Yard and Garden Column for the Week Beginning March 25, 2005

By Betsy Matos
Entomologist
Iowa State University Extension

There is more to flowers than meets the eye. While we enjoy these colorful beauties something more important is at work: sexual reproduction that will ensure continuance of the plant species. As you remember from your childhood lessons on "the-birds-and-the-bees," pollination is what we call it when plants "make babies." A flower's "babies" are seeds, the primary method of survival and dispersal for most plants. Pollination must happen for plants to produce seeds and pollination is also necessary for proper development of many fruits and vegetables. Without pollination most fruits and vegetables will not set fruit, the fruit will be incomplete or misshapen or yield will be low.

What is pollination?
Technically speaking, pollination is the transfer of pollen from the anther (male organ of the flowers) to the stigma (female organ of the flower). The culmination of pollination is fertilization, the union of the male sperm nucleus contained in the pollen with a female egg located inside the ovary. To achieve fertilization the pollen grain that landed on the stigma must grow downward through a structure called the style and arrive in the ovary where the eggs are located. If pollination and fertilization are successful the result is a fertilized egg that may grow into a seed.

Different plants have different ways of accomplishing pollination. Some plants such as corn, grape, soybean, tomato, eggplant and pepper have female and male organs close together in the same or adjacent flowers. Wind releases pollen that falls onto the stigma without difficulty and no aid or assistance is needed. For other plants the pollen will not fall easily to the stigma and another means of transporting pollen is necessary. This is where pollinators play an important role. Pollinators are the animals that carry pollen from one flower to another. Specifically, pollinators carry pollen from the male flower parts to the female flower parts.

Common pollinators in Iowa
Insects that commonly serve as pollinators in Iowa include honey bees, bumble bees, flower beetles, hover flies and butterflies. These insects are not selflessly doing a favor to the flowers. The insects are enticed to do the job with a reward of food. The food is usually nectar (sugar and water) and pollen (protein). For example, in a honey beehive, all the members of the colony are nourished by the pollen and nectar brought in by the worker bees. Even birds such as hummingbirds are pollinators, as are certain bats and a few other animals.

Attracting pollinators to your garden
Although we appreciate flowers for their beauty, the colors, shapes and even fragrances first evolved for only one purpose; to attract pollinators. Flowers with bright colors such as blue, yellow, red and violet are the most often visited by pollinators during the daytime. At night, colors are less important and smells take on new importance. Co-evolution of flowers and pollinators has resulted in spectacular differences among flowers. Some flowers have variations in colors, lines, or 'roads' that point the pollinator toward the nectar. Other flowers have traps, twists and other devices that force pollinators into contact with the pollen.

Once the pollinator flies or crawls to the flower, the mouth, body, antennae or legs may collect pollen that is dispersed to the next flowers visited by the pollinator. Many structures of insects are highly modified to accomplish the dual jobs of obtaining the reward and carrying the pollen.

For example, the mouth parts of some pollinators are adapted for reaching the reward (food) that may be concealed just past the anthers and stigmas. Butterflies and moths have a long tubular proboscis for reaching to the bottom of the flower. Similarly, certain bees have 'tongues' for drawing up nectar as the insects walk on the exposed anthers. Other insects such as beetles and ants have short mouth parts and are unable to reach nectaries located deep in the flower without crawling completely inside. Everything about the flower (shape, size, structural arrangement and fragrance) is important for attracting pollinators and maximizing pollination efficiency. Consequently, different flowers attract different pollinators.

Managing pollinators
Pollinators are essential for high yield and high quality of many vegetables and fruits in the garden. Plants such as strawberries, squash, melons, sweet cherries and apples rely heavily on pollinators to reproduce. Therefore it is important that you have a plan to preserve, protect, and attract pollinators. Here are some general ideas:

* Diversify your garden plantings so you have different flower colors, fragrances, and shapes throughout the growing season. Consider leaving flowering weeds such as clover, wild parsnip and dandelions to serve as alternate nectar sources for pollinators.
* Limit pesticide use.
* When insecticides are necessary use those that are least disruptive to pollinators.
* Apply insecticides when pollinators are least active (very early morning, late evening or after dark).
* Do not apply insecticides to plants that are blooming or when it is windy.