Varroa has been the main enemy of beekeepers for decades. Another mite, Tropilaelaps mercedesae, is now moving west from Asia and has already been reported in southwestern Russia and in Georgia.1,2
A study published in March 2026 in the Journal of Economic Entomology looked at how this mite moves between colonies in temperate conditions. The answer is not reassuring: mites left with a swarm, settled in the new colony and reproduced there.1,2
Tropilaelaps is a genus of four mite species, all from Asia.7 Their natural hosts are the giant honey bees, Apis dorsata and Apis laboriosa, common across South and Southeast Asia.2 At some point, T. mercedesae switched to the western honey bee, Apis mellifera, the species kept by beekeepers worldwide.2
Its biology looks a lot like varroa: it reproduces in capped brood7, but it also feeds on uncapped larvae [de Guzman et al. 2019], and recent laboratory word suggests it may survive on adult bees for longer than previously thought [Gill et al. 2025]. It damages larvae directly and carries several well-known bee viruses, including deformed wing virus (DWV), black queen cell virus (BQCV), acute bee paralysis virus (ABPV) and sacbrood virus (SBV). Viral loads increase significantly in infested colonies.3
The host switch is recent. Apis mellifera and Tropilaelaps have not evolved together, so our bees have no co-evolved defenses against it. Work by Gao and colleagues, published in PLoS Pathogens in 2021, showed that the mite changes worker behavior and reduces foraging efficiency.4
For a long time, the mite stayed in Asia. It has now been found in southwestern Russia and in Georgia and more recently in Turkey, which worries researchers and beekeepers alike.2 The western honey bee is kept by beekeepers worldwide, and bees, queens and equipment move a lot between regions and countries. To slow the mite down, we first need to know how it spreads. Until now, most dispersal studies were carried out in the tropics, where colony dynamics and beekeeping practices are very different from ours.2
The team, from institutes in Europe and China, started from a simple question: how does Tropilaelaps naturally move from one colony to another? The authors include Aleksandar Uzunov, professor at Ss. Cyril and Methodius University in Skopje (North Macedonia), and Maggie Gill, UK-based researcher, founder of PHIRA-Science and senior author of the study. The work was done in Georgia, in temperate conditions typical of western-style beekeeping.2
First, the researchers followed a natural swarming event. Eight female mites left the original colony with the swarm. Four died shortly afterwards. The other four were found in capped brood cells in the new colony, and two of them produced three offspring each. The mites did more than survive the trip: they restarted their life cycle in the new colony. According to Uzunov, this route of transmission had never been shown before in Apis mellifera colonies.2
They then looked at drifting, which happens when a forager returns to the wrong hive. Two artificial colonies were set up with foragers collected as they returned to infested hives. The mites made the move: 23 in one colony, 17 in the other. These colonies reared brood at a similar rate to a natural swarm, yet no mite reproduced, and all died within 4 to 6 days.2
That does not make drifting harmless. For Uzunov, the number of mites found on foragers shows that moving between colonies within an apiary, through drifting or robbing, is likely and common. Gill adds that foragers could also pass mites on when they meet bees from neighboring apiaries.2
These results still need to be read with care. The study rests on one natural swarm and eight mites. It shows that transmission through swarming can happen, not how often it does. Measuring the real risk will take work on more colonies.
The usual view was that Tropilaelaps cannot survive more than three days away from brood. A swarm leaves without brood, so it looked like a low-risk route. This study shows otherwise: some mites lived long enough to reach brood in the new colony.2
It fits with other recent work. An in vitro study published in late 2025 in the journal APIS, led by Gill, tested how long the mite survives without brood and pointed to a possible survival mechanism outside the brood. The authors flagged routes once seen as low risk, such as the queen and package bee trade and second-hand beekeeping equipment.5
One question remains open: why did mites that came with the swarm outlive those carried by foragers? The researchers do not know yet, and wonder whether the physiological state of the colony plays a part.2
Swarming is usually seen as a sign of a healthy colony. This study shows it can also carry the mite, whether the swarm is natural or made by the beekeeper.2 Several points follow:
Our picture of Tropilaelaps is still being drawn. Assumptions that seemed solid a few years ago are now being questioned, and new studies will likely refine what we know. In the meantime, the best defense is vigilance: check your colonies, limit the arrival of bees, queens and equipment from at-risk regions, and stay informed as research moves forward.
In September, we hosted a French-language webinar on Tropilaelaps with Delphine Panziera, researcher at Wageningen University & Research. We are running it again in January, this time in English. Subscribe to our newsletter to get the registration link as soon as it’s out in December.
8. An in vitro investigation into the survival of Tropilaelaps mercedesae on a range of matrices. bioRxiv preprint. biorxiv.org
