Pollination Efficiency of Bumblebee Queens and Workers in the Alpine Shrub Rhododendron aureum

Rhododendron aureum is a weakly self-compatible alpine shrub pollinated by bumblebees. Flower visitation frequency and foraging behavior differed between queen bumblebees and workers. Queens exhibited less frequent flower visits, longer flight distances between inflorescences, and a smaller number of sequential inflorescence visits within patches than did worker bees. Flowering in early-snowmelt populations occurred in June, when only overwintered queens were available, while flowering in late-snowmelt populations occurred from late July to August, when frequent visits by workers were common. The fruit to flower ratio per inflorescence was lower in early-snowmelt populations due to quantitative pollen limitation, while the seed to ovule ratio per fruit was lower in late-snowmelt populations. This is because geitonogamous pollination by workers was common later in the season, which resulted in a high abortion rate of self-pollinated seeds, that is, qualitative pollen limitation. Thus, pollination efficiency, or the type of pollen limitation, shifts during the flowering season, reflecting the seasonal life cycle of bumblebees in the ecosystem. Many of the self-fertilized seeds aborted due to high inbreeding depression, and the outcrossing rate was ∼80% regardless of season