Nurturing Bumblebees

Expert details on what makes magnificent bumblebees the perfect pollinators.

| Summer 2014

  • Like palace guards, the stamens stand in a circle around the pistil (with ovary within) forming a tube-like structure with an opening only at the exposed end. The pollen is firmly attached inside the anthers, capping the stamens. Something has to shake the stamens to release the pollen through pores in the anthers.
    Photo by Lew Stamp
  • A bumblebee investigates tomato flowers. Tomato flowers hang from the plant, with stamens bearing pollen pointing outward or downward; consequently the pollen doesn’t naturally fall on the stigma (which is surrounded by the stamens and is the gateway to the floral ovary).
    Photo by Lew Stamp
  • A bumblebee hangs onto the stamens with its jaws, holds its wings still, and vibrates its thorax muscles to shake pollen from the stamens.
    Photo by Lew Stamp
  • The pollen is captured by the bumblebee’s fur. By grooming in flight, she’ll store it on her hind legs (visible as a tan lump in this photo) and carry it home to her young. The flowers have tell-tale brown bruising on the stamens, left by earlier bumblebees grasping with their jaws while buzzing for pollen.
    Photo by Lew Stamp

Bumblebees, those big furry, burly bees — otherwise known as Bombus (the bumblebee genus, meaning "booming") — have been ignored by farmers until recently because bumblebee colonies produce very little honey surplus.

But the truth is, far from being useless annoyances, bumblebees are vital pollinators of native plants and as well as many crops. In North America, bumblebees are pollinators of clover, alfalfa, beans, blueberries and cranberries. Heirloom tomatoes and many cherry tomato cultivars — plus their relatives in the Solanaceae family, such as peppers and eggplant — need assistance from such buzz pollinators for pollination to occur. Bumblebees are the pollinator of choice.

What makes a bumblebee such a good pollinator? Bumblebees fly at lower temperatures than smaller bees because their large bodies covered with dense fur retain body heat better. Queens can fly at near-freezing temperatures and the smaller workers at 50 degrees Fahrenheit. A bumblebee averages 450 flower visits per hour — more than 7 flowers per minute — and works several hours more each day than a honey bee.

The bumblebee’s larger body size also means a longer tongue to obtain food from tubular flowers, and more hair to hold pollen dust. Bumblebees depend heavily on pollen for food and so will visit nectar-less flowers, such as tomato, that honey bees shun.

Because bumblebees are such reliable pollinators, moving so much pollen around, more pollen is deposited on flowers. Up to point, this means more plant eggs are fertilized; in turn more seeds develop and the plants yield larger and juicier fruits. For a flower like the tomato, with a floral design making it difficult to self-pollinate, the bumblebee is the perfect match.

Bumblebees are also the champs of buzz pollination. Some flowers, such as the tomato, require quite fast vibration (equivalent to that of an electric toothbrush) to release their pollen. Honey bees are too small to create that, but bumblebees can. Bumblebees grab hold of the flowers with their jaws and then shiver their flight muscles while their wings are folded against their body. With no drag created by flapping wings, the bumblebees can vibrate twice as fast as honey bees, between 300-400 Hz. This causes the flower to shake enough to shoot pollen out, dusting the bumblebees. Then the bumblebees groom themselves of the pollen, even during flight, and store it on their hind legs in a spoon-like indentation where it is taken back to the nest to feed the bumblebee larvae. However, the bumblebees cannot groom themselves of all of the pollen. The pollen deposited at the base of the bee’s legs or between the thorax and abdomen is too difficult to groom, especially during flight. Consequently, some of the pollen dust ends up on the next flowers visited.



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