From Pollen to Pathogen Defense: How Pollen Chemical Quality Impacts Deformed Wing Virus Infection and Survival in Honey Bees

1. Introduction

The health of honey bees (Apis mellifera L.) is essential for the functioning of terrestrial ecosystems and global agricultural production because of their central role in the pollination of both crops and wild plants. Pollination by animals underpins the reproduction of nearly 90% of wild flowering plants, contributes to more than 75% of global food crop types, and supports an estimated US$235–577 billion in annual crop production worldwide (Priyadarshana et al., 2024). However, over the past decades, a sustained decline in colony health has been documented across different regions of the world, largely driven by the interaction of multiple stressors, including pathogens, agricultural intensification, pesticide exposure, and reduced floral diversity (Goulson et al., 2015).

Among the biological factors compromising colony stability, viral infections are particularly important because of their high prevalence and cumulative sublethal effects. Deformed wing virus (DWV) has emerged as one of the most important viral pathogens of A. mellifera due to its widespread distribution, persistence, and close association with the mite Varroa destructor, which promotes its transmission and amplification within colonies (Grozinger & Flenniken, 2019). DWV infection has been associated with alterations in development, physiology, behavior, and lifespan, thereby contributing to the progressive weakening of colonies (Penn et al., 2022).

As a defense against pathogens, honey bees lack adaptive immunity and rely exclusively on innate immune mechanisms to cope with infection. At the colony level, these defenses are complemented by forms of social immunity, such as grooming behavior and the removal of compromised individuals, which help limit pathogen transmission (Evans et al., 2006). Nevertheless, the effectiveness of these defenses depends strongly on the physiological condition of bees and on the nutritional context in which infection occurs (Ricigliano et al. 2022). Within this framework, nutrition has been proposed to influence not only the activation of immune responses but also the host capacity to tolerate damage associated with persistent viral infections (Dosch et al., 2021).

In this context, the nutrition of honey bees encompasses the acquisition and utilization of essential macro- and micronutrients that sustain individual growth, development, and vital functions. Carbohydrates, obtained from nectar, represent the primary energy source and contribute to oxidative balance and antimicrobial defense (Fernandes et al., 2022). Pollen, on the other hand, provides proteins, amino acids, lipids, sterols, vitamins, minerals, and bioactive compounds that support tissue development, cellular regulation, and immune competence (Ansaloni et al., 2025). However, pollen quality varies according to botanical origin and environmental conditions, which can in turn influence bee physiology and host–pathogen interactions.

Despite growing interest in the links between nutrition and honey bee health, the specific role of pollen chemical quality in shaping DWV infection outcomes remains insufficiently integrated. Available evidence is dispersed across studies focused on nutrition, immune function, oxidative stress, gut microbiota, and seasonal ecology, and is often difficult to compare because of differences in experimental systems, life stages, and response variables. In this systematic review, the available evidence linking pollen chemical quality to DWV infection dynamics and survival in Apis mellifera is examined across molecular, physiological, ecological, and seasonal scales. Attention is given to the role of bioactive pollen compounds, especially flavonoids and phenolic acids, while explicitly distinguishing evidence derived from honey bee studies from mechanistic hypotheses proposed in other viral systems. This review aims to integrate these fragmented lines of evidence, identify the main knowledge gaps, and provide a critical framework for understanding how pollen chemical quality may shape DWV outcomes and honey bee survival under different biological and ecological contexts.

2. Methodology

This review was conducted as a structured narrative synthesis given the substantial heterogeneity in viral inoculation methods, host age, diet composition, and outcome metrics in the literature. Comprehensive research was performed in Web of Science, Scopus, PubMed, Google Scholar, and journal databases using combinations of terms related to bee nutrition, DWV infection, immune gene expression, pollen chemistry, phytochemicals, and microbiota. The primary search window covered studies published between 2000 and 2026, with older references retained only when they provided foundational information on honey bee immunity or viral biology. Priority was given to peer-reviewed experimental studies evaluating pollen, pollen substitutes, bee bread, floral phytochemicals, or defined dietary compounds in relation to DWV or other honey bee viruses. Studies were grouped by experimental scale, dietary treatment, pathogen context, and response variables, allowing mechanistic and ecological evidence to be integrated qualitatively and comparatively.

