Importance of pollen grain volumes for calculating bee diets

Pollen grains harvested by bees differ greatly in volume, thus pollen grains contribute differentially to larval and imaginal nutritional ecology, and ultimately to bee fitness. Simple proportions are inadequate when disentangling the importance of various pollen taxa found on foraging bees, their scopal loads or in their nest provisions. Disparate volumes of pollen grains are an essential feature to be considered in any foraging or dietary study. To document the importance of pollen volume on diet we mixed equal amounts (by weight) of ten morphologically diverse pollen taxa commonly collected by honey bees in the Sonoran desert of Arizona. These taxa were: Cereus giganteus, Ephedra trifurca, Fouquieria splendens, Helianthus annuus, Prosopis juliflora, and Simmondsia chinensis. Additionally, a small grain, Solanum rostratum, and two large grains, Cucurbita foetidissima and Opuntia phaeacantha, rarely harvested by honey bees, were included in the mixture. The mixture was inoculated with calibrated spore tablets, acetolyzed, counted, and % volumes calculated. Pollen grain numbers obtained from the middle of the coverslip were compared with those along the coverslip edge (x²) with no apparent statistically significant difference. The percent by pollen grain number was compared with percentage by pollen grain volume using a Chi-square test.