0. Abstract

Beeswax has traditionally been regarded as a simple structural material for the comb. Recent scientific data indicate, however, that it constitutes an active biological compartment capable of influencing colony health through structural, toxicological, and epidemiological mechanisms. This synthesis analyses the effects of comb ageing, the accumulation of contaminants, the persistence of pathogens, and the implications of the wax cycle.

Morphometric studies demonstrate that comb age leads to a progressive reduction in cell volume and in the weight of emerging workers, with documented correlations for brood area, longevity, and colony performance. Simultaneously, beeswax acts as a cumulative reservoir for lipophilic substances (acaricides, pesticides) and heavy metals. Sublethal effects on worker longevity and on the sperm viability of drones have been documented under developmental exposure conditions. Combs may also constitute a persistence compartment for highly resistant bacterial spores such as Paenibacillus larvae if sterilisation procedures do not comply with validated parameters. Data on viral presence in wax indicate measurable contamination, although the relative epidemiological significance of this transmission route remains to be quantified.

Wax recycling, a central practice in beekeeping, thus emerges as a strategic risk-management node. Open systems promote the inter-apiary redistribution of residues and potentially of pathogens, whereas closed systems limit external introduction but do not eliminate internal accumulation. The available data support the value of periodic comb renewal in order to limit the cumulative build-up of structural, chemical, and biological stressors.

In conclusion, management of the wax cycle constitutes a major strategic lever for colony health and for the quality of bee products. The realistic objective is not the total elimination of contaminants, but the progressive minimisation of risks through controlled comb rotation, wax-cycle traceability, and a clear distinction between biological sterilisation and chemical decontamination.

1. Foundations and current state of the scientific debate

Wax management in beekeeping has long been approached primarily from a technical and economic perspective. Combs were regarded mainly as support structures for the brood nest and for storing reserves, their renewal being a matter of empirical practice rather than structured biological reasoning.

Scientific work over the past two decades has profoundly changed this perception. Combs now appear as a dynamic compartment in which structural, chemical, microbiological, and epidemiological factors interact. Comb ageing, the progressive accumulation of contaminants, the possible persistence of pathogens, and the physiological responses they induce in larvae constitute interdependent dimensions.

The objective of this chapter is to examine these different aspects in the light of available experimental and observational data, in order to clarify the biological role of wax in colony health. The aim is not to identify a single factor, but to understand a cumulative system in which wax acts as an exposure matrix and as a medium for the progressive transformation of the brood microenvironment.

1.1 Wax as a biological compartment

The current scientific consensus converges on a clear conclusion: beeswax is not a simple, inert structural material, but an active biological compartment endowed with physicochemical and ecological properties that directly influence colony health.

From a physicochemical standpoint, wax is a highly lipophilic matrix composed primarily of fatty acid esters and long-chain alcohols. This characteristic confers on it a high capacity for the adsorption and accumulation of lipid-soluble substances present in the environment or introduced into the hive, notably synthetic acaricides (Calatayud-Vernich et al., 2018; Marti et al., 2022). Unlike honey, which is regularly renewed, wax persists for several years in the brood nest and acts as a cumulative reservoir.

However, reducing wax to a mere "chemical store" would be incomplete. Combs evolve structurally over successive brood cycles. Meng et al. (2025), in a recent systematic review, describe this process as a progressive transformation of the original material into a composite matrix associating wax, successive layers of larval cocoons, organic debris, and incorporated residues. This dynamic produces measurable morphological modifications:

• progressive darkening linked to the adsorption of pigments and larval faecal matter,
• significant increase in cell wall thickness,
• increase in the areal weight of the comb,
• reduction in cell internal volume,
• modification of cell geometry and of the macro-architecture of the comb.

Quantitative data illustrate the scale of the phenomenon: an increase in areal weight from 0.26 g/cm² to 1.32 g/cm² after seven years of use, an increase in wall thickness from 88 µm to nearly 300 µm in two years, and a reduction in cell volume from 0.31 ml to 0.18 ml over six years (Meng et al., 2025).

These modifications are not merely aesthetic. They alter the microclimate of the brood nest, gas diffusion, local thermal capacity, and potentially the distribution of chemical signals within the colony. They also influence the contact area between the larva and its immediate environment, which may modulate exposure to accumulated residues.

Earlier work confirms that comb ageing is accompanied by a progressive decrease in cell internal diameter (Berry & Delaplane, 2001; Al-Kahtani, 2021). The consistency of these results across different geographical and experimental contexts reinforces the robustness of this observation.

Beyond the structural aspect, wax also constitutes a biological microhabitat. Analysis of the specific microbiomes associated with the different compartments of the hive shows that combs exhibit a profile distinct from that of honey, pollen, or propolis (Grubbs et al., 2015). Samples cluster primarily by compartment type rather than by colony or site, suggesting the existence of an ecological signature specific to combs. Although these analyses, based on lipid biomarkers, do not allow precise identification of the microbial species involved, they demonstrate that wax participates in the internal microbial ecology of the colony.

