Pollination of Plants by Insects

by Rudy Scheibner,
Extension Entomologist

Pollination is the process whereby plants receive pollen from other plants of the same species so that they can reproduce by forming seeds. Some plants are pollinated by the wind, and some are pollinated by insects or other small animals. When plants are pollinated by insects, it seems like some kind of agreement was made whereby the plant will provide the insect with something, if the insect will pollinate the plant's flowers in return. Although the plant and the insect may benefit because of their relationship with each other, the insect visiting a flower usually does not purposefully pollinate the flower.

Honey bees are very important insect pollinators. Most of the time, both honey bees and the plants they visit are benefited. The honey bee gets some food and the plant gets pollinated. When a honey bee is collecting pollen from the anthers (male reproductive part) of a flower, it puts the pollen in special pollen baskets on its hind legs. All that pollen will be taken back to the hive for the bees' use. The honey bee is a messy gatherer of food, and some pollen gets stuck on the hairs of its body. When the bee visits the next flower, some of that pollen brushes off onto the flower and if it sticks to the stigma (female reproductive part) of the flower, pollination will take place. The bee does not make any effort to put the pollen in the right place.

Bees also gather pollen from corn, but the corn gets nothing in return. Corn is pollinated by the wind. When honey bees try to get nectar from an alfalfa flower, the flower has a tripping device that slaps the bee in the face with an anther (the pollen-bearing part of a flower). That's the flower's way of assuring that its pollen gets attached to an insect for transfer to another flower. The bees don't like being slapped in the face, so they usually find some other kind of plant to get nectar. Some of the bees learn to chew a hole in the side of the flower and get to the nectar through this side door. When they do that, pollination does not occur because the pollen on the bee does not reach the top of the stigma. So it seems that the only reason a bee visits a flower is to get what it wants and not to perform a pollination service. Pollination occurs as a happy accident for the flower.

Many other kinds of insects, such as wasps, flies, beetles, thrips and other kinds of bees that feed on nectar or pollen also pollinate flowers by accident. However there are at least some insects that intentionally see to it that the flowers they visit get pollinated. The way a yucca flower is built, it cannot be pollinated in the ordinary way, and it depends on the special service of a yucca moth to do it. When the moth comes to a yucca flower it gathers up a ball of pollen, and then goes to the ovary of the female part of the flower where it lays some eggs. It then climbs to the top of the female part of the flower and places the ball of pollen on the stigma where it needs to be for pollination to occur. Now that it is pollinated, the flower can produce seeds in its ovary. The yucca moth caterpillars eat some of the seeds, but many are left over to produce new yucca plants. Because the yucca moth caterpillars eat only yucca seeds, and the yucca plant is pollinated only by yucca moths, both the moths and the plant depend on each other for survival.