Co-evolution of flowers and bees

On Monday, July 11^th at High Elms Country Park, Pam Hunter gave us a fascinating insight into the variety and complexity of the interactions between insects, particularly bees, and flowering plants

This process of pollination is, of course, the transfer of the male gametes contained within pollen grains to the female gametes contained within carpels and this transfer may be abiotic, for example carried out by wind or water, or biotic, carried out by animals.

Amongst the animals known to carry out pollination in some plants are vertebrates such as bats and birds – though not in this country, and invertebrates such as butterflies, moths, beetles, flies, parasitic wasps and, of course, bees.

Of these pollinators the insects probably evolved first, possibly as early as the Silurian (435 – 410 mya) though at that early date they probably had no role in pollination as the first plants, such as the liverworts and early ferns, were probably pollinated by motile gametes swimming over the damp surface of the plants in search of an archegonium.

Then, during the late Carboniferous (355 - 295 mya) there evolved fern-like plants that were wind-pollinated. However it was not until the Triassic (250 – 203 mya) that there is evidence of insects such as beetles and flies visiting these wind-pollinated plants, presumably to harvest their pollen but then inadvertently carrying this pollen from plant to plant.

However it was not until the Cretaceous (135 – 65 mya) that there is the first evidence of nectaries, possibly signalling the arrival of a symbiotic relationship between the angiosperms and their pollinating insects.

Bee on Crocus                                                                                                          © Gvision | Dreamstime.com

The main pollinators in Britain are possibly the bees, with some 250 species of Miners, Carpenters, Masons, leafcutters and other solitary bees and the Social bees such as the Bumble and the honey bees.

These bees all belong to the Order Hymenoptera which includes Ants, Bees, Wasps and Sawflies, all terrestrial insects, usually with two pairs of membranous wings and with the rear wings linked to the fore wings by hooked bristles or hamuli. In addition they usually have well developed mandibles and often a lengthy proboscis through which they drink nectar. They also have large compound eyes which in bees see in the UV as well as the visible, following UV-reflecting guides to the nectar or pollen sources.  

Pollen is a protein and fat source for the pollinating insects, though its amino acid content varies with species and so pollen from one source may not suit all pollinating insects. Pollen also contains minerals, organic acids, polyphenols, vitamins and other nutrients and is converted by workers within the hive into brood food including Royal jelly.

However the main energy source for bees is the nectar which contains sugars such as glucose, sucrose and fructose plus trace amounts of other sugars. In the hive this nectar is distributed amongst the members and used for the synthesis of honey; an energy source to enable the colony to survive the winter.

However the nectar flow can vary with the time of year, temperature, humidity, soil moisture and pH, time of day etc.

Pam then went on to talk about the adaptations of the flowering plants to insect pollination with examples such as the deadnettle in which, as the bee enters the flower, the stamens move to deposit pollen on the back of the bee, and Sweet pea and Broom in which the pollen is deposited on the underside of the bee.

In addition she discussed modifications to flowers to prevent self fertilisation, for example the pin-eyed and thrum-eyed primroses, and the foxglove where the stigma matures before the stamen.

Finally Pam explained the Bee waggle dance first described by the Austrian ethologist and Nobel laureate Karl von Frisch.

Overall a most interesting and informative afternoon.