3. Deformed Wing Virus (DWV) in Apis Mellifera: Biology, Transmission, and Pathological Effects

Deformed wing virus (DWV) is one of the most widely distributed viral pathogens in Apis mellifera and is currently recognized as one of the central components within the set of factors underlying colony weakening and collapse at a global scale (Martin & Brettell, 2019). Its high prevalence, even in colonies that do not show clinical signs, reflects its remarkable ability to establish persistent and subclinical infections, which can be maintained for prolonged periods within bee populations (Gisder & Genersch, 2021). This characteristic distinguishes DWV from other pathogens and helps explain why its impact is often expressed progressively and cumulatively, rather than as isolated events of massive mortality (Lamas et al. 2026).

DWV belongs to the family Iflaviridae and corresponds to a non-enveloped, positive-sense single-stranded RNA virus, whose genomic organization follows the characteristic pattern of picorna-like viruses (de Miranda & Genersch, 2010). Its genome is translated as a single polyprotein that is subsequently processed to give rise to structural and non-structural proteins, including an RNA-dependent RNA polymerase, a helicase, and a 3C-like protease, all of which are indispensable for viral replication and virion assembly (Reuscher et al. 2023). In recent years, advances based on reverse genetics have made it possible to deepen the understanding of the replicative biology of DWV; however, relevant gaps remain regarding its cellular tropism and the host factors that modulate the efficiency of viral replication (Woodford & Evans, 2021).

At the population level, DWV should not be understood as a homogeneous entity, but rather as a complex of closely related genetic variants. To date, at least three main variants have been described, DWV-A, DWV-B, and DWV-C, in addition to multiple recombinant forms whose relative frequencies vary as a function of space and time (Mordecai et al., 2016).

DWV transmission can occur through multiple routes, including vertical transmission from the queen to the offspring, through drone semen, horizontal transmission between individuals by trophallaxis, and through contact with infected pupae (Amiri et al., 2016; Posada-Florez et al. 2021). Nevertheless, the most consequential change in the epidemiology of this virus has been its association with vector-mediated transmission by Varroa destructor, a process that significantly increases transmission efficiency and promotes abrupt increases in viral load. In addition, this route is associated with a greater probability of clinical disease manifestation (Martin & Brettell, 2019; Piou et al., 2022). The association between DWV and Varroa has therefore redefined not only the intensity of infection, but also its pathological and epidemiological consequences within colonies (Doublet et al. 2024). In this sense, it has been described that DWV replication can occur in diverse bee tissues, and that tissue tropism constitutes a key component for understanding the breadth of the physiological effects associated with infection (Gusachenko et al., 2021). When high viral loads are reached during pupal development, particularly in the presence of Varroa parasitism, evident clinical manifestations may appear, including the characteristic deformed wing phenotype (Wu et al., 2021). However, in most adult bees, DWV persists as a covert infection, with sublethal consequences that include reduced longevity, lower survival, and impaired foraging performance (Benaets et al., 2017). Therefore, the biological importance of DWV lies not only in its capacity to generate visible clinical signs, but also in its ability to silently and persistently compromise colony physiology and functioning.

4. Immune System of Honey Bees and Its Modulation by Diet Quality

Since the pathological manifestation of DWV depends not only on viral load but also on the host’s ability to resist and tolerate infection, it is essential to consider the organization of the immune system in A. mellifera and the nutritional factors that modulate its function. Honey bees rely exclusively on an innate immune system to combat pathogens, as they lack antibody-based adaptive immunity. This system is composed of humoral and cellular mechanisms highly conserved in insects, allowing bees to recognize, limit, and tolerate infections caused by bacteria, fungi, and viruses (Evans et al., 2006). The effectiveness of these responses is not constant but is closely linked to the physiological state of the individual, which depends largely on the availability and quality of nutritional resources (Alaux et al., 2010; Di Pasquale et al., 2013). Without adequate dietary inputs, even genetically intact immune pathways may be insufficient to control DWV replication.