The epidemiological dimension further reinforces this interpretation. International health manuals explicitly recognise that spores of Paenibacillus larvae (the causative agent of American foulbrood) can survive for years in hive products including wax, and that the manufacture of foundation from contaminated wax can contribute to the spread of the disease if sterilisation procedures are inadequate (WOAH, 2023a). Even in the absence of sporulation, European foulbrood can persist via mechanical contamination of combs (WOAH, 2023b). The comb is thus not only a passive substrate: it can become a pathogen persistence compartment.

It is also demonstrated that fungal spores, notably Ascosphaera apis (the causative agent of chalkbrood), present in contaminated foundation, can infect the brood reared on those foundations (Flores et al., 2005). This experimental observation confirms that wax can constitute a transmission vector when infectious agents are incorporated within it.

Regarding viruses, several studies have detected the presence of viral RNA in wax (De Guzman et al., 2019; Colwell et al., 2024). Although RNA detection does not necessarily demonstrate the presence of active, infectious particles, this work suggests that combs may contain viral material and potentially contribute to the ecology of infections, even if the relative importance of this route compared with transmission via Varroa destructor or nurse bees remains to be quantified.

Finally, wax plays a role in parasite dynamics. Experimental comparisons have shown that old combs may exhibit significantly higher infestation rates by Varroa destructor than new combs, independently of simple cell width (Piccirillo & De Jong, 2004). This suggests that chemical or structural factors associated with comb age could influence the attractiveness of the comb or the reproduction of the parasite.

Furthermore, reviews on the biology of Galleria mellonella indicate that the greater wax moth develops preferentially in old, dark combs rich in larval cocoons and organic debris (Kwadha et al., 2017). Old wax not only provides a structurally more favourable matrix for gallery excavation, but also offers an increased nutritive value linked to the accumulation of proteins and debris from brood cycles (Kwadha et al., 2017). Although strong colonies can generally limit larval development through their cleaning and defensive activity, stored material — notably old brood box frames — remains particularly vulnerable to mass infestations (Charrière & Imdorf, 1999; Ellis et al., 2013). Regular renewal of old frames thus contributes not only to reducing the chemical and microbiological load, but also to reducing the pressure exerted by the greater wax moth.

Taken together, these elements lead to viewing wax as an integrated biological matrix in which structure, chemistry, and microbiology interact. It does not constitute a simple material substrate for the brood nest, but an evolving environment that durably influences the physiological and sanitary balances of the colony.

This conceptual evolution transforms the beekeeping perspective: comb management becomes a strategic biological parameter, rather than merely a technical or economic decision.

1.2 Structural ageing of combs and impacts on workers

The ageing of combs is not limited to a visual change or to a simple accumulation of residues. It corresponds to a progressive transformation of the microenvironment in which the brood develops, with measurable consequences for the morphology and performance of adult bees.

Morphometric data from controlled experimental series show that the reduction in cell volume associated with comb age results in measurable differences in emerging workers. Meng et al. (2025) report that bees reared in old combs have a significantly lower emergence weight than those from recent combs (e.g. 114.9 mg versus 88.0 mg depending on the series compared). This decrease in body weight is accompanied by morphological changes such as a shortening of the tongue (proboscis), a reduction in thoracic and abdominal dimensions, and a reduction in tarsal size and in certain wing parameters.

These results are consistent with earlier work demonstrating a negative correlation between comb age and the weight of newly emerged workers (Berry & Delaplane, 2001; Al-Kahtani, 2021). In Berry and Delaplane's (2001) study, colonies placed on new combs produced heavier bees and larger brood areas than those placed on old combs. The study nevertheless highlights an important complexity: at certain sampling times, short-term brood survival could be slightly higher in old combs, suggesting that the effects are neither uniform nor strictly linear.

The decrease in weight and body dimensions does not constitute a simple, isolated morphological parameter. It may influence individual physiology and the functional capacities of workers. Meng et al. (2025) report associations between comb ageing and:

• a reduction in the frequency of return with pollen,
• a decrease in total brood area,
• a reduction in worker longevity,
• a decline in honey and pollen accumulation at colony level.

These correlations suggest that modification of the larval microenvironment may feed back on collective performance.

It must be stressed, however, that precise causal attribution remains complex. Several mechanisms may coexist:

1. Geometric constraints: reduction in cell diameter and depth potentially limits the larval development space (Al-Kahtani, 2021).
2. Accumulation of chemical residues: the increasing presence of lipophilic substances in the wax-cocoon matrix may alter the chronic exposure of larvae (Wu et al., 2011).
3. Microclimate modification: thickening of the walls and organic densification may influence thermal properties and gas circulation.
4. Behavioural factors: the queen's oviposition preferences and the workers' maintenance behaviours may be affected by the condition of the combs.

The experimental study by Wu et al. (2011) provides an important element by showing that workers reared in combs heavily contaminated with pesticide residues exhibit an approximately four-day reduction in average adult longevity, together with impaired larval development. Although this study concerns chemical contamination specifically, it underlines that the comb can act as a vector of chronic exposure throughout the entire larval period.

