Drifting and other inter-colony pathways of pest and pathogen transmission in apiaries

The European honey bee (Apis mellifera, L., Hymenoptera: Apidae) faces multiple threats that cause colony mortality individually and synergistically. Of these, pests and pathogens play a significant role in honey bee health decline and are often reported as the main cause of colony death. Research efforts have focused on understanding the biology of these stressors, but information gaps remain concerning the mechanisms of transmission between colonies, or inter-colony transmission. Inter-colony transmission can occur through both vertical, parent-to-offspring, and horizontal, individual-to-individual, pathways. Behaviors like drifting and robbing, as well as general beekeeping activities, are the main modes of horizontal inter-colony transmission, but have not yet been comprehensively reviewed. Drifting specifically is identified as an important mechanism of spread within apiaries, but an incomplete understanding of this behavior impedes the development and improvement of management actions to better honey bee health. This review compiles the literature of inter-colony, horizontal transmission pathways and how they affect pest and pathogen spread, with a specific emphasis on the nuances of drifting, drifting methodology, and current knowledge gaps.