Due to their pollinating activity, honey bees (Apis mellifera L.) are of prime importance for the maintenance of agricultural and natural ecosystems. In the last decade, extensive colony losses have been reported all over the northern hemisphere, which represent a major concern for the beekeeping industry and the whole agriculture. It is nowadays accepted that colony losses have a multifactorial origin, with parasites and pathogens playing an active role reinforced by the effects of pollution, pesticides and depletion of natural ecosystems. Among the above listed stress factors, the mite Varroa destructor Anderson and Trueman, together with the strictly associated deformed wings virus (DWV), play a major role. The aim of this work is to investigate how two stressors, the Varroa mite and the DWV, affect honey bees' health under different both conditions and levels. A first study of the transcriptome of bees under different mite infestation levels, environments and scenario, allows the individuation of a group of differently expressed genes affected by the parasitization, which can potentially be strong biomarkers of imminent collapse. In particular, the whole genome analysis, combined with previous observations, highlighted the possible influence of the mite on honey bee's behaviour, deeper investigated by means of a set of experiments aiming at understand how the mite alters the in-hive behaviour of nurse bees and what are the possible underlying mechanisms behind the observed effects. Honey bees can rely on resources from the environment to contrast the detrimental effect of parasites and pathogens., e.g. a complete and high quality nutrition. Furthermore, particularly interesting is the prophylactic use of pollen by Varroa mite infested bees, described in the last part of this work, which revealed that pollen represents an essential component of honey bees' nutrition and whose properties go well beyond the supply of essential amino acids or metabolic energy.