It is also notable that the ageing comb structure may interact with parasite dynamics. Piccirillo and De Jong (2004) observed that old combs presented significantly higher infestation rates by Varroa destructor than new combs, independently of simple cell width. This observation suggests that comb age may influence chemical or structural signals affecting the attractiveness or reproduction of the parasite, which could indirectly modulate viral pressure within the colony.

Comb ageing must thus be understood as a multifactorial process involving:

• measurable geometric modifications,
• a progressive accumulation of organic and inorganic compounds,
• a transformation of the larval microenvironment,
• potential effects on individual physiology and colony dynamics.

It would be scientifically imprecise to assert that "old combs are always harmful". The available data show rather a set of interconnected effects whose magnitude depends on context (subspecies, environmental conditions, parasite pressure, treatment history). Nevertheless, the convergence of morphometric and functional results reinforces the idea that periodic comb renewal constitutes a coherent biological lever for limiting the accumulation of structural and chemical stressors.

In summary, an old comb is not simply a worn structure: it progressively becomes a transformed microenvironment whose physical, chemical, and biological properties can influence the quality of the bees produced and, by extension, the performance of the colony.

1.3 Environmental contaminants, residues, and accumulation in wax

Beeswax constitutes one of the principal accumulation compartments for contaminants within the colony. Its lipophilic nature favours the adsorption and retention of many hydrophobic substances, notably the acaricides used against Varroa destructor, as well as various pesticides of environmental origin (Calatayud-Vernich et al., 2018; Marti et al., 2022). Unlike honey, which is harvested and renewed regularly, wax in the brood nest can remain in place for several years, making it a long-term cumulative reservoir.

Monitoring studies confirm the high frequency of residues in commercial wax. In an analysis of Swiss foundation collected from nine manufacturers (98 samples), Marti et al. (2022) detected between 7 and 14 active substances per sample, including acaricides commonly used in beekeeping such as coumaphos and tau-fluvalinate. Similarly, analyses in Germany revealed up to 16–20 substances simultaneously present in commercial foundation, with a predominance of persistent lipophilic acaricides (Alkassab et al., 2020). These results illustrate that wax in the commercial circuit is frequently exposed to complex residue mixtures.

It is important to distinguish two analytical dimensions:
(1) the presence of residues, which is empirically well documented;
(2) the biological impact of these residues, which depends on concentration, combination of substances, route of exposure, and developmental stage.

Regarding biological effects, Wu et al. (2011) demonstrated experimentally that workers reared in brood combs with high pesticide residue loads showed a significant reduction in adult longevity (approximately four days on average) as well as impaired larval development. Of particular importance, the authors observed a transfer of residues to initially clean combs introduced subsequently, confirming that wax can act not only as a direct source of exposure, but also as a vector for internal redistribution.

However, the presence of residues does not necessarily imply immediate lethal effects. A dose-response study conducted under real in-hive conditions showed that coumaphos concentrations up to 62 mg/kg in foundation did not significantly increase brood mortality compared with controls, while a higher concentration (132 mg/kg) strongly reduced the emergence rate (Kast et al., 2023). This study provides an essential quantitative anchor: not all concentrations detected in wax automatically produce lethal effects. Sublethal effects and interactions between substances nevertheless constitute a major area of uncertainty.

The issue of mixtures ("cocktails" of residues) is central. Multiple exposures are the rule rather than the exception in commercial circuits (Marti et al., 2022). Work on male reproduction shows that developmental exposure to acaricide mixtures can significantly reduce sperm viability in drones (Fisher & Rangel, 2018). Although these results do not formally demonstrate synergy in the strict toxicological sense, they indicate that combinations of substances can affect sensitive reproductive parameters.

The effects on queens are more nuanced. McAfee et al. (2021) showed that topical adult exposure to pesticides commonly found in wax, even at concentrations above typical levels, did not impair body mass or sperm viability in queens in their experimental protocol. By contrast, Rangel and Tarpy (2015) demonstrated that queens reared in acaricide-contaminated wax cells showed a significant decrease in stored spermatozoa and their viability. These results underline a fundamental distinction between adult contact exposure and developmental exposure during the larval phase, the latter potentially implying different physiological mechanisms.

Beyond organic pesticides, the accumulation of heavy metals constitutes a complementary dimension. Meng et al. (2025) observed a significant increase in concentrations of Cd, Cr, Ni, Pb, and Mn in old combs compared with recent combs. Metals accumulate particularly in the successive cocoon layers. Larvae reared in these environments exhibit increased activation of genes involved in detoxification, notably those of the CYP450 family. This molecular response indicates a physiologically relevant exposure, even if it does not automatically imply a measurable clinical effect.

Wax must therefore not be viewed as a simple passive substrate, but as a compartment of chronic exposure. It progressively concentrates substances arising from:

• veterinary treatments applied within the colony,
• environmental pesticides brought in by foragers,
• atmospheric or agricultural inorganic contaminants,
• mixtures resulting from industrial recycling.

