The wonders (and cheaters!) of insect pollination

article written by Caitlin Coombs, Berkshire Biodiversity Officer

Pollination is defined as the transfer of pollen grains from the anthers of one flower to the stigma of another. A range of methods exist, involving wind, water, invertebrates, mammals, birds, and even elephants!

Insect pollination, also known as entomophily, is by far the most successful system and occurs in 80% of flowering plants.

Why is it so evolutionarily advantageous?

Well, in comparison to wind, it enables species-specific cross-pollination and isn’t weather dependent. It also means that pollen grains can travel further, and more precisely. In the plants point of view, they don’t need to produce vast amounts of pollen (to make up for wastage) and it is less energetically costly than producing larger rewards for pollinators such as birds.

Plants attract insects by tempting them with various ‘rewards’ such as nectar. Insects visit the flower to reap the rewards, and usually pollinate the flowers in return.

There are many methods used by plants to attract and reward pollinators:

  • Flower shape: some plants (like umbellifers) have flowers which act as large, landing platforms which can attract a range of species, whereas some plants have long, tube-shaped flowers which can only be pollinated by long-tongued bees
  • Flower colour: some plants have ‘nectar guides’, which are UV patterns on petals which ‘point’ the pollinator towards the nectar
  • Smell/ fragrances: often present in night-blooming flowers
  • Reward type and amount: can be nutritional (eg nectar, pollen) or non-nutritional (eg heat)

Most plant-pollinator relationships are mutualistic: the plant gets pollinated, and the pollinator gets rewards. One example is the fig wasp:

The story of the fig wasp:

For each species of fig in the world (around 900!) there is a unique species of parasitic wasp which pollinates it. The species have co-evolved to be entirely dependent on each other. First, pollen-laden females find a fig plant and lay their eggs in the flowers, pollinating them in the process. Flowers in which larvae hatch turn into special structures, while the others become seeds. Males hatch, wander around the fig to find females, and fertilise them. They then build a sacrificial tunnel for the female to escape, and die inside the fig. The female wasp then clambers out, picking up pollen on her way, to find another fig and repeat the process. Meanwhile, the carcass of the dead male is broken down by an enzyme. Beautiful!

However, there are ‘cheaters’ around! Some pollinators take the reward from the plant without pollinating it, and some plants get pollinated without rewarding the pollinator!

‘Nectar thieves’ remove nectar by piercing into the flower corolla, taking the nectar, but avoiding contact with reproductive structures. Some carpenter bees, bumblebees, solitary bees, wasps and ants do this. This obviously isn’t ideal for the plant, but the damage actually goes a step further, as the ‘piercing’ destroys the reproductive parts of plants and reduces their overall seed set!

And- there’s more! ‘Secondary thieves’, such as some honey bees (eg Apis mellifera), collect nectar from perforations previously made by the initial thieves!

Botanists- don’t worry. Plants are also getting their revenge, in a process known as ‘floral mimicry’. Some species, such as the bee orchid (Ophrys apifera), have evolved flowers which mimic the females of a pollinating insect; thus enticing in the male pollinator under false hope. The male visits the flower, gets covered in pollen and pollinates the plant, but is left disappointed.

Other plants have evolved to mimic food sources. For example, the stinking corpse lily (Rafflesia arnoldi) mimics the smell of rotting meat to attract carrion flies. Other plants mimic the flowers of other rewarding plants which grow nearby, and others mimic egg deposition surfaces. Clever, right?

In the UK alone, there are approximately 1,500 species of pollinators and they come in all shapes and sizes. There is always more to discover!

 

 

Posted: February 9, 2021