How plants communicate with pollinators
Since between 75 to 90% of flowering plants require pollinators for reproduction, and since they’re clearly immobile, they must have strategies for attracting pollinators to themselves.
Pollinator syndrome describes the way plants have developed over time to attract specific pollinators. The more we learn about a flower’s appearance as well as its food rewards of nectar and pollen, the more nature amazes us.
The most obvious way flowers entice pollinators is by sight. And pollinators do not value the same flowers as we do. Part of the reason for the proliferation of flowering weeds is due to their visual appeal to bees, for example.
Many flowers use visual cues to attract pollinators: shape, size, showy petals and sepals, guides to nectar and color. Bees, butterflies and moths, bats and birds see these features while flying from above.
The bell shape of the blossoms of the native azalea attracts native ruby-throated hummingbirds. As the bird reaches the nectar, it contacts the reproductive parts of the plant and performs pollination service inadvertently.
On the other hand, beetles and other non-hovering pollinators are attracted to open and flat flowers that serve as landing areas, such as echinacea (purple coneflower).
Irises use the energy from their underground rhizomes to produce very large flowers. Their flower size helps them compete with surrounding flowering plants for the pollination services of bumblebees.
Plants also use coloration and patterns to signal the right pollinators. Nocturnal pollinators such as bats and moths locate night-blooming flowers that are white or very pale.
Aromatic aster, a fall-blooming plant, communicates its pollination status vistually to monarch butterflies. Monarchs need to fill up on nectar before their long winter migration to Mexico. The florets in the center of the aster are yellow when they are full of pollen; once pollinated, the center turns red. Butterflies are considered to have the widest visual range of any form of wildlife.
Nectar guides (honey guides or floral guides), which are patterns to guide pollinators to nectar and pollen, can be visible to humans as well as to pollinators. Those include subtle lines, spots, swaths of color, or streaks such as those directing a bumblebee within penstemon (beardtongue).
Some nectar guides, unseen by our human eyes, are visible to bees who can see ultraviolet color. About 7% of all flowers show floral patterns in ultraviolet light that are not evident in visible light. In bee-pollinated flowers, there is a region of low ultraviolet reflectance near the center of each petal called the “blue halo.” Data have shown that the blue halo is the key visual signal that attracts bees. The subtle effect of the blue halo is produced from nanoscale ridges on the surface of flower petals that change the light when viewed from certain angles.
One flower that shows a bullseye effect in the ultraviolet range is rudbeckia (black-eyed Susan). The petals of the black-eyed Susan appear plain yellow to humans, but insects can see a larger and darker center than we can.
In the native flowering dogwood (our state flower and tree) as well as the kousa dogwood, what appears as a four-petaled flower is actually four bracts surrounding many small flowers. The bracts look white in visible light with small yellowish green central flowers. However, in ultraviolet light, the central flowers appear highly reflective, and the large bracts look dark.
Bribes with food
The majority of animals involved in pollination do so because the flower provides food (nectar and/or pollen) in exchange for unintentional pollination services. Nectar is primarily sugar water that contains amino acids, minerals, and vitamins in concentrations best suited to meet the nutritional needs of the pollinator.
Researchers studying different species of Impatiens flowers discovered that nectar varied, even within a genus. For example, the nectar of some species included more amino acids, which helped butterflies who lacked protein in their diet. Plants dependent on pollinators with high-energy needs, such as hummingbirds, produce highly-concentrated sugar nectar. Bees have been shown to be capable of perceiving differences in sugar concentration as small as 1 to 3%.
Plants have changed over time to get the amount of nectar just right. If the flower offers too little, pollinators won’t be attracted to it. On the other hand, too much nectar will sate visitors quickly and they won’t have an incentive to look for more (effectively putting a halt to the pollination process). A large volume of nectar typically attracts bird pollinators.
Another food reward, pollen, is high in protein, which meets the needs of many bees and beetles. Some flowers produce two types of pollen: normal pollen that is involved in cross-pollination and sterile pollen that is a tempting food source.
Additionally, plants have evolved flowering times throughout the growing season to decrease competition for pollinators and to provide pollinators with a constant supply of food.
Plant native plants to entice the pollinators to your own back yard.
Note: This is part one of a two-part series. [Read part two here]
Lela Martin is a Master Gardener with the Chesterfield County office of the Virginia Cooperative Extension.