Continuing with a series of blogs focused on varroa mite biology, and the host-parasite interactions of mites and honey bees, this week’s blog will focus on virus transmission. Virus transmission is one of the ways that mites weaken colonies, and impact colony loss. Varroa mites are responsible for vectoring a multitude of viruses to honey bees, and this week’s blog will explore the biology of how mites transmit viruses to their host.

The Host-Parasite Interactions of Varroa Mites and Honey Bees: Virus Transmission

Varroa mites function as a viral vector and are responsible for the transmission of various honey bee viruses such as deformed wing virus, Israeli acute paralysis virus, and acute bee paralysis virus¹. The reproductive cycle of Varroa mites facilitates vertical and horizontal transmission of these viruses, weakening developing bees and impacting the overall health of the colony. Viruses can be transmitted horizontally in a colony through ingestion of food or fecal matter, topical contact such as grooming behavior of bees, during mating between a queen and drones, and indirectly through Varroa mites feeding on more than one bee. Viruses can also be transmitted vertically from the queen to her offspring.

Varroa mites have been demonstrated to induce both persistent circulative transmission of viruses and persistent non-circulative transmission of viruses. Persistent circulative transmission occurs when a pathogen is acquired, circulates internally within the mite such as through the gut and salivary glands but does not replicate in the vector². Varroa induces persistent circulative transmission of deformed wing virus B, infecting both the salivary glands and gut of the mites, allowing direct injection of the virus into honey bees during feeding². Alternatively, persistent non-circulative transmission involves viruses that are carried on the surface of the mite’s body, generally without internalization of the virus³. Viruses such as deformed wing virus A, acute bee paralysis virus, and Israeli acute paralysis virus, accumulate on the mite’s cuticle, but also can reside within the gut and salivary glands of Varroa mites³. Although these viruses do not replicate within the mite they can be introduced into the host through feeding wounds the mite creates, which leads to immune suppression and viral replication within the host^3,4.

In addition to being a virus vector, Varroa mites actively influence viral diseases in honey bees⁴. Mite parasitism reduces the immune response on honey bees which helps facilitate virus replication⁵. During feeding, the saliva of Varroa mites reduces the expression of host antimicrobial peptide genes and enzymes which are linked to honey bee immune responses, allowing viruses to replicate more aggressively.  This has been shown to be the case with deformed wing virus⁵.

Honey bee with deformed wing virus and varroa mites on back (ATTTA©2023).

In addition to suppressing the immune system of honey bees from factors in their saliva, Varroa mites can also facilitate virus replication through physical mechanisms. As mites feed on honey bees they remove antiviral molecules such as peptides and immune related proteins. Research demonstrates that increased hemolymph loss correlates with increased viral load of deformed wing virus, suggesting that physical extraction of immune factors impacts viral defences⁶. Both salivary components and hemolymph removal contribute to increased viral replication and the development of viral diseases such as deformed wing virus⁴.

To learn more about the impacts of various honey bee viruses read ATTTA’s past blogs: “Deformed Wing Virus: A Persistent Pathogen of Honey Bees” (August 5, 2021); “Sacbrood Virus: The First Honey Bee Virus Discovered” (August 12, 2021); “Bee Paralysis: The Common Ground of 3 Honey Bee Viruses” (August 19, 2021); “Black Queen Cell Virus: One of the most prevalent viral pathogens of honey bees” (August 26, 2021); and, “Kashmir Bee Virus: Last, but Not Least Virulent” (September 2, 2025).

Overall, Varroa mites serve as a vector for several viruses and actively influence viral replication and related diseases in honey bees, which weakens developing bees and impacts the health of the colony. Next week’s blog will discuss the cascade impacts Varroa mites have on the overall health of a honey bee colony, and the management steps beekeepers can utilize to help keep their colonies strong and healthy.

References

1. Doublet, V., Oddie, M.A., Mondet, F., Forsgren, E., Dahle, B., Furuseth-Hansen, E., Williams, G.R., De Smet, L., Natsopoulou, M.E., Murray, T.E. and Semberg, E., 2024. Shift in virus composition in honeybees (Apis mellifera) following worldwide invasion by the parasitic mite and virus vector Varroa destructor. Royal Society Open Science, 11(1), p.231529.
2. Gisder, S. and Genersch, E., 2021. Direct evidence for infection of Varroa destructor mites with the bee-pathogenic deformed wing virus variant B, but not variant A, via fluorescence in situ hybridization analysis. Journal of Virology, 95(5), pp.10-1128.
3. Shen, M., Yang, X., Cox-Foster, D. and Cui, L., 2005. The role of varroa mites in infections of Kashmir bee virus (KBV) and deformed wing virus (DWV) in honey bees. Virology, 342(1), pp.141-149.
4. Jeyapriya, G., Sumathi, E., Saminathan, V.R., Renukadevi, P., Sasikala, R., Priya, S.S., Kowsika, S. and Pradeep, S., 2025. Parasitic Mites of Honey Bees (Apis Spp.): A Detailed Review of Varroa destructor in Parasitism, Pathogen Transmission and its Management. Acta Parasitologica, 70(5), pp.1-25. 
5. Yang, X. and Cox-Foster, D.L., 2005. Impact of an ectoparasite on the immunity and pathology of an invertebrate: evidence for host immunosuppression and viral amplification. Proceedings of the National Academy of Sciences, 102(21), pp.7470-7475.
6. Mockel, N., Gisder, S. and Genersch, E., 2011. Horizontal transmission of deformed wing virus: pathological consequences in adult bees (Apis mellifera) depend on the transmission route. Journal of General Virology, 92(2), pp.370-377.

