Parasitic mites transmit a lethal bacterium to honey bees

Jim Crocker
24th October, 2025

Parasitic mites transmit a lethal bacterium to honey bees

To trace the path of a deadly infection in bees, scientists engineered the M. morganii bacterium (A) to glow green (B), revealing that while mites on healthy bees remain dark (C), those feeding on infected bees light up (D) as they acquire the lethal pathogen.

Image adapted from: Chen, Huang. / CC BY (Source)

Key Findings

  • Researchers identified a new bacterium, Morganella morganii, within Varroa mites collected from honey bee colonies in Jiangxi, China
  • Morganella morganii is highly lethal to bees, causing over 30% mortality in both pupae and adults in lab tests
  • Varroa mites are the primary vector for Morganella morganii, efficiently transmitting it between bees with a 92.1% bee-to-mite and 68.49% mite-to-bee transmission rate
Honey bee populations are declining globally, posing a threat to both agriculture and the wider ecosystem due to their crucial role in pollination. While numerous factors contribute to these losses, the parasitic mite Varroa destructor is considered a major driver of colony failure. Historically, the understanding of how Varroa harms bees focused on its direct feeding habits and its role as a vector for viruses, particularly the deformed wing virus[2]. However, recent research suggests a more complex picture, one involving bacterial pathogens transmitted by the mite. Researchers at the Honeybee Research Institute, Jiangxi Agricultural University and the Medical College of Wisconsin[1] have identified a previously unknown bacterium, Morganella morganii, found within Varroa mites. This bacterium proved highly lethal, causing over 30% mortality in both pupae (developing bees) and adult bees in laboratory settings. This discovery is significant because it points to a new pathway through which Varroa mites can devastate honey bee colonies – not just through direct feeding or viral transmission, but through bacterial infection. The study went beyond simply identifying the bacterium; it investigated how Morganella morganii spreads. Contrary to expectations, direct bee-to-bee transmission through typical social contact appeared minimal. Instead, the research demonstrated a strong link between the mites and the bacterium, with a 92.1% transmission rate from infected bees to mites and a 68.49% transmission rate from infected mites to healthy bees. This suggests the mite acts as the primary vector, efficiently moving the pathogen between individuals. The researchers utilized fluorescent protein tagging to track the bacterium’s spread, providing visual evidence of this transmission pathway. This finding helps explain patterns of colony collapse, particularly during winter. As[2] has shown, winter losses fluctuate, and are often correlated with honey flow dynamics – the amount of nectar bees collect. The study proposes that the expanding Varroa mite population during winter creates a larger reservoir of Morganella morganii, accelerating the spread of the lethal bacterium and contributing to increased mortality rates. This aligns with observations of higher colony losses when mite infestations are severe. Previous research[3] highlighted the importance of honey flow in colony health, demonstrating that early and abundant honey flow can reduce winter losses. This could be because stronger colonies are better able to withstand parasitic pressure, including Varroa infestations. The discovery of Morganella morganii adds another layer to this understanding. A healthy colony may not only be better equipped to fight off Varroa mites directly, but also less susceptible to the secondary bacterial infections they carry. Interestingly,[4] established that Varroa mites don’t feed on hemolymph as previously thought, but instead consume fat body tissue – a critical organ involved in immune function, pesticide detoxification, and overwinter survival. This finding is particularly relevant in the context of Morganella morganii. Damage to the fat body weakens the bees’ immune system, potentially making them more vulnerable to bacterial infections. The combined effect of fat body depletion by Varroa and subsequent infection with Morganella morganii could be a significant factor in colony collapse. The work by the Honeybee Research Institute and Medical College of Wisconsin underscores the need to re-evaluate strategies for managing Varroa mites. Current approaches often focus on reducing mite numbers, but this research suggests that controlling the spread of associated pathogens, like Morganella morganii, may be equally important.

WildlifeEcologyAnimal Science

References

Main Study

1) Pathogenicity and transmission of Morganella morganii in honey bees

Published 22nd October, 2025

https://doi.org/10.1371/journal.ppat.1013613

Related Studies

2) First large-scale study reveals important losses of managed honey bee and stingless bee colonies in Latin America.

https://doi.org/10.1038/s41598-024-59513-6

3) Annual Fluctuations in Winter Colony Losses of Apis mellifera L. Are Predicted by Honey Flow Dynamics of the Preceding Year.

https://doi.org/10.3390/insects13090829

4) Varroa destructor feeds primarily on honey bee fat body tissue and not hemolymph.

https://doi.org/10.1073/pnas.1818371116


Related Articles

An unhandled error has occurred. Reload 🗙