Development and Population Dynamics of Bees and Varroa Mites Throughout the Year
Throughout the year, the bee colony is constantly changing: it grows, reaches its peak, and then gradually prepares for winter. At the same time, Varroa destructor follows its own dynamic, often less visible but crucial to the health of the hive. Understanding these two processes helps us better understand what is happening in the apiary and why certain times of the year are particularly critical. This article provides a clear overview of the possible trajectories throughout the season.
Abstract
The seasonal development of an Apis mellifera colony in a temperate climate follows a structured annual dynamic: a winter survival phase with a reduced population and thermoregulation in the winter cluster, a spring recovery conditioned by temperature and pollen availability, a summer peak of brood and adult bees, followed by a gradual transition to the production of long-lived winter bees. It is precisely this last phase that determines the colony's capacity to survive the following winter.
Varroa destructor is directly embedded in this dynamic. The parasite reproduces only in capped brood, with a marked preference for drone brood, which emerges on day 24 after capping. Its total population, largely hidden in the brood during summer, is structurally underestimated by the standard indicators of phoresis on adult bees and natural mite drop. The biologically most critical period falls in late summer and autumn, when a high parasite load coincides with the production of winter bees. This internal pressure is compounded by re-infestation from the apicultural environment, documented in both Switzerland and Germany. Without adequate control, the subsequent deterioration is often silent: the colony may still appear functional while its capacity to overwinter is already compromised. The practical natural mite drop thresholds proposed by the Bee Health Service are useful intervention tools, provided they are interpreted in relation to the season and not confused with universal biological thresholds.
1. Seasonal development of a bee colony
 |
In a temperate climate such as that of Switzerland and much of central Europe, the development of a bee colony follows a fairly regular annual dynamic, even if its intensity varies from year to year depending on forage conditions, altitude, genetics, and colony management. |
In winter, the colony is not inactive, but operates in a survival mode. Its population is reduced, egg laying is zero or very low, and the bees cluster together when the outside temperature drops below 10 to 14 °C. This winter cluster is not simply a mass of motionless bees: it limits heat loss, while the core of the brood nest, once present, must be maintained at around 33 to 36 °C. The colony's entire winter strategy rests on this coexistence of a cold environment, a tight cluster, and a thermally stable zone at the centre that is essential for the brood (Imdorf et al., 2010; Stabentheiner et al., 2010).
For a production colony in a Swiss or central European context, the orders of magnitude proposed by Agroscope are particularly useful. At the onset of winter, a colony typically numbers 8,000 to 15,000 bees. After normal winter losses, the post-winter population is typically around 5,000 to 13,000 bees. This phase of low population is followed by rapid spring growth, then a summer peak that generally reaches 25,000 to 40,000 bees. Over the entire season, a colony can thus rear approximately 130,000 to 200,000 young bees. These figures should not be read as rigid norms, but as robust orders of magnitude for describing the dynamics of a typical colony in the Swiss context (Imdorf et al., 2010).
The resumption of egg laying often occurs between late January and early spring, therefore before external conditions become truly favourable. It may begin while the colony is still clustered. Experimental work shows that temperature acts as the primary driver here, while photoperiod plays more of a modulating role. It is therefore more accurate to speak of a window for resumption than of a fixed date. Under Swiss conditions, this resumption remains strongly dependent on the state of food stores, temperature, and the availability of early pollen collection (Imdorf et al., 2010; Nürnberger et al., 2018).
In spring, the colony enters a phase of rapid expansion. Worker brood increases first, then the adult population follows with the delay imposed by the development period. The Liebefeld observations illustrate this logic well: in the colonies monitored, brood volume at its peak reached approximately 34,000 to 36,000 cells before declining noticeably towards autumn. At the same time, the adult population increases to the summer maximum. This growth depends not only on the queen's egg laying but also on the longevity of the workers. This is why two colonies with fairly similar brood volumes can nevertheless show different adult populations (Imdorf et al., 1987; Imdorf et al., 2010).