A crucial point, often misunderstood, concerns the distinction between sterilisation and decontamination. Thermal treatments effective for inactivating bacterial spores, notably in the context of American foulbrood (≥121 °C for 30 minutes at 2 bar pressure, according to Swiss official recommendations), do not suppress persistent lipophilic residues (Kast et al., 2023; Swiss Federal Food Safety and Veterinary Office, 2016). Heat destroys biological pathogens but does not "reset" the chemical composition of the native wax produced by wax-secreting bees.

In summary, the available data permit the following assertions with a high level of confidence:

• wax is frequently contaminated by residue mixtures;
• certain developmental exposures can affect longevity, reproduction, or morphological parameters;
• effects depend on dose, mixture, and biological context;
• wax acts as a long-term cumulative reservoir;
• routine thermal procedures do not eliminate chemical contaminants.

The question is therefore not only that of residue presence, but of their management over time. The accumulation dynamic progressively transforms wax into a compartment of chronic exposure, conferring strategic importance on comb renewal and wax cycle management within an integrated colony health approach.

1.4 Molecular adaptation and physiological response: adaptation does not imply absence of risk

Chronic exposure to the contaminants present in old combs is not limited to morphological or reproductive effects observable at the individual or colony level. It also induces measurable molecular responses that reflect activation of detoxification and stress-regulation systems.

Meng et al. (2025) report that larvae reared in old combs exhibit a significant increase in the expression of genes involved in detoxification, notably those of the cytochrome P450 family (CYP450), glutathione S-transferases (GST), and carboxylesterases. These enzymes play a central role in the biotransformation of xenobiotic compounds, including lipophilic acaricides and certain environmental pesticides.

Activation of these systems is regulated by several molecular signalling pathways, notably the AhR/ARNT, MAPK, and CncC/Keap1 axes, known to be involved in the response to oxidative stress and persistent organic contaminants. Induction of these pathways constitutes a biological indicator of exposure, even in the absence of clinically visible short-term effects.

The metabolic mobilisation it implies may generate indirect physiological costs, liable to affect individual or collective performance in the medium term. Molecular adaptation thus reflects a genuine biological interaction with the wax environment, but does not guarantee the absence of delayed effects. Accordingly, the observation of activation of detoxification pathways should be interpreted as a biological signal of exposure, and not as evidence of harmlessness. It indicates that the wax compartment acts as a source of measurable environmental constraint at the cellular level.

In summary, the molecular data reinforce the view that old and/or contaminated combs constitute a biologically active environment. The adaptive response of larvae demonstrates genuine physiological interaction with the contaminants present. However, adaptation does not mean complete neutralisation of risks. Constant mobilisation of detoxification mechanisms may generate subtle biological costs, liable to affect individual performance and colony dynamics in the long term.

1.5 Quality of honey and wax: implications of comb ageing

The effects of comb ageing are not confined to bee biology and colony health dynamics. They also extend to the quality of hive products, in particular honey and wax itself. This dimension is often underestimated in beekeeping practice, yet it is of relevance from economic, sensory, and toxicological perspectives.

Meng et al. (2025) report that honey stored in old combs shows measurable changes in certain physicochemical parameters. Among the differences observed are a significant increase in the hydroxymethylfurfural (HMF) content of honey from old combs (28.8 mg/kg versus 18.4 mg/kg in recent combs), as well as changes in the carbohydrate profile. HMF is a classical indicator of thermal degradation or ageing of honey; its increase in old combs suggests that the storage microenvironment could influence the chemical stability of the product.

It is important to note that these values remain dependent on environmental context, harvesting practices, and storage conditions. Nevertheless, the correlation observed between comb age and certain qualitative parameters of honey indicates that the wax matrix does not constitute a simple neutral container, but an environment capable of interacting with the stored product.

The same work also reports higher concentrations of heavy metals in honey from old combs (Meng et al., 2025). Although the exact transfer mechanisms have not been fully elucidated, prolonged physical proximity between honey and a wax-cocoon matrix enriched with contaminants could contribute to these differences. These observations should be interpreted with caution, but they underline that comb management may have indirect implications for food quality.

As regards wax itself, ageing leads to a progressive modification of its chemical composition. Meng et al. (2025) describe a relative increase in shorter-chain components and an alteration of the overall lipid profile in old combs. These changes may influence the mechanical properties, thermal stability, and plasticity of the material. They may also modify the interaction between the wax and the lipophilic substances adsorbed within it.

The issue of commercial wax adulteration constitutes a distinct but closely related element of product quality. Alkassab et al. (2020) demonstrated experimentally that the addition of stearin to beeswax could cause significant brood disturbance. At approximately 20% adulteration, colonies exhibited delayed acceptance of foundation, repeated larval removal behaviour, and a marked reduction in capped brood. These effects differ qualitatively from the impacts observed for acaricide residues at realistic concentrations, which manifest more as chronic and sublethal effects.

This distinction is essential:
– adulteration represents an acute problem linked to material quality;
– residue accumulation corresponds rather to a progressive phenomenon of chronic loading.

In open recycling circuits, these two dimensions can combine: mixing of waxes from multiple operations, presence of persistent residues, and risk of introduction of adulterated waxes. Monitoring analyses (Marti et al., 2022) confirm that commercial foundation may contain complex residue profiles. Traceability and transparency of circuits thus become determining factors in final quality.

