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).