Phenological differences in pollinator visitation, pollen deposition and seed set in the sticky catchfly, Viscaria vulgaris

(1) The mechanisms behind seasonal differences in pollinator visitation and resulting seed set in the bumblebee-pollinated, perennial, protandrous herb, Viscaria vulgaris (Caryophyllaceae) were studied in S. W. Sweden. (2) Seed set per flower decreased over time because of (i) decrease in ovule number, (ii) decrease in pollen availability due to protandry, and (iii) decrease in pollinator visitation and pollen deposition. Hand pollination increased late seed set nine-fold one year, 1.1 times one other year and 1.3 times during mid-season a third year, indicating temporal pollination limitation. Late seed set per flower was not increased by adding nutrients. Late-produced seeds germinated less well than early ones, with a success similar to self-fertilized seeds, suggesting a higher proportion of self-pollination among late-produced seeds. (3) Pollen deposition on receptive V. vulgaris stigmas was highest during the first half of the flowering season, with the pollen to ovule ratio highest after approximately one week of flowering. Stigmas of bagged flowers commonly bore some self-pollen grains despite protandry. Pollen of wind-dispersed species dominated the alien grains on V. vulgaris stigmas. (4) Both standing crop per flower and sugar content of nectar changed over the course of the season. (5) A few long-tongued bumblebee queens made most of the visits to V. vulgaris. Bumblebees deposited up to 500 conspecific pollen grains on a stigma during single visits. However, they abandoned V. vulgaris prior to its flowering peak because more rewarding species began to flower. The bumblebees subsequently concentrated on one pollen plant (Lupinus polyphyllus) and one nectar plant (Trifolium pratense).