Another often neglected aspect concerns sensory perception. Meng et al. (2025) mention organoleptic deterioration of honey stored in old combs. Although these observations require further validation and may depend on floral origin, they reinforce the idea that the quality of the final product depends not only on floral resources and extraction techniques, but also on the structural condition of the storage support.

In summary, the available data suggest that:

• comb age can influence certain physicochemical parameters of honey;
• contaminants accumulated in wax may potentially interact with the stored product;
• the chemical composition of wax evolves over time;
• wax adulteration represents a risk distinct from residues but equally relevant;
• management of the wax cycle has implications that are not only sanitary but also qualitative.

These elements indicate that the structural condition of the comb can influence certain physicochemical parameters of honey. Wax must therefore not be regarded solely as a passive container, but as an environment capable of interacting with the stored product.

Management of comb renewal thus acquires a wider scope: it concerns not only colony biology, but also the quality of bee products.

2. The optimal wax cycle

The preceding analyses have shown that wax evolves over time under the combined influence of structural modifications, chemical accumulations, and possible biological persistence. The central question therefore becomes that of managing this dynamic. The wax cycle does not represent a simple logistical flow, but an organisational system that can either limit or amplify these processes. Recycling, batch mixing, traceability, and sterilisation parameters directly influence the accumulation trajectory. This chapter examines the different possible circuit configurations — open, closed, or intermediate — in order to evaluate their sanitary and toxicological implications in the light of the available scientific data.

2.1 The wax recycling issue

Wax recycling has historically been a pillar of beekeeping. In the light of current knowledge, however, it must be analysed as a mechanism for the redistribution of accumulated loads. From an economic and ecological standpoint, recovering old combs and transforming them into new foundation appears as a rational and sustainable practice. In the light of current knowledge, recycling must be analysed as a redistribution mechanism. The substances adsorbed in old combs do not disappear during melting; they are incorporated into the new wax matrix and may be homogeneously distributed throughout the processed batch.

The first dimension of the problem concerns the cumulative accumulation of lipophilic residues. As demonstrated above, wax acts as an adsorption compartment for many acaricides and environmental pesticides (Calatayud-Vernich et al., 2018; Marti et al., 2022). When old combs are melted and then reintroduced into the circuit as new foundation, the accumulated substances do not disappear. On the contrary, they may be redistributed homogeneously throughout the recycled batch.

Meng et al. (2025) explicitly point out that recycling old combs results in the reintroduction of heavy metals, persistent acaricides, and organic pollutants into the new wax matrix. This dynamic creates a phenomenon of "chemical memory" in the wax cycle: even if treatment practices evolve, historical residues may persist over several recycling cycles.

Analyses of commercial wax corroborate this reality. Marti et al. (2022) showed that commercial foundation regularly contained several acaricide residues, sometimes at non-negligible concentrations. Alkassab et al. (2020) also highlighted the simultaneous presence of numerous residues in German commercial wax samples. These observations indicate that the mixing of waxes from multiple operations mechanically amplifies the diversity of residues present in the final product.

This issue is accentuated in so-called "open" systems, in which waxes from different beekeepers are collected, melted together, and redistributed without strictly traceable batch separation. In such a system, contamination from a single source can theoretically be diluted, but can also be disseminated on a large scale.

The second dimension concerns the persistence of pathogens. International sanitary standards recognise that spores of Paenibacillus larvae can survive for long periods in wax (WOAH, 2023a). In the absence of adequate sterilisation (≥121 °C for 30 minutes under pressure, according to Swiss recommendations), standard thermal processing of wax does not guarantee complete inactivation of spores (Swiss Federal Food Safety and Veterinary Office, 2016). Recycling without validated thermal control can therefore constitute a dissemination vector.

It is essential to distinguish two levels here:

• simple melting of the wax (removal of solid impurities),
• validated sterilisation (inactivation of resistant spores).

Confusion between these two processes remains common in practice (see section 3.5).

The recycling issue is not limited to residues and spores. Adulteration constitutes a third risk factor. Alkassab et al. (2020) demonstrated that the addition of stearin to wax could cause acute brood disturbances, including delayed acceptance of foundation and significant larval losses. In open circuits under high economic pressure, the temptation to adulterate wax to reduce costs represents an additional risk.

The analysis should, however, be nuanced. Recycling as such is not inherently problematic. In a closed system with strict traceability, batch separation, and validated sanitary control, wax recirculation can be managed. The problem arises when:

• the origin of the waxes is heterogeneous,
• traceability is absent,
• thermal treatments are not validated,
• no residue analyses are performed.

Meng et al. (2025) emphasise that intensive recycling without sufficient renewal promotes the progressive increase in contaminant load. It is a cumulative phenomenon rather than a one-off event.

Finally, it should be noted that recycling can also influence the physical structure of new foundation if the source wax is severely degraded. Although melting homogenises the material, certain mechanical and compositional properties may evolve over time, modifying plasticity or thermal stability.

