Little footprints on petals

I find it difficult not to write about bumblebees these days. We've had a week of showers, and bumblebees do not seem to mind much, going about their business rain or shine. They have the advantage of being warm-blooded insects, they keep their body temperature above the environment, and therefore, they do not need the sun to warm up and forage, like other insects. When the flowering bushes are buzzing, how do bumblebees decide what flowers to visit and when? Do they follow a pattern or do they choose specific flowers? If you watch a single bumblebee visiting a plant, say, a foxglove or a Stachys spike, it is easy to see that they follow a pattern: from the bottom up, and that might suffice to avoid visiting the same flowers twice; but what if there are many bumblebees feeding together? Surely they will end up visit flowers that have just been sucked up dry of nectar? Well, research shows us an unexpected aspect of bumblebee's - and bee's - natural history. Bumblebees actually mark, with scents detectable by other bumblebees, each flower they visit with glands located on their tarsi - at the end of their legs. Some of these marks are repellent, so that bumblebees avoid flowers that themselves - or others - have recently visited and, as the chemicals are volatile, by the time they have faded, the flower is full with nectar again. Interestingly, flower scent marks seem to be recognised not only by bumblebees of the same species, but also by several other bumblebee species, and bees visiting the same flowers, despite being different chemical compositions. Jane Stout and Dave Goulson carried out simple, but labour intensive, experiments by with Bombus lapidarius and Apis mellifera (the honey bee) feeding on a large patch of Melilotus officinalis, showing that both species recognise each other marks on flowers and avoided recently visited flowers. What I found really interesting around this topic is that, in a different study, Gawleta and coworkers showed that bumblebees avoid to a larger extent the flowers previously visited by Anthidium manicatum, the Wool-Carder bee. This could be a response to the aggressive behaviour of males of this territorial bee.

Bombus pascuorum leaving a Stachys flower. Wool-carder bees have been quite late this year and it is the first time I see B. pascuorum feeding on these flowers.

Female Wool-Carder bee feeding on Stachys. Males have not emerged yet and both bumblebees and Wool-Carder bees seem happy to feed together.