It has been proposed that pollen nutritional content is the most important factor in the selection of flowers by bees. We hypothesized that pollen and nectar quality affect bee visitation among co-flowering plant species. We also hypothesized that increased soil fertility increases pollen nutritional content and subsequently floral visitation. We conducted two field studies to test these hypotheses. In the first experiment, we recorded the following taxa groups of bees: 1) Apis mellifera (L.), 2) Bombus ternarius Say, 3) Bombus spp., and 4) Other Bees (solitary bees); visiting the following plants: Helianthus annuus L., Gaillardia aristata Pursh, Borago officinalis L., and Phacelia tanacetifolia Bentham. We measured the number of open flowers, corolla depth, flower diameter and nutritional content of pollen and nectar to determine which characteristics were correlated with bee visitation to flowers. We found that bees in different taxa foraged preferentially on one or several of the four plant species, and that pollen amino acid profile was associated with the amount of visitation for honey bees, B. ternarius, and Other Bees; and plants with higher percent nectar sugar content was positively related to visitation for honey bees and Bombus spp.

In the second experiment, we varied soil fertility for Impatiens capensis Meerb. (Jewelweed, Family: Balsaminaceae) by adding fertilizer to potted plants and measured subsequent floral production, flower morphological characteristics, pollen amino acids, and bumble bee, Bombus impatiens, visitation from an adjacent commercial bumble bee hive. We found that fertilizer application had a positive effect on plant growth, flower production, and amino acid content, which positively influenced bumble bee visitation rate.