In summary, wax recycling represents a strategic node in colony sanitary management. It can be:

• a tool for managed circular valorisation,
• or a vector for the cumulative recirculation of contaminants.

The question is therefore not "to recycle or not to recycle", but "how to recycle". The answer depends on the structure of the wax cycle, the level of sanitary control, traceability, and the sterilisation procedures implemented.

2.2 Open and closed systems: risk architectures of the wax cycle

The recycling question cannot be dissociated from the organisational structure of the wax cycle. The distinction between open and closed systems is not a mere logistical preference. It corresponds to two fundamentally different risk architectures in terms of chemical and biological contaminants.

An open system is characterised by the collection and melting of waxes from multiple operations, followed by redistribution in the form of standardised foundation. This model is historically widespread in Europe and economically efficient. It allows volume pooling and access to industrial processing equipment, including autoclaves capable of reaching the temperatures required for inactivation of Paenibacillus larvae spores (Swiss Federal Food Safety and Veterinary Office, 2016).

However, from an epidemiological and toxicological standpoint, the open system introduces a phenomenon of inter-colony mixing. Each final batch becomes the resultant of a heterogeneous set of sanitary and chemical histories. If even a single initial input contains a high load of residues or non-inactivated spores, this load can be redistributed on a large scale.

Analytical data confirm that commercial foundation frequently contains complex residue mixtures (Marti et al., 2022; Alkassab et al., 2020). This finding does not necessarily mean that concentrations reach toxic thresholds, but it demonstrates that open systems structurally tend to homogenise and redistribute the loads present.

From a microbiological standpoint, international standards explicitly recognise that contaminated wax can constitute a dissemination vector for American foulbrood if it is not correctly sterilised (WOAH, 2023a). Simple thermal processing (melting) does not guarantee inactivation of spores, which requires validated parameters (≥121 °C for 30 minutes under pressure, in Swiss recommendations). Actual efficacy depends on mastery of industrial processes, equipment calibration, and documentary control.

A closed system, by contrast, rests on the strict separation of wax flows at the level of the operation. Cappings wax and removed combs are melted and reused exclusively within the same apiary or a group of operations sharing a controlled sanitary history. This model presents several structural advantages:

• complete batch traceability,
• limitation of inter-operation mixing,
• direct control of treatment practices,
• consistency between exposure history and reuse.

In a closed system, any contamination remains confined to the production unit. There is no inter-regional amplification. A closed system does not automatically eliminate internal accumulation problems, however. If old combs are systematically recycled without sufficient renewal, the residue load may continue to increase progressively (Meng et al., 2025).

In other words, the closed system reduces the risk of external introduction but does not neutralise the risk of internal accumulation.

The comparison between open and closed systems must therefore be formulated in terms of risk architecture:

• the open system presents a higher risk of horizontal diffusion (between operations);
• the closed system concentrates risk within a unit but offers better traceability and better capacity for targeted management.

A third, intermediate model, sometimes described as a semi-closed or controlled system, consists of using an external processor while requiring:

• strict batch separation,
• documentation of sterilisation parameters,
• periodic residue analyses,
• contractual guarantee of absence of adulteration.

This model seeks to combine the technical advantages of industrial infrastructure with the traceability principles of the closed system.

The temporal dimension must also be incorporated. In open circuits, dilution can sometimes temporarily reduce the concentration of a specific contaminant, but it increases the overall diversity of residues present. In closed circuits, diversity may be lower, but the concentration of a historically used compound may remain stable or increase if no renewal is implemented.

Thus, no architecture is intrinsically "perfect". Assessment depends on:

• the regional prevalence of diseases,
• historical treatment practices,
• the degree of industrial control,
• available analytical capacity.

The current scientific literature permits the following assertions with a high level of confidence:

• wax can carry persistent spores if it is not correctly sterilised (WOAH, 2023a);
• commercial wax frequently contains residue mixtures (Marti et al., 2022; Alkassab et al., 2020);
• intensive recycling promotes cumulative accumulation (Meng et al., 2025).

Consequently, the choice of wax cycle model should be considered as a strategic risk-management decision, and not as a simple logistical question.

2.3 Renewal intervals and biological thresholds

The question of comb renewal intervals is one of the most sensitive points in wax cycle management. Historically, renewal recommendations rested primarily on empirical or practical considerations (darkening, mechanical strength, comb aesthetics). More recent work now makes it possible to approach this question from a more structured biological standpoint.

Comb ageing corresponds to a cumulative process involving structural, chemical, and microbiological changes. Morphometric and toxicological data suggest that these changes follow a progressive trajectory rather than an abrupt threshold. However, certain studies attempt to identify functional break points.

Meng et al. (2025), in their systematic review, propose temporal benchmarks drawn from the comparative literature: for Apis mellifera, renewal after more than three years of use would be biologically justified; for Apis cerana, the proposed threshold is considerably shorter (more than six months or beyond eight brood cycles). These values are specific to the contexts studied and should not be transposed mechanically to all European conditions. They do indicate, however, that there are biologically relevant thresholds linked to the cumulative accumulation of stressors.

