Skip to main content
NCBI home page
Ecology and Evolution logoLink to Ecology and Evolution
. 2026 Feb 17;16(2):e73107. doi: 10.1002/ece3.73107

Sunflower Pollen and Bumble Bee Health: Mechanisms, Modifiers and Trade‐Offs

Richard Odemer 1,✉
PMCID: PMC12910244  PMID: 41710518

ABSTRACT

Bumble bees face increasing pressure from interacting stressors, including pathogens, nutritional limitations, and agricultural intensification. Among natural dietary factors that modulate disease, Asteraceae pollen—particularly sunflower ( Helianthus annuus )—has repeatedly been shown to reduce infection by the trypanosomatid Crithidia bombi in bumble bees under laboratory conditions. Yet the mechanisms, generality, and ecological relevance of these effects remain incompletely resolved, and field‐based evidence from European systems, particularly for Bombus terrestris, is scarce. Here, I synthesise current knowledge on how Asteraceae pollen traits influence bumble bee health, focusing on the interplay between pollen morphology, phenolamide chemistry, nutrient composition, gut microbiota, and host physiology. I evaluate evidence for three non‐exclusive mechanistic pathways—mechanical abrasion, chemical activity, and microbiome‐associated effects—and review emerging evidence for nutritional, immunological, and colony‐level trade‐offs associated with medicinal pollen. To place these mechanisms in a field‐relevant context, I integrate pollen‐trap data from B. terrestris and Apis mellifera colonies foraging in Central European agricultural landscapes, indicating strong seasonal reliance on Solanaceae pollen, no uptake of sunflower pollen by B. terrestris, and moderate use of Silphium perfoliatum , a perennial Asteraceae crop of growing agroecological interest. Together, these patterns highlight a mismatch between laboratory efficacy and field‐level pollen use, indicating that sunflower pollen is unlikely to function as a standalone medicinal resource under realistic foraging conditions. Instead, potential health effects of Asteraceae pollen appear context dependent and embedded within diverse nutritional landscapes. I identify key knowledge gaps—including cultivar‐level chemical variation, species‐specific responses, and interactions with co‐occurring stressors—and outline research priorities for evaluating when and how medicinal pollen may contribute to pollinator‐supportive cropping systems.

Keywords: agroecological interventions, Bombus, Crithidia bombi, Helianthus annuus , medicinal pollen, phenolamides, pollen foraging

This review integrates mechanistic evidence and field foraging data to examine how Asteraceae pollen affects Crithidia bombi infection in bumble bees. While laboratory studies show strong antiparasitic effects, field exposure is often limited, revealing a mismatch between efficacy and ecological relevance. Medicinal pollen effects therefore emerge as highly context dependent.

graphic file with name ECE3-16-e73107-g003.jpg

1. Introduction

Pollinator health is shaped by the interplay of nutrition, pathogens, and environmental stressors, yet the mechanistic links between these factors remain incompletely understood (Stevenson et al. 2022; Cameron and Sadd 2020). Among dietary components that modulate disease, pollen from the Asteraceae—particularly sunflower ( Helianthus annuus )—has become a focal point of research due to repeated reports that it reduces gut infections by the trypanosomatid Crithidia bombi in bumble bees (Giacomini et al. 2018; Fowler et al. 2020; Figueroa et al. 2023). C. bombi is a widespread parasite of bumble bees and has been linked to reduced colony performance and reproductive success under natural conditions, indicating that even sublethal infections can have population‐relevant consequences (Imhoof and Schmid‐Hempel 1999; Cameron and Sadd 2020). However, evidence for these antiparasitic effects has been demonstrated mainly under controlled laboratory conditions, and they appear to depend on pollen traits such as echinate morphology (Figueroa et al. 2023; Knoerr et al. 2024), phenolamide‐rich chemistry (Gekière et al. 2022; Palmer‐Young et al. 2017, 2023), and unusual nutrient profiles (Nicolson and Human 2013; Filipiak et al. 2022). Despite these advances, direct in vivo evidence for the relative contribution of these pathways remains limited, and their generality, ecological relevance, and species specificity continue to be debated (Cameron and Sadd 2020; Figueroa et al. 2023).

The majority of experimental work to date has been conducted with Bombus impatiens , a North American species that may not fully represent the physiology, foraging behaviour, or nutritional ecology of European species such as B. terrestris (LoCascio, Aguirre, et al. 2019; LoCascio, Pasquale, et al. 2019; Giacomini et al. 2021; Figueroa et al. 2023). Moreover, most studies rely on monofloral diets or purified pollen presented at high concentrations, conditions that do not reflect the diverse and heterogeneous pollen environments experienced by colonies in agricultural landscapes (Malfi et al. 2023; Husband et al. 2025). Reported benefits also coexist with potential costs: several studies indicate that Asteraceae pollen can be nutritionally suboptimal or even detrimental when it constitutes a major share of the diet, affecting growth, reproduction, or survival (Nicolson and Human 2013; Giacomini et al. 2021, 2023; Fowler et al. 2020). Taken together, these findings suggest that any antiparasitic benefit of sunflower pollen is likely context‐dependent rather than universally beneficial across dietary environments (Cameron and Sadd 2020; Giacomini et al. 2023).

