For honey bees (Apis mellifera), colony maintenance and growth are highly dependent on worker foragers obtaining sufficient resources from flowering plants year round. Despite the importance of floral diversity for proper bee nutrition, urban development has drastically altered resource availability and diversity for these important pollinators. Therefore, understanding the floral resources foraged by bees in urbanized areas is key to identifying and promoting plants that enhance colony health in those environments. In this study, we identified the pollen foraged by bees in four developed areas of the U.S., and explored whether there were spatial or temporal differences in the types of floral sources of pollen used by honey bees in these landscapes. To do this, pollen was collected every month for up to one year from colonies located in developed (urban and suburban) sites in California, Texas, Florida, and Michigan, except during months of pollen dearth or winter. Homogenized pollen samples were acetolyzed and identified microscopically to the lowest taxonomic level possible. Once identified, each pollen type was classified into a frequency category based on its overall relative abundance. Species richness and diversity indices were also calculated and compared across states and seasons. We identified up to 64 pollen types belonging to 39 plant families in one season (California). Species richness was highest in CA and lowest in TX, and was highest during spring in every state. In particular, “predominant” and “secondary” pollen types belonged to the families Arecaceae, Sapindaceae, Anacardiaceae, Apiaceae, Asteraceae, Brassicaceae, Fabaceae, Fagaceae, Lythraceae, Myrtaceae, Rhamnaceae, Rosaceae, Rutaceae, Saliaceae, and Ulmaceae. This study will help broaden our understanding of honey bee foraging ecology and nutrition in urban environments, and will help promote the use of plants that serve the dual purpose of providing aesthetic value and nutritious forage for honey bee colonies placed in developed landscapes.

Poor nutrition and landscape changes are regularly cited as key factors causing the decline of wild and managed bee populations. However, what constitutes ‘poor nutrition’ for bees currently is inadequately defined. Bees collect and eat pollen: it is their only solid food source and it provides a broad suite of required macro- and micronutrients. Bees are also generalist foragers and thus the different pollen types they collect and eat can be highly nutritionally variable. Therefore, characterizing the multidimensional nutrient content of different pollen types is needed to fully understand pollen as a nutritional resource. Unfortunately, the use of different analytical approaches to assess pollen nutrient content has complicated between-studies comparisons and blurred our understanding of pollen nutrient content. In the current study, we start by reviewing the common methods used to estimate protein and lipids found in pollen. Next, using monofloral Brassica and Rosa pollen, we experimentally reveal biases in results using these methods. Finally, we use our collective data to propose a unifying approach for analysing pollen nutrient content. This will help researchers better study and understand the nutritional ecology—including foraging behaviour, nutrient regulation and health—of bees and other pollen feeders. This article is part of the theme issue ‘Natural processes influencing pollinator health: from chemistry to landscapes’.

The importance of dietary salt may explain why bees are often observed collecting brackish water, a habit that may expose them to harmful xenobiotics. However, the individual salt preferences of water-collecting bees were not known. We measured the proboscis extension reflex (PER) response of Apis mellifera water foragers to 0-10% w/w solutions of Na, Mg, and K, which provide essential nutrients,. We also tested phosphate, which can deter foraging. Bees exhibited strong preferences: the most PER responses for 1.5-3% Na and 1.5% Mg. However, K and phosphate were largely aversive and elicited PER responses only for the lowest concentrations, suggesting a way to deter bees from visiting contaminated water. We then analyzed the salt content of water sources that bees collected in urban and semi-urban environments. Bees collected water with a wide range of salt concentrations, but most collected water sources had relatively low salt concentrations, with the exception of seawater and swimming pools, which had >0.6% Na. The high levels of PER responsiveness elicited by 1.5-3% Na may explain why bees are willing to collect such salty water. Interestingly, bees exhibited significant high individual variation in salt preferences: individual identity accounted for 32% of PER responses. Salt specialization may therefore occur in water foragers.

Pollinator nutrition is a highly complex subject that we are just starting to unravel, from the multidimensional nature of bee forage (pollen and nectar) to how the abiotic environment can affect the resources available to bees. Doing so is of utmost importance, as improving pollinator resource availability and nutrition is one of the proposed mechanisms to improve populations and pollinator health. However, landscape change has changed the resources naturally available for pollinators. Farmland and cropping systems create a unique nutritional landscape for pollinators, with agroecosystems typically containing few crops dominating a landscape along with natural corridors containing noncrop plants. The types of crops planted and the surrounding landscape will ultimately affect the nutritional landscape bees have access to. Even the management practices in agriculture and how pests are controlled will, directly and indirectly, affect bee health and nutrition. Hence, a better understanding of bee nutrition in agricultural ecosystems is warranted. This review synthesizes research on bee nutritional ecology and the agricultural landscapes to advance our understanding of bee health in agriculture.

Pollen is the primary source of dietary protein for honey bees. It also includes complex polysaccharides in its outer coat, which are largely indigestible by bees but can be metabolized by bacterial species within the gut microbiota. During periods of reduced availability of floral pollen, supplemental protein sources are frequently provided to managed honey bee colonies. The crude proteins in these supplemental feeds are typically byproducts from food manufacturing processes and are rarely derived from pollen. Our experiments on the impact of different diets showed that a simplified pollen-free diet formulated to resemble the macronutrient profile of a monofloral pollen source resulted in larger microbial communities with reduced diversity, reduced evenness, and reduced levels of potentially beneficial hive-associated bacteria. Furthermore, the pollen-free diet sharply reduced the expression of genes central to honey bee development. In subsequent experiments, we showed that these shifts in gene expression may be linked to colonization by the gut microbiome. Lastly, we demonstrated that for bees inoculated with a defined gut microbiota, those raised on an artificial diet were less able to suppress infection from a bacterial pathogen than those that were fed natural pollen. Our findings demonstrate that a pollen-free diet significantly impacts the gut microbiota and gene expression of honey bees, indicating the importance of natural pollen as a primary protein source.

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.