Few things in life are more enjoyable than to watch a patch of colorful flowers swaying in the breeze, giving off pretty scents and with insects—such as bees and butterflies—alighting from plant to plant. Although it may be relaxing for humans to watch, the activity is a sort of "business relationship;" your plants use at least three things to attract insects to flowers. It has a practical purpose, because to reproduce, most plants need to spread pollen to other flowers to create seeds. The flowers attract insects and forge a mutually beneficial relationship.
Color: Fluttering In the Air
The ways flowers attract insects evolved over time into an efficient means of pollination. To draw an insect's attention, most flowers advertise themselves by being brightly colored and sitting atop long stems, so they wave in the air and are closer to where insects are flying, rather than on the ground. Beyond color, a flower's petal sizes and shapes also attract insects. Some insects are attracted more to certain flowers than to others. For example, tiny wasps like tiny flowers such as alyssum.
Food: Nectar and Pollen
Insects need energy and protein that come from food. Deep in a flower's center is a "nectary" which produces nectar, a sugary solution insects like that provides carbohydrates. Plant pollen is rich in protein, which insects need for building tissues. Bees, for example, carry back pollen and nectar to their hives to help their young bees develop.
Scent: Flowers and Fragrance
Plants also produce scents to attract insects, probably as a way to advertise that food—nectar and pollen—is available. As the insect is drinking nectar or gathering pollen, it moves around in a flower and pollen grains, which sit atop long thin stalks in the flower's center, collect on its legs or underside. When it moves to another flower, pollen can travel down a tube called a stigma to where the plant's ovules are. The ovules contain eggs. When a grain of pollen reaches it, an egg is fertilized and develops into a seed.
Flower fragrance probably evolved from chemicals meant to fend off plant-eating animals. But once insects who visited the flowers learned that the chemicals were not irritating or a deterrent to them, they began to selectively visit flowers with these fragrances.
Flower Markings: Nectar Guides
Petals sometimes have lines or other markings, called nectar guides. These help lead insects to a flower's center, where both the flower's nectar and reproductive systems lie. In flowers that are popular with bees, the nectar guide is sometimes a region of low ultraviolet reflectance near the center of each petal. The reflectance is invisible to human eyes but not to bees.
Plant to Attract Beneficial Insects
To have a top garden, gardeners can plant flowers that draw "good" insects that eat those insects that can cause damage—aphids, the Colorado potato beetle, Harlequin bugs, green cabbage worms, tomato hornworms, flea beetles, corn ear worms and many other caterpillars, and Japanese beetles. To prevent the devastation caused by voracious pests, attract the beneficials, such as bees, green lacewings, lady bugs, wasps and flower flies, which parasitize or eat the "bad" bugs.
There are a myriad of flower choices that attract these beneficial insects, including daisies, marigolds, passionflowers and sunflowers. In general, flowers of red, yellow or blue are attractive to both humans and insects.