At the same time, a key unresolved question is whether bees actually collect meaningful amounts of sunflower pollen under field conditions. Foraging preference, floral architecture, nectar rewards, and competitive landscape context may limit natural exposure to Asteraceae pollen, even when medicinal properties are demonstrable in laboratory bioassays (LoCascio, Aguirre, et al. 2019; Bergonzoli et al. 2022; Figueroa et al. 2023; Ferguson et al. 2024). This mismatch between laboratory efficacy and field uptake is central to understanding the actual potential of medicinal pollen in real‐world settings and is particularly relevant for European landscapes where B. terrestris dominates wild and managed bumble bee communities. At present, individual‐level pollen diet composition is rarely quantified in field studies, limiting our ability to assess whether experimentally effective doses are reached under natural foraging conditions or sustained over ecologically relevant timescales. In addition, evidence in this field may be shaped by publication bias towards positive outcomes, making critical evaluation of null or context‐dependent results essential (Møller and Jennions 2001; Nakagawa et al. 2022).

To clarify how Asteraceae pollen—and H. annuus in particular—may influence disease dynamics in bumble bees, I integrate mechanistic, nutritional, and behavioural perspectives. This includes synthesising hypotheses on mechanical, chemical, and microbiome‐associated pathways; evaluating evidence for nutritional trade‐offs across castes and contexts; and analysing field‐relevant foraging patterns based on pollen‐trap data from B. terrestris and A. mellifera colonies in Central Europe. Where mechanistic evidence remains indirect—particularly for microbiome‐associated effects inferred largely from nectar or correlative studies—interpretations are treated as provisional (Koch et al. 2022; Yost et al. 2023). In addition, I examine Silphium perfoliatum , an increasingly cultivated perennial Asteraceae crop, as a potential complementary or contrasting resource (Mueller et al. 2020; Häfner et al. 2023). Together, these perspectives help identify when and how medicinal pollen may mitigate pathogen pressure in bumble bees and outline research priorities for pollinator‐friendly cropping systems.

2. Literature Identification and Scope

The synthesis presented here is based on a focused, narrative literature survey targeting Bombus–Asteraceae interactions, with emphasis on H. annuus and S. perfoliatum , and on outcomes related to foraging behaviour, palynology/chemistry, and bee health—particularly C. bombi infection. Searches were conducted in Google Scholar (last search: 12 October 2025) using combinations of terms for bumble bees or Apis, Asteraceae taxa, pollen/foraging/diet, and pathogens (e.g., “Bombus AND (sunflower OR Helianthus annuus OR Silphium perfoliatum ) AND pollen AND (Crithidia OR pathogen)”). Titles and abstracts were screened for relevance, followed by backward and forward citation chasing from key papers (e.g., Giacomini et al. 2018; Figueroa et al. 2023), and complemented by additional sources identified through field, chemical, and palynological expertise. Inclusion criteria were peer‐reviewed studies reporting Bombus or Apis foraging on focal Asteraceae, pollen palynology or chemistry, and/or bee health outcomes after exposure to Asteraceae pollen. Items lacking relevant outcomes or restricted to greenhouse assays without foraging or health endpoints were excluded. No formal risk‐of‐bias scoring or meta‐analysis was performed. Methods S1 provide a reproducible search record (queries, dates, counts, and deduplication notes). A semantic overview of the screened literature is shown in Figure S1, highlighting the dominance of B. impatiens studies and the scarcity of colony‐level development endpoints.

3. Pollen Traits of Asteraceae and Relevance for Bee Nutrition

The Asteraceae family produces pollen that is distinctive both morphologically and chemically, with important implications for bee foraging, digestion, and health outcomes. Two genera, Helianthus and Silphium, are of particular interest because they are increasingly used in agriculture and agri‐environmental schemes and have been linked to potential medicinal effects.

Asteraceae pollen is characterised by a thick, echinate exine bearing pronounced spines. This morphology is well documented for both H. annuus and S. perfoliatum (Halbritter et al. 2020; Auer and Koelzer 2021; Knoerr et al. 2024). Standardised PalDat plates depict medium‐sized, isopolar, tricolporate, echinate grains for both taxa and serve as a useful reference framework for discussing shared morphological traits. Although this distinctive architecture has been proposed to influence digestion and pathogen dynamics, direct in vivo evidence that spine morphology alone causes mechanical damage or detachment of parasites remains limited (Figueroa et al. 2023).

