Protective pollination properties

John Hale, of Dunedin, asked:

In early August I seem to have seen more early flowers than usual that are pink. What is the attraction of pink to bees and insects?

Janice Lord, a botanist at the University of Otago, responded:

Pink, purple, red and blue colours in flowers and fruit are generally due to common plant pigments called anthocyanins. These pigments have been called nature’s ‘‘Swiss Army knife’’ because they perform a variety of defensive and protective functions in stems, leaves, flowers and fruit, as well as producing colours that attract pollinators, or deter herbivores. In relation to their defensive and protective properties, anthocyanins are often produced in response to damage or environmental stress; for example, from low temperatures or high light levels. Increased anthocyanin production is linked to increased freezing tolerance and also an enhanced ability to recover from tissue damage as anthocyanins are highly effective antioxidants. So early blossoms appearing more pink than usual might reflect a cold snap that has stimulated the production of these protective pigments in developing petals.

But would an increase in pink coloration make the flowers especially attractive to pollinating bees and other insects? All bees, including our many small native bees, have trichromatic colour vision, with the ability to perceive light in the UV range, as well as colours that appear purple, blue, green and yellow to humans. Bumblebees in particular have a strong preference for blue and purple flowers. However, bees are not sensitive to light at the red end of the human colour spectrum, so a purely red flower would not be particularly obvious to them unless it had, for example, a bright yellow centre such as in dahlias and polyanthus. Just like bees, flowervisiting flies such as native hoverflies can also perceive colours in a similar range, from UV to blue and yellow.

A pink flower with a purplish rather than red hue would therefore be perceived as coloured to bees and hoverflies, and would attract their attention. If that flower also contained rewarding nectar or nutritious pollen, these insects would quickly learn to search for the pink colour to gain the reward. However, if the flowers offered little reward, as is the case with many ornamental blossom trees, the colour might initially attract the attention of naive pollinators, however, they would quickly learn to avoid these pink flowers, due to their false advertising.

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