Pathogen recognition in A. mellifera occurs through pattern recognition receptors (PRRs), which detect conserved molecular structures of microorganisms and activate intracellular signaling cascades (Larsen et al., 2019). Among the main immune pathways described in bees are Toll, Imd, JAK/STAT, and JNK, which regulate the expression of immune genes and the production of antimicrobial peptides (AMPs) such as defensin, abaecin, apidaecin, and hymenoptaecin (Evans et al., 2006; Evans & Spivak, 2010).

Against RNA viruses such as DWV, the primary antiviral defense in bees is RNA interference (RNAi), a mechanism highly conserved in insects (Swevers et al., 2018). During viral replication, double-stranded RNA (dsRNA) intermediates are generated, recognized by the enzyme Dicer-2 and processed into small interfering RNAs (siRNAs) (Sabin et al., 2013). These siRNAs are incorporated into the RISC complex, where Argonaute-2 directs the specific degradation of viral RNA, thereby reducing viral replication (Brutscher et al., 2015). The activity of the RNAi pathway is energetically demanding, and its efficiency is thus directly sensitive to nutritional status. A non-specific antiviral response induced by dsRNA has also been described in A. mellifera, capable of activating immune genes without exact sequence matching (Niu et al., 2014).

Along with humoral responses, bees possess cellular immunity mechanisms mediated by hemocytes, which participate in phagocytosis, encapsulation, and melanization (Strand, 2008). Melanization, regulated by the enzyme phenoloxidase, generates reactive oxygen species as by-products, implying a risk of collateral tissue damage if not properly controlled (González-Santoyo & Córdoba-Aguilar, 2012). This balance between immune activation and damage control is particularly relevant during persistent infections such as DWV and represents an interface where pollen-derived antioxidants may play a meaningful physiological role.

The activation and maintenance of immune responses represent a significant energetic cost (Roger et al., 2017). In this context, poor or low-diversity pollen is associated with reduced immune capacity and increased susceptibility to viral infections, whereas access to high-quality pollen improves general physiological condition and tolerance to pathogenic stress (Belsky & Joshi, 2019). The effectiveness of these immune pathways, both humoral and cellular, depends not only on their genetic integrity, but on the availability of the energetic and molecular substrates that pollen provides. Dietary quality thus emerges as a transversal determinant of immune function in A. mellifera, ultimately shaping the margin of resistance and tolerance available against DWV infection.

5. Pollen Chemical Quality as a Determinant of Defense Against Deformed Wing Virus (DWV) in Apis Mellifera

In Apis mellifera, pollen constitutes the primary dietary source of proteins, lipids, sterols, vitamins, minerals, and a wide range of bioactive compounds (Ansaloni et al., 2025). These elements provide the chemical substrates necessary to sustain the vital processes related to the health and defense of the host organism (Walton & Dolezal, 2021). Unlike nectar, whose role is primarily energetic, pollen supplies the essential compounds that become especially critical under pathogen pressure (Vaudo et al., 2016). The magnitude of this nutritional dependence becomes evident when considering the scale at which colonies consume pollen. A colony can consume between 15 and 55 kg annually, with individual adult bee daily consumption ranging from 3.4 to 5.4 mg depending on age (Fernandes et al., 2022). The period of greatest pollen demand corresponds to the phases of active brood rearing and population growth. To rear a single worker larva, between 124 and 187.5 mg of pollen are required, providing between 25 and 37.5 mg of protein depending on the available protein content (Li et al., 2014). This demand falls primarily on nurse bees, which during their first 10 days of age consume large quantities of pollen to drive the development and secretory activity of their hypopharyngeal glands (Lazarov et al., 2025). These glands produce the main protein fraction of royal jelly, distributed via trophallaxis to larvae and the queen. Therefore, the nutritional status of nurse bees is not merely an individual matter, but a determining factor of population dynamics.