Chemically, H. annuus pollen is notable for its high concentrations of phenolamides, particularly di‐ and tricoumaroyl spermidines and related conjugates, which have been implicated in antimicrobial activity and antiparasitic effects in bee‐pathogen systems (Gekière et al. 2022; Palmer‐Young et al. 2017, 2023; Fitch et al. 2022). These compounds may influence parasite viability directly or act indirectly via modulation of host immunity or gut microbiota, but empirical support for microbiome‐associated effects of pollen (as opposed to nectar metabolites) remains limited and largely correlative (Koch et al. 2022; Yost et al. 2023). Asteraceae pollen also contains distinctive sterol profiles and variable lipid fractions, which can shape both nutritional quality and physiological responses. Legacy data suggest that sunflower pollen sterols are dominated by isofucosterol, β‐sitosterol, and 24‐methylene‐cholesterol, but modern cultivar‐level updates are largely lacking (Husband et al. 2025).

Nutritionally, Asteraceae pollen is often considered suboptimal, especially for A. mellifera . Sunflower pollen typically exhibits a protein‐to‐lipid ratio that deviates from the preferred range for honey bees and is associated with challenging digestibility and high K:Na ratios (Nicolson and Human 2013; Filipiak et al. 2022). Experimental feeding studies show reduced survival or poor performance of honey bee workers when confined to nutritionally constrained or low‐diversity pollen diets, including sunflower‐based regimes (Schmidt et al. 1995; Muturi et al. 2022). Bumble bees, by contrast, appear more flexible: some Bombus species tolerate or even selectively exploit Asteraceae pollen in certain contexts, although responses vary widely among species and environments (LoCascio, Aguirre, et al. 2019; Giacomini et al. 2018). This flexibility does not preclude nutritional trade‐offs, particularly under conditions of low floral diversity or high reliance on a single pollen type (Giacomini et al. 2021, 2023).

For S. perfoliatum , pollen‐specific chemical and nutritional data remain surprisingly scarce. Most work has focused on leaves, inflorescences, and essential oils, documenting phenolic acids and complex volatile blends (Kowalski 2003; Kowalski and Wolski 2005; Kowalski and Kędzia 2007). Pollen chemistry is therefore largely inferred rather than directly measured, and its potential medicinal or nutritional roles for bees remain hypothetical. Nevertheless, both H. annuus and S. perfoliatum share the characteristic echinate Asteraceae pollen morphology depicted in Figure 1, which underpins several mechanistic hypotheses discussed below.

FIGURE 1.

FIGURE 1

Open in a new tab

PalDat reference plates for Asteraceae pollen used in this review. A–C: H. annuus (A: LM overview, hydrated; B: SEM polar view; C: SEM exine surface). D–F: S. perfoliatum (D: LM overview, hydrated; E: SEM polar view; F: SEM exine surface). All panels illustrate medium‐sized (~26–30 μm), isopolar, tricolporate, echinate pollen typical of Asteraceae. Preparation details follow the original legends: LM (light microscopy) panels show hydrated pollen in glycerine (unstained); SEM (scanning electron microscopy) panels are rehydrated and critical‐point dried (Helianthus) or acetolyzed (Silphium) and sputter‐coated with gold. Images reproduced from PalDat—A palynological database (AutPal), entries accessed 03 September 2025: H. annuus ID 304619 (images 3, 6, 9); S. perfoliatum ID 305920 (images 1, 7, 11). Scale bars as in the original plates (Halbritter et al. 2020; Auer and Koelzer 2021).

A concise comparison of key traits for H. annuus and S. perfoliatum is provided in Table 1, emphasising well‐characterised features for sunflower and current gaps for Silphium.

TABLE 1.

Key pollen traits of Helianthus annuus and Silphium perfoliatum relevant for bee nutrition and antiparasitic mechanisms (Morphological traits from PalDat; chemistry based on published pollen/vegetative data).

Trait category Helianthus annuus Silphium perfoliatum
Pollen morphology Echinate (spiny), thick exine; tricolporate Echinate (spiny), thick exine; tricolporate
Size class (LM) ~26–30 μm (medium) ~26–30 μm (medium)
Protein content Generally low for Apis; broad ranges reported Likely moderate; pollen‐specific data largely lacking
Phenolamides High; hallmark tricoumaroyl spermidine; di/tri‐coumaroyl + feruloyl spermidines; tetracoumaroyl spermine (Palmer‐Young et al. 2017; Gekière et al. 2022) Unknown; pollen‐specific chemistry unreported (phenolics documented mainly for leaves/inflorescences)
Sterols Legacy: isofucosterol ~42%, β‐sitosterol ~20%, 24‐methylene‐cholesterol ~18%; modern cultivar updates lacking (Husband et al. 2025) No pollen‐specific sterol data available
Lipids/fatty acids Variable; α‐linolenic acid often 20%–42%; lipid fraction 1.5%–8.3% (bee‐collected pollen) No consolidated pollen lipid profile published
Minerals ~17 elements; Ca, K, P consistently major (Filipiak et al. 2022) Sparse pollen‐specific mineral data
Digestibility (Apis) Often poor; associated with reduced performance (Nicolson and Human 2013; Schmidt et al. 1995) Unknown
Attractiveness/uptake Low in Apis; context‐dependent in Bombus (LoCascio, Aguirre, et al. 2019) Used in some contexts; quantitative field data limited
Relevance for antiparasitic mechanisms Echinate pollen morphology and phenolamide‐rich chemistry have been proposed as contributors to antiparasitic effects, although their relative contributions remain unresolved Morphology is similar to Helianthus; chemical mechanisms remain speculative due to the absence of pollen‐specific chemical data
Open in a new tab

