Flower Trap

European Skipper with pollinia on feet © Kent McFarland

Like a Chinese finger trap, the European Skipper was struggling to free itself from the milkweed flower. On another flower nearby only a leg remained from a previous struggle. Survey enough milkweed flowers and eventually you’ll find a few dead insects, usually small species, hanging from a leg or two.

European Skipper nectaring © Kent McFarland

They are not a carnivorous plant; trapping and death are just an accident. Instead, milkweed has solved the problem of pollination in a unique manner. There are five pinkish hoods with horns where the nectar is located. Milkweed produces copious amounts of sweet smelling nectar in the hoods to attract insect pollinators. Between each of the hoods is a dark spot with a long slit leading down from it. With most of the flowers in the umbel hanging downward, these slits are a natural place for insects to grab with their feet while syphoning nectar upside-down.

Legs with pollinia © Kent McFarland

Unlike most flowers, milkweeds don’t produce tiny grains of pollen to be carried away piece by piece. Instead, the flower produces sticky, orange packets of pollen, called pollinia, which are designed to stick to an insect’s leg. In each of the five slits are two pollinia waiting to be accidentally snagged and carried off.

Common Milkweed © Kent McFarland

In order for an insect to pick up one of the pollinia from a milkweed flower, its leg has to slip into a tiny slit between the anthers along the side of the flower.  As the insect struggles to pull its leg back out of that tiny opening, it might emerge with a pollinia or two stuck to it. If the insect is too small or too weak, the only way it can escape the flowers grip is to leave its leg behind. Worker bumblebees, much smaller than spring queens, that forage on milkweed often are missing a claw or leg part.

Common Milkweed © Kent McFarland

If the flower is lucky, the insect will travel to another milkweed flower in search of nectar and deliver the pollinia. To do this successfully, they must again pass their leg through one of the anther slits in another flower and have the pollinia come into contact with a very small area at the base of the stigma lobe. There’s a cost to the insect for this delivery service. Bumblebees with pollinia attached forage about 25 percent more slowly.

Common Milkweed © Kent McFarland

The odds for successful pollination are slim. Seed set in milkweeds is often quite low with only a flower or two in the entire umbel producing seed. But a few flowers are enough to produce clouds of drifting seeds each autumn to sow a new generation in some far off field.

Close up of a European Skipper on Milkweed with its legs in pollinia © Kent McFarland

Lost leg on Milkweed © Kent McFarland

Kent McFarland (New England)

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One Response to “Flower Trap”

  1. Julie Craves says:

    I have tried for years to photograph this process! Great job.