How do flowers attract pollinators?

Flowers can be choosy about pollinators. Flowers use attractants such as color patterns, aromas, conspicuous anthers, and the flavors of nectar. They use deterrents such as folded petals, narrow passages, tough anthers that must be peeled open or shaken (buzz-pollination),

and nectar that is distasteful or toxic to some visitors. The shape of many flowers forces a visitor to push its head past the stigma and anthers in order to reach the nectaries deep inside. Some flowers form a tube that restricts access to an insect or hummingbird with a long tongue. A complex folded flower selects for bumblebees, which are clever and use their hooked claws to unlock the puzzle. Long anthers favor larger bees that can grasp and chew them one by one whereas

the composites (Asteraceae) and umbels (Apiaceae) produce broad carpets of tiny flowers on which small bees and flies can walk. Flowers that close at night (nyctinasty) may exclude moths in favor of day-flying insects.

Flower color is a signal to pollinators. Insects see yellow, green, blues and violet, as well as the near ultraviolet (which humans cannot see) but not red. Most flowers that we tend to see as red or pink are actually a pale violet that bees can see. True red flowers attract hummingbirds rather than insects. The large round eyes of flies afford them a wider field of view but no greater range of colors.

Sphaerophoria philanthus,

Iceberg Point, Lopez Island

Bees as well as flies also have three tiny simple eyes on the tops of their heads (ocelli) that detect brightness and moving edges, rather than color, so they are useful for orientation during flight, and early detection of

approaching predators, but not in choosing flowers. Flower aroma is another signal. When their pollen is ripe many flower species secrete volatile compounds that broadcast aromas. Insects have sensitive chemical detectors in their antennae, their mouthparts, and their feet that enable them to “smell” a flower while they are flying some distance from it, and then to “taste” it when they land. Bees and flies quickly learn

to associate the scent of a flower with the size of the nectar reward they find when they alight on it. Their ability to learn to recognize new food sources during a single lifetime makes them adaptable to change in their environment.

Next: Pollinator networks and plant communities