Bumble bee selection of Mimulus guttatus flowers: The effects of pollen quality and reward depletion

The ability of flower visitors to monitor returns when collecting pollen fr om flowers has been seldom studied despite the importance of pollen as a fo od resource, particularly for bees. Californian populations of Mimulus gutt atus are polymorphic for pollen quality: many plants produce a high proport ion of cytoplasmless pollen grains that render the grains incapable of fert ilizing ovules or of supporting bees nutritionally. We found that different genotypes maintained a consistent proportion of inviable pollen within a g enotype and over time. The number of pollen grains per flower was also cons istent within a plant at each date but declined over time. We studied the a bility of British bumble bees (Bombus spp.) to discriminate among plants of Mimulus guttatus on the basis of pollen quality and quantity at three scal es: indoors with choices of two genotypes, in outdoor plots of several geno types that varied in pollen quality, and outdoors at a whole-patch scale wh ere two patches of plants differed in quality. We found that bees could discriminate among plants on the basis of pollen q uality provided that flowers still retained most of the pollen. In the two- genotype trials, bees chose genotypes primarily on the quantity of viable p ollen, and nectar was much less important. Similarly, where patches of low- and high-pollen quality plants were established, bees responded by visitin g the high-quality patch more often and by visiting more flowers within the patch. However, the results from the outdoor plots that contained genotype s of varying phenotypes were inconsistent. A meta-analysis of a large numbe r of separate plots showed that the overall correlation between visitation rate and pollen quality was significant, but variation among plots was also significant. A possible explanation for this inconsistency was suggested i n a greenhouse trial in which we showed that, when foraging density was hig h, depletion of the standing crop of pollen happened quickly, and this redu ced the ability of the foragers to choose the higher-quality genotypes. The results have implications for the evolution of pollen production in Mimulu s guttatus and reward production in other plants.