These figures reflect not only the energetic investment that pollen represents for the colony, but also its indispensable role in sustaining all major physiological functions (Qiao et al., 2024). However, pollen composition is far from uniform. Protein levels ranging from 2.5% to 61% have been reported in pollen collected by honey bees (Roulston et al., 2000). This variability in quantity and quality can represent a challenge for the nutritional homeostasis of the colony (Rodríguez-Pólit et al., 2023). Nutritional deficits are directly reflected in lifespan, disease susceptibility, and body weight. In this regard, Di Pasquale et al. (2013) found that bees fed higher-quality protein pollen exhibited more developed hypopharyngeal glands, higher vitelogenin levels, and greater pathogen tolerance, suggesting that the nutritional quality of pollen directly modulates both the physiology and defensive capacity of nurse bees. Similarly, Branchiccela et al. (2019) found that nutritional stress associated with a diet based primarily on Eucalyptus grandis pollen significantly compromised colony strength, as evidenced by a reduction in brood and adult bee populations. They also observed increased Nosema spp. infection levels in nutritionally restricted colonies compared to those supplemented with polyfloral pollen. Although supplemented colonies showed slightly higher RNA virus infection levels, these were generally low and without an evident negative impact. In the long term, nutritional stress prevented population recovery in spring, while supplemented colonies maintained better growth dynamics and seasonal resilience. These findings suggest that pollen quality not only determines the nutritional status of the colony, but may also condition its capacity to respond to specific viral pathogens. In this context, a previous study from our research group (García Domínguez et al., 2025) evaluated the effect of diets based on pollen of different botanical compositions on DWV-A viral load, survival, and immune gene expression in experimentally inoculated bees. The results showed that native Eucryphia cordifolia pollen drastically reduced viral load from 1.0×10¹³ to 1.0×10⁵ copies per bee, along with a survival rate of 91%. Furthermore, pollen diets modulated the expression of immune-related genes, reducing Cactus expression while upregulating Dorsal, Relish, and Dicer-like.

At the colony level, DeGrandi-Hoffman et al. (2020) demonstrated that colonies supplemented with pollen developed larger populations and survived longer than unsupplemented ones, although Varroa destructor and DWV levels increased similarly in both groups throughout the season. This suggests that the primary benefit of pollen does not lie in directly reducing pathogen load, but in strengthening colony resilience against infection. Along the same lines, Frizzera et al. (2022) demonstrated that the beneficial effect of pollen in Varroa-infested bees is associated with the reversal of parasite-induced accelerated behavioral maturation, accompanied by improved immune gene expression and reduced DWV load. As shown in Table 1., these findings indicate that pollen quality conditions susceptibility to DWV across multiple levels of biological organization: from the individual physiology of nurse bees and glandular development to immune gene expression, brood quality, population dynamics, and the seasonal resilience of the colony. Consequently, pollen must be understood not only as a nutritional resource, but as a key modulator of host–pathogen interactions in A. mellifera, where its quality and diversity determine the physiological margin of tolerance against viral infections such as DWV.

6. Pollen Phytochemicals as Context-Dependent Modulators of DWV Outcomes in Apis Mellifera

Pollen is not only a source of essential nutrients for Apis mellifera, but also a chemically complex matrix of secondary metabolites with potentially important effects on host physiology and pathogen responses. Among the most relevant compounds are polyphenols, terpenoids, phytosterols, alkaloids, and other aromatic metabolites, whose abundance and composition vary markedly with botanical origin, floral diversity, and environmental conditions (Gámbaro et al., 2025; Tlak Gajger & Cvetkovikj, 2025). Phenolic compounds are particularly prominent in this context because they are frequently detected in pollen and have been associated with antioxidant activity and other biological properties that may influence bee health (Rodríguez-Pólit et al., 2023).