Spring growth depends largely on pollen availability. Honey and nectar supply energy, but pollen provides the proteins essential for brood development, nurse bees, and the constitution of young bees. Available estimates indicate that a colony produces approximately 130,000 to 200,000 bees per season, corresponding to a requirement of around 17 to 34 kg of pollen per year. Per individual, this represents approximately 160 to 180 mg of pollen over the entire cycle. These figures remain balance-sheet estimates, but they clearly show that the spring population dynamic is closely dependent on pollen availability, far more than is sometimes perceived when observing entrance activity alone (Imdorf et al., 2010; Keller et al., 2005).
During this expansion phase, drones also appear. In a temperate climate, they are present mainly from spring to summer, typically from May to August, and remain a minority in the adult population (generally less than 10% of the adult population, (Czekońska et al., 2015)). Their presence accompanies the reproductive phase of the colony. Conversely, when resources diminish in late summer and autumn, workers progressively cease to tolerate their presence: drones are fed less, expelled more often, and temperate colonies then enter winter without them. This phenomenon of drone eviction, linked to the gradual cessation of brood rearing, is well documented and provides a useful counterpart to the worker cycle: appearance in spring, maintenance during the reproductive season, then eviction as overwintering approaches (Winston, 1987; Bogaert et al., 2020, citing Langowska & Zduniak, 2019).
During the active season, summer workers have a relatively short lifespan, of around 3 to 6 weeks. The colony then operates on a logic of rapid renewal: many births, much activity, but also high mortality linked to the intensity of work. This logic changes progressively at the end of the season. The summer solstice can serve as a practical reference point for identifying the turning point, as growth often loses its intensity after late June. However, it should not be assigned too simple a causal role on its own. The most experimentally well-supported mechanism is rather the diminishing of pollen, which progressively reduces investment in brood rearing and promotes the transition to long-lived workers (Imdorf et al., 2010; Mattila & Otis, 2007; Nürnberger et al., 2018).
This transition is not merely a quantitative slowing of egg laying. It also corresponds to a physiological reorganisation. When brood rearing decreases, nurse bees have fewer larvae to feed and therefore consume less of their vitellogenin reserves for royal jelly production. This key protein can then accumulate in the workers' fat bodies, which is precisely one of the physiological bases for the increased longevity of winter bees. These live much longer than summer bees, generally 3 to 6 months, sometimes longer depending on conditions. The end of the season therefore does not simply correspond to a decline in bee numbers, but to the production of a specialised population capable of surviving winter and enabling the resumption of development in the following spring (Amdam & Omholt, 2002; Mattila et al., 2001; Mattila & Otis, 2007).
The seasonal development of a bee colony can thus be summarised as a succession of interconnected phases: a reduced but specialised winter population, a resumption of egg laying still under thermal constraints, rapid spring growth highly dependent on pollen, a peak of brood and adult bees in late spring or early summer, then a gradual transition to the production of winter bees. This general framework is robust. However, the exact dates, the amplitude of the peak, and the rate of slowdown vary according to local climate, altitude, forage resources, and beekeeping management. For this reason, it is more appropriate to reason in terms of orders of magnitude and seasonal trends than in fixed monthly figures (Imdorf et al., 2010; Keller et al., 2005; Mattila & Otis, 2007).
2. Seasonal development of Varroa destructor
 |
The seasonal development of Varroa destructor is closely linked to that of the bee colony. The parasite does not multiply freely on adult bees, but reproduces in capped brood. |
The phoretic phase on adult bees enables its dispersal and maintenance within the colony, but population growth depends above all on the quantity of brood available. This is why the annual development of varroa cannot be understood independently of that of worker brood and, in spring, of drone brood (Rosenkranz et al., 2010).
The difference between worker brood and drone brood is central. Drone cells are not only more attractive to varroa, but also offer better reproductive conditions. In the experiment by Boot et al. (1995), varroa females invaded drone cells 11.6 times more often than worker cells under the tested conditions. Moreover, viable offspring per foundress is higher in drone brood than in worker brood: the orders of magnitude commonly cited in the literature are approximately 1.3 to 1.45 viable daughters per foundress in worker brood, compared with 2.2 to 2.6 in drone brood (Boot et al., 1995; Rosenkranz et al., 2010). This difference explains in large part why the spring phase, marked by the presence of drone brood, favours an acceleration of the parasite's dynamics (Fig. 1).
