Where Did Varroa Mites Come From and How Did They Spread?

Varroa destructor, the parasitic mite responsible for devastating honeybee colonies worldwide, originated on the Eastern honeybee (Apis cerana) in Asia, where it lived as a relatively controlled parasite for millions of years before jumping to the Western honeybee (Apis mellifera) in the mid-twentieth century. That host switch, which happened when European-style beekeeping brought Western honeybees into the mite’s native range, set off a global invasion that reached every inhabited continent within a few decades. The story of how a regional parasite became a worldwide crisis involves genetics, trade routes, bee behavior, and a host species that had no evolutionary preparation for what hit it.

The Original Host and Its Defenses

Varroa mites evolved alongside the Eastern honeybee, Apis cerana, across mainland and island Asia. On this original host, the mites exist at low levels and rarely kill colonies. Early research in the 1980s and 1990s attributed this to grooming behavior, brood removal, and high rates of mite infertility in A. cerana, though a more recent review of those resistance traits found surprisingly little hard evidence for some of those conclusions.

What does seem clear is that A. cerana worker brood creates a hostile environment for reproducing mites. When researchers allowed mites to infest A. cerana worker brood under controlled conditions, the mites were attracted to larvae and initiated reproduction at rates similar to those seen in Western honeybees. But a large proportion of the infested A. cerana brood developed abnormally and was removed by adult workers, killing the developing mites in the process. Successful production of mite offspring in A. cerana worker brood was rare, though it remained occasionally possible.¹ One research group proposed a “mutual destruction” dynamic: the worker larvae block mite reproduction while the mite’s saliva toxin kills larvae, making the whole interaction costly enough that mites largely avoid worker brood in the field.²

In A. cerana colonies, mites reproduce mainly in drone brood, which is larger, has a longer development time, and is produced in limited quantities. This effectively caps the mite population. Western honeybees lack most of these evolved checks. Their worker brood does not trigger the same removal response, and mites reproduce freely in both worker and drone cells, allowing populations to explode.

The Host Switch That Started It All

The mite’s jump to Western honeybees happened when European beekeepers introduced A. mellifera colonies to Asia for commercial honey production. The two bee species were brought into close proximity, and Varroa mites crossed over.³ The first documented host-switching event occurred around 1952 in eastern Russia, producing what researchers call the Korea haplotype. A second event followed around 1957, giving rise to the Japan haplotype.⁴

These dates matter because they show how recent the invasion is. Beekeepers had been managing Western honeybees for thousands of years without Varroa. The entire crisis is roughly seventy years old. Once mites established themselves in A. mellifera colonies, they spread westward from Asia through Europe, the Middle East, Africa, and eventually the Americas.

Not One Species, But Two

For decades, all Varroa mites parasitizing honeybees were classified as a single species, Varroa jacobsoni. That changed in 2000 when researchers demonstrated that what had been called V. jacobsoni was actually a complex of at least two species. The mites infesting A. cerana across mainland Asia were reclassified as a new species: Varroa destructor. The original V. jacobsoni name was retained for mites found on A. cerana in the Malaysia-Indonesia region. Adult females of V. destructor are larger and less round than V. jacobsoni females, and the two are reproductively isolated.⁵

This reclassification was more than an academic exercise. Of the eighteen or so distinct genetic variants (haplotypes) found infesting A. cerana across Asia, only two have proven capable of reproducing on Western honeybees. Both belong to V. destructor, not V. jacobsoni. The Korea haplotype is by far the more widespread, found on A. mellifera in Europe, the Middle East, Africa, Asia, and the Americas. The Japan haplotype is less common and has been identified on A. mellifera mainly in Japan, Thailand, and parts of the Americas.⁵ The fact that the vast majority of haplotypes cannot exploit Western honeybees at all suggests that the host switch required specific adaptations that most lineages never acquired.

A Genetic Bottleneck and Its Consequences

When the Korea and Japan haplotypes jumped to Western honeybees, they went through a severe genetic bottleneck. The founding populations were tiny, and their descendants carry almost no genetic diversity. Microsatellite analysis shows that variation within each haplotype is virtually absent, while the two types remain distinct from each other with nearly complete separation of their nuclear and mitochondrial markers.⁶

This stands in sharp contrast to the genetic diversity seen in Varroa populations on their native host in Asia. The mites that colonized A. mellifera worldwide are essentially two near-clones, each descended from a single or very few founding females.⁷ Given that mite populations can grow roughly twelvefold per year in unmanaged colonies, the lack of diversity is not from gradual erosion over time; it was locked in from the start. Researchers have also found additional haplotypes in Asia that can reproduce on A. mellifera but have so far remained regional, raising the possibility that new host-switching events could introduce more genetically diverse mites into the global population in the future.