Note: Values for H. annuus compile data from bee‐collected and hand‐collected pollen across multiple geographies and cultivars (see Husband et al. 2025, for detailed tables). For S. perfoliatum , pollen chemistry and nutritional composition remain a major knowledge gap; most available chemical studies concern vegetative organs or essential oils rather than pollen (Kowalski 2003; Kowalski and Wolski 2005; Kowalski and Kędzia 2007).

4. Mechanistic Evidence for Antiparasitic Effects

Laboratory and semi‐field studies have repeatedly shown that sunflower pollen can reduce C. bombi infections in bumble bees, particularly in B. impatiens (Giacomini et al. 2018; Fowler et al. 2020; Figueroa et al. 2023). Comparable but often weaker effects have been observed for other echinate Asteraceae pollens. Three broad mechanistic classes have been proposed: mechanical abrasion, chemical activity, and microbiome‐ or digestion‐associated processes. Current evidence suggests that these pathways are not mutually exclusive, but their relative contributions remain incompletely resolved.

4.1. Mechanical Gut Abrasion

The echinate exine of Asteraceae pollen has long been hypothesised to inflict mechanical damage on gut parasites or host tissues. Both H. annuus and S. perfoliatum exhibit dense spines on their exine surface (Figure 1), which can interact physically with the intestinal epithelium (Halbritter et al. 2020; Auer and Koelzer 2021; Knoerr et al. 2024). Figueroa et al. (2023) provided compelling support for an abrasion‐based mechanism by showing that sunflower pollen continued to suppress C. bombi infection after chemical extracts had been removed, and that other echinate pollens could elicit similar reductions. These results suggest that physical contact between spiny grains and the gut lumen can disrupt parasite attachment or damage parasite cells directly. However, direct visualisation of abrasion events, parasite detachment, or cell damage in vivo remains scarce, and the magnitude of such effects is likely modulated by gut morphology, peristalsis, and mucus properties, all of which differ among Bombus species and developmental stages (LoCascio, Aguirre, et al. 2019; Husband et al. 2025). Consistent with a role of tissue‐level responses, sunflower pollen consumption has also been shown to upregulate immune transcripts linked to the maintenance and repair of the gut epithelium (Giacomini et al. 2023), suggesting that abrasion‐related processes may be accompanied by compensatory epithelial responses. An alternative possibility is that spines primarily influence digestion or transit rather than mechanically injuring parasites, a distinction that has not yet been experimentally resolved.

4.2. Chemical Activity of Phenolamides and Related Metabolites

Sunflower pollen is unusually rich in phenolamides, including di‐ and tricoumaroyl spermidines and structurally related conjugates that can exhibit antimicrobial and antiparasitic activity (Gekière et al. 2022; Palmer‐Young et al. 2017, 2023; Fitch et al. 2022; Barberis et al. 2023). These compounds may interfere with parasite metabolism or membrane integrity, alter gut epithelial properties, or modulate host immune responses. Experimental studies have shown that phenolamide‐rich fractions can reduce pathogen load or affect immune markers in bees, although results are often context dependent and vary with dose and exposure regime (Adler et al. 2020; Gekière et al. 2022; Giacomini et al. 2023).

A major complication is the substantial variation in phenolamide composition across sunflower cultivars, years, and growth conditions (Husband et al. 2025; Ferguson et al. 2024). Cultivars bred for oil yield or ornamental traits may differ markedly in pollen chemistry, and studies rarely report cultivar identity or quantify phenolamide profiles. This variability makes it difficult to generalise across experiments and may explain inconsistencies in infection outcomes and fitness metrics. For S. perfoliatum , pollen phenolamides have not yet been systematically quantified, although vegetative tissues contain abundant phenolic acids and bioactive extracts (Kowalski 2003; Kowalski and Wolski 2005; Kowalski and Kędzia 2007). Whether Silphium pollen shares sunflower's phenolamide‐based antiparasitic potential is therefore an open question.

4.3. Digestibility, Nutrient Ratios, and Microbiome‐Associated Processes

Asteraceae pollen's relatively low protein content, unusual mineral composition, and challenging digestibility introduce a second layer of mechanistic complexity. Poorly digestible pollen may accelerate gut transit, reducing the time available for C. bombi replication in the gut lumen. Studies on diet choice and digestion in Bombus indicate that sunflower pollen is digested less efficiently than nutritionally preferred pollen types and can increase excretion rates, which may contribute to reduced parasite establishment (LoCascio, Aguirre, et al. 2019; Giacomini et al. 2022). At the same time, low protein or imbalanced amino‐acid profiles can reduce colony performance when sunflower pollen constitutes a large fraction of the diet (Giacomini et al. 2021, 2023).