|
Fig. 1 — The life cycle of Varroa destructor in a capped brood cell Source: Animal and Plant Health Agency (APHA). Managing Varroa. York Biotech Campus, Sand Hutton, York. |
In spring, the total varroa population is still relatively moderate, but can already grow rapidly. This phase is often misleading for the beekeeper, as the increase in the bee population and brood can produce an apparent dilution effect: the proportion of phoretic varroa on adult bees may seem moderate while the total parasite population is already increasing. The phoresis signal should therefore never be interpreted in isolation, especially when brood becomes abundant. This dissociation between total population and phoretic infestation is an essential methodological point (Dietemann et al., 2013; Rosenkranz et al., 2010).
In summer, varroa reproduction reaches its maximum potential as there are numerous capped cells. During this phase, a large proportion of the parasite population is hidden in the brood. The literature reports that at the height of the season, the majority of varroa mites may be found in the brood, with values reaching up to approximately 90% in certain situations. This has a direct consequence for monitoring: neither phoresis nor natural mite drop alone reflects the total population present in the colony. The same value measured in summer and in autumn therefore carries a completely different biological meaning (Rosenkranz et al., 2010; Dietemann et al., 2013).
The biologically most critical period generally falls in late summer and autumn. This is not only because the varroa population is then high, but above all because this phase coincides with the production of winter bees. High parasite pressure at this point affects the physiological quality of these long-lived bees and increases the risk of winter mortality. In the Swiss study by Hernandez et al. (2022), infestations were measured in August and October on samples of adult bees taken from the brood area using a standardised wash method. In the context of this comparison between colonies compliant, nearly compliant, and non-compliant with the recommended treatment schedule, the authors report that in cases of non-compliance, a level of 10 mites per 100 adult bees in October corresponded to a probability of approximately 50% of winter mortality. This result should therefore not be read as a universal biological threshold, but as a risk relationship observed within a precise field context (Hernandez et al., 2022). Results from German monitoring point in the same direction: the risk of collapse increases strongly with autumn phoretic infestation, yet does not behave as an absolute threshold but rather as a risk gradient (Genersch et al., 2010).
This internal dynamic is compounded by a second phenomenon that is particularly important at the end of the season: re-infestation. The growth observed at the end of summer does not stem solely from reproduction within the colony. Varroa flows can arrive from outside, through drifting, robbing, or exchanges between neighbouring colonies. In Germany, Frey and Rosenkranz (2014) showed that late invasion could average between 126 ± 16 and 462 ± 74 mites per colony over approximately three and a half months, depending on the site. In Switzerland, Guichard et al. (2024) showed that 17 to 48% of the varroa mites present at the time of the final summer treatment could originate from inter-colony re-infestation since mid-spring. This means that at the end of the season, the increase in parasite load should not be attributed solely to internal reproduction: the colony also experiences pressure from the apicultural environment.
In winter, the logic changes again. When brood disappears, reproduction stops and the varroa population becomes almost entirely phoretic. This situation explains the value of treatments during the broodless period, but it also shows why values measured at this time are not comparable to those of summer. In the absence of brood, the parasite no longer hides in cells; however, the bee population is smaller, which can make high phoretic percentages appear without the total population necessarily being at its annual maximum (Rosenkranz et al., 2010; Dietemann et al., 2013).
The natural mite drop constitutes an important practical tool in this context, particularly in Switzerland. The Bee Health Service (Apiservice) recommends measuring it on a varroa floor insert protected by a mesh, sheltered from ants, for at least seven days, counting only dark adult varroa mites. The result is expressed in mites per day. This method is useful in practice, but it must be made clear that it is an intervention indicator, not a direct measure of the total population nor a biological damage threshold. In the current Swiss standard system, the proposed values are: more than 3 mites per day at end of May, more than 10 mites per day at end of June to beginning of July, more than 5 mites per day at end of October, and more than 10 mites per day for the rest of the season as a signal for immediate action (BGD/apiservice, Practical Guide 1.5.1, version V2401).
- more than 3 mites per day at the end of May,
- more than 10 mites at end of June to beginning of July,
- more than 5 mites at end of October,
- and more than 10 mites for the rest of the season as a signal for rapid action.
These values are useful for deciding when to intervene, but they must remain what they are: seasonal practical intervention thresholds, and not universal biological thresholds.