In honey bees, the relevance of pollen-derived phytochemicals extends beyond their simple presence in floral resources. Current evidence suggests that these compounds may modulate immune function, oxidative balance, and physiological resilience, thereby affecting the host context in which viral infection develops. At the molecular level, phytochemicals derived from nectar and pollen have been associated with changes in immune-related gene expression, including the regulation of antiviral effectors and signaling pathways linked to Toll- and Imd-related responses (Barroso-Arévalo et al., 2019; Parekh et al., 2021). These observations support the idea that floral secondary metabolites may influence antiviral defense in bees, although the magnitude and biological significance of these effects likely depend on the compound involved and the physiological condition of the host.

Some of the strongest evidence in bees comes from studies showing that dietary phytochemicals can alter responses associated with DWV infection. Palmer-Young et al. (2017) reported that phytochemicals naturally present in nectar and pollen stimulated antimicrobial peptide expression in adult bees and reduced DWV levels in young bees under experimental conditions. Likewise, Lu et al. (2020) showed that caffeine increased the expression of immune-related genes and reduced DWV copy number in A. mellifera. More broadly, dietary phytochemicals have also been linked to increased longevity and improved tolerance to pathogens, reinforcing the view that floral chemistry may shape infection outcomes not only through effects on pathogen load, but also through enhanced host resilience (Bernklau et al., 2019).

Beyond immune modulation, pollen-derived phytochemicals may influence host–pathogen interactions through several non-exclusive mechanisms. They may contribute to antioxidant protection, help maintain redox homeostasis during infection, and support physiological functions that are otherwise compromised under viral challenge (Liao et al., 2021). They may also interfere more directly with viral processes, although evidence for this possibility remains much stronger in non-bee systems than in honey bees themselves (Brutscher et al., 2015; Hsieh et al., 2020). Because direct mechanistic evidence in the honey bee–DWV system is still limited, much of the current functional interpretation relies on studies from other RNA virus models, where polyphenols have been shown to act at different stages of the viral cycle. For example, polyphenol-rich extracts can reduce viral infectivity at the pre-entry stage by altering capsid integrity or viral adsorption (Magnavacca et al., 2022), whereas compounds such as quercetin and luteolin have been reported to inhibit RNA-dependent RNA polymerase activity in positive-sense RNA viruses, including SARS-CoV-2 (Munafò et al., 2022; Agrawal et al., 2020). In the context of DWV, however, these mechanisms should be regarded as plausible hypotheses rather than demonstrated effects.

Evidence generated directly in bees nevertheless suggests that plant-derived compounds can meaningfully alter DWV-related outcomes. Boncristiani et al. (2021) highlighted the capacity of natural products, including flavonoids, to modulate viral infections in pollinators through both immunomodulatory and putative antiviral effects. Pascual et al. (2022) further showed that supplementation with grape pomace rich in polyphenols was associated with reduced DWV load, improved survival, and activation of immune-related genes in adult honey bees. These findings are especially relevant because they indicate that pollen-associated phytochemicals may affect DWV not only indirectly through general improvements in host condition, but also through more specific changes in infection-associated physiology.

At the same time, the biological relevance of pollen phytochemicals cannot be inferred solely from their occurrence in pollen. Their effects are shaped by compound identity, dose, post-ingestion transformation, and the broader biological context in which they are consumed. Structurally related compounds may differ substantially in their antipathogenic activity, and floral products can affect bee pathogens through both direct and host-mediated pathways (Fitch et al., 2022). Host-plant chemistry may also alter pathogen susceptibility through interactions with immune regulation and microbiota composition (Sanaei & de Roode, 2025). This complexity is reinforced by evidence showing that the phytochemical profiles of larvae and adult bees differ from those of their pollen diet, suggesting that metabolism, selective transfer, and biotransformation strongly influence actual host exposure (Vidkjær et al., 2024).

Thus, the role of pollen-derived phytochemicals should not be viewed as a uniform antiviral effect, but rather as a context-dependent modulation of infection outcomes emerging from the interaction among floral chemistry, host physiology, oxidative balance, microbiota, and environmental stress. From this perspective, the novelty of current research lies not simply in identifying phytochemicals in pollen, but in understanding how these compounds shape the physiological environment in which DWV persists, replicates, and causes damage. Pollen phytochemicals therefore emerge as candidate modulators of infection outcome rather than universally acting antiviral agents (Figure 1). This interpretation provides a more integrative framework for explaining why chemically distinct diets may lead to different patterns of viral load, survival, and tolerance in A. mellifera and highlights the need for bee-specific mechanistic studies that move beyond extrapolation from non-apicultural viral systems.