Sunflower pollen and co‐occurring floral resources can also influence gut microbial communities, either via direct effects of secondary metabolites or through changes in gut environment and substrate availability (Palmer‐Young et al. 2017; Fitch et al. 2022; Koch et al. 2022). Microbiome composition has been shown to modulate susceptibility to parasites and to shape the effectiveness of dietary interventions in Bombus (Koch et al. 2022). However, evidence that sunflower pollen per se induces microbiome shifts that directly mediate C. bombi suppression remains limited. Notably, gut‐transplant experiments indicate that microbiota from bees fed antipathogenic pollen diets do not necessarily confer resistance to recipients, suggesting that microbiome effects may be secondary or context dependent rather than a primary mechanism (Yost et al. 2023). Some studies nonetheless report that microbiome state explains substantial variance in infection outcomes (Fowler et al. 2020), implying that pollen chemistry and digestion may act partly by altering microbial communities rather than through direct antiparasitic activity alone.

4.4. Integrating Mechanisms: A Multimodal Framework

Given the diversity of pollen traits and the complexity of the bumble bee gut environment, it is unlikely that a single mechanism explains the full range of observed antiparasitic effects. Mechanical abrasion, phenolamide chemistry, digestion dynamics, and microbiome‐associated processes are best viewed as interacting components of a mechanistic suite rather than independent pathways. Spiny pollen grains may disturb gut surfaces and parasites, while simultaneously releasing phenolamides and other metabolites that act locally or systemically. Digestive inefficiencies may shorten transit time and alter nutrient landscapes for both parasites and microbiota, and microbiome shifts may further influence immune priming, pathogen competition, and gut physicochemistry.

These interactions are summarised in Figure 2 using directional arrows (↑, ↓) to indicate increases or decreases in specific processes along the infection pathway. The model situates mechanical, chemical, and microbiome‐associated actions at the level of parasite adhesion and replication in the ileum and hindgut, while highlighting the importance of exposure context (pollen purity, co‐flowering resources, nectar‐only foraging) in modulating dose and mechanism strength.

FIGURE 2.

FIGURE 2

Open in a new tab

Conceptual model of how Asteraceae pollen could reduce Crithidia bombi load in bumble bees. Three non‐exclusive mechanisms are proposed to act during parasite establishment and persistence in the ileum and hindgut. Mechanical abrasion arises from echinate pollen traits (medium‐sized, ~26–30 μm, tricolporate grains with dense spines), increasing abrasive interactions and thereby reducing parasite adhesion. Chemical activity involves phenolamides, flavonoids, and sterols with direct antiparasitic effects on C. bombi and potential immunomodulatory effects on the host. Microbiome‐associated processes include shifts in gut microbial community composition and pH that may enhance competitive exclusion and reduce parasite load. Exposure context, including pollen purity, presence of co‐flowering species, and nectar‐only foraging, modulates pollen dose and thus the strength of these mechanisms. The schematic highlights the ileum as the principal site of C. bombi adhesion and replication (not to scale). Species identity, life stage, exposure regime, and landscape or seasonal context are shown as modifiers that influence the strength and relevance of all pathways.

Species‐specific responses further complicate this picture. Several Bombus species show reduced C. bombi infections after consuming sunflower pollen, whereas A. mellifera generally avoids Asteraceae pollen, digests it poorly, and does not exhibit comparable health benefits (Nicolson and Human 2013; Giacomini et al. 2018; Palmer‐Young et al. 2023). Even within Bombus, castes and sexes differ in physiology and may respond differently to the same dietary interventions (Fowler et al. 2020; Giacomini et al. 2023). The resulting framework is therefore strongly context dependent, and mechanistic interpretations must be evaluated alongside fitness outcomes and real‐world foraging patterns.

5. Fitness Trade‐Offs and Host‐Level Constraints

While sunflower pollen has robust antiparasitic effects in many laboratory assays, growing evidence suggests that these benefits may come at nutritional or physiological costs, particularly when sunflower constitutes a dominant diet component. In controlled colony‐feeding experiments, B. impatiens colonies maintained on sunflower‐rich diets showed reduced growth and lower queen production compared with colonies receiving more protein‐rich mixed pollen (Giacomini et al. 2021, 2023). These results indicate that sunflower pollen's composition, including its relatively low protein content and unbalanced amino‐acid profile, can constrain colony performance, even if parasite loads decline. In this sense, a resource that is medicinal at the individual level may become nutritionally limiting at the colony level when not complemented by diverse floral inputs.

