A Pollinator's View: Flower Attractants

Nectar Guides
Prairie Phlox, Phlox pilosa
Dark pink lines near the corolla opening
 act as nectar guides for pollinators.
Nectar guides are stripes or spots inside the flower corolla that guide the insect visitor past the anthers to the nectar reward. Since bees have compound eyes with receptors that can discern different wavelengths from the human eye, some guides on flowers are not visible to humans. Bees have the ability to see intensity, color and polarization of light. They also have a high sensitivity to UV and red-blindness on the other end of the color spectrum. Because their perception of flower color is different from humans, many floral visual cues such as nectar guides and color contrasts in flowers are visible only to bees.
Black-eyed Susan flowers, Rudbeckia hirta, look a lot different to bees than humans. The central disc florets on the cones along with the bases of the ray florets absorb ultraviolet light. The outer tips of the ray florets reflect ultraviolet light.

Illustration approximating the
bee's view of the flower rays
absorbing and reflecting
ultraviolet light.
The overall appearance to bees is two rings, light and dark from the outer ray floret tips towards the central disc florets. The differences in color of the outer and inner rays are believed to act as nectar guides for visiting bees. These potentially help guide and orient pollinators towards the floral rewards (food).

Flower Color & Color Change
Heptica flower buds
are often a different
color than the sepals,
and once open, flower
color changes as it
On many plants, the flower color changes from the beginning to the end of the flowering period. These color changes can coincide with the flower phase, male or female, the depletion of the flower rewards or indicate that the flower has been pollinated. Changes in flower color can redirect flower visitors to other flowers that are still offering a reward. Lighter flower colors, whites, creams and yellows, have the advantage of being more visible to nocturnal pollinators. Bees have shown a preference for flowers that are pink, purple or blue, their second choice white or yellow flowers (as the colors appear to humans).

Flower Shape & Color Contrast
The shape of a flower can influence the types of floral visitors it attracts. Bees like symmetrical flowers, with simple outlines. They also like a landing pad, such as the bottom lip of bilabiate flower.

Color contrast between more than one color on the corolla, or between the color of the anthers and petals also serves as an attractant to flower visitors.

New England aster, Symphyotrichum novae-angliae (left),  has contrasting lavender rays and yellow-orange disc florets in the center. The open, flat-topped flowers are any easy landing pad for bees, flies, beetles and butterflies.

McCrea, K. D., & Levy, M. (1983). Photographic visualization of floral colors as perceived by honeybee pollinators. American journal of botany, 369-375.

Willmer, P. (2011). Pollination and floral ecology. Princeton University Press.