The influence of nectar and pollen availability on pollen transfer by individual flowers of oil-seed rape (Brassica napus) when pollinated by bumblebees (Bombus lapidarius)

1 Levels of nectar and pollen were manipulated in individual flowers of oil-seed rape, Brassica napus, which were then offered to foraging bumblebees (Bombus lapidarius) for a single visit. 2 The durations of flower visits by bumblebees increased with the amount of nectar, bur were unaffected by the level of available pollen, The bees' flower-handling routine may have become behaviourally fixed by the expectation of low pollen rewards, which prevailed in the experimental garden. 3 The effects of nectar provision on both pollen import (deposition on stigma) and export (removal from anthers) were studied. The amount of pollen transfected during a bumblebee's visit was affected only by the amount of pollen that the flower presented. 4 Manipulated flowers that presented a large amount of available pollen received approximately three times more pollen on their stigmas during a bee's visit than flowers with little available pollen, which suggests that autogamous pollen transfer contributes substantially to pollen deposition when pollen availability is high. The gradual dehiscence of the anthers in this species means that available pollen accumulates only if pollinator visits are infrequent, when the increased autogamy may act to ensure adequate pollination. 5 There was no relation between the quantity of nectar residue that a bee left after visiting a manipulated flower and the mean duration of its visits to unmanipulated flowers. Therefore, variation among individuals in the duration of visits to unmanipulated flowers apparently resulted from differences in foraging speed rather than from variation in the thoroughness of nectar removal.