.