These values are biologically coherent, even if they may seem counterintuitive. The fact that the end-of-October intervention threshold (>5 mites/day) is lower than the early-summer threshold (>10 mites/day) does not mean that October is a less dangerous period. On the contrary, this stricter threshold reflects the fact that at that point the correction time before winter is very short, that re-infestation may have increased the parasite load, and that a high residual population can still jeopardise the colony's survival. Natural mite drop thresholds must therefore be understood as practical intervention thresholds, interpreted season by season, and not as simple equivalents of the total population nor as universal biological thresholds.
The seasonal development of Varroa destructor in a temperate climate can thus be summarised as follows: a growth phase in spring, a marked acceleration in summer due to the abundance of brood and the high reproductive output in drone brood, a biologically critical moment in late summer and autumn when the production of winter bees coincides with a high parasite load, then a winter phase during which the population becomes phoretic and ceases to reproduce. This general framework is robust. However, monitoring figures only acquire meaning when they are consistently related to their measurement level — total population, phoresis, infested brood, natural mite drop, or re-infestation — and to the season at which they were obtained (Rosenkranz et al., 2010; Dietemann et al., 2013; Hernandez et al., 2022).
3. Two possible trajectories over the course of the season
 |
Over the course of a season, two colonies may appear comparable in spring and then follow very different trajectories when it comes to Varroa destructor. |
This divergence does not depend on a single figure, but on a set of biological and practical processes: the parasite's rate of reproduction in the brood, the timing of treatment, the actual efficacy of that treatment, the residual pressure after intervention, and, at the end of the season, the extent of re-infestation from the apicultural environment (Rosenkranz et al., 2010; Frey & Rosenkranz, 2014; Guichard et al., 2024).
The diagrams proposed in this section should therefore be read as plausible didactic trajectories, not as universal curves or direct measurement series. The varroa curve here represents a total estimated population within the colony. It must not be confused with phoresis measured on adult bees, with the brood infestation rate, with natural mite drop expressed in mites per day, or with the share of re-infestation originating from other colonies. These indicators describe different realities and are only meaningful in relation to the season in which they are observed (Dietemann et al., 2013; Rosenkranz et al., 2010; Hernandez et al., 2022).
The general logic is nevertheless robust. In spring, the presence of drone brood in particular favours an acceleration of the parasite's dynamics, even when the apparent pressure may still seem moderate (Boot et al., 1995; Rosenkranz et al., 2010).
The divergence between trajectories appears mainly from summer onwards. In the first case, the colony is treated and the varroa population declines without disappearing entirely. Control is therefore partial: residual reproduction remains possible as long as brood is present, and a rebound can then occur under the combined effect of this persistent reproduction and re-infestation. In the second case, the absence of adequate control allows the parasite population to continue growing until the most sensitive moment of the season — that is, the period when the colony is producing its winter bees (Hernandez et al., 2022; Genersch et al., 2010).
It is in fact in late summer and autumn that interpreting the trajectories becomes most biologically important. A high parasite load at this point does not merely mean "more varroa"; it means above all that the bees destined to live for several months are produced in a compromised health environment. The risk does not operate like a binary switch with a single universal threshold, but rather like a gradient: the higher the parasite load and autumn infestation, the greater the probability of damage to winter bees and winter colony losses (Genersch et al., 2010; Hernandez et al., 2022).
3.1 Scenario A: treated colony with partial control
 |
In this first scenario, the colony is treated, but varroa pressure decreases without disappearing. |
This is a central point for reading the diagram: after a summer treatment, the total estimated varroa population may decline noticeably while still leaving a biologically plausible residual load. As long as brood remains, part of the parasite population escapes treatments that only reach the phoretic phase, and reproduction can therefore continue at a certain level. A decline after treatment therefore means neither eradication nor a return to a risk-free situation (Fig. 2).
|
Fig. 2 — Scenario A: treated colony with partial control |
In spring and early summer, the total varroa population increases with the expansion of brood. The presence of drone brood favours an early acceleration of this dynamic. The rise observed up to July simply reflects the fact that parasite reproduction accompanies the kinetics of the colony's biological maximum — a colony still strong in July is not in itself a sign of low parasite pressure.