Direct mechanistic evidence for the effects of pollen-derived phytochemicals in the A. mellifera–DWV system remains limited. Therefore, much of the current functional interpretation is informed by studies conducted in other RNA virus models. Table 2 summarizes selected pollen-derived phytochemicals with reported antiviral activity in non-bee systems, highlighting their proposed mechanisms of action and their plausible relevance to DWV infection in honey bees. In this context, these mechanisms should be interpreted as working hypotheses rather than as demonstrated antiviral effects in bees.

7. Seasonal and Ecological Influences on Diet Quality, DWV load, and Bee Survival

Honey bees exhibit seasonal variation in their tolerance to viral infections. The biological impact of DWV depends not only on the presence of the virus or on viral load, but also on the health status of the host and the environmental context in which the infection occurs (Dowell, 2001). This perspective is consistent with general frameworks in disease ecology, according to which seasonality simultaneously modulates pathogen dynamics, host susceptibility, and the environmental conditions that mediate their interaction (Altizer et al., 2006). In this sense, seasonality implies changes in the body condition of individuals and in colony organization. Differences between summer and winter bees include variations in fat body reserves, metabolism, microbiota, transcriptomic profiles, and immunocompetence (Dostálková et al., 2021). This suggests that the response to infections does not occur on a biologically constant basis throughout the year (Bresnahan et al., 2022). In this context, tolerance to DWV should be understood as a dynamic trait, conditioned by the moment of the annual cycle in which infection occurs. This framework is especially relevant for DWV, whose epidemiology is closely linked to V. destructor, but is not explained exclusively by the presence of the mite. The interaction between Varroa and DWV has driven the amplification, spread, and changes in virulence of this pathogen system on a global scale (de Miranda & Genersch, 2010; Wilfert et al., 2016; Traynor et al., 2020). However, the manifestation of infection is not uniform over time. Longitudinal studies have shown that viral abundance, pathogen co-occurrence, and colony vulnerability change seasonally, with overwintering being a particularly critical period (Faurot-Daniels et al., 2020). Seasonality can also influence the population structure of DWV itself. Temporal changes have been documented in the relative prevalence of viral variants and in their association with colony losses, especially in relation to the dynamics between DWV-A and DWV-B (Grindrod et al., 2021; Paxton et al., 2022). Given that some genotypes differ in virulence, transmission, and persistence, these transitions can modify not only the epidemiology of the virus, but also the biological consequences of infection at the colony level (Norton et al., 2021). Within this scenario, the floral landscape and the seasonal availability of pollen form part of the ecological context that modulates the host-pathogen relationship. The composition and diversity of collected pollen change throughout the year and according to land use, affecting the nutritional basis that sustains both individual bee and the colony (Lau et al., 2019). In intensive agricultural landscapes, colonies often face an alternation between floral abundance and scarcity, with effects on their health and their ability to cope with stressors (Zhang et al., 2020). Thus, seasonal tolerance to DWV should not be interpreted only as an intrinsic property of the host, but rather as the result of the interaction among the time of year, the context of infection, and the quality of the nutritional environment.

This way of understanding tolerance is aligned with the distinction between resistance and tolerance as alternative defense strategies against disease (Medzhitov et al., 2012). Applied to A. mellifera, this suggests that colonies may pass through periods of the year in which they do not necessarily control infection more effectively but do differ in their ability to maintain vital functions and survive in the face of viral infection. In this sense, seasonality emerges as an ecological organizer of DWV pathogenicity and raises relevant applied implications, since if tolerance to the virus changes throughout the year and depends on the ecological context of the host, then floral landscape management, resource availability during critical periods, and timely Varroa control could contribute not only to limiting viral transmission, but also to reducing the probability that infection results in severe damage or colony loss (Gregorc & Sampson, 2019).

