Parasites alter the physiology and behaviour of their hosts. In domestic honey bees, the microsporidia Nosema ceranae induces energetic stress that impairs the behaviour of foragers, potentially leading to colony collapse. Whether this parasite similarly affects wild pollinators is little understood because of the low success rates of experimental infection protocols. Here, we present a new approach for infecting bumblebees (Bombus terrestris) with controlled amounts of N. ceranae by briefly exposing individual bumblebees to parasite spores before feeding them with artificial diets. We validated our protocol by testing the effect of two spore dosages and two diets varying in their protein to carbohydrate ratio on the prevalence of the parasite (proportion of PCR-positive bumblebees), the intensity of parasites (spore count in the gut and the faeces), and the survival of bumblebees. Overall, insects fed a low-protein, high-carbohydrate diet showed the highest parasite prevalence (up to 70%) but lived the longest, suggesting that immunity and survival are maximised at different protein to carbohydrate ratios. Spore dosage did not affect parasite infection rate and host survival. The identification of experimental conditions for successfully infecting bumblebees with N. ceranae in the lab will facilitate future investigations of the sub-lethal effects of this parasite on the behaviour and cognition of wild pollinators.
15Parasites alter the physiology and behaviour of their hosts. In domestic honey bees, 16 the microsporidia Nosema ceranae induces an energetic stress and impairs the 17 behaviour of foragers, potentially leading to colony collapse. Whether this emerging 18 parasite similarly affects wild pollinators is little understood because of the low 19 success rates of experimental infection protocols. Here we established a new 20 apporach for infecting bumblebees (Bombus terrestris) with controlled amounts of N. 21 ceranae, by briefly exposing individual bumblebees to a sucrose solution containing 22 parasite spores, before feeding them with artificial diets. We validated our protocol by 23 testing the effect of two spore dosages and two diets varying in their protein to 24 carbohydrate ratio, on the prevalence of the parasite (proportion of infected bees), 25 the intensity of infection (spore count in the gut), and the survival of bumblebees. 26 Insects fed a low-protein high-carbohydrate diet showed highest parasite prevalence 27 (up to 70%) but lived longest, suggesting that immunity and survival of bumblebees 28 are maximised at different protein to carbohydrate ratios. Spore dosage had no effect 29 on parasite infection rate and host survival. The identification of experimental 30 conditions for successfully infecting bumblebees with N. ceranae in the lab will 31 facilitate investigations of the sub-lethal effects of this parasite on the behaviour and 32 cognition of wild pollinators. 33 34
Over the past few decades, honey bees have been facing an increasing number of stressors. Beyond individual stress factors, the synergies between them have been identified as a key factor in the observed increase in colony mortality. However, these interactions are numerous and complex and call for further research. Here, in line with our need for a systemic understanding of the threats that they pose to bee health, we review the interactions between honey bee viruses. As viruses are obligate parasites, the interactions between them not only depend on the viruses themselves but also on the immune responses of honey bees. Thus, we first summarise our current knowledge of the antiviral immunity of honey bees. We then review the interactions between specific pathogenic viruses and their interactions with their host. Finally, we draw hypotheses from the current literature and suggest directions for future research.
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