Abstract
Agricultural intensification has drastically altered foraging landscapes for bees, with large-scale crop monocultures associated with floral diversity loss. Research on bumblebees and honeybees has shown individuals feeding on pollen from a low richness of floral sources can experience negative impacts on health and longevity relative to higher pollen source richness of similar protein concentrations. Florally rich landscapes are thus generally assumed to better support social bees. Yet, little is known about whether the effects of reduced pollen source richness can be mitigated by feeding on pollen with higher crude protein concentration, and importantly how variation in diet affects whole colony growth, rearing decisions and sexual production. Studying queen-right bumblebee (Bombus terrestris) colonies, we monitored colony development under a polyfloral pollen diet or a monofloral pollen diet with 1.5-1.8 times higher crude protein concentration. Over 6 weeks, we found monofloral colonies performed better for all measures, with no apparent long-term effects on colony mass or worker production, and a higher number of pupae in monofloral colonies at the end of the experiment. Unexpectedly, polyfloral colonies showed higher mortality, and little evidence of any strategy to counteract the effects of reduced protein; with fewer and lower mass workers being reared, and males showing a similar trend. Our findings (i) provide well-needed daily growth dynamics of queenright colonies under varied diets, and (ii) support the view that pollen protein content in the foraging landscape rather than floral species richness per se is likely a key driver of colony health and success.
Highlights
Bees are essential insect pollinators of many wild flowers and crops, making reported declines an issue of global importance [1,2,3,4,5,6,7]
Two experimental replicates (ERs) were conducted: (i) for ER1, colonies arrived at the approximate size requested with monofloral assigned colonies (n = 6 colonies) having a mean (±s.e.m.) of 34.2 ± 3.9 pupae and 24.7 ± 2.0 workers, and polyfloral colonies (n = 6) having 26.8 ± 2.8 pupae and 22.8 ± 1.2 workers; (ii) for ER2, colonies arrived slightly larger than requested with monofloral colonies (n = 6) having 42.2 ± 5.7 pupae and 47.0 ± 4.8 workers, and polyfloral colonies (n = 6) having 37.2 ± 7.9 pupae and 50.5 ± 1.6 workers
Our experiment simulated a hypothetical scenario whereby bee colonies had access to a set quantity of pollen resource with half the colonies having access to pollen from multiple floral species and the remaining half restricted to pollen from a single species but with a 1.5–1.8 times higher relative crude protein concentration
Summary
Bees are essential insect pollinators of many wild flowers and crops, making reported declines an issue of global importance [1,2,3,4,5,6,7]. Large scale crop monocultures leading to fragmentation and loss of wild floral resources [12, 13] are thought to have degraded the “nutritional landscape” by lowering nectar and pollen availability to bees [14,15,16]. Having large swathes of single plant species/varieties can reduce the diversity of florally sourced nectar and pollen available [19] leading. Is focusing on floral diversity the most important criterion when informing such schemes? It may be important to understand how the amount and/or quality of nectar and pollen provisioned in these floral habitats contributes to bee reproductive success [22,23,24,25] Is focusing on floral diversity the most important criterion when informing such schemes? For instance, it may be important to understand how the amount and/or quality of nectar and pollen provisioned in these floral habitats contributes to bee reproductive success [22,23,24,25]
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