How Mites Evade Their New Host

One of Varroa’s most remarkable tricks is chemical camouflage. The mites coat themselves in cuticular hydrocarbons that match those of their host bee, effectively becoming invisible to the colony’s hygienic defenses. Research showed that mites artificially transferred from one honeybee species to another can adjust their hydrocarbon profile to mimic the new host, demonstrating that the camouflage is not species-fixed but actively adaptive.⁸ This mimicry is not perfect, but it is good enough to let mites move through colonies without being targeted for removal.⁹

This ability to rapidly tune chemical camouflage likely played a role in the original host switch. A mite landing on an unfamiliar bee species that could quickly adjust its surface chemistry would survive long enough to reproduce, while one that could not would be detected and killed. The fact that V. destructor can perform this adjustment when experimentally moved between A. cerana and A. mellifera suggests the mechanism may have been pre-adapted for exploiting new hosts, not something that evolved after the switch.

How Mites Spread Between Colonies

Once Varroa reaches a region, it spreads between colonies through two main mechanisms: drifting and robbing. Drifting happens when bees from one hive accidentally enter another, carrying mites with them. Robbing happens when foragers from healthy colonies raid the honey stores of weakened ones. A detailed study of how mites move from collapsing colonies to their neighbors found that robbing was the primary driver. When heavily infested colonies weakened to the point of collapse, workers from neighboring hives robbed them intensely. The collapsing colonies’ mite loads plummeted while the neighbors’ mite loads surged simultaneously, indicating that the robbers were picking up mites and bringing them home. Drifting of mite-carrying bees also contributed, particularly to the nearest colonies.¹⁰

This creates a vicious cycle that beekeepers sometimes call the “mite bomb” phenomenon. A single untreated colony in a neighborhood can collapse and seed mites into every hive within foraging range. The effect is not limited to managed apiaries. Feral honeybee populations, which receive no mite treatments, can serve as persistent reservoirs. In areas where Varroa has been present for decades, feral colony populations have declined substantially, though some persist.¹¹ The interaction between managed and feral bees means that even diligent beekeepers treating their own hives can face reinfestation from wild colonies and vice versa.

The Role of Migratory Beekeeping

Commercial beekeeping in many countries involves trucking hives hundreds or thousands of miles to pollinate crops. Almonds in California, blueberries in Maine, canola in the Canadian prairies: these crops depend on pollination services that concentrate hives from many different regions in one place, then scatter them again. You might expect this to turbocharge mite spread, and historically it probably did help Varroa colonize new areas. But the research on whether migratory colonies carry higher mite loads than stationary ones is surprisingly mixed. Some studies report higher Varroa loads in migratory operations, others find no difference, and at least one found that stationary colonies actually had higher mite levels a month after migratory hives arrived nearby.¹²

The inconsistency likely reflects the fact that commercial migratory beekeepers tend to treat aggressively for mites, since losing colonies is expensive. Meanwhile, stationary hobby beekeepers may treat less consistently. What migratory beekeeping unquestionably does is mix bee and mite populations from distant areas, creating opportunities for pathogen exchange that would not occur if hives stayed put.

The Virus Connection

Varroa’s damage goes well beyond blood feeding. The mites act as vectors for a suite of honeybee viruses, most notably Deformed wing virus (DWV). Before Varroa arrived, DWV existed in bee populations at low levels and rarely caused visible symptoms. The mite changed that equation by injecting virus directly into developing pupae as it feeds, bypassing the bees’ oral infection route and dramatically amplifying viral loads. A landmark study tracking the mite’s arrival in Hawaii documented in real time how DWV went from a diverse, low-level background infection to a dominant, virulent strain as Varroa swept across the islands. Despite quarantine measures, the mite spread from Oahu to the Big Island by early 2009 and had colonized the southern region of the island by that November.¹³

Research into the mechanics of this relationship has revealed that DWV does not actually replicate inside the mite. Instead, the mite appears to function as a passive carrier, picking up virus from one bee and injecting it into the next. The selective pressure this creates favors viral strains that are stable inside the mite and transmit well through injection, rather than strains adapted to replicate in the mite itself.¹⁴ Researchers have also identified recombinant viruses, hybrids between DWV and a related virus called Varroa destructor virus-1, that appear better adapted to transmission between mites and bees than either parent virus alone.¹⁵ This viral dimension is a large part of why Varroa is so destructive: the mite does not just weaken bees directly but reshapes the entire viral landscape of every colony it enters.

