Determining the minimum number of pollen grains needed for accurate honey bee (Apis mellifera) colony pollen pellet analysis

Honey bee (Apis mellifera) colony maintenance depends on foraging workers to obtain food resources from flowering plants year round. Because diverse floral diets have a positive impact on honey bee health, identifying the plants preferred by foragers provides valuable information to manage bee-friendly habitats. Recent studies have utilized palynology to better understand honey bee nectar foraging preferences. Futhermore, the International Honey Commission has established standards for analyzing honey samples. However, standards for studying the plant taxonomic composition of honey bee pollen pellets have not been established. The goal of this project was to determine the minimum number of pollen grains that need to be counted to obtain an accurate floral taxonomic representation in a pollen pellet sample. To do this, pollen samples were collected from pollen traps placed outside honey bee hives, and a pollen subsample foraged by each colony was acetolyzed and identified to the lowest taxonomic level possible. Cohorts of 100 pollen grains obtained from homogenized pollen samples from three different colonies were counted successively 5 times for a total count of 500 pollen grains per colony. This was repeated for each of the replicates from the three separate colonies. We found no statistically significant differences in the number or proportion of floral taxa found between the 200 and 500 pollen grain counts in two out of the three colonies sampled. Species diversity index analysis suggested that the higher number of floral taxa found in some 500-grain counts were attributed to a relatively low presence of minor pollen types. Thus, a 200 pollen grain count seems sufficient to assess the predominant, secondary and important minor plant taxa present in a pollen sample, while a 500-grain count may be needed to elucidate a more specific taxonomic assessment of additional minor taxa floral types to determine a sample's geographic origin.