Flowers and insects... what’s the big attraction?

The colourful centre of narcissi flowers help attracts insects.

The colourful centre of narcissi flowers help attracts insects. - Credit: Archant Ltd.

Nature: Grace Corne looks at how flowers use colours, patterning and strength in numbers to attract insects.

We have had long periods of sunshine this autumn, which have made the dull days perhaps a little more memorable. Whilst travelling to a nearby town recently on just such a day it was impossible to miss the white flower heads of yarrow and white deadnettle which really stood out against the background of autumn leaves and largely brown foliage.

In the world of plants everything happens for a reason, so it is not surprising that these plants, which are insect-pollinated, have flowers which are most likely to be visible from a distance. Calculations done by earlier botanists suggested that at least 60% of plants had either white or yellow blooms for precisely that reason. Closer inspection reveals that some of the blooms may be more brightly coloured on their upper surfaces and their under surfaces may simply be green or a darker colour. However if the flower naturally hangs downwards the outside upper parts near the stem may be more brightly coloured.

Some plants have naturally small flowers and the roadside yarrow is one of these, so to ensure the plant is more visible the little blooms are bunched together to make a bigger impact. The white-flowered wild carrot uses a similar ploy but adds to the attraction by having a fly-like lure at its centre.

There are many other ways plants use colour to attract pollinators. For example, the cornflower has flowers of an outstanding brilliant blue, and the larger funnel-shaped florets surrounding the flower head are a great temptation, but they are totally sterile. Insects flying in to investigate will look in vain for honey in them, but they will soon realise that it is the inner, inconspicuous blooms which contain the honey, so in actual fact the plant is almost flag-waving to advertise itself.

Many plants have more than one colour and sometimes a dark colour on a white or yellow flower will act as a 'honey guide' for visiting insects. For an example think of a narcissus which is a pure white flower, with a red ring at the centre, which focuses the attention of any visiting insect and increases the chance of successful pollination.