Immunological responses to sunflower pollen further complicate this picture. Fowler et al. (2020) reported paradoxical patterns in immune markers: some genes associated with immune defence were upregulated, suggesting immune activation or priming, whereas others indicated potential immune suppression or reallocation of energetic resources. This immune activation–suppression paradox implies that sunflower pollen may not simply “boost” immunity but instead induces nuanced, context‐dependent shifts in immune function that could have both protective and detrimental consequences depending on exposure duration, parasite pressure, and colony condition. Such responses are therefore better interpreted as immune reconfiguration rather than uniformly beneficial immune enhancement.

Caste‐ and sex‐specific responses further shape the fitness landscape. Fowler et al. (2020) found that queens and workers benefitted from reduced C. bombi infections after consuming sunflower pollen, whereas males did not show comparable advantages. Given that queens represent the primary reproductive bottleneck in annual bumble bee life cycles, such caste‐specific effects could influence population dynamics, particularly in landscapes where sunflower is abundant. At the same time, negative impacts on worker longevity or brood development might offset queen‐level benefits under certain conditions.

Interactions with additional stressors are likely to modulate the cost–benefit balance of sunflower pollen. Malfi et al. (2023) showed that sunflower plantings reduced C. bombi infection in queens and increased queen production in B. impatiens , but overall outcomes depended on landscape complexity and pesticide exposure. Sublethal pesticide residues can alter immunity and microbiome composition (Castelli et al. 2020; Rivest et al. 2024), potentially amplifying or diminishing the effects of medicinal pollen. Husband et al. (2025) emphasise that dietary interventions cannot be evaluated in isolation because host genotype, parasite strain variation, and environmental factors jointly determine immune and health outcomes.

Taken together, these findings underscore a central paradox in the sunflower‐for‐health narrative. Antiparasitic effects are often strong and reproducible under controlled, simplified conditions, yet the same dietary traits that suppress parasites can impose nutritional constraints or immunological trade‐offs at the colony level. Under resource‐poor or low‐diversity conditions, sunflower pollen could therefore function less as a “cure” than as a context‐dependent or even potentially maladaptive resource. Resolving this paradox requires integrating mechanistic studies with realistic diets, mixed floral resources, multiple stressors, and colony‐level metrics, rather than focusing solely on pathogen load in individual workers.

6. Field Foraging Patterns in Europe: Integrating New Pollen‐Trap Data

A major open question is whether bumble bees, particularly B. terrestris , actually consume sunflower pollen at levels sufficient to realise the medicinal effects observed in laboratory assays. To address this, we analysed trap‐collected pollen from Apis mellifera and B. terrestris colonies placed at experimental sites in Niedersachsen, Germany (Elm region near Königslutter) (Odemer et al. 2025). Colonies were positioned at the margins of large sunflower fields (~40 ha) and smaller stands of S. perfoliatum (~2 ha) within the typical foraging distance of B. terrestris (Wolf and Moritz 2008). Pollen traps were operated during a comparable late‐summer window in both years (mid‐July to late August 2024 and 2025). Sunflower was present only in 2024, whereas Silphium flowered only in 2025. On each sampling date, trap pollen was pooled across all colonies of a species and identified to at least family level. For analysis, pollen was grouped into H. annuus , S. perfoliatum , Solanaceae (dominated by Solanum spp.), other Asteraceae, and all remaining taxa (“other pollen”; see Table S1), and normalised to 100% per sample; analytical details and raw data are provided in Methods S2. Because pollen was pooled across colonies and sampling dates were limited—especially in 2025—these data describe qualitative patterns of resource use rather than population‐level estimates.

Across both years, A. mellifera foraging patterns were remarkably stable. In 2024, sunflower pollen accounted for only ~3%–6% of total corbicular pollen; in 2025, Silphium contributed similarly low proportions of ~2%–3% (Figure 3). The relative share of “other Asteraceae” also changed little between years, indicating that shifts in which Asteraceae crop dominated the landscape did not substantially reconfigure honey bee diets. These findings align with previous work showing that honey bees often treat Asteraceae pollen as a low‐priority resource and exhibit comparatively weak macronutrient‐driven selectivity for such pollen when alternative sources are available (Nicolson and Human 2013; Weiner et al. 2010; Castelli et al. 2020; Stephen et al. 2024). From an applied perspective, even if sunflower or Silphium pollen possess medicinal properties, their low uptake by A. mellifera in diverse landscapes suggests that they are unlikely to become major pollen sources for honey bee colonies.

FIGURE 3.

FIGURE 3

Open in a new tab

Pollen composition in honey bee ( Apis mellifera ) and bumble bee ( Bombus terrestris ) colonies foraging in landscapes differing in Asteraceae crop availability (Niedersachsen, Germany; mid‐July to late August 2024–2025). Stacked bars show proportional contributions of major pollen groups in trap‐collected corbicular pollen. In 2024, pooled pollen samples were obtained on three sampling dates for A. mellifera (3 pooled samples) and four sampling dates for B. terrestris (4 pooled samples). In 2025, pollen was collected on one sampling date for A. mellifera (1 pooled sample) and two sampling dates for B. terrestris (2 pooled samples). Sunflower ( H. annuus ) occurred only in 2024, whereas Silphium ( S. perfoliatum ) was available only in 2025. Solanaceae pollen is dominated by Solanum spp.; “Other Asteraceae” includes all remaining Asteraceae taxa; “Other pollen” comprises all non‐Asteraceae types. All values are normalised to 100% per sample.