8. Gut Microbiota as an Intermediary Between Pollen Chemical Quality and DWV Outcomes in Apis Mellifera

In A. mellifera, the relationship between pollen chemical quality and deformed wing virus (DWV) infection should be interpreted considering the gut microbiota, which may act as an important intermediary between diet and host response (Motta & Moran, 2024). The honey bee gut microbiome is relatively simple and is concentrated mainly in the hindgut, where a core set of bacterial taxa predominates, including Lactobacillus, Bifidobacterium, Bombilactobacillus, Gilliamella, and Snodgrassella, whereas other taxa such as Bartonella, Commensalibacter, and Frischella are often considered more variable or transient members (Bonilla-Rosso & Engel, 2018). This microbial community contributes to digestion, metabolism, and immune regulation, and its composition is shaped by diet, season, and environmental conditions (Meehan & O’Toole, 2025). In general, diets derived from diverse floral sources tend to support more stable and functionally diverse gut communities, whereas monofloral or artificial diets are more often associated with simplified microbiota (Powell et al., 2023). Within this framework, pollen-derived bioactive compounds may influence host–pathogen interactions not only at the systemic level but also through changes in the intestinal environment. Experimental evidence shows that dietary phytochemicals such as caffeine, gallic acid, p-coumaric acid, and kaempferol can increase gut microbial diversity and alter the abundance of taxa such as Snodgrassella and Lactobacillus, although these effects appear to be compound-specific (Geldert et al., 2021). Other studies have also linked dominant symbionts, particularly Gilliamella apicola, to improved digestive efficiency, intestinal barrier function, and overall physiological condition (Kwong & Moran, 2016). Although these studies do not directly assess DWV infection, they provide a mechanistic basis for considering microbiota as a mediator of diet-dependent variation in infection outcome (Kwong et al., 2017). This interpretation is especially relevant because the gut microbiota appears to contribute more to infection tolerance than to direct viral elimination. Microbiota stability and functionality are strongly influenced by diet, and pollen phenolics may reach the gut in partially transformed forms that selectively affect microbial composition and metabolism (Ozdal et al., 2016). Similar interactions have been described in other organisms, although their specific outcome depends on compound identity, concentration, and the pre-existing microbial community (Bešlo et al., 2023). In honey bees, microbial biotransformation of dietary compounds has been documented mainly in relation to nutrient metabolism and host physiology, rather than as a direct antiviral mechanism (Khumalo et al., 2025). From the perspective of DWV infection, microbiota therefore seems more likely to support infection tolerance than to directly suppress viral replication (Kwong et al., 2017). This framework may help explain why bees fed chemically richer diets often show lower viral loads and improved survival. At least part of this pattern could reflect a better basal physiological state mediated by the microbiota, rather than a direct antiviral effect exerted by gut bacteria themselve. This interpretation is consistent with studies showing that disruption or absence of the gut microbiota is associated with broad changes in host gene expression affecting immunity, metabolism, development, and behavior (Raymann & Moran, 2018). Nevertheless, in the honey bee–DWV system, current evidence does not support a direct causal role of the microbiota in controlling viral replication. Rather, the gut microbiome should be viewed as part of a broader diet-dependent physiological network through which pollen chemical quality may shape tolerance to infection, survival, and colony health in honey bees.

9. Limitations and Gaps in Knowledge

Despite increasing evidence that pollen chemical quality can influence DWV infection outcomes in honey bees, important methodological and conceptual limitations still prevent the development of robust and predictive models. One of the main challenges is the high phytochemical variability of pollen, which arises from taxonomic, environmental, and seasonal differences (Bryś et al., 2021). This variability complicates comparisons among studies and limits reproducibility. Even within the same plant species, the concentration of flavonoids and other metabolites can fluctuate substantially, making it difficult to establish clear causal relationships between specific compounds and infection-related effects (Gercek et al., 2021; El Ghouizi et al., 2023).

