Spring: Season of bursting buds and buzzing bees

April's Charms
by William Henry Davies (1871-1940)

When April scatters charms of primrose gold
Among the copper leaves in thickets old,
And singing skylarks from the meadows rise,
To twinkle like black stars in sunny skies;

When I can hear the small woodpecker ring
Time on a tree for all the birds that sing;
And hear the pleasant cuckoo, loud and long --
The simple bird that thinks two notes a song;

When I can hear the woodland brook, that could
Not drown a babe, with all his threatening mood;
Upon these banks the violets make their home,
And let a few small strawberry blossoms come:

When I go forth on such a pleasant day,
One breath outdoors takes all my cares away;
It goes like heavy smoke, when flames take hold
Of wood that's green and fill a grate with gold.

Red-tailed bumblebee queen (Bombus lapidarius)

Common carder bee queen (Bombus pascuorum)

Siberian bugloss (Brunnera macrophylla)

Mediterranean spurge (Euphorbia characias)

Early bumblebee queen (Bombus pratorum)

A little hoverfly in an azalea flower

Bee-fly (Bombylius major)

Alpine clematis (Clematis alpina)

Mining bee (Andrena sp.) in a tulip flower

Spotted deadnettle (Lamium maculatum)

A Horse chestnut tree

Buff-tailed bumblebee queen (Bombus terrestris)

Cowslip (Primula veris)

Hairy-footed flower bee female (Anthophora plumipes)

Japanese maple (Acer palmatum)

A Japanese maple leaf

Ornamental cherry tree (Prunus sp.) blossom

Ornamental cherry trees in April

Cowslips, daffodils and cherry blossom

A little crab spider hiding in a daffodil flower

Crap apple tree flowers

Drumstick primulas (Primula denticulata)

Snake`s head fritillary (Frittilaria meleagris)

A green-bottle fly in a Lesser celandine flower

Small tortoiseshell sipping nectar from a Grape hyacinth

Daffodils on the University of Reading campus

A Comma butterfly on ornamental cherry flowers

A little mining bee (Andrena sp.)

A honeybee

Young Sycamore leaf

Cowslip in early morning sunshine

Hairy-footed flower bee male

Wild daffodil (Narcissus pseudonarcissus)

A little fly sitting on a Snake`s head fritillary flower

An ornamental cherry tree in full flower

Bumblebees like the cherry tree flowers

A little Common toad, perfectly camouflaged

Feel the buzz: How bumblebees use vibration to collect pollen

Most bee-pollinated flowers offer not only pollen but also nectar as a reward to ensure frequent visits by bees. But some flowers have a different strategy and only offer pollen as a sole reward. Poppies (Papaver) are typical pollen flowers, offering an abundance of pollen which bees avidly collect. Other pollen flowers are rockroses (Cistus) and sunroses (Helianthemum), although rockroses do produce some nectar.

Poppies are pollen flowers, offering abundant pollen but no nectar

 

Bittersweet flower with reflexed petals and central anther cone

Even more specialised are a group of largely unrelated pollen flowers adapted to vibratory pollen collection (also called buzz pollination) by bumblebees. These flowers all have a similar shape with reflexed petals and a prominent anther cone in the centre which contains the small, dry and light pollen. The flowers produce no nectar.

Typical examples for these flowers are Bittersweet (Solanum dulcamara), Tomato (Solanum lycopersicum) and the North American Shooting star (Dodecatheon spp.). Interestingly Borage (Borago officinalis) has a very similar flower form but produces abundant nectar. Honeybees, which do not use vibratory pollen collection, visit borage flowers mainly to collect nectar and bumblebees also for collecting pollen.

Shooting star (Dodecatheon)

Borage flowers look similar but produce abundant nectar

Buzz-pollinated flowers are mainly visited by bumblebee workers which land on the flower and curl the ventral side of their body around the anther cone while grabbing the base with their mandibles. They then start (after decoupling the wings so they do not beat) to rapidly vibrate their thoracic muscles. The vibration releases the small pollen grains from pores at the tip of the anthers which land on the bumblebee`s body. The bumblebee then either collects the pollen grains to transport them back to the nest or carries them to the next flower and by repeating the above process pollinates the flower.

 

Bumblebees vibrate their thoracic muscles to release the pollen

This solitary bee is trying to release some pollen but is not very successful

Modern tomato varieties are mostly self-fertile and in outdoor-grown tomatoes sufficient pollination can occur when the wind is shaking the flowers and releases the pollen. But other methods must be employed in large commercial greenhouses which now use cultured colonies of the Buff-tailed bumblebee (Bombus terestris) to pollinate the tomato flowers which is a lot more economical than humans pollinating the flowers with a “vibrating wand”.

Bumblebees are efficient pollinators of tomato flowers

Other flowers which do not have the typical flower shape (reflexed petals and a prominent anther cone) but also benefit from buzz pollination are blueberries, cranberries and kiwi-fruit (Actinidia).