The turning point comes after the summer treatment. The total estimated population declines, but a residual pressure persists. It may originate from several cumulative mechanisms: incomplete treatment efficacy, retention of part of the varroa population in the brood at the time of intervention, resumption of reproduction in the brood still present after treatment. The slight autumn rebound is also biologically plausible: it reflects this prolonged residual reproduction, to which re-infestation from the apicultural environment may be added. In the Swiss and central European context, this phenomenon is documented — a significant proportion of the varroa mites present at the time of the final summer treatment may derive from inter-colony re-infestation (Guichard et al., 2024; Frey & Rosenkranz, 2014).
In this scenario, the colony enters autumn with a lower probability of damage than a heavily infested colony — but it must not be considered out of danger. The decisive point remains the health quality of the winter bees: an excessively high residual pressure in late summer can still compromise the physiology of these long-lived bees.
3.2 Scenario B: colony without adequate control
 |
In this second scenario, the colony is described as one where varroa control is absent or insufficient to effectively slow the parasite's dynamics. |
The curve should therefore be read as a trajectory of increasing pressure. At the start of the season, this scenario can appear deceptively similar to scenario A — the total varroa population remains moderate in absolute terms, while the colony is rapidly expanding its brood and adult population (Fig. 3).
|
Fig. 3 — Scenario B: colony without adequate control |
The divergence becomes clear from late spring onwards. Without adequate control, varroa benefits from a large number of capped brood cells and, earlier in the season, from a particularly favourable reproductive output in drone brood. The parasite population accumulates month by month and reaches a level in summer that already constitutes a major biological risk factor. This is also the period when a large proportion of varroa mites is found in the brood: the actual colony load may therefore be significantly underestimated if only a single indicator is used.
The critical point falls in late summer and autumn, when this high pressure coincides with the production of winter bees. Heavy autumn infestation is associated with a substantial risk of winter mortality — not as a universal biological threshold, but as a risk relationship documented in precise Swiss and German contexts (Hernandez et al., 2022; Genersch et al., 2010). This internal dynamic is compounded by re-infestation: in a Swiss or central European context with densely spaced apiaries, an already inadequately controlled colony may see its situation further worsened by varroa re-infestation from the apicultural environment. The late-summer increase should therefore not be attributed mechanically to a single cause (Guichard et al., 2024; Frey & Rosenkranz, 2014).
The final points of the curve call for particularly cautious interpretation. If the total estimated population declines in November and December, this decline must under no circumstances be read as a spontaneous return to a healthy situation. On the contrary, it may correspond to an already severely compromised colony: declining brood, reduction in bee numbers, disruption of colony functioning, or even entry into a collapse trajectory. The descending end of the curve does not describe an improvement — it may reflect a biological system that has already lost part of its capacity for renewal. It is precisely for this reason that this trajectory should be read not as a normal development, but as a plausible worst-case scenario.
3.3 From imbalance to the worst-case scenario: towards possible collapse
 |
The transition of a still-functional colony to a collapse syndrome does not occur at a single tipping point. |
It builds as a progressive — often silent — deterioration. What makes this situation particularly difficult is that the deterioration can be well advanced before external signs become visible.
The decisive point is not only the total estimated number of varroa mites, but the biological moment at which this pressure is exerted. In summer, a large proportion of the parasite population is located in the capped brood and escapes the usual indicators. To this internal dynamic, re-infestation may be added: towards the end of the season, varroa flows from neighbouring colonies can increase pressure at the most critical moment, accelerating an already initiated imbalance.
This does not imply that every heavily infested colony will inevitably collapse. Between a still-viable colony and one on a collapse trajectory, there exists a zone of progressive deterioration, modulated by the colony's initial strength, the chronology of infestation, beekeeping management, and the intensity of re-infestation. But when parasite pressure remains high at the time of winter bee production, these moderating factors often prove insufficient to compensate for the accumulated damage. The transition from imbalance to collapse should therefore be presented not as an instantaneous inevitability, but as a possible, biologically coherent trajectory when several unfavourable factors combine.