The work of Berry and Delaplane (2001) demonstrates that significant differences in brood area and bee weight appear when comparing colonies placed on new versus old combs. Although this work does not define a precise temporal threshold, it confirms that the changes become functionally detectable.

Al-Kahtani (2021) shows that the reduction in cell internal diameter and worker weight follows an almost monotonic progression with comb age. This continuity suggests that this is not a binary phenomenon (new versus old), but a biological gradient.

The central question therefore becomes: from what level of accumulation do the cumulative effects become sufficiently significant to justify renewal?

Several dimensions must be taken into account:

1. Structural accumulation
Wall thickening and reduction in cell volume progress with each brood cycle (Meng et al., 2025). Even if a colony can tolerate these modifications over several cycles, successive accumulation may durably alter the larval microenvironment.

2. Chemical accumulation
Heavy metals and persistent acaricides increase over time in combs (Meng et al., 2025; Marti et al., 2022). Recycling without external renewal sustains this accumulation. Even in the absence of an immediate lethal effect, chronic exposure constitutes a continuous physiological stressor.

3. Parasite dynamics
Piccirillo and De Jong (2004) observed significantly higher infestation rates by Varroa destructor in old combs than in new combs. Although the exact mechanism has not been fully elucidated, this suggests that comb age may indirectly influence parasite pressure.

4. Colony resilience
The documented sublethal effects (Wu et al., 2011; Fisher & Rangel, 2018) show that developmental exposures can affect longevity or reproductive quality without causing immediate collapse. The question then becomes that of cumulative resilience: a colony subjected to moderate but persistent stressors may present increased vulnerability in the face of additional stress.

It is important to stress that no study currently makes it possible to define a universally valid threshold applicable to all regions and all practices. Environmental conditions, parasite pressure, treatment history, and bee subspecies strongly influence the dynamics.

Rather than a rigid threshold, the literature therefore suggests a logic of progressive management:

• the older the comb, the greater the accumulation of structural and chemical stressors;
• these stressors potentially interact with one another;
• regular renewal mechanically reduces cumulative accumulation.

From this perspective, the three-year interval frequently mentioned for Apis mellifera appears as a biologically coherent benchmark rather than an absolute norm (Meng et al., 2025). It corresponds to a compromise between practical feasibility and limitation of accumulation.

It is also appropriate to distinguish between comb types. Brood nest combs, subjected to repeated brood cycles and successive cocoon deposits, accumulate structural and chemical modifications more rapidly than super frames used primarily for honey storage. Differentiated management may therefore be scientifically justified.

In summary, the question of renewal intervals should be understood as a mechanism for temporal regulation of the wax compartment. Periodic renewal does not aim to achieve absolute purity, but to limit the cumulative accumulation of structural, chemical, and biological stressors.

No universal threshold can be set, as dynamics vary according to environmental and sanitary context. Nevertheless, the data converge to show that periodic renewal mechanically reduces cumulative load and constitutes a rational management lever.

3. Practical recommendations

The preceding sections have shown that wax evolves under the combined influence of structural transformations, chemical accumulations, and possible biological persistence. The following recommendations derive directly from these findings. They aim to reduce the identified progressive risks in a coherent manner, without claiming to achieve absolute purity.

3.1 Active management of comb age

Comb ageing is associated with:

• a progressive reduction in cell volume (Al-Kahtani, 2021; Meng et al., 2025),
• a decrease in the weight of emerging workers (Berry & Delaplane, 2001; Meng et al., 2025),
• an increasing accumulation of heavy metals and lipophilic residues (Meng et al., 2025; Marti et al., 2022).

Beyond these structural and toxicological effects, old combs also present increased vulnerability with regard to certain pests of beekeeping equipment. Reviews on Galleria mellonella show that dark combs rich in cocoons and organic debris constitute a particularly favourable substrate for larval development. Regular renewal of frames thus also contributes to limiting the pressure from the greater wax moth, especially during storage of equipment. These observations justify active management of comb age.

In practice:

• Implement a systematic rotation of brood frames (e.g. 3 to 5 frames per year), corresponding approximately to an annual renewal of around one-third of the brood box frames and a fraction of the super frames.
• Critically assess combs that have exceeded approximately three years of use for Apis mellifera (Meng et al., 2025), taking account of the sanitary and environmental context.
• Give priority to removing combs that are heavily darkened, thickened, or showing marked wall densification.

The objective is not aesthetic but biological: to limit the cumulative accumulation of structural and chemical stressors.

3.2 Management of residues and heavy metals

Wax acts as an accumulation compartment for acaricides and environmental contaminants (Calatayud-Vernich et al., 2018; Marti et al., 2022). Residues can exert sublethal effects on longevity or development (Wu et al., 2011), while high concentrations can impair brood emergence (Kast et al., 2023).

In practice:

• Avoid repeated and prolonged use of persistent lipophilic acaricides without concurrent comb renewal.
• Integrate frame renewal into the overall Varroa destructor management strategy.
• Consider wax as a potential bioindicator of the environment: high loads may reflect external pressures.