A second major limitation lies in the heterogeneity of experimental designs used to evaluate the effects of pollen and bioactive compounds on DWV. Studies differ in viral inoculation procedures, developmental stages examined, laboratory versus field settings, control diets, and concentrations of phenolic compounds, often preventing direct comparison of results (Dubois et al., 2020). In addition, many experiments rely on complex plant extracts or pollen mixtures whose chemical composition is only partially characterized, which limits the identification of the metabolites responsible for the observed responses (Qiao et al., 2024).

Knowledge of the bioavailability and metabolism of flavonoids and phenolic acids in bees also remains surprisingly limited. It is still largely unknown how these compounds are absorbed, biotransformed, distributed, or stored, and which concentrations they reach in physiologically relevant tissues such as the hypopharyngeal glands, brain, or gut (Braglia et al., 2025). This uncertainty makes it difficult to connect dietary composition with actual host exposure and biological effect. The interaction between phenolic compounds and the gut microbiota adds further complexity, since microbial biotransformation may generate secondary metabolites capable of influencing host physiology and immunity, yet the underlying mechanisms remain poorly resolved (Motta & Moran, 2024). The lack of integrated metabolomic approaches therefore remains a key obstacle to establishing causal links among pollen chemistry, microbial activity, and DWV modulation.

Another unresolved issue concerns the dose-dependent effects of polyphenols. Although these compounds are often discussed in relation to antioxidant and protective functions, some studies indicate that high concentrations may act as pro-oxidants and induce oxidative or cytotoxic effects (Andrés et al., 2023). This suggests that the biological activity of pollen phytochemicals cannot be interpreted independently of dose, diet matrix, or physiological context. In parallel, intercolony variability in responses to supplemental diets indicates that genetic, and possibly epigenetic, factors may also influence antiviral responses, although these sources of variation are rarely incorporated into the same experimental framework (Penn et al., 2022).

A further gap concerns the translation of laboratory findings to field conditions. Many studies report beneficial or antiviral-associated effects of supplemental diets under controlled conditions, but their expression in real colonies is shaped by seasonality, floral variability, varroa mite level, pathogen co-occurrence, and environmental stress (Noordyke et al., 2021). As a result, effects detected in caged-bee assays cannot be directly extrapolated to colony-level resilience in complex landscapes.

Overall, these limitations indicate that the relationship between pollen chemical quality and DWV cannot yet be understood through single-factor explanations. Progress in this field will require more standardized experimental designs, chemically well-characterized diets, integrated metabolomic and microbiome analyses, and field-scale studies capable of capturing seasonal and ecological variation. Addressing these gaps will be essential not only to clarify how pollen chemistry shapes DWV outcomes, but also to support the development of evidence-based nutritional strategies to improve colony resilience under increasingly challenging environmental conditions.

10. Conclusions

The available evidence indicates that the outcome of deformed wing virus (DWV) infection in A. mellifera is shaped not only by viral pressure, but also by the nutritional and physiological context in which infection occurs. Within this framework, pollen chemical quality emerges as an important determinant of host condition, influencing immune function, oxidative balance, and the capacity of bees to tolerate infection. Chemically rich and diverse pollen diets are more consistently associated with improved survival and physiological performance than with uniform reductions in viral load, suggesting that their main contribution may lie in enhancing host resilience rather than in acting as direct antiviral factors. This interpretation also highlights the potential role of the gut microbiota as an intermediary through which pollen-derived compounds may influence host physiology and infection outcomes, although the specific mechanisms involved remain insufficiently resolved.

At the same time, current knowledge is constrained by the high phytochemical variability among pollen sources, the heterogeneity of experimental designs, the limited understanding of compound bioavailability and metabolism in bees, and the difficulty of extrapolating laboratory findings to field conditions. Future research should therefore combine chemically characterized diets, bee-specific mechanistic studies, microbiota and metabolomic approaches, and field-scale experiments that incorporate seasonal and ecological variation. Such advances will be essential to clarify how pollen chemical quality shapes DWV outcomes and to support the development of evidence-based nutritional strategies aimed at strengthening colony resilience under increasingly challenging environmental conditions.