Pollen Transport and Deposition by Bumble Bees in Erythronium: Influences of Floral Nectar and Bee Grooming

Abstract

(1) The transport of pollen from donor flowers by bumble bees was measured by examining deposition on stigmata of sequences of recipient flowers. The rate of decay of grain deposition was estimated as a measure of pollen carryover. (2) Bombus bifarius was a much less effective pollinator of Erythronium grandiflorum than was the larger Bombus occidentalis. (3) The numbers of pollen grains deposited by bumble bees on the stigmata of Erythronium americanum vary greatly from flower to flower. (4) The time spent by a bee on a flower is positively related to the nectar concentration and volume. (5) Flowers with large volumes of nectar receive more pollen grains per visit than those with small volumes, presumably because the visits are longer. The results are insufficient to show a parallel increase in deposition with nectar concentration. (6) Measures of pollen carryover are presented. Most deposition of grains from a particular donor flower occurs on the first several recipient flowers subsequently visited by the bee, but a few grains travel much farther. (7) Pollen carryover in E. grandiflorum is reduced by bee grooming. (8) The negative effect of grooming on carryover is increased when the recipient flowers have undehisced anthers. (9) Erythronium americanum and E. grandiflorum were similar in carryover, but Linaria vulgaris showed much higher carryover. The reasons for this are discussed.