Seasonal pattern of viral load in colonies of Apis mellifera from Italian and French apiaries

Among the pathogens of the honey bee Apis mellifera, viruses are receiving increasing attention. Indeed, immunosuppressing agents, such as the reduction of pollen variety, the agrochemical pollution, and the parasitic mite Varroa destructor, which also plays a role in virus transmission, may turn latent viral infections into overt ones. We present here the results of a two-year survey carried out in several apiaries located in Piemonte (Italy) and Provence-Alpes-Côte d'Azur (France). The work was carried out in the frameshift of the Interreg-Alcotra project "Innov'api". Viral loads of the most prevalent viruses (DWV, ABPV complex, CBPV, BQCV, SBV) were estimated in adult populations by Real-time qPCR. Data were collected at the colony level, from a total of 150 hives, both in the production season (summer) and in the pre- and post-wintering periods. At the same dates, the gene expression of three markers of the physiological status of the colonies (vitellogenin, insulin-receptor 1, adipokinetic hormone receptor) was recorded. The same colonies were monitored for varroa infestation, by counting the phoretic mites in the hives all over the sampling period. Besides the plurality of environmental and climatic conditions, this survey embraced different management practices: stationary and nomadic beekeeping, as well as chemical or biotechnical control (coupled with organic treatments) of V. destructor. With this experimental plan, we could depict the sanitary status of the apiaries under different conditions. In particular, we gained insights into the seasonal variations of the titre of the main viruses infecting A. mellifera.