Certain biological mechanisms — grooming behaviour, hygienic behaviour, brood break during swarming — can partially slow the parasite's progression and explain why not all colonies follow exactly the same trajectory (Mondet et al., 2020b; Nazzi & Le Conte, 2016). These partial resistance traits are real, but they do not constitute an alternative to adequate control: when parasite pressure becomes too high at the time of winter bee production, they are generally no longer sufficient to compensate for the accumulated damage (Rosenkranz et al., 2010; Genersch et al., 2010; Hernandez et al., 2022) (overview: Fig. 4).
|
Fig. 4 — Overview: comparative trajectories of the colony and Varroa destructor over the year |
4. Practical implications for apiary management
 |
The practical logic that follows from the preceding chapters is simple in principle, but demanding in execution: it is not enough to "treat against varroa". |
The entire season must be planned, the different indicators distinguished, and above all the period during which the future winter bees are being reared must not be missed. It is at that moment that a large part of the colony's survival until the following spring is determined.
4.1 Monitoring varroa pressure throughout the season
The first practical consequence is that an apiary should not be managed on the basis of a general impression of "strong colonies" or "weak colonies", but on the basis of regular monitoring of parasite pressure. In Switzerland, the Bee Health Service recommends measuring natural mite drop on a varroa floor insert protected by a mesh, sheltered from ants, for at least seven days, counting only dark adult mites. The result is expressed in mites per day. The reference values in the current standard system are as follows:
- more than 3 mites per day at the end of May,
- more than 10 mites at end of June to beginning of July,
- more than 5 mites at end of October,
- and more than 10 mites for the rest of the season as a signal for rapid action.
These values are useful for deciding when to intervene, but they must remain what they are: seasonal practical intervention thresholds, and not universal biological thresholds.
See also:
4.2 Slowing the spring dynamic early
The second practical consequence is that the varroa dynamic must be slowed before summer. Within the framework of the Swiss concept, this is achieved in particular through the drone frame. The Bee Health Service recommends introducing it as soon as colonies start developing and the wild cherry is in bloom, then removing or cutting out the capped drone brood two to three times. According to the apiservice practical guide, this measure can reduce the varroa load by up to 50% in spring. This figure should not be read as a universal promise, but as a practical order of magnitude that highlights one essential point: slowing the parasite's multiplication early improves the trajectory of the season.
See also:
4.3 After the solstice: protecting future winter bees without delay
The summer solstice can serve as a practical reference point. It does not in itself constitute a biological switch, but it serves as a reminder that from late June onwards the colony progressively enters a different phase: growth no longer follows the same dynamic, the role of pollen becomes even more decisive, and the priority is no longer merely sustaining the current season but already protecting the cohort of bees that will need to survive until the following spring. This is why the first summer intervention must not be delayed. In the current Swiss concept, the first summer treatment begins in July: during the first half of July with brood-break-based methods not involving formic acid, or, with formic acid, before the end of July. The earlier this pressure reduction is achieved, the better the chances that future winter bees will be reared under acceptable health conditions.
See also:
4.4 Feeding without losing the late-summer health logic
After the summer honey harvest, feeding must be planned together with varroa control, not separately. The Swiss practical guide on feeding notes that a production colony requires approximately 16–20 kg of winter food stores, to be built up after harvest in the form of suitable liquid feed. In practice, a gradual build-up of reserves is often more appropriate than excessively rapid filling: the aim is to help the colony reconstitute its stores without interrupting the residual dynamics of the brood nest too early. The central idea remains that feeding and treating must serve the same objective: bringing the colony into autumn with adequate reserves, but also with already reduced parasite pressure.
This phase also requires explicitly factoring in re-infestation. A late varroa increase does not necessarily stem from internal reproduction alone. In the study by Frey and Rosenkranz, cumulative invasion over 3.5 months ranged from 266 to 1,171 varroa mites per colony at a site with a high density of neighbouring colonies, compared with 72 to 248 at a low-density site; in untreated colonies, the mean final infestation in November reached 2,082 mites at a high-density site versus 340 at a low-density site. More recently, Guichard et al. showed that these inter-colony flows can also bias the assessment of individual colony infestation. In practice, this means that a correctly managed apiary is not necessarily "safe" in late summer: what is actually happening must still be verified.