Connecting with ATTTA Specialists

If you’d like to connect with ATTTA specialists or learn more about our program, you can:

visit our website at https://www.perennia.ca/portfolio-items/honey-bees/

Email attta@perennia.ca

As beekeepers we often see the damage inflected by Varroa mites (Varroa destructor) at the colony level. When colonies are not managed vigilantly for Varroa mites, and populations of mites reach a critical level, they colony is significantly weakened and is at high risk of dying before or during the winter months. To better understand why Varroa mites cause colony loss it is important to explore the biology of Varroa mites including their host-parasite interaction. This week’s blog will specifically focus of Varroa mite feeding and the direct impacts feeding has on their honey bee host.

The Host-Parasite Interactions of Varroa Mites and Honey Bees: Varroa Mite Feeding

Varroa mites remain the most widespread and economically damaging pest of honey bees¹. It is important to continue researching how Varroa mites target and parasitize honey bees to develop new chemical treatments and improve management practices. An aspect of Varroa mite biology that is worthwhile to discuss is how their feeding practices directly impact honey bees.

Varroa parasitizes both the adult bee and developing brood by feeding on both hemolymph and fat bodies. However, more recent studies have identified that the fat body is the primary feeding source for Varroa mites as this tissue plays a critical role for honey bee immune function, pesticide detoxification, energy metabolism, and overall health of the honey bee². This means that damage inflicted by Varroa mites to the fat body impacts the bee’s ability to protect itself from pathogens such as viruses and bacteria, and to detoxify pesticides, which overall weakens the bee and shortens their lifespan².

Figure 1. Research by Ramsey et al. (2019) demonstrates the preferred feeding location of Varroa destructor on honey bee hosts, with the majority of mites (95.2%) feeding underneath the metasoma (an area predominated by fat body tissue) (n = 104) (Diagram created by Ramsey et al. 2019).

Beekeepers should consider the fact that the preferred feeding location of Varroa mites (underneath the metasoma) is in an area not visible to them when inspecting colonies. Therefore, when a beekeeper does identify a mite dorsally (on top) of a bee, this often indicates the ventral (underneath) feeding locations are already occupied by additional mites, and mite levels may be critically high within the colony.

The feeding process involves puncturing the bee’s cuticle using toothed chelicerae, and then using their pharyngeal pump to extract nutrients¹. Continuous feeding weakens the host, reduces longevity, and facilitates viral infections¹.

Exploring other impacts of Varroa mite feeding, brood infested with mites leads to a reduction in the body weight of newly emerged bees and malformed adult bees, which indicates developmental stress and compromised health³. Reduced body weight and malformation is caused by a combination of loss of nutrients, viral transmission and suppressed immune function during development³. Bees with compromised development have a reduced lifespan, which contributes to weakened colonies and overall colony decline.

One of the primary ways Varroa mite feeding directly impacts honey bees is their saliva composition. The saliva of Varroa mites contains a complex mixture of bioactive proteins and enzymes that disrupt host immunity and wound healing⁴. Typically, when an insect’s cuticle is punctured the wound will naturally heal to prevent hemolymph loss. However, when Varroa punctures the cuticle of a honey bee the wound inflicted does not close properly suggesting that the saliva interferes with normal healing⁴. Approximately 356 proteins have been identified in the saliva of Varroa mites that can either interfere with the immune response to heal wounds, or function as virulence factors, enhancing the mite’s ability to infect the honey bee host⁵. Furthermore, the saliva of mites contains various enzymes which protect the mite against harmful compounds produced by the honey bee during feeding^2,6. These enzymes break down toxic compounds that are produced during feeding, and other harmful compounds existing within the honey bee host, which provides the Varroa mites an adaptive advantage to parasitism^2,6.

The direct damage inflicted by the Varroa mite through feeding is severe, and contributes to weakened colonies and colony loss. However, there are other aspects of the host-parasite interaction between Varroa mites and honey bees that contribute to the overall colony decline associated with high Varroa mite levels. Next week’s blog will further explore Varroa mite biology and how this parasite transmits viruses to their host.

References

1. Jeyapriya, G., Sumathi, E., Saminathan, V.R., Renukadevi, P., Sasikala, R., Priya, S.S., Kowsika, S. and Pradeep, S., 2025. Parasitic Mites of Honey Bees (Apis Spp.): A Detailed Review of Varroa destructor in Parasitism, Pathogen Transmission and its Management. Acta Parasitologica, 70(5), pp.1-25.
2. Ramsey, S.D., Ochoa, R., Bauchan, G., Gulbronson, C., Mowery, J.D., Cohen, A., Lim, D., Joklik, J., Cicero, J.M., Ellis, J.D. and Hawthorne, D., 2019. Varroa destructor feeds primarily on honey bee fat body tissue and not hemolymph. Proceedings of the National Academy of Sciences, 116(5), pp.1792-1801.
3. Martin, S.J., 2001. The role of Varroa and viral pathogens in the collapse of honeybee colonies: a modelling approach. Journal of Applied Ecology, 38(5), pp.1082-1093.
4. Kanbar, G. and Engels, W.J.P.R., 2003. Ultrastructure and bacterial infection of wounds in honey bee (Apis mellifera) pupae punctured by Varroa mites. Parasitology research, 90(5), pp.349-354.
5. Zhang, Y. and Han, R., 2019. Insight into the salivary secretome of Varroa destructor and salivary toxicity to Apis cerana. Journal of Economic Entomology, 112(2), pp.505-514.
6. Morfin, N., Goodwin, P.H. and Guzman-Novoa, E., 2023. Varroa destructor and its impacts on honey bee biology. Frontiers in Bee Science, 1, p.1272937.

Connecting with ATTTA Specialists

If you’d like to connect with ATTTA specialists or learn more about our program, you can:

visit our website at https://www.perennia.ca/portfolio-items/honey-bees/

Email attta@perennia.ca