Floral Cues for Bee Pollination

We're all aware of the amazingly complex lifestyles and abilities of bees that are so important for pollinating many of the important crops of the world. Just to mention a few of these, bees in flight can recognise objects by their three dimensional shapes, and they can also learn the non-natural task of string pulling for a reward, a skill they can then pass onto other members of their colony. The latest revelation published in a 2018 issue of the journal Science shows that honeybees can even understand the concept of 'zero', an ability that parallels animals such as the African grey parrot, nonhuman primates, and even preschool children. When it comes to pollination, how do scout foragers efficiently locate flowers that will reward them with pollen or nectar? Social bees behave as a superorganism and we know that once identified, they then pass on the message (ie direction, distance) to the rest of the foragers in the hive through variations in their waggle dance. The average horticultural reference source regarding insect pollination will say when bees approach a plant, where foliage is usually more prominent than flowers, they are guided by odour and contrasting petal/leaf colours. However it's known they can also use shape, humidity, carbon dioxide emission, UV-absorbing pigmentation, fluorescence, electrostatic fields and even nectar colour. Now it's been reported (Harrap et al. eLife (2017) 6, e31262) they can also make use of variations in the spatial temperature of flowers, as the following summary describes:

Bees experience the world in a different way to humans, and plants exploit the bees’ senses to facilitate their ability to find and pollinate flowers. Some flowers are warmer than others when they grow in their natural environment, and bumblebees can learn to choose between flowers with different spatial temperatures by using heat as a means of identifying the best ones. Recent advances in technology mean that scientists can now document such temperature variations better than previously. A broad species of plant taxa (118) were considered in this study, and over half had flowers with complex heat patterns across their petals, echoing the colourful and often beautiful visual patterns we see with our own eyes. On average, some parts of the petals were 4–5°C warmer than elsewhere. In experiments designed to explore this phenomenon, we used artificial flowers with different patterns and sizes to demonstrate that bumblebees can learn to distinguish different temperatures across petals. These patterns appear widespread in nature, and for a number of insect pollinators it’s likely they’re a hidden signal along with other cues that help them locate those flowers that will give a reward of pollen or nectar. These findings are potentially important given current concerns about climate change. If pollinators are partly reliant on subtle differences in temperature across the surface of a petal, then even small changes in the temperature of the environment might have a large and unanticipated influence on how efficient insect pollinators are when visiting flowers with heat patterns not readily apparent to us.

Together with the other search modalities mentioned above, these temperature cues may be jointly or individually used to improve the efficiency of foraging. The authors focussed on bumblebees (Bombus terrestris) because they’re present globally and are a generalist pollinator group across a wide species range, but honeybees and stingless bees have similar capabilities. Within-flower temperature differences as little as 2°C could be detected, and this ability was independent of flower size. In the same way that we have temperature-sensitive receptors in our fingers and elsewhere, bees use receptors in their legs and antennae to sense likely flowers. Across the 118 species studied, spatial flower temperature differences were more commonly influenced by leaf structure and morphology, sunlight interception (heliotropism), and pigment absorption than by internally-generated thermogenesis. Temperatures were more commonly hotter towards the flower centre than the periphery, and there was a wide variation in the pattern of these differences. These temperature patterns appear to be a general phenomenon across flora that are pollinated biotically, rather than a specific co-evolutionary adaptation between a particular pollinizer and flower.

All up, we now know about another impressive ability in these fascinating little creatures that we should nurture and protect, particularly with colony collapse disorder causing mayhem elsewhere round the world. Losing their services would be catastrophic for a rapidly expanding world population.

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