See also:
4.5 Securing autumn and winter
The second late-summer treatment is not an optional supplement: it forms part of the protection of winter bees. In the current Swiss concept, the second summer treatment begins no later than mid-September. The aim is not simply to bring a number down, but to further reduce the parasite load at a time when the correction window before winter becomes short. This logic then continues with the winter oxalic acid treatment when the colony is broodless, in November or December. Post-treatment monitoring remains essential: if more than 500 mites fall in the two weeks following the winter treatment, the Bee Health Service recommends repeating it.
See also:
5. Conclusion
 |
The seasonal dynamics of a bee colony and those of Varroa destructor are inseparable. |
Understanding one without the other inevitably leads to misinterpretation — in particular to underestimating the actual parasite pressure in summer, or to misreading the significance of a monitoring value depending on the season in which it was obtained.
Two points deserve to be retained as guiding principles for beekeeping practice. The first is that field indicators — phoresis, natural mite drop, brood infestation — do not measure the same reality and can only be interpreted in relation to the time of season. The second is that the critical window is not summer, but late summer and autumn: it is at that point that the quality of winter bees is determined, and it is there that the consequences of inadequate control become irreversible before overwintering.
These partial resistance mechanisms are real, but structurally insufficient in the vast majority of European beekeeping contexts. Control of Varroa destructor therefore remains a biological necessity, not a management option. It must be conducted in coherence with the seasonal biology of the parasite and the colony, and not on the basis of decontextualised fixed thresholds.
Overall, the practical approach that emerges from this article is not that of a rigid calendar, but that of a coherent sequence of actions: slow the spring dynamic early, monitor pressure with correctly interpreted indicators, do not miss the first pressure reduction before the end of July, account for re-infestation in late summer, then secure the autumn and the onset of winter. It is this seasonal coherence — more than an isolated figure or a treatment considered in isolation — that truly protects the colony.
See also:
- The recommended varroa treatment regimen improves colony survival
- Integrated varroa control through the seasons
- Winter survival of honey bee colonies
- Vitellogenin and the keys to the colony
- Practical Guide: 1.5.1 Measuring the natural varroa mite drop
- Preserving the bees' life capital
Bibliography
Amdam, G. V., & Omholt, S. W. (2002). The regulatory anatomy of honeybee lifespan. Journal of Theoretical Biology, 216(2), 209–228. https://doi.org/10.1006/jtbi.2002.2545
BGD/apiservice. (n.d.). 1.5.1 Natürlichen Milbenfall messen (Version V2401). Bienengesundheitsdienst/apiservice. https://bienen.ch/wp-content/uploads/2023/04/1.5.1_natuerlichen_milbenfall_messen.pdf
Bogaert, J.-B., Coppée, A., Misson, C., Defrance, T., & Verheggen, F. (2020). The honey bee (Apis mellifera L., 1758) and the seasonal adaptation of productions. Livestock Science, 235, 104023. https://doi.org/10.1016/j.livsci.2020.104023
Boot, W. J., Schoenmaker, J., Calis, J. N. M., & Beetsma, J. (1995). Invasion of Varroa jacobsoni into drone brood cells of the honey bee, Apis mellifera. Apidologie, 26(2), 109–118. https://doi.org/10.1051/apido:19950204
Czekońska, K., Chuda-Mickiewicz, B., & Samborski, J. (2015). Quality of honeybee drones reared in colonies with limited and unlimited access to pollen. Apidologie, 46(1), 1–9. https://doi.org/10.1007/s13592-014-0296-z
Dietemann, V., Nazzi, F., Martin, S. J., Anderson, D. L., Locke, B., Delaplane, K. S., Wauquiez, Q., Tannahill, C., Frey, E., Ziegelmann, B., Rosenkranz, P., & Ellis, J. D. (2013). Standard methods for varroa research. Journal of Apicultural Research, 52(1), 1–54. https://doi.org/10.3896/IBRA.1.52.1.09
