A multifactorial approach to honeybees health: from multiple causes to consequences and possible actions

In the last decades, large-scale losses of honey bee (Apis mellifera L.) colonies have been recorded all over the world. After years of intense investigation, no specific causal agent for the widespread colony losses has been found but rather a multifactorial origin has been proposed for this syndrome. Biotic and abiotic factors contribute to this phenomenon, but several studies indicate that the ectoparasitic mite Varroa destructor and the Deformed Wing Virus (DWV) play an important role. Recent research has shown that DWV infections at low viral levels are asymptomatic because the honey bee immune competence is able to contain virus replication. As soon as any stress factor interferes with this equilibrium, competing for metabolic resources or negatively acting on immunity, uncontrolled viral replication is promoted, resulting in the transition from a benign covert infection to a devastating overt disease. Xenobiotics, abiotic stressors, malnutrition and other factors can further contribute to complicate the situation. The aim of this thesis was to investigate, at the individual level, how different stress factors and nutrition interact to influence the survival of honey bees. To this aim, we subdivided the study in two phases; in the first one, we assessed how several stress factors as well as pollen influence honey bee health. In a second phase of this study, we investigated how some of these stressors act in combination with the others and in combination with nutrition. We selected seven factors that are possibly implicated in the multifactorial syndrome related to colony losses: pollen, as a natural supply of amino acids and lipids for the honey bee; hydroxymethylfurfural (HMF), a toxic compound contained in additional sugars syrups; acidity, which normally characterizes the sugar syrups mentioned above; nicotine, which is a toxic alkaloid that bees can encounter in the environment and with a mode of action similar to that of neonicotinoid insecticides; a temperature 2-3 degrees below that normally found within the hive; V. destructor, the most dangerous ectoparasite of honey bees and Deformed wing virus (DWV), a key pathogen of honey bees. We confirmed the beneficial effect of pollen nutrition on honey bees as negatively affected by parasites and pathogens. To gain insight into the mechanisms underlying these effects, we interfered with the energetic pathway of mite infested bees using rapamycin, a chemical inhibitor of mTOR, a protein complex that regulates cell growth and other key cellular processes. We observed that rapamycin, just like pollen, decreases DWV load in a manner that could be related to the stimulation of autophagy. More in general, we noted the important role played by nutrition in the interactions between honey bee and other stressors. We also documented for the first time a kind of physiological anorexia triggered by V. destructor infestation, which leads to a reduced energy availability that influences the capacity of honey bees to cope with other stressors. Moreover, we shed light on the possible detrimental side effects of supplementary nutrition administered to bees by beekeepers with homemade sugar syrups. In fact, acidity and HMF that are normally present in such syrups are toxic for honey bees. Lastly, with this work we showed how unpredictable it can be the relationship between stress factors. Since an analytic study of all the factors that can affect honey bee health is unimaginable, the attention should be focused on the metabolic process accounting for the observed interactions in order to develop one or several models that could help to predict the outcome of such interactions.