A Bumblebee's Shaking Job

In a greenhouse, tomato plants may have plenty of leaves and bright yellow flowers but produce few tomatoes. Water and sunlight are important, yet they do not solve every problem. The flowers also need successful pollination: pollen must reach a flower part called the stigma. A bumblebee can help by giving each flower a powerful shake.

Pollen in a Tube A tomato flower's pollen forms inside narrow structures called anthers. The anthers cluster around the center of the flower. Instead of splitting open along their sides, they release pollen through tiny openings at their tips. Think of a saltshaker: turning it over may not be enough to make much salt come out. Shaking helps the grains move through the holes.

The same idea helps explain why a light breeze may release too little pollen from a tomato flower. Outdoors, stronger movements from wind or visiting insects can help. Inside a greenhouse, however, the air may be quite still. Flowers can have plenty of pollen waiting inside their anthers, but not enough reaches a stigma. The problem is moving the pollen, not making more of it.

A Shake from a Bee A bumblebee lands on a tomato flower and grips the anthers. Then it uses muscles in its chest to make rapid vibrations, or back-and-forth movements. These are the same muscles that power flight, but the bee can use them without flapping its wings. The vibrations travel through the flower and shake pollen loose. Some grains land on the bee's fuzzy body. The bee gathers pollen as food for young bees.

As the bee works, some loose pollen may reach the flower's stigma. Pollen carried on the bee can also reach another tomato flower's stigma during a later visit. Tomato flowers can use pollen from their own flower, so a second plant is not required. After pollen reaches the stigma, a process called fertilization can follow. Seeds begin developing, and the flower's ovary, the part that holds them, grows into a tomato.

Helping in a Greenhouse Some greenhouse growers use managed bumblebee colonies to help pollinate tomato plants. The bees' visits provide the shaking that still greenhouse air may not provide. Honeybees visit many kinds of flowers, but they do not perform this special vibrating behavior, called buzz pollination. A bee that is useful for one crop is not necessarily the best helper for another.

A close-up photograph under the heading "A Shake from a Bee" could show a bumblebee gripping a flower. Its caption might read, "Vibrations release pollen from tiny openings." That picture would capture an important step, but not the whole process. Pollen still needs to reach a stigma. Even after successful pollination, a plant needs suitable water, light, and temperatures to grow fruit.