Close Encounters

Indirect effects of drought on bumblebee pollination of common charlock

How does drought influence floral traits of Sinapis arvensis and how does this affect visitation by bumblebee pollinators?

Flowers are multimodal displays that plants use to attract pollinators and ensure pollination. Under dry conditions, plants can reduce their flower number and size to save water. However, reducing the size and number of flowers (or the scent or colour of flowers) in response to water deficit may in turn reduce attractiveness to potential pollinators. Understanding such interactions will be increasingly important in the future, with predicted changes in precipitation regimes reducing water availability in many regions across the globe.

In their new study published in AoBP, Kuppler et al. explored how drought affects floral traits of Sinapis arvensis (Common Charlock, Brassicaceae) and how this affects pollinator visitation. They conducted a flight cage experiment with an automated visitation recording system observing bumblebee interactions with drought stressed and well-watered plants.

Bumblebee (Bombus terrestris) pollinating a lavender plant.

S. arvensis is a wide-spread annual self-incompatible plant. It is native to Southern and Middle Europe, occurs mainly on fields, field margin and ruderal areas and is visited by a broad range of flower visitors, including hoverflies, bees and bumble bees. Buff-tailed bumblebees (Bombus terrestris) were used for this work as they are a common flower visitor of S. arvensis. The bees were obtained from a self-reared colony at the Institute of Evolutionary Ecology and Conservation Genomics, Ulm University, Germany.

The results of the study show that decreasing flower number and size under drought conditions result in decreasing bumblebee visits. Decreasing flower visitation with lower soil moisture availability might be explained by lower numbers of flowers and thus a reduced attractiveness and/or by increased difficulties experienced by bumblebees in handling smaller flowers. This indicates that indirect effects of drought on pollinators can modify plant–pollinator interactions and potentially change pollen transfer and, thus, pollination in plant communities.

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