Frey, E., & Rosenkranz, P. (2014). Autumn invasion rates of Varroa destructor (Mesostigmata: Varroidae) into honey bee (Hymenoptera: Apidae) colonies and the resulting increase in mite populations. Journal of Economic Entomology, 107(2), 508–515. https://doi.org/10.1603/EC13381
Genersch, E., von der Ohe, W., Kaatz, H., Schroeder, A., Otten, C., Büchler, R., Berg, S., Ritter, W., Mühlen, W., Gisder, S., Meixner, M., Liebig, G., & Rosenkranz, P. (2010). The German bee monitoring project: A long term study to understand periodically high winter losses of honey bee colonies. Apidologie, 41(3), 332–352. https://doi.org/10.1051/apido/2010014
Guichard, M., von Virag, A., Droz, B., & Dainat, B. (2024). Do Varroa destructor (Acari: Varroidae) mite flows between Apis mellifera (Hymenoptera: Apidae) colonies bias colony infestation evaluation for resistance selection? Journal of Insect Science, 24(4), 3. https://doi.org/10.1093/jisesa/ieae068
Hernandez, J., Hattendorf, J., Aebi, A., & Dietemann, V. (2022). Compliance with recommended Varroa destructor treatment regimens improves the survival of honey bee colonies over winter. Research in Veterinary Science, 144, 1–10. https://doi.org/10.1016/j.rvsc.2021.12.025
Imdorf, A., Bühlmann, G., Gerig, L., Kilchenmann, V., & Wille, H. (1987). Examination of the method for estimating the brood area and number of worker bees in free-flying bee colonies. Apidologie, 18(2), 137–146. https://doi.org/10.1051/apido:19870204
Imdorf, A., Ruoff, K., & Fluri, P. (2010). Le développement des colonies chez l'abeille mellifère (ALP forum, 68). Agroscope Liebefeld-Posieux.
Keller, I., Fluri, P., & Imdorf, A. (2005). Pollen nutrition and colony development in honey bees: Part I. Bee World, 86(1), 3–10. https://doi.org/10.1080/0005772X.2005.11099641
Mattila, H. R., Harris, J. L., & Otis, G. W. (2001). Timing of production of winter bees in honey bee (Apis mellifera) colonies. Insectes Sociaux, 48(1), 88–93. https://doi.org/10.1007/PL00001764
Mattila, H. R., & Otis, G. W. (2007). Dwindling pollen resources trigger the transition to broodless populations of long-lived honey bees each autumn. Ecological Entomology, 32(5), 496–505. https://doi.org/10.1111/j.1365-2311.2007.00904.x
Mondet, F., Rau, A., Klopp, C., Working Group Behavioural Genetics of Apis mellifera, Vautrin, S., Le Conte, Y., & Alaux, C. (2020). Transcriptome profiling of natural Varroa resistant honeybees shows differential expression of immune, neural, and chemical signaling genes. Scientific Reports, 10, 7279. https://doi.org/10.1038/s41598-020-64144-6
Mondet, F., Beaurepaire, A., McAfee, A., Locke, B., Alaux, C., Blanchard, S., Danka, B., & Le Conte, Y. (2020b). Honey bee survival mechanisms against the parasite Varroa destructor: A systematic review of phenotypic and genomic research efforts. International Journal for Parasitology, 50(6–7), 433–447. https://doi.org/10.1016/j.ijpara.2020.03.005
Nazzi, F., & Le Conte, Y. (2016). Ecology of Varroa destructor, the major ectoparasite of the western honey bee, Apis mellifera. Annual Review of Entomology, 61, 417–432. https://doi.org/10.1146/annurev-ento-010715-023731
Nürnberger, F., Härtel, S., & Steffan-Dewenter, I. (2018). The influence of temperature and photoperiod on the timing of brood onset in hibernating honey bee colonies. PeerJ, 6, e4801. https://doi.org/10.7717/peerj.4801
Rosenkranz, P., Aumeier, P., & Ziegelmann, B. (2010). Biology and control of Varroa destructor. Journal of Invertebrate Pathology, 103(Suppl. 1), S96–S119. https://doi.org/10.1016/j.jip.2009.07.016
Stabentheiner, A., Kovac, H., & Brodschneider, R. (2010). Honeybee colony thermoregulation — Regulatory mechanisms and contribution of individuals in dependence on age, location and thermal stress. PLoS ONE, 5(1), e8967. https://doi.org/10.1371/journal.pone.0008967
Winston, M. L. (1987). The biology of the honey bee. Harvard University Press.