Interspecific Variation in Bumblebee Performance on Pollen Diet: New Insights for Mitigation Strategies

Abstract

Bumblebees (i.e. Bombus genus) are major pollinators of flowering wild plants and crops. Although many species are currently in decline, a number of them remain stable or are even expanding. One factor potentially driving changes in bumblebee distribution is the suitability of plant communities. Actually, bees probably have specific nutritional requirements that could shape their floral choices and constraint them in the current context of global change. However, most studies primarily focus on one bumblebee species at a time, making comparative studies scarce. Herein we performed comparative bioassays on three bumblebee species (i.e. Bombus hypnorum, B. pratorum and B. terrestris) fed on three different pollen diets with distinct nutritive content (Cistus, Erica and Salix pollen diets). Micro-colony performance was compared through different developmental and resource collection parameters for understanding the impact of change in pollen diet on different bumblebee species. The evidence suggests that B. terrestris is by far the most competitive species because of its performance compared to the other species, regardless of pollen diet. Our results also highlight a Bombus species effect as pollen diet impacts the micro-colonies in different ways according to the actual bumblebee species. Such interspecific variation in Bombus performance in response to a dietetic change underlines the importance of considering different bumblebee species in mitigation strategies. Such comparative studies are good advice for developing appropriate suites of plant species that can benefit threatened species while supporting stable or expanding ones.

Fig 1. Pollen collection per offspring for micro-colonies of the three bumblebee species (B. hypnorum, B. pratorum and B. terrestris) reared on the three pollen diets (Salix, Cistus and Erica).

Differences across species were significant (GLMMs: F_2,64.09 = 31.39, p < 0.001; see Results). Majuscule letters indicate interspecific significant differences and minuscule letters indicate intraspecific significant differences.

Fig 2. Pupal mean mass for micro-colonies of the three bumblebee species (B. hypnorum, B. pratorum and B. terrestris) reared on the three pollen diets.

Differences across diets were significant for B. hypnorum as it produced smaller pupae than B. terrestris (GLMMs: F_2,64.32 = 4.78, p = 0.012; see Results). Micro-colonies of B. terrestris produced higher mass pupae on Salix diet versus the Cistus diet (t = -3.10, df = 64, p = 0.003). Majuscule letters indicate interspecific significant differences and minuscule letters indicate intraspecific significant differences.

Fig 3. Percentage of pupae, larvae and eggs in the total number of offspring for the three species (B. hypnorum, B. pratorum and B. terrestris) reared on the three pollen diets (Salix, Cistus and Erica).

No differences were observed in the dynamics of growth (perMANOVA: F_2,71 = 1.00, p = 0.38).

Fig 4. Fat body content of the three bumblebee species (B. hypnorum, B. pratorum and B. terrestris) reared on the three pollen diets.

Differences across diets species were significant for B. terrestris (GLMMs: F_2,296 = 10.11, p < 0.001; see results). No impact from diet on the fat body content was detected (F_2,296 = 2.29, p = 0.103). Majuscule letters indicate interspecific significant differences and minuscule letters indicate intraspecific significant differences.