Honeybee colonies play a vital role in the continuity of ecosystems and the productivity of agricultural production through their pollen and nectar collection activities. However, these valuable creatures face complex problems threatening the apiculture industry worldwide. Foremost among these issues is the infestation known as Varroa disease, caused by an external parasitic mite called Varroa destructor. This pest not only weakens colonies through direct feeding but also transmits deadly viruses, leading to reduced performance and, if left untreated, complete colony collapse.
What is Varroa?
Varroa is an obligate external parasite that lives on both adult honeybees (Apis mellifera) and their developing brood (pupae). Instead of the bee’s blood (hemolymph), this mite feeds directly on the bee’s vital fat body tissue. This feeding behavior suppresses the bee’s immune system and shortens its lifespan. While the primary destructive species is V. destructor, V. jacobsoni causes less harm in its natural host.
Life cycle and host preferences
The mite’s life cycle is perfectly synchronized with the honeybee colony’s brood cycle and consists of two main phases. The first phase is the “phoretic” (transport) period. During this time, female mites live on adult worker bees, often hiding between the abdominal segments. They feed on the bee’s fat body and move around the hive. This stage ensures the parasite’s survival during broodless periods and allows for transfer between colonies.
The second and most destructive phase is the “reproductive” period. The foundress female mite invades a cell containing a bee larva just before the cell is sealed (approximately 15-20 hours before for worker bees, 40-50 hours for drones). About 60 to 70 hours after the cell is sealed, the female mite begins feeding on the pupating bee and lays her first egg (unfertilized, male). Subsequently, she lays fertilized female eggs every 30 hours. These offspring also feed on the pupa, mature within the cell, and mate with each other (usually their male sibling). When the young bee emerges from the cell, the foundress mite and her mated female offspring emerge with it.
Mites are highly selective regarding host preference. Research shows that the parasite prefers drone brood for reproduction 8 to 12 times more than worker bee brood. The reason is that the development time for drone brood (about 24 days) is longer than for worker bees (21 days). This extra 3-day period allows the parasite to produce a larger number (approximately 2-3) of mature and mated female offspring within the cell. In a worker cell, this number is usually limited to 1-2 offspring.
V. destructor vs. V. jacobsoni Differences
Although these two names are often used interchangeably in beekeeping circles, there is a significant difference between them regarding the global beekeeping crisis. Varroa jacobsoni is a mite that lives on its natural host, the Asian honeybee (Apis cerana). Because the Asian honeybee has undergone thousands of years of co-evolution with this parasite, it has developed effective defense mechanisms against it (e.g., intense grooming behavior, removing and discarding parasitized pupae). Therefore, V. jacobsoni rarely causes fatal problems in its natural host.
The real problem began in the mid-20th century when Varroa destructor jumped species from Apis cerana to Apis mellifera (the European honeybee). The European honeybee has almost no natural defense against this new and aggressive parasite. Although they are morphologically very similar (flat, oval, reddish-brown, about 1.1 mm long and 1.6 mm wide), the biology of V. destructor makes it more dangerous. The most critical difference is that V. destructor can reproduce with a high success rate even in the worker brood of the European honeybee. Most strains of V. jacobsoni cannot accomplish this. This reproductive capability has turned the Varroa disease outbreak into a global catastrophe.
How does Varroa disease spread?
The spread of Varroa disease is accelerated by both human activities and the natural instincts of bees. Modern agricultural practices like migratory beekeeping transport the parasite across continents, while behaviors like robbing and bee drift within the apiary cause inter-hive transmission. Swarming also plays a role in moving the disease to new locations.
The impact of migratory beekeeping on spread
The practice of moving colonies over long distances (migratory beekeeping) to meet pollination needs in modern agriculture or to exploit different nectar flows plays a primary role in the parasite’s spread. Transporting infested bee colonies from one region to another rapidly disseminates the parasite geographically. Especially in large agricultural areas (e.g., almond or sunflower fields), placing thousands of hives in very close proximity (sometimes 1-2 meters apart) maximizes the risk of transmission between apiaries.
