Spring’s arrival is marked by days that start to grow longer, temperatures that begin to rise, and the many plants that awake from their winter sleep. Certain species of flowering plants start to develop beautiful blooms and love is in the air. At first look, one might think nature is simply showing off with a wonderful display of color. When examined further, however, it becomes evident that flowers serve a much greater purpose.
Producing flowers is a huge drain on a plant’s energy resources, yet nearly 400,000 different species of plants produce flowers. Since plants do not expend resources looking and competing for a mate like animals do, they expend those resources on creating flowers. Flowers are plants’ reproductive parts, which help them spread their genetic information and create offspring. Pollination occurs when pollen (male genetic information or gamete) is transferred to the stigma, and then travels down the style to reach the ovary, where the ovules (female gamete) await. See Fig. 1.
Each ovule fertilized by a pollen grain will become a seed. While self-pollination is possible in some flowering plants, cross-pollination (between 2 different plants of the same species) allows for greater genetic variation in a species. Flowers have evolved many adaptations to allow for cross-pollination.
Some flowers are aided by the wind to spread pollen to other plants. However, this method is very random. The pollen has an extremely small chance of landing on a stigma, so the flower must produce a lot of pollen. Other flowers have coevolved with different animals to aid in the transport of pollen from one plant to another. This coevolution has made flowering plants the most successful plant group on earth.
For a plant to use an animal to help spread its pollen, it must first attract the animal to its flowers. But what makes a flower attractive to pollinators? Its smell? Its color? Its taste? The answer depends on the pollinator in question. Plants have coevolved with many different animals to create symbiotic relationships. Pollinating animals include bees, butterflies, beetles, moths, hummingbirds, and bats. In most cases, the plant offers food (nectar and/or pollen) in exchange for help spreading its genetic information.
Bees and butterflies are important pollinating insects. Honey bees are the best known pollinators due to the important role they play in pollinating numerous food crops. Many of the farming practices we have developed are dependent on managed honey bee hives. Perhaps less known is that lots of plants, not just food crops, need pollinators and that other species of bees and butterflies play a crucial role in their pollination.
Bees and flowers
Bees are the most prolific pollinators in nature. They spend the majority of their time searching for pollen and nectar as they are the main sources of food for themselves and their young. There are over 4,000 different species of native bees in the United States alone. Surprising to most, the honey bee is not one of them. Honey bees were imported to North America by English settlers.
Flowers that have evolved to attract bees as their main pollinators often are full of nectar and colored bright white, yellow, or blue. Bees cannot see the color red, which may be why flowers with red colors do not tend to attract bees. Many flowers even have regions that reflect ultraviolet (UV) light, invisible to the human eye but not to a bee’s, that guide the bee to the flower’s nectar and pollen.
Bees have branched hairs that pick up pollen while they are feeding. Some bees have even developed basketlike structures on their hind legs that allow them to carry pollen. When a bee buzzes from flower to flower of the same species, grains of pollen are transferred to a flower’s stigma, pollinating the plant.
A number of plants have evolved mechanisms that only allow certain bees to receive their nectar and pollinate them. For example, different species of bees have many different lengths of tongues. Some flowers store their nectar in areas inaccessible to bees with short tongues.
Other plants have evolved even more complex structures to keep certain pollinators from getting to their nectar. Snapdragons produce irregularly shaped flowers that keep nectar and pollen closed away. Only bees of the correct weight are able to open the flower to expose the nectar and pollen when they land on its landing pad.
Butterflies and flowers
Unlike bees, butterflies can see the color red, so many of the flowers they are attracted to are colored bright red, pink, or purple. Similar to bees, butterflies can see light in the UV spectrum and lots of the flowers that attract butterflies have areas that reflect UV light to guide the butterfly to the nectar. Butterflies are also lured to a flower by its fragrance. They use their feet to taste and need to land to feed. The flowers that often attract butterflies have larger landing pads near the source of nectar.
A butterfly drinks nectar through its proboscis, a long strawlike tube that is part of its mouth. The nectar of flowers visited by butterflies is often deeply hidden where only butterfly proboscises can reach. Because nectar is frequently butterflies’ only source of food, they tend to prefer nectar that is a mix of carbohydrates, vitamins, proteins, and minerals. As butterflies feed, they may also pick up pollen on their legs, mouth, and wings. When they travel to another flower, there is a chance the pollen will be transferred and reproduction will take place.
Decline of pollinators
Plants have evolved to depend on pollinators to reproduce. Without the animals that carry pollen from plant to plant, genetic variation would be greatly decreased and the survival of many species would be in question. Over the past few decades we have seen a measurable loss in the number of both managed and native bee populations, along with a decline in butterfly populations. We depend on pollination for many of our staple foods, as do many other organisms. Approximately one-third of the food produced globally is dependent on pollinating insects. It is estimated that pollinators contribute $24 billion to the agricultural economy.
The decline in pollinators can be attributed to many different factors. Land development often greatly reduces or eliminates habitats needed by pollinators. Insecticides are highly toxic to bees and butterflies. Herbicides used by farmers and homeowners kill many of the flowering plants bees and butterflies depend on. Inadequate diets, loss of genetic diversity, and parasitic mites in managed honey bee colonies may also play a role in their decline.
Butterfly larvae are very susceptible to many of the factors listed above, resulting in dwindling populations. Many butterfly larvae have very specialized diets. For example, the famous monarch butterfly only lays its eggs on a milkweed plant. If milkweed is removed or controlled through the use of herbicides, the number of plants available to adult butterflies for egg laying is decreased along with the availability of food for the caterpillars.
What you can do to help
Schoolyards can provide great spaces for plantings that promote pollinating insects. Talk with your local agricultural extension office or a knowledgeable garden supply center about which native species to plant. You want species that will be easy to maintain and that will attract local pollinating bees and butterflies. Make sure to select flowering perennials that bloom during different times of the year. They provide pollinators with food throughout the year and you won’t have to replant them each season. Choose a variety of colors, sizes, and shapes of flowers to accommodate many different types of pollinators. You may also want to include a source of clean water.
When selecting the area for your garden, consider the insects you are hoping to attract. Do not place a bee garden in close proximity to high traffic areas such as playgrounds. Select a space that is sheltered from the wind but receives ample sunlight. Make sure the grounds crew at your school knows to leave the area undisturbed. If attempting to attract butterflies, you may want to consider the plants needed for their larvae to feed on.