Australia and the Last Frontier

For decades, Australia was the only continent with managed Western honeybee populations that remained free of Varroa destructor. That changed in June 2022, when the mite was detected in surveillance hives at the Port of Newcastle in New South Wales.¹⁶ Australian authorities mounted an aggressive eradication effort, establishing movement restrictions and destroying infested hives. By mid-2023, however, the decision was made to shift from eradication to management, acknowledging that the mite had spread too widely to eliminate.

Australia’s experience underscores how difficult it is to keep Varroa out once global trade in bees and bee products brings it to a port. The Hawaii example told the same story: quarantine delayed but did not prevent inter-island spread.¹³ Once a small founding population of mites establishes itself, the exponential reproduction rate and the movement of bees between foraging sites make containment extremely challenging.

Climate and the Expanding Suitable Range

Varroa’s geographic limits are partly set by climate. Habitat modeling using global climate data found that the mite’s occurrence is best predicted by temperature and precipitation variables, with optimal conditions below about 30°C and moderate rainfall. Precipitation thresholds of around 150 mm in the wettest month and 45 to 50 mm in the driest month mark the sweet spot.¹⁷ Extremely hot, arid regions are less favorable.

Rising temperatures create a more nuanced picture than simple range expansion. A study examining long-term weather data alongside colony monitoring found that warmer springs and autumns reinforced autumn Varroa infestations, not by extending the mite’s active season as previously assumed, but by promoting more bee brood production during those periods, which gives mites more reproductive opportunities.¹⁸ In other words, warming does not help the mite directly so much as it helps the bees produce more brood, which indirectly helps the mite. Under future climate projections, some currently marginal areas may become more suitable for Varroa, while extremely hot regions could become less so.

Signs of Resistance in Western Honeybees

The picture is not entirely grim. In several parts of the world, populations of A. mellifera have shown signs of developing natural resistance to Varroa after years of exposure without chemical treatment. A study comparing resistant and susceptible bee populations in Uruguay found that the resistant bees exhibited stronger hygienic and grooming behaviors, maintained lower mite loads by the end of summer, and carried lower levels of Deformed wing virus. Genetic analysis revealed these resistant bees were Africanized hybrids, while the susceptible bees were closer to European subspecies.¹⁹

Similar resistance has been documented in isolated populations in Europe, including one near Toulouse, France, where bees showed the ability to inhibit Varroa reproduction in drone brood.²⁰ These populations are small and sometimes depend on specific local conditions, but they demonstrate that A. mellifera is not genetically incapable of coexisting with Varroa. The challenge is that natural selection for resistance is brutal: it requires letting colonies die until only tolerant ones remain, which is economically devastating for beekeepers and ecologically costly for pollination services. Breeding programs that try to speed up the process without mass die-offs are underway in several countries, but scaling those efforts to the global beekeeping industry remains a formidable task.

Potential for New Host-Switching Events

Researchers monitoring Varroa diversity in Asia have flagged additional haplotypes that can reproduce on A. mellifera but have not yet spread beyond the region. At least three of these, found in Thailand, Vietnam, China, and Japan, successfully reproduce on Western honeybees under local conditions.⁷ If any of these genetically distinct mites were to escape Asia through the same trade routes that spread the Korea and Japan types, they could introduce new genetic diversity into the global Varroa population. That would complicate breeding efforts aimed at resistance and could potentially alter the mite’s virulence, reproductive rate, or susceptibility to chemical treatments. Given the mite’s demonstrated ability to switch its chemical camouflage to match novel hosts, the barrier to such an event may be lower than it first appears. Biosecurity at ports and in the international bee trade is the primary line of defense, but Australia’s 2022 breach is a reminder of how thin that line can be.