Importance of Queen Bumble Bees as Pollinators Facilitating Inter-morph Crossing in Primula sieboldii

Abstract Flower-visiting insects, the distribution of pollen on an insect's body, and fruit and seed sets of Primula sieboldiiE. Morren were investigated in a maritime deciduous forest habitat of the Hidaka region in Hokkaido. Queens of Bombus diversus tersatus Smith (Hymenoptera: Apidae), Bombus schrencki albidopleuralis Skorikov, and Bombus deuteronymus deuteronymus Schulz were observed to visit the flowers of the species. Bumblebee claw marks were found on the flower petals of 68% of the flowering ramets of the species. Scanning-electronmicroscopy revealed that pin and thrum pollens of P. sieboldii were deposited in different positions along the proboscis of a B. diversus tersatus queen (n=38400 grains). On the glossa, all the observed pollen was of the pin type, while more than half of the pollen grains observed on the upper part of the proboscis, galea, stipes, and maxillary palpus were of the thrum type. In the field, high fruit and seed sets were recorded for both pin and thrum morphs, indicating the occurrence of sufficient legitimate pollination during the flowering season. In addition to the effectiveness of queen bumblebees as pollinators for P. sieboldii suggested in this study, phenological and morphological matches, i.e., early spring flowering during the queen bee emergence season and the similar lengths of the corolla tube and the bee proboscis, also support the inference that queen bumble bees play an important role in legitimate pollination of P. sieboldii.