Mutual complementarity among diverse pollinators as a mechanism underlying open insect pollination in Japanese pear orchards

Cultivated Japanese pear varieties, mostly showing self-incompatibility, require pollen transfer from other varieties. To elucidate the underlying mechanisms of open Japanese pear insect pollination, we first collected flower-visiting insects using plastic vials and sticky traps in orchards located at different regions in Japan. Results showed that insects assigned to the families Andrenidae (Hymenoptera), Apidae (Hymenoptera), Halictidae (Hymenoptera), Syrphidae (Diptera) and Empididae (Diptera) are abundant in the orchards. Second, we restricted the flower-visiting insects to access Japanese pear flowers using bags with mesh sizes of 0, 2 and 3.5 mm. Results indicated that insects which allowed to pass through bags with 3.5-mm mesh size but not through bags with 2-mm mesh size contribute primarily to pollination, represented as the fruit-set ratio and seed number. Third, we measured head and thorax widths of the flower-visiting insects and counted pollen grains on their body surfaces to estimate their pollination potential. Results indicated that insects assigned to the families Andrenidae, Halictidae, Syrphidae, Bibionidae (Diptera) and Muscidae (Diptera), including species with both widths smaller than 3.5 mm, harbour large quantities of Pyrus pollen grains, in addition to Apis mellifera (Apidae) with both widths greater than 3.5 mm. Consequently, the families Andrenidae, Apidae, Halictidae and Syrphidae might be the most important insect families for Japanese pear pollination. However, species identification of the flower-visiting insects showed no common key species that contribute remarkably to pollination services other than A. mellifera. Consecutive insect collection using plastic vials and sticky traps demonstrated that compositions of the flower-visiting insects are fluctuating continuously in the orchards. Nevertheless, year-to-year fluctuation of the fruit-set ratio was less pronounced in open-pollinated orchards than in hand-pollinated orchards. These results suggest that mutual complementation among diverse pollinator species might be the mechanism underlying open Japanese pear insect pollination.