Pollen Removal and Deposition by Pollen- and Nectar-Collecting Specialist and Generalist Bee Visitors to Iliamna bakeri (Malvaceae)

Abstract

Up to 60% of the bee species of a region are oligolectic; they collect pollen only from a closely related group of plants though nectar-collecting choices are often broader. Bee specialists are expected to be superior to generalists in gathering pollen from their host plants and perhaps in transferring pollen to host stigmas. We used the oligolege Diadasia nitidifrons and its pollen-host Iliamna bakeri to ask if specialists 1) were more efficient than generalists as pollen-collectors; 2) deposited more pollen on stigmas than generalists; and 3) if pollen-collectors removed and deposited more pollen than did nectar-collectors. We found support for the first and third hypotheses. Diadasia pollen- and nectar-collectors removed more pollen per flower-visit than did their primary generalist competitors (Agapostemon spp.). The superior pollen-gathering efficiency of Diadasia exceeded differences that might be attributed to size: although Agapostemon females are, on average, 12.5% smaller than Diadasia females, pollen-collecting Agapostemon left 22.9% more pollen in flowers than did Diadasia. We found no difference between taxa in time spent foraging on a single flower. Diadasia and Agapostemon pollen-collectors deposited significantly more pollen on I. bakeri stigmas than did nectar-collectors; there was no difference between taxa in pollen deposition. Diadasia was superior to generalists as a pollinator in two ways: Diadasia was 1) a more reliable presence in I. bakeri populations; and 2) always most abundant at I. bakeri flowers. The association between D. nitidifrons and I. bakeri appears to be another example of a highly specialised bee affiliated with an unspecialised host-plant.