The impacts of almond pollination on honeybee viral dynamics

Honeybees provide critical pollination services, including the remarkable annual mass human-mediated intercontinental ‘migration’ where over half of all commercial honeybees in the United States are shipped to California’s Central Valley to pollinate most of the world’s almond supply. Clearly, this ‘mass mixing event’ with hives from across the country has the risk of spreading highly virulent pathogens, including viruses. In addition to their economic benefits, almonds also provide the first major food source for the honeybees emerging from winter. It is essential to weigh the advantages of the almond bloom against the risks of disease in honeybees, which may also impact native pollinators. We conducted an observational longitudinal RNAseq study of viral diversity across multiple honeybee colonies from a California-based beekeeping operation as they participated in almond pollination, compared to control hives that did not. Viral diversity in honeybee colonies increased transiently during almond pollination but returned to pre-bloom levels afterward, indicating a short-lived bloom-associated effect. Directly after the bloom, viral communities became more homogeneous across colonies, consistent with enhanced viral exchange through shared floral resources and inter-colony interactions. Viruses exhibited seasonal patterns in prevalence and abundance, yet no single species dominated the community. Instead, viral composition varied with site location and time, with hives in closer proximity displaying more similar viral dynamics, suggesting local mixing within orchards through drift. These findings indicate that the resource superabundance during almond bloom increases viral diversity and homogenization, but with a short-term effect as diversity and community structure return to pre-bloom conditions once hives leave the orchards.