In this density, the phenomenon known as “bee drift” is inevitable. Worker bees returning from the field, disoriented, may enter a neighboring hive instead of their own. If these drifting bees are carrying phoretic mites, they easily transmit the parasite to a healthy colony. Furthermore, the commercial sale of infested queen bees, package bees, or hives supports this commercial spread.
Transmission via robbing and swarm transfer
Bees’ natural behaviors also play a critical role in parasite transfer. Robbing is particularly observed during dearth periods (like autumn) when the nectar flow slows or stops. Strong colonies attack weak colonies to steal their honey stores. A colony weakened by a high parasite load from Varroa disease, with its defensive strength diminished, is an easy target for robbers. Robber bees enter this weak hive, steal the honey, and return to their own hives. During this process, phoretic mites on the adult bees in the weak hive attach to the robber bees and are transferred to the healthy, strong colony. This causes the parasite level to rise rapidly throughout the entire apiary.
Swarm transfer, on the other hand, is the bee colony’s natural method of reproduction. When a colony swarms, the old queen and about 40% to 60% of the colony leave the hive to find a new home. This departing swarm inevitably takes a portion of the phoretic mites (the infection) from the parent colony with it. When this infested swarm settles in a new nest or is captured by a beekeeper, it establishes a new focal point for Varroa disease infection.
Managing Varroa
Successful management of Varroa disease does not rely on a single silver bullet but on an integrated management plan (IPM) maintained throughout the year. The goal is not to eradicate the parasite completely, but to keep its population below the economic injury threshold, where it does not threaten the colony’s health and productivity. The core principle for residue-free production is to avoid treatments during the honey flow and to prefer natural compounds that do not accumulate in beeswax.
Annual integrated management plan
Integrated Pest Management (IPM) involves continuous monitoring of parasite levels and intervening only when established thresholds are exceeded. This plan requires a rational combination of biotechnical methods and chemical treatments. The strategy varies seasonally. Spring: This is the colony build-up period. Biotechnical methods like the “drone brood trap” are prioritized. This leverages the mites’ tendency to prefer drone brood (8-12 times more). Special frames placed in the hive are removed and destroyed along with the parasites inside after they are sealed. This significantly suppresses the population without using chemicals.
Summer’s End / Autumn: This is the most critical period for treatment. A strong treatment must be applied immediately after the main honey harvest, but before the healthy “winter bees” that will survive the winter are raised. Since brood rearing is still active during this period, applications that can penetrate sealed brood cells (e.g., formic acid) or long-release treatments may be necessary. Winter: This is the period when brood rearing has completely stopped or is at a minimum. Treatments applied during this time (e.g., oxalic acid dribble or vaporization) achieve the highest success (efficacy can exceed 95%). This is because all mites are on the adult bees (phoretic) and cannot hide in brood cells. Rotating the chemicals used (rotation) is mandatory to prevent resistance development.
Core principles for residue-free production
Some synthetic acaricides used in beekeeping tend to be lipophilic (fat-soluble).This causes them to accumulate in beeswax, where they can remain for years, contaminating both honey and wax. This situation endangers product quality, food safety, and bee health. Residue-free production aims to minimize these risks. The fundamental principle is to absolutely avoid medication during the honey flow (nectar collection) period. All treatments should be planned for after the main honey harvest.
In this approach, “organic” or “natural” substances are prioritized over synthetic chemicals. These are often natural components of honey or substances that break down quickly in nature. Oxalic acid and formik asit are the most common in this group. These organic acids do not accumulate in beeswax and show high efficacy when applied correctly. Some essential oils, like thymol, are also in this category. However, the effectiveness of these natural methods is very sensitive to external factors like temperature (e.g., formic acid is ideal between 15°C and 25°C) and hive humidity. Success depends on using approved products at the right time and the right dose.
Varroa damage and colony effects
The damage Varroa disease causes to a colony is far more than just the weakness from the parasite’s direct feeding. The mite’s primary destructive impact is its action as a vector for viruses. It transmits and activates numerous pathogens, especially Deformed Wing Virus (DWV). This shortens bee lifespan, reduces performance, and is considered the number one cause of colony overwintering failure.
