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.
Highlights
Oligolecty describes an inherent preference by all members of a bee population or species for the pollen of a circumscribed taxon of plants (Minckley & Roulston 2006)
Pairwise mean comparisons showed significantly more pollen remained in control flowers than in flowers visited by Diadasia and Agapostemon pollen collectors (DP, AP; P < 0.001) or by Diadasia and Bombus nectar collectors (P = 0.002 and < 0.001, respectively) but not in Agapostemon nectar collectors (P = 0.545)
A two-factor analysis of pollen remaining showed that flowers visited by pollen-collectors had significantly less pollen remaining than those visited by nectar collectors (F1,64 = 14.82, P < 0.001) and that flowers visited by Diadasia had fewer pollen grains remaining than those visited by Agapostemon (F1,64 = 3.63, P = 0.061)
Summary
Oligolecty describes an inherent preference by all members of a bee population or species for the pollen of a circumscribed taxon of plants (Minckley & Roulston 2006). Up to 60% of the bee species of a region may be oligolectic (Minckley & Roulston 2006). The commonest explanation for oligolecty is related to one advanced by Darwin (1876) for flower constancy (Cane & Sipes 2006; Raine & Chittka 2007). Darwin thought that individuals that restricted their visits to a few closely related plants would learn from frequent use to collect pollen from those flowers more effectively than their generalist competitors. At least some of those learned foraging behaviors are presumed to have become instinctive
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