Varroa-Vectored Viral Pathogens and Miticide Resistance as Drivers of Honey Bee Colony Collapse: A brief synopsis

Bill gartner
Feb 6
4 min read

Link to paper: https://www.biorxiv.org/content/10.1101/2025.05.28.656706v1.full

In this paper, the researchers investigated the causes behind the extreme honey bee colony losses reported by U.S. commercial beekeepers in early 2025. According to the study, losses exceeded 60% in many operations, which is unusually high and economically devastating, especially because these losses occurred just before the California almond pollination season. The authors focused on whether pathogens—particularly Varroa-vectored viruses—and failures in mite control were responsible for these losses.

The main questions I took from this paper were whether viral pathogens were directly contributing to honey bee morbidity and mortality, and whether resistance to the commonly used miticide amitraz had compromised Varroa control. To answer these questions, the researchers examined colonies from six large commercial beekeeping operations that experienced heavy losses. They collected pooled adult bee samples from colonies as well as individual bees that were visibly sick or dying, allowing them to compare colony-level diagnostics with individual-level disease processes.

Pathogen prevalence across colonies is summarized in Table 1, which shows that Deformed Wing Virus (DWV) was the most common virus detected, appearing in 78% of colonies, while Acute Bee Paralysis Virus (ABPV) was found in 72% of colonies. Despite these high detection rates, Figure 1 demonstrates that there was no statistically significant difference in pathogen loads between strong and weak colonies when pooled samples were analyzed. This result suggests that colony-level sampling alone may not be sensitive enough to detect imminent collapse.

To better understand real-time disease effects, the researchers analyzed individual bees displaying abnormal behaviors. The results shown in Figure 2 indicate that symptomatic bees had significantly higher viral titers of DWV-A and DWV-B compared to asymptomatic controls. DWV-B was detected in all symptomatic bees but not in any asymptomatic individuals. This finding clearly links viral load at the individual level to morbidity.

Variation in viral patterns among individual bees is further illustrated in Figure 3, which presents a hierarchical clustering analysis. This figure shows that while DWV-A and DWV-B often occurred together, some morbid bees had extremely high levels of DWV-B alone, suggesting multiple viral pathways leading to death. Statistical modeling showed no significant effect of beekeeping operation on viral loads, indicating that this problem was widespread rather than localized.

To test whether the detected viruses were actually causing disease, the researchers conducted laboratory infection experiments. Survivorship curves shown in Figure 3a and 3b demonstrate rapid and significant mortality in bees injected with viral inoculate derived from symptomatic individuals. These results are supported numerically in Table 2, which shows strong viral replication and mortality compared to heat-inactivated and PBS controls. One viral isolate, CV5, was especially virulent and caused high mortality even at extreme dilutions.

The role of Varroa mites is further emphasized by genetic screening results showing that 100% of tested mites carried a marker associated with amitraz resistance. Although this result is presented textually rather than graphically, its biological relevance is reinforced by Figure 4, which shows a heavily parasitized emerging adult bee. This image highlights how Varroa feeding during development can severely weaken bees and facilitate viral transmission.

Overall, the figures and tables in this paper strongly support the authors’ conclusions. Table 1 and Figure 1 show the limitations of pooled sampling, Figures 2 and 3 link viral load to morbidity, Figure 3a/b and Table 2 demonstrate viral causation through experimental infection, and Figure 4 visually reinforces the role of Varroa as a key stressor.

From a beekeeping perspective, one of the most surprising findings was that colonies could appear similar in pooled diagnostics while individual bees were already experiencing lethal viral infections. This reinforces the importance of early detection, diversified Varroa control strategies, and reduced reliance on a single miticide. The study aligns with what we have learned in class about the central role of Varroa in colony health and the dangers of miticide resistance.

In conclusion, this paper provides strong evidence that Varroa-vectored RNA viruses, combined with widespread amitraz resistance, were major contributors to the 2025 colony collapse event. The integration of field observations, molecular diagnostics, experimental infections, and visual data makes this study convincing and highly relevant to modern beekeeping management.

References

Project Apis m. (2025). Colony loss information.
Genersch, E., & Aubert, M. (2010). Emerging and re-emerging viruses of the honey bee (Apis mellifera). Veterinary Research, 41(6), 54.
Rosenkranz, P., Aumeier, P., & Ziegelmann, B. (2010). Biology and control of Varroa destructor. Journal of Invertebrate Pathology, 103, S96–S119.
Wilfert, L., et al. (2016). Deformed wing virus is a recent global epidemic in honey bees driven by Varroa mites. Science, 351(6273), 594–597.
Traynor, K. S., et al. (2020). Miticides and honey bee health: The role of Varroa resistance. Current Opinion in Insect Science, 46, 9–15.
VanEngelsdorp, D., et al. (2009). Colony collapse disorder: A descriptive study. PLoS ONE, 4(8), e6481.