Monitoring bee health in European agro-ecosystems using wing morphology and fat bodies

Changes in Vitellogenin, Abdominal Lipid Content, and Hypopharyngeal Gland Development in Honey Bees Fed Diets with Different Protein Sources

Honey bees play an important role in the pollination of flowering plants. When honey bee colonies are deficient in pollen, one of their main nutrients, protein supplements are required. In this study, the effects of diets with six different protein sources on the physiological characteristics of worker bees (vitellogenin (Vg), abdominal lipid content (ALC), hypopharyngeal gland (HPG)) and consumption were investigated. The protein sources of the diets (diet I, …, diet VI) included pollen, spirulina dust (Arthrospira platensis Gomont), fresh egg yolk, lyophilized lactose-free skimmed milk powder, active fresh yeast, and ApiProtein. It was identified that consumption by worker bees was highest in the diet group supplemented with spirulina (diet II). Although there was no statistical difference regarding the Vg content in the hemolymph, numerically, the highest content was found in diet group IV (lyophilized lactose-free skimmed milk powder) (4.73 ± 0.03 ng/mL). ALC and HPG were highest in the group fed diet II. These results suggest that offering honey bees diets with certain protein sources can support their physiological traits.

Pollen as Bee Medicine: Is Prevention Better than Cure?

To face environmental stressors such as infection, animals may display behavioural plasticity to improve their physiological status through ingestion of specific food. In bees, the significance of medicating pollen may be limited by their ability to exploit it. Until now, studies have focused on the medicinal effects of pollen and nectar after forced-feeding experiments, overlooking spontaneous intake. Here, we explored the medicinal effects of different pollen on Bombus terrestris workers infected by the gut parasite Crithidia bombi. First, we used a forced-feeding experimental design allowing for the distinction between prophylactic and therapeutic effects of pollen, considering host tolerance and resistance. Then, we assessed whether bumble bees favoured medicating resources when infected to demonstrate potential self-medicative behaviour. We found that infected bumble bees had a lower fitness but higher resistance when forced to consume sunflower or heather pollen, and that infection dynamics was more gradual in therapeutic treatments. When given the choice between resources, infected workers did not target medicating pollen, nor did they consume more medicating pollen than uninfected ones. These results emphasize that the access to medicating resources could impede parasite dynamics, but that the cost-benefit trade-off could be detrimental when fitness is highly reduced.

Using physiology to better support wild bee conservation

There is accumulating evidence that wild bees are experiencing a decline in terms of species diversity, abundance or distribution, which leads to major concerns about the sustainability of both pollination services and intrinsic biodiversity. There is therefore an urgent need to better understand the drivers of their decline, as well as design conservation strategies. In this context, the current approach consists of linking observed occurrence and distribution data of species to environmental features. While useful, a highly complementary approach would be the use of new biological metrics that can link individual bee responses to environmental alteration with population-level responses, which could communicate the actual bee sensitivity to environmental changes and act as early warning signals of bee population decline or sustainability. We discuss here through several examples how the measurement of bee physiological traits or performance can play this role not only in better assessing the impact of anthropogenic pressures on bees, but also in guiding conservation practices with the help of the documentation of species' physiological needs. Last but not least, because physiological changes generally occur well in advance of demographic changes, we argue that physiological traits can help in predicting and anticipating future population trends, which would represent a more proactive approach to conservation. In conclusion, we believe that future efforts to combine physiological, ecological and population-level knowledge will provide meaningful contributions to wild bee conservation-based research.

Bumble Bee Breeding on Artificial Pollen Substitutes

Bumble bees are important pollinators for many temperate crops. Because of the growing demand for food from entomophilous crops, bumble bee colonies are commercially reared and placed in fields or greenhouses to guarantee sufficient pollination services. Besides, commercial colonies are increasingly used in laboratories for various bioassays under controlled conditions. For both usages, bumble bee colonies are commonly provided with sugar solution and honey bee-collected pollen pellets. However, the latter display several disadvantages since they may contain pollutants, pathogens, or toxic phytochemicals. Consequently, companies have developed pollen-free artificial diets to sustain colonies. Such diets are designed to boost worker health in the field, in complement of floral pollen collected by workers outside the colonies, but their suitability in 'closed' systems without access to floral pollen, such as in laboratory bioassays, is arguable. Here, we used microcolonies of the commercially important bumble bee Bombus terrestris L. (Hymenoptera: Apidae) to assess the suitability of five artificial pollen substitutes and three mixed diets. We also assessed the evaporation rate of the different diets as it could impact their suitability. At the end of the bioassays, microcolonies fed the artificial diets showed a reduced offspring development when compared to microcolonies fed natural pollen, which was partly offset by mixing these diets with natural pollen. By contrast, the artificial diets did not have deleterious effects on worker's health. We discuss the potential nutritional and physical causes of artificial diets unsuitability for offspring development and encourage further research to accordingly establish appropriate pollen-free diets for bumble bee breeding.

Poison or Potion: Effects of Sunflower Phenolamides on Bumble Bees and Their Gut Parasite

Simple Summary

Bee declines have been reported worldwide, partly due to parasite spread induced by human activities. However, bees may forage on specific floral resources to face parasite infection. Such natural resources are comparable to ‘natural pharmacies’ and may be favoured in bee conservation strategies. Consumption of sunflower pollen, despite being detrimental for larval development, has been recently shown to reduce the load of a widespread bumble bee gut parasite in the common eastern bumble bee. Although the underlying mechanisms remain unknown, it has been suggested that sunflower phenolamides—a family of molecules found in most flowering plants—may be responsible for such a reduction in parasite load. Here, we tested the impacts of sunflower phenolamides on healthy and infected buff-tailed bumble bees. Expectedly, sunflower pollen had harmful consequences on bumble bee development but surprisingly, it did not alter parasite load. By contrast, sunflower phenolamides had milder effects on bumble bee development but unexpectedly increased parasite load. Phenolamide effects may stem from the physiological stress they induced or the gut microbial community alteration they may have triggered. Since biological models and experimental framework differ greatly in related studies tackling plant–bee–parasite interplays, we challenged the definition of medicinal effects and questioned the way to assess them in controlled conditions.

Abstract

Specific floral resources may help bees to face environmental challenges such as parasite infection, as recently shown for sunflower pollen. Whereas this pollen diet is known to be unsuitable for the larval development of bumble bees, it has been shown to reduce the load of a trypanosomatid parasite (Crithidia bombi) in the bumble bee gut. Recent studies suggested it could be due to phenolamides, a group of compounds commonly found in flowering plants. We, therefore, decided to assess separately the impacts of sunflower pollen and its phenolamides on a bumble bee and its gut parasite. We fed Crithidia-infected and -uninfected microcolonies of Bombus terrestris either with a diet of willow pollen (control), a diet of sunflower pollen (natural diet) or a diet of willow pollen supplemented with sunflower phenolamides (supplemented diet). We measured several parameters at both microcolony (i.e., food collection, parasite load, brood development and stress responses) and individual (i.e., fat body content and phenotypic variation) levels. As expected, the natural diet had detrimental effects on bumble bees but surprisingly, we did not observe any reduction in parasite load, probably because of bee species-specific outcomes. The supplemented diet also induced detrimental effects but by contrast to our a priori hypothesis, it led to an increase in parasite load in infected microcolonies. We hypothesised that it could be due to physiological distress or gut microbiota alteration induced by phenolamide bioactivities. We further challenged the definition of medicinal effects and questioned the way to assess them in controlled conditions, underlining the necessity to clearly define the experimental framework in this research field.