In B. terrestris , by contrast, we observed pronounced discrimination among pollen types. In the sunflower year (2024), H. annuus pollen was completely absent from bumble bee loads, despite high sunflower availability. In the Silphium year (2025), S. perfoliatum reached moderate levels of approximately 5%–10% of total pollen (Figure 3). Thus, the same species that entirely avoided sunflower under one landscape configuration did incorporate Silphium pollen when it became available. Given that H. annuus and S. perfoliatum pollen share similar echinate, medium‐sized, tricolporate morphology (Figure 1; Halbritter et al. 2020; Auer and Koelzer 2021), and that sunflower is rich in phenolamides with antiparasitic potential (Gekière et al. 2022; Palmer‐Young et al. 2023; Husband et al. 2025), these differences in uptake are unlikely to be explained by palynology alone. Instead, they point to functional differences in floral architecture, nectar rewards, phenological overlap with preferred resources, and broader competitive context. Other factors—such as local floral diversity or potential chemical deterrence—cannot be excluded with the present dataset.

The most conspicuous pattern in our data concerns Solanaceae pollen. In both years, B. terrestris pollen loads were dominated by Solanaceae, often exceeding 65%–90% of total pollen, while A. mellifera collected no Solanaceae pollen at all (Figure 3). This extreme divergence reflects the buzz‐pollination syndrome of many Solanum species: bumble bees can sonicate poricidal anthers to release pollen, whereas honey bees cannot (Buchmann and Cane 1989). Nutritional studies show that Bombus workers actively track pollen with favourable protein: lipid ratios and amino‐acid profiles and adjust foraging to maintain these macronutrient targets (Stabler et al. 2015; Vaudo et al. 2016; 2020; Adler et al. 2020, 2025; Stephen et al. 2024). In our system, Solanaceae therefore functioned as the primary core pollen source for B. terrestris , with Helianthus and Silphium playing secondary roles despite their potential medicinal traits.

These field patterns dovetail with the wider literature. Sunflower pollen is often under‐represented in corbicular loads or actively avoided when other resources are available, even where antiparasitic effects have been demonstrated in the laboratory (Nicolson and Human 2013; LoCascio, Aguirre, et al. 2019; LoCascio, Pasquale, et al. 2019; Giacomini et al. 2018; Filipiak et al. 2022). Most laboratory studies demonstrating such effects relied on monofloral or high‐proportion sunflower diets, and the minimum dietary fraction required to achieve meaningful pathogen suppression under mixed‐field conditions remains unresolved. Recent experimental work confirms this context dependence: McCormick and Adler (2026) showed that sunflower pollen reduced C. bombi infection across closely related Bombus species, but only under controlled feeding regimes with substantial sunflower pollen fractions, underscoring that effects observed under experimental conditions may not translate directly to mixed diets in the field. Accordingly, reports of reduced pathogen prevalence in sunflower‐dominated agroecosystems may reflect indirect, habitat‐level mechanisms rather than sunflower becoming a dominant pollen resource per se; for example, Pluta et al. (2024) documented health effects in A. mellifera in organic sunflower systems without corresponding evidence of high sunflower pollen intake. At the same time, Asteraceae pollen is frequently nutritionally constrained or difficult to digest, especially for Apis (Nicolson and Human 2013; Filipiak et al. 2022), and Bombus responses remain strongly context dependent (LoCascio, Aguirre, et al. 2019; LoCascio, Pasquale, et al. 2019; Giacomini et al. 2018; Cohen et al. 2021; Rivest et al. 2024).

From a broader perspective, our field data reinforce a central message of this review. Sunflower pollen is best viewed as a context‐dependent medicinal supplement within a diverse diet rather than a standalone therapeutic resource. Silphium appears somewhat more compatible with B. terrestris foraging than sunflower, yet both remain clearly secondary to preferred pollens such as Solanaceae. Mass‐flowering crops like sunflower may still shape pathogen dynamics indirectly—for example via pollinator aggregation or interactions with landscape floral diversity—but any health benefits must be evaluated against nutritional and behavioural constraints and realistic exposure levels.

7. Outstanding Mechanistic Uncertainties

Despite substantial progress, several critical mechanistic questions remain unresolved. One of the most important concerns the relative contribution of gut microbiome modulation versus direct mechanical or chemical effects. Koch et al. (2022) showed that nectar metabolites can alter parasite susceptibility via host–microbiome interactions in Bombus, suggesting that similar mechanisms might operate for pollen‐derived phenolamides. However, the causal chain linking sunflower pollen ingestion, microbiome shifts, and C. bombi suppression remains incomplete. Recent gut‐transplant experiments indicate that microbiome changes alone are insufficient to transfer pathogen resistance between hosts (Yost et al. 2023), implying that any microbiome‐associated effects of sunflower pollen likely depend on concurrent physical or chemical properties of the pollen itself rather than on community restructuring in isolation. Whether such interactions persist under field conditions or under co‐exposure to agrochemicals remains unknown.

