Pollination, the process by which animals transfer pollen, allows flowering plants to reproduce. Pollinators include insects, birds, and bats. Plants attract these partners using visual and chemical signals. Flower color is the most obvious advertisement, serving as the primary signal to announce the availability of nectar and pollen rewards.
How Pollinators Perceive Color
Pollinator vision is fundamentally different from human sight, which dictates which flower colors are attractive. Humans possess trichromatic vision, using three types of photoreceptors keyed to red, green, and blue. Many insects, such as bees, also have trichromatic vision, but their spectral range is shifted toward shorter wavelengths of light.
A bee’s three color receptors are tuned to ultraviolet (UV), blue, and green light, granting them a broader range of color perception than humans, especially in the UV spectrum. Because bees lack a photoreceptor for red, true red flowers often appear as black or gray to them, unless the petals reflect UV light.
The visual systems of other pollinators vary widely. Some insects, like certain flies and beetles, possess dichromatic vision, relying on only two types of photoreceptors. Butterflies, in contrast, can have up to 15 different types of photoreceptors, giving them an exceptionally complex color sense. This variation explains why a single flower color can be highly attractive to one pollinator but ignored by another.
Specific Color Preferences by Pollinator
Pollinator color preferences result from coevolution, where floral traits and animal senses have adapted to maximize successful pollen transfer. These preferences are categorized by pollinator groups, each seeking hues that signal a high-quality food source.
Bees are strongly drawn to flowers in the blue, purple, and violet range, as well as yellow and white. These colors stand out clearly against the green foliage within their UV-blue-green color space. Although they generally avoid true red, they are attracted to flowers that reflect UV light, making them appear blue or purple.
Butterflies, which are active during the day, have a wider range of color preferences, including red, orange, yellow, and pink. Unlike bees, butterflies perceive red light effectively, making bright red flowers a strong visual target. Their reliance on sight means they are attracted to vivid, conspicuous hues.
Hummingbirds possess excellent color vision and are famous for their attraction to strong reds and oranges. Flowers relying on hummingbirds often display these colors, which helps deter insect visitors like bees that cannot see red well. The intensity of the color is a strong signal, and these flowers typically offer copious amounts of nectar.
For nocturnal pollinators, such as moths and bats, pale colors are favored because they are most visible in low light. Moths are attracted to white or light-colored flowers that stand out brightly against the night sky. Similarly, flowers pollinated by bats tend to be large and light-colored to ensure visibility in the dark.
Beyond Color: Visual Cues and Patterns
Although the main petal color is the initial draw, flowers employ additional visual signals to enhance attraction and guide pollinators precisely to the reproductive structures. One significant cue is the nectar guide, which consists of patterns on the petals that direct the visitor toward the pollen and nectar reward. These guides often manifest as lines, dots, or a central ring of contrasting color.
For many insects, these nectar guides are visible only in ultraviolet light, appearing as a bullseye pattern or landing strip. The guide reduces the time a pollinator spends searching for the reward, making the flower more efficient to visit. This efficiency benefits the plant by increasing the speed and accuracy of pollen transfer.
Flower shape is another specialized visual cue that dictates which pollinator can access the reward. Tubular flowers, for example, are suited for the long beaks of hummingbirds or the elongated proboscis of butterflies, allowing them to reach the deeply hidden nectar. Conversely, flies and beetles prefer open and flat flowers, which provide an easy landing surface and allow for crawling. Larger, clustered flowers also present a more conspicuous visual target, which benefits pollinators like butterflies.