The Importance of Time and Place: Nutrient Composition and Utilization of Seasonal Pollens by European Honey Bees (Apis mellifera L.)

Abstract

Honey bee colonies have a yearly cycle that is supported nutritionally by the seasonal progression of flowering plants. In the spring, colonies grow by rearing brood, but in the fall, brood rearing declines in preparation for overwintering. Depending on where colonies are located, the yearly cycle can differ especially in overwintering activities. In temperate climates of Europe and North America, colonies reduce or end brood rearing in the fall while in warmer climates bees can rear brood and forage throughout the year. To test the hypothesis that nutrients available in seasonal pollens and honey bee responses to them can differ we analyzed pollen in the spring and fall collected by colonies in environments where brood rearing either stops in the fall (Iowa) or continues through the winter (Arizona). We fed both types of pollen to worker offspring of queens that emerged and open mated in each type of environment. We measured physiological responses to test if they differed depending on the location and season when the pollen was collected and the queen line of the workers that consumed it. Specifically, we measured pollen and protein consumption, gene expression levels (hex 70, hex 110, and vg) and hypopharyngeal gland (HPG) development. We found differences in macronutrient content and amino and fatty acids between spring and fall pollens from the same location and differences in nutrient content between locations during the same season. We also detected queen type and seasonal effects in HPG size and differences in gene expression between bees consuming spring vs. fall pollen with larger HPG and higher gene expression levels in those consuming spring pollen. The effects might have emerged from the seasonal differences in nutritional content of the pollens and genetic factors associated with the queen lines we used.

Keywords: Apis mellifera nutrition; fat body; hex 110; hex 70; hypopharyngeal glands; nutrients in pollen; vg expression.

Figure 1

Proportions of pollens from plant species collected in pollen traps attached to honey bee colonies in Arizona (Pima County) or Iowa (spring—Story County, Fall—Story and Polk Counties). Pollen was collected in the spring and fall. Identification of pollen types was conducted using ITS gene sequencing [28,30]. For each seasonal pollen mix, ‘N’ represents the number of quality ITS DNA sequences obtained from each library.

Figure 2

Essential (E), conditional (C), and nonessential (NE) amino acids in polyfloral mixtures of spring and fall pollens collected in Arizona (AZ) (Pima County, AZ, USA) and Iowa (Spring-Story County, Fall-Story and Polk Counties, IA, USA). Separate F-tests were conducted for each amino acid to compare mean concentrations among pollen types. Means for each amino acid followed by the same letter are not significantly different at p = 0.05. Histidine (HIS) and tryptophan (TRP) had no significant differences among the means.

Figure 3

Fatty acid concentrations in polyfloral mixtures of spring and fall pollens collected in Arizona (AZ) (Pima County, AZ, USA) and Iowa (Spring-Story County, Fall-Story and Polk Counties, IA, USA). Fatty acids found in lowest concentrations are shown in (A), medium concentrations in (B), and highest concentrations in (C). Separate F-tests were conducted for each fatty acid to compare mean concentrations among pollen types. Means for each fatty acid followed by the same letter are not significantly different at p = 0.05.

Figure 4

Average (±SE) amount of protein consumed (A) hemolymph protein concentration (B) and size of hypopharyngeal gland acini (C) in 7-day old honey bees that are offspring of queens from California (CAq) or Iowa (IAq) that were fed pollen collected in Arizona (AZp) or Iowa (IAp) in either the spring of the fall. Comparisons among queen/pollen type within the season were made using analysis of variance (protein consumption—spring: F_3,12 = 7.73, p = 0.004, fall: F_3,12 = 4.88, p = 0.019, hemolymph protein concentration—spring: F_3,12 = 8.24, p = 0.003, fall: F_3,12 = 2.39, p = 0.119, acini area—spring: F_3,12 = 3.93, p = 0.036, fall: F_3,12 = 1.40, p = 0.291). Means with the same letter within the same plot are not significantly different at the p = 0.05 level as determined by analysis of variance followed by Fisher pairwise comparisons.