Density-dependent negative responses by bumblebees to bacteria isolated from flowers

Flowers offer habitats for bacterial communities that are often characterized by low diversities but high densities. The composition of these communities and the dissemination of bacteria between flowers receive increasing attention, whereas the ecological functions of flower-associated but non-phytopathogenic bacteria remain understudied. We screened bacteria isolated from nectar, petals and leaves of two plant species for their potential to affect flower–visitor interactions. We took advantage of the proboscis extension reflex (PER) of bumblebees evoked by sugar and investigated whether bacteria associated with the reward may interrupt this reflex. Cultivated bacteria were transferred into a watery glucose solution in increasing densities and their effect on the proportion of bumblebees displaying the PER after antennal contact with glucose solutions and bacteria was scored. In all but one trial, the proportion of bumblebees that accepted the watery glucose solution was negatively correlated with the bacterial density. Nearly half of the bacteria tested evoked avoidance at naturally occurring densities. Our results suggest that bacteria colonizing flowers have the potential to negatively affect the reproduction of plants via reduced visits by pollinators.