Opposite seasonal and spatial dynamics of DWV-A and DWV-B suggest distinct transmission pathways in managed honey bees (Apis mellifera L.) colonies

Abstract

Deformed wing virus (DWV) is a major viral pathogen of Apis mellifera, existing mainly as two genotypes, DWV-A and DWV-B, which differ in transmission dynamics and virulence. This study presents a three-year national molecular surveillance (2021–2023) of Italian honey bee colonies to investigate the spatio-temporal distribution of both variants in relation to apiary density, geographical gradients, and land-use patterns. Quantitative PCR (qPCR) and Bayesian spatio-temporal models were applied to assess viral prevalence and environmental predictors. DWV-B was the dominant variant (overall 73.7%) and displayed a marked autumnal peak in November, followed by a winter decline. In contrast, DWV-A showed a complementary trend, peaking in summer and decreasing with apiary density, suggesting an environmentally mediated transmission pathway. Spatial analysis revealed higher DWV-B prevalence in southern and insular regions, whereas DWV-A predominated in central and northeastern regions. Land-use effects further indicated that DWV-B is linked to anthropogenic landscapes with intensive beekeeping, while DWV-A is associated with more heterogeneous environments. These findings highlight distinct ecological dependencies between DWV variants: DWV-B is probably more Varroa-associated, colony-driven virus favoured by warm, stable climates, while DWV-A reflects diffuse environmental persistence. Integrating climatic and management factors is essential to predict DWV epidemiological shifts under global change.