Abstract
Bumblebees (Bombus spp.) are efficient pollinators of many flowering plants, yet the pollen deposition performance of individual bees has not been investigated. Worker bumblebees exhibit large intraspecific and intra-nest size variation, in contrast with other eusocial bees; and their size influences collection and deposition of pollen grains.Laboratory studies with B. terrestris workers and Vinca minor flowers showed that pollen grains deposited on stigmas in single visits (SVD) were significantly positively related to bee size; larger bees deposited more grains, while the smallest individuals, with proportionally shorter tongues, were unable to collect or deposit pollen in these flowers. Individuals did not increase their pollen deposition over time, so handling experience does not influence SVD in Vinca minor.Field studies using Geranium sanguineum and Echium vulgare, and multiple visiting species, confirmed that individual size affects SVD. All bumblebee species showed size effects, though even the smallest individuals did deposit pollen, whereas there was no detectable effect with Apis with its limited size variation. Two abundant hoverfly species also showed size effects, particularly when feeding for nectar.Mean size of foragers also varied diurnally, with larger individuals active earlier and later, so that pollination effectiveness varies through a day; flowers routinely pollinated by bees may best be served by early morning dehiscence and visits from larger individuals.Thus, while there are well-documented species-level variations in pollination effectiveness, the fine-scale individual differences between foragers should also be taken into account when assessing the reproductive outputs of biotically-pollinated plants.
Highlights
Bees are the most plentiful and successful of the pollinators, and depend entirely on flowering plants since they feed only on pollen and nectar throughout their lives
Pollen grain deposition overall varied from 0 to 300 grains per flower, with zero deposition on the first flower visit and means of 39.8 ± 2.6 and 38.0 ± 4.5 grains on the second and third visits (after pollen had been acquired from the previous flower(s)) within a trial
In more complex flowers handling time should decrease, but whether this leads to an increase or a decrease in Single Visit Deposition (SVD) is hard to predict: we previously found little or no relation between visit duration and SVD across 13 plant species (King, Ballantyne & Willmer 2013, though longer visits on nectar-rich flowers may improve pollen deposition in some cases (e.g. Thomson & Plowright 1980)
Summary
Bees are the most plentiful and successful of the pollinators, and depend entirely on flowering plants since they feed only on pollen and nectar throughout their lives. Bumblebees show substantial inter-specific differences in worker size (Benton 2006), but unlike most other eusocial bees demonstrate large intra-specific and intra-nest size variation (Plowright & Jay 1968; Peat et al 2005); workers can exhibit a ten-fold size variation within a single nest (Alford 1975) compared with less than two-fold variation in honeybees and stingless bees (Waddington et al 1986; Roulston & Cane 2000) This is not genetically controlled as workers within a nest are normally full sisters, but instead probably stems from unequal larval feeding (Sutcliffe & Plowright 1988; Couvillon & Dornhaus 2009); Persson & Smith (2011) have shown that adult size of bumblebee workers is significantly related to the availability of floral resources. There has been little investigation of the pollen-depositing abilities of individuals, and the influence of visitor size variation on plant pollination
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