*Protocol and Literature Search*

This systematic review was carried out according to the Preferred Reporting Items for Systematic Review and Meta-Analysis (PRISMA) protocols [209]. The research question to be reviewed was: "Which honey bee pathogens may generate spillover to managed and wild Hymenoptera species and, more in general, to the arthropofauna?"

The search intentionally excluded arthropods living in close contact with the honey bees that, like *V. destructor*, are obligate parasites.

The article search was carried out in PubMed, Web of Science, Science Direct, Google Scholar, and Scopus scientific databases for studies aimed to assess the detection of spillover cases of honey bee pathogens. Filters were used to select articles published from January 1960 to April 2021. The last search date was 31 May 2021.

The following search strategy was designed and utilized: "Honey Bee Pathogens" OR "Bumblebee Pathogens" OR "Spillover" OR "Spill-over " OR "Inter species transmission" OR "Inter taxa transmission" OR "Host species transmission" OR "*Apis mellifera*" OR "Honey Bee Diseases" OR "Honey Bee Virus" OR "Honey Bee Bacteria" OR "Honey Bee Microsporidia" OR "Honey Bee Protozoa" OR "Managed Bees" OR "Wild Bees" OR "Commercial Bees" OR "Artificial Infection" OR "Replicative Virus" OR "Bumblebees" OR "Colony Collapse Disorder" OR "Deformed Wing Virus" OR "Acute Bee Paralysis Virus" OR "Israeli Acute Paralysis Virus" OR "Black Queen Cell Virus" OR "Sacbrood Virus" OR "*Apis mellifera* Filamentous Virus" OR "Kashmir Bee Virus" OR "Slow Bee Paralysis Virus" OR "Lake Sinai Virus" OR "*Varroa destructor*" OR "Macula-like Virus" OR "*Nosema apis*" OR "*Nosema ceranae*" OR "*Nosema bombi*" OR "*Spiroplasma*" OR "*Ascosphaera*" OR "*Apicystis*" OR "Arthopods" OR "Entomofauna" OR "Hive Hosts" OR "Hive" OR "Free-Ranging Insect" OR "Bee Interaction" OR "*Varroa destructor*". The logical operator "OR" was used to combine the descriptors.

Studies carried out both in field and laboratory conditions were selected. Besides, studies that did not assess whether the presence of the honey bee pathogens could be related to external contamination were not included. The detected active replication of honey bee viruses was also reported in the Supplementary Materials with an asterisk.

Duplicate studies were excluded. The search and screening for titles, abstracts and results were carried out independently by the authors, including all articles, letters, notes, scientific notes and communications aimed to assess a spillover case of honey bee pathogens and excluding reviews, books, book chapters and theses.

The potentially eligible research articles were read and reviewed independently by the authors and the data were compared to ensure integrity and reliability.

For each article included in this review, relevant information related to the authors, publication year, host species, host conditions, host stage, pathogens and prevalence were extracted. The data from the eligible studies are expressed in the Supplementary Materials

and Figures. The authors provided a narrative synthesis of the results for each pathogen capable to generate a spillover case, according to the main characteristics and results related to the topic addressed.

#### **5. Conclusions**

This review shows that, in recent years, the frequency of recorded spillover cases of honey bee pathogens to other arthropods, including wild bees, has dramatically increased. Certainly, human movements and globalization have fostered the inflow of novel pathogenic microorganisms, often with detrimental consequences. However, it should also be considered that the analytical methods currently available give impulse to the research on bee pathology, increasing the chance to identify interspecific transmission events.

The host plasticity shown by some honey bee pathogens raises ecological concern for the potential negative consequences on the pollinating entomofauna and ecosystems in general. Despite the fact that research on these pathogens has significantly improved, we have limited knowledge of their potential impact on other bees, insects, and arthropods in general and the cascade of environmental effects. Laboratory studies are not sufficient to cover this gap, for the intricate interaction of the involved biotic and abiotic factors. For the same reasons, the exploitation of these pathogens in the control of arthropods considered as pests (e.g., *A. tumida*, *G. melonella*, *V. velutina*, *L. humile*) should be considered with extreme carefulness.

The tight interaction between honey bees and the other environmental components suggests a holistic approach to the study of bee diseases, including their control. Indeed, pathogens may survive in alternate hosts, generating spillback events and possibly jeopardizing the efficacy of the treatments. This emphasizes the beekeeper's responsibility to maintain healthy colonies to benefit both their production and the environment.

Spillover of honey bee pathogens may have undetected yet important repercussions on the health and functioning of an ecosystem. Health management of honey bee colonies is of high importance in this context. Honey bees and the beekeeping industry should, therefore, undertake an essential role in the One Health concept. This requires the adoption of dedicated research actions.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/ 10.3390/pathogens10081044/s1, Table S1: Bee pathogen spillover and prevalence identified in hymenopteran hosts, of which are reported condition, stage, geographical area and year, Table S2: Bee pathogen spillover and prevalence identified in arthropod hosts, of which are reported condition, stage, geographical are and year [210–232].

**Author Contributions:** Conceptualization, A.N., L.B. and G.C.; methodology, G.C.; writing—original draft preparation, G.C.; writing—review and editing, A.N., L.B. and G.C.; project administration, L.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the project BeeNet (Italian National Fund under FEASR 2014- 2020 from MIPAAF "Ministero delle politiche agricole alimentari e forestali" to CREA "Consiglio per la Ricerca in agricoltura e l'analisi dell'economia agraria").

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** MDPI Research Data Policies.

**Acknowledgments:** The Authors acknowledge Laura Zavatta (CREA-AA) for her kind support in generating some of the charts published in this paper.

**Conflicts of Interest:** The authors declare no conflict of interest.