It should be recalled that thermal treatments targeting biological inactivation do not eliminate persistent chemical residues (Kast et al., 2023). This distinction is addressed in detail in point 3.5.

3.3 Distinguishing residues from adulteration

Problems linked to residues and those linked to wax adulteration must be distinguished.

Acaricide residues at concentrations typically observed in commercial circuits are associated primarily with chronic or sublethal effects (Wu et al., 2011; Fisher & Rangel, 2018).

By contrast, adulteration by the addition of stearin can cause acute effects on the brood nest, including larval removal behaviour and reduction of capped brood (Alkassab et al., 2020).

In practice:

• In the event of a severely patchy brood nest without obvious infectious or parasitic cause, consider wax quality as a possible factor.
• Demand greater transparency and traceability when purchasing foundation.

3.4 Choice of wax circuit

The configuration of the wax cycle directly influences the management of chemical and microbiological risks.

Open systems involve mixing of waxes from multiple sources, with potential redistribution of residues and — in the event of inadequate thermal control — of spores (WOAH, 2023a; Marti et al., 2022).

Closed systems offer better traceability and limit external inputs, while requiring regular renewal to prevent internal accumulation.

In a closed system, it is biologically pertinent to give preference, where possible, to the priority reintroduction of cappings wax from supers. This wax, not having undergone repeated brood cycles or successive accumulation of larval cocoons, generally presents a lower structural and chemical load than brood nest combs. Its preferential use for the manufacture of new foundation contributes to limiting internal cumulative accumulation.

In practice:

• Where possible, prefer a closed or semi-closed circuit with batch separation.
• If using an external processor, verify documentation of sterilisation parameters and separation of flows.

3.5 Sterilisation versus decontamination

It is fundamental to distinguish between two technically and biologically different objectives: inactivation of pathogens and reduction of chemical contaminants.

Reliable inactivation of Paenibacillus larvae spores requires, according to Swiss official recommendations, a treatment of at least 121 °C for 30 minutes under controlled pressure (Swiss Federal Food Safety and Veterinary Office, 2016; Agroscope, 2018). This parameter corresponds to autoclave sterilisation using saturated steam under pressure.

Classical beekeeping melting devices — solar wax extractors, water baths, open steam boilers — do not exceed 100 °C at normal atmospheric pressure. They allow liquefaction and clarification of the wax but do not guarantee inactivation of highly resistant spores. Prolonged melting therefore does not constitute validated sterilisation.

A domestic pressure cooker can reach temperatures close to 120 °C. However, it guarantees neither precise temperature control within the wax mass nor thermal homogeneity comparable to a professional autoclave. Due to the specific thermal properties of molten wax, insufficiently heated zones may persist. Sporicidal efficacy cannot therefore be considered validated.

It should also be recalled that correctly performed sterilisation does not eliminate persistent chemical residues. Lipophilic acaricides and certain environmental contaminants remain present after thermal treatment (Kast et al., 2023). Biological inactivation and chemical decontamination rely on distinct mechanisms.

Thus, sanitary management of wax rests on a clear understanding:

• Do not equate "melted wax" with "sterilised wax".
• Do not equate "sterilised wax" with "chemically clean wax".

3.6 Integrating parasite dynamics

Old combs may exhibit higher infestation rates by Varroa destructor than new combs (Piccirillo & De Jong, 2004).

In practice:

• Integrate comb renewal into an overall strategy for reducing parasite pressure.
• Consider that management of the wax cycle may indirectly influence viral dynamics, even if the exact magnitude of this effect remains to be quantified (De Guzman et al., 2019; Colwell et al., 2024).

3.7 Strategic objective

The strategic objective is not total elimination of contaminants, but progressive reduction of structural, chemical, and biological stressors.

Coherent management rests on regular comb rotation, control of the wax cycle, distinction between biological sterilisation and chemical decontamination, and vigilance regarding the quality of foundation.

5. Conclusion

Scientific data converge on a clear recognition of the role of combs as a biologically active compartment within the colony. Comb ageing modifies the structure of the brood nest, influences the development of workers, and is accompanied by a cumulative increase in contaminants. To this are added indirect sanitary implications, notably regarding parasite pressure and the management of stored material.

The question is therefore not aesthetic but systemic: management of the wax cycle constitutes a central lever for biosecurity and beekeeping sustainability. Planned frame renewal, combined with a controlled wax cycle, represents today the most coherent approach in the light of the available evidence.

See also:

• Six scientific reasons not to use old frames
• Wax and combs
• Beeswax contaminations
• Practical Guide: 4.4.1 Melting frames
• Practical Guide: 2.1 American foulbrood

 

Bibliography

Agroscope. (2018). Leitfaden Bienengesundheit – Zentrum für Bienenforschung (Agroscope Transfer Nr. 245/2018). Agroscope, Bern, Switzerland.

Al-Kahtani, S. N. (2021). Effect of comb age on cell measurements and worker body size of the honey bee (Apis mellifera). Journal of King Saud University – Science, 33(6), 101567. https://doi.org/10.1016/j.jksus.2021.101567