An interesting paper examining differing flower phenology and the visiting pollinator community.
Flower-visiting insect communities on two closely related Rhododendron species ﬂowering in different seasons
Arthropod-Plant Interactions (2012) 6:333–344
Abstract To determine the effects of ﬂowering season on the community structure and dynamics of ﬂower visitors, I examined annual and hourly variation in ﬂower-visiting insects on two Rhododendron species in a deciduous secondary forest in central Japan. Rhododendron reticulatum ﬂowers from late March to mid-April, whereas R. macrosepalum ﬂowers from late April to late May. Bagged and hand-pollination experiments indicated that outcrossing by ﬂower visitors was important for the pollination of both R. reticulatum and R. macrosepalum. Pollinator and pollen limitation were detected in both Rhododendron species, although the extent of both differed between the two species. The ﬂowers of both Rhododendron species were visited by insects of diverse taxa, including Hymenoptera, Diptera and Lepidoptera. The composition of ﬂower visitors differed between R. reticulatum and R. macrosepalum, although a few species visited both of them. The beeﬂy Bombylius major (Diptera: Bombyliidae) most frequently visited R. reticulatum, whereas the bumblebee Bombus ardens ardens (Hymenoptera: Apidae) most frequently visited R. macrosepalum. Hourly changes in the number of ﬂower visitors also differed between R. reticulatum and R. macrosepalum; insects visited R. reticulatum ﬂowers less frequently in the morning than in the afternoon, whereas insect visitations to R. macrosepalum ﬂowers did not differ between morning and afternoon. Differences in both insect community structure and hourly changes of visitations between the two Rhododendron species occurred annually and may be related to the life history of the abundant visitor species as well as ﬂowering phenology.