Honey bee (Apis mellifera) nurses do not consume pollens based on their nutritional quality

Abstract

Honey bee workers (Apis mellifera) consume a variety of pollens to meet the majority of their requirements for protein and lipids. Recent work indicates that honey bees prefer diets that reflect the proper ratio of nutrients necessary for optimal survival and homeostasis. This idea relies on the precept that honey bees evaluate the nutritional composition of the foods provided to them. While this has been shown in bumble bees, the data for honey bees are mixed. Further, there is controversy as to whether foragers can evaluate the nutritional value of pollens, especially if they do not consume it. Here, we focused on nurse workers, who eat most of the pollen coming into the hive. We tested the hypothesis that nurses prefer diets with higher nutritional value. We first determined the nutritional profile, number of plant taxa (richness), and degree of hypopharyngeal gland (HG) growth conferred by three honey bee collected pollens. We then presented nurses with these same three pollens in paired choice assays and measured consumption. To further test whether nutrition influenced preference, we also presented bees with natural pollens supplemented with protein or lipids and liquid diets with protein and lipid ratios equal to the natural pollens. Different pollens conferred different degrees of HG growth, but despite these differences, nurse bees did not always prefer the most nutritious pollens. Adding protein and/or lipids to less desirable pollens minimally increased pollen attractiveness, and nurses did not exhibit a strong preference for any of the three liquid diets. We conclude that different pollens provide different nutritional benefits, but that nurses either cannot or do not assess pollen nutritional value. This implies that the nurses may not be able to communicate information about pollen quality to the foragers, who regulate the pollens coming into the hive.

Fig 1. Venn diagram representing the types of plant taxa identified in honey bee-collected pollens.

The number of plant taxa from pollens from the Southeastern United States (“SE”) and the desert are shown and were catalogued using ITS sequences and light microscopy.

Fig 2

Hypopharyngeal gland (HG) sizes differed according to diet when consumption was not (A) or was (B) accounted for. Gland size is represented by the least square (LS) mean estimate of (A) the square root transformed average acinus size (mm²) or (B) the average acinus size (mm²) relative to pollen type (see methods). Bees were fed for 8d on each of the three diets. “SE” indicates pollen from the Southeastern United States. Bars with different letters are significantly different (p<0.05). Error bars represent S.E. around the LS mean values.

Fig 3. Pollen (g) consumed by bees in each cage in choice assays.

Consumption is shown for each cage for pairwise comparisons between (A) almond and desert, (B) desert and SE, or (C) almond and SE pollens. Average (AV) consumption over all cages is also shown. For each panel, both types of pollen were provided ad libitum and the total amount of pollen consumed over 7d was recorded. For panels A and B, there were significant differences in consumption (p<0.001). In panel C, consumption did not differ. Experiments in panel C were conducted in two different trials at two different times.

Fig 4. Average pollen (g) consumed by bees in each in supplemented pollen choice assays.

Both types of pollen were provided ad libitum and the total amount of pollen consumed over 7 days was recorded. The error bars represent the S.E. around the mean consumption. Significant differences (p<0.0001) were observed in all comparisons. “SE” indicates pollen from the Southeastern United States.