Relationship with viruses (DWV, etc.) and performance loss
The mite itself weakens a bee and suppresses its immune system by consuming its fat body. But the fatal blow comes from its role as a virus vector. Many viruses exist at low levels (latent infections) in bee populations. Varroa acts like a “dirty needle” during feeding, transmitting these viruses from one bee to another and, most importantly, activating them. The most dangerous virus is Deformed Wing Virus (DWV).
When the mite feeds on a pupating bee, it injects this virus directly into the bee’s system. This causes the virus to replicate rapidly and results in the bee being born crippled. As the name suggests, these bees emerge with shriveled, non-functional wings. These bees cannot fly, cannot forage, and die quickly after crawling in front of the hive. Even without visible wing deformity, bees exposed to the parasite during their pupal stage have significantly shortened lifespans. A worker bee that would normally live 6 weeks may only live 3-4 weeks. This leads to a rapid erosion of the colony’s foraging population, insufficient food collection, and a collapse in the colony’s overall performance.
Impact on overwintering success
The moment Varroa disease determines a colony’s fate is during the overwintering period. Colonies’ ability to survive the winter healthily depends on a special generation of “winter bees” raised in the autumn (usually August-September). These bees are physiologically different from summer bees; they store abundant protein and fat (vitellogenin) in their bodies, necessary to keep the winter cluster warm and to feed the new generation in the spring. These bees can live for 6-8 months.
If the colony enters autumn with a high Varroa population, these parasites target the very winter bee pupae that are developing. These bees, fed upon by mites, their fat bodies consumed, and exposed to high doses of viruses (especially DWV), cannot store the necessary physiological reserves. Even if they emerge, they do not have the qualities of “winter bees.” When the colony enters the winter cluster, these unhealthy and short-lived bees begin to die off rapidly. The cluster population dwindles, becoming unable to maintain the internal temperature (approx. 20°C to 30°C). Even if there is sufficient honey in the hive, the shrinking cluster cannot reach the food, freezes, and starves. This is the main reason for the winter losses beekeepers experience.
Varroa detection: symptoms and first signs
Early detection of Varroa infestation is the key to successful management and colony health. Wingless bees in front of the hive or irregularities in the brood are initial suspicions. However, quantitative measurement of the parasite level is necessary to make a treatment decision. Shake tests (powdered sugar or alcohol) provide an infestation rate percentage, while a sticky board count helps in decision-making by monitoring natural mite drop.
Direct observation and shake tests
Symptoms of advanced infestation are visible to the naked eye. Bees crawling in front of the hive with deformed wings (DWV) are the clearest sign. When inspecting brood frames, “perforated brood” (gaps in sealed cells due to bees removing infected pupae), holes in sealed cell cappings, or a collapsed, unhealthy brood area (Parasitic Mite Syndrome) may be observed. Sometimes, the red-brown adult mites can be seen on the thorax of worker bees or drones.
However, by the time these symptoms appear, it is often too late for intervention. Active monitoring tests are essential for early diagnosis. Shake tests determine the infestation rate (percentage). In the powdered sugar method, a standard measure (e.g., 300 bees or a half-cup) is taken from a brood frame. The bees are gently shaken in a jar with powdered sugar. The sugar causes the mites to lose their grip (on their adhesive pads) and fall off. The bees (alive) are returned to the frame, and the mites remaining in the jar are counted. For example, if 9 mites are found on 300 bees, the infestation rate is calculated as 3% (9/300). This rate is compared to the intervention threshold (usually 2-3%).
Counting fallen mites with a sticky bottom board
Another common and non-harmful (passive) method for determining the parasite level in the colony is the use of a sticky bottom board. In this method, a sticky (or Vaseline/oil-coated) sheet is placed on the bottom board of the hive, under a wire screen that bees cannot reach. This sheet catches mites that die naturally or are groomed off by bees and fall during normal hive activities.
The board is left in the hive for a specific period (usually 24, 48, or 72 hours). At the end of the period, the board is removed, and the total number of mites on it is counted. The resulting number is divided by the number of days the board was in place to find the “average daily natural mite drop.” For example, if 30 mites were counted in 72 hours (3 days), the daily drop is calculated as 10 mites/day. This figure allows for an estimate of the total mite population in the colony. Intervention thresholds vary by season; however, a daily drop exceeding 10-15 mites in the autumn is generally considered an alarm level requiring immediate action.
