Protein/Lipid ratio of pollen biases the visitation of bumblebees (Bombus ignitus Smith) to male-fertile cultivars of the Japanese pear (Pyrus pyrifolia Nakai).

Bees have been known to visit the male-fertile cultivars of self-incompatible flowering plants more frequently than the male-sterile cultivars, but the origin of this preference is poorly understood. Here, we demonstrate that this preference is driven by the higher protein/lipid ratio of male-fertile pollen compared with male-sterile pollen by way of two caged-behavioral assays with six cultivars. In the first assay, flower-naïve bumblebees (Bombus ignitus Smith) showed a significantly higher flower-visitation rate to male-fertile cultivars (pollen germination rate > 55%; > 14 visits/10 min) of the Japanese pear (Pyrus pyrifolia Nakai) than male-sterile cultivars (pollen germination rate ≤ 20%; > 6 visits/10 min).