Visitation frequency of Melampyrum roseum var. japonicum (Scrophulariaceae) by three bumblebee species and its relation to pollination efficiency

The efficiency of pollination of Melampyrum roseum Maximowicz var. japonicum Franchet et Savatier by three bumblebee species, Bombus consobrinus Vogt, Bombus diversus Smith, and Bombus honshuensis Tkalcu, which differ especially in proboscis length, was examined under experimental conditions where visitation frequencies were controlled. First, we found that B. honshuensis, with the shortest proboscis, deposited the most pollen per stigma, resulting in the highest seed set in single visit experiments among the three bumblebee species, while B. diversus removed the most pollen from the anthers. The morphological match between pollinators and flowers was found to be an important factor affecting the per-visit pollination intensity mentioned above. Second, we found that seed set and pollen removal increased when flowers were visited twice by any of the three bumblebee species and that the differences among the three bumblebee species disappeared. When flowers were visited three or four times, the seed set and pollen removal did not increase further. These findings indicate that pollination effectiveness may differ among the bumblebee species only when flowers are infrequently visited by any of the bumblebee species.