Another major gap involves the uptake, metabolism, and bioavailability of pollen phenolamides. Palmer‐Young et al. (2023) and Gekière et al. (2022) emphasise that these compounds may act locally in the gut or be absorbed and transported to other tissues, but direct evidence for their pharmacokinetics in bumble bees is scarce. It is unknown whether phenolamides accumulate in specific tissues, how quickly they are cleared, and whether they require metabolic activation or conjugation to exert antiparasitic effects. These uncertainties limit attempts to quantitatively link measured pollen chemistry to observed infection outcomes or fitness effects.

A further challenge is distinguishing mechanical damage from immunological priming as drivers of infection reduction. The hypothesis that echinate pollen spines physically disrupt gut epithelium or parasite structures is compelling and supported by indirect lines of evidence (Knoerr et al. 2024; Figueroa et al. 2023), yet immune gene expression changes have also been observed after sunflower exposure (Fowler et al. 2020; Giacomini et al. 2023). Disentangling the relative contributions of physical abrasion, local tissue damage, and repair, and systemic immune activation will require multifactorial experiments combining microscopy, histology, transcriptomics, and metabolomics, ideally under ecologically realistic diets.

Finally, experimental standardisation remains a major limitation. Many studies rely on commercial sunflower pollen without reporting cultivar, storage history, or phenolamide content; infection protocols, diet composition, and exposure duration also differ widely. Targeted experiments with panels of well‐characterised cultivars, standardised infection assays, and harmonised endpoints are needed to determine how cultivar‐level variation in pollen chemistry maps onto parasite suppression and fitness outcomes.

Table 2 summarises selected knowledge gaps and suggests experimental approaches to address them.

TABLE 2.

Key mechanistic knowledge gaps and experimental approaches.

Knowledge gap Proposed hypothesis Suggested experimental approach Reference context
1. Microbiome‐associated effects Sunflower pollen modulates gut microbiota, altering parasite susceptibility Gnotobiotic bees with defined microbiomes; 16S rRNA sequencing before/after pollen diets; gut‐transplant experiments to test whether microbiome shifts alone can transfer resistance Koch et al. (2022); Yost et al. (2023)
2. Phenolamide uptake and metabolism Phenolamides are absorbed and act systemically or locally on parasites Isotope‐labelled phenolamides; metabolomic profiling of bee tissues; pharmacokinetic tracking of accumulation, conjugation, and clearance Palmer‐Young et al. (2023); Gekière et al. (2022)
3. Mode of action: gut damage vs. immune priming Pollen spines damage epithelial cells; phenolamides stimulate immune response Histological gut analyses; qPCR of immune‐related genes; in vitro gut epithelium assays; combined multi‐omics (transcriptomics + metabolomics) under mixed diets Figueroa et al. (2023); Knoerr et al. (2024); Fowler et al. (2020)
4. Cultivar‐specific variation Helianthus cultivars differ in phenolamide levels and antiparasitic activity Comparative chemical analysis of pollen across cultivars; standardised infection assays; report cultivar identity and quantify phenolamide profiles in all feeding trials Figueroa et al. (2023); Husband et al. (2025)
5. Interaction with pesticides or co‐pathogens Phenolamide or microbiome effects are modulated by co‐exposures Full‐factorial bioassays: pollen × pathogen × pesticide; survival, immunity, and pathogen load endpoints Malfi et al. (2023); Castelli et al. (2020); Rivest et al. (2024)
6. Bee caste‐ or sex‐specific responses Effects vary between workers, queens, and males due to physiological differences Separate trials per caste; measure survival, reproduction, immunity, and infection Fowler et al. (2020); Giacomini et al. (2023)
7. Species bias and transferability of medicinal pollen effects Antiparasitic effects of Asteraceae pollen demonstrated primarily in B. impatiens may differ in magnitude, mechanism, or fitness consequences in other Bombus species and ecological contexts Comparative infection and diet assays across multiple Bombus species (e.g., B. terrestris , B. pascuorum ), including colony‐level endpoints under mixed‐pollen diets; explicit cross‐species replication of existing laboratory protocols. Giacomini et al. (2018); Fowler et al. (2020); Figueroa et al. (2023); Cameron and Sadd (2020)
Open in a new tab

8. Limitations of Current Research

The current evidence base on medicinal Asteraceae pollen is shaped by several systematic limitations. The most prominent is a pronounced model bias towards B. impatiens , which dominates experimental work, particularly in North American systems (Giacomini et al. 2018; Fowler et al. 2020; Figueroa et al. 2023). The keyword co‐occurrence network in Figure S1 underscores this pattern and simultaneously reveals the relative underrepresentation of B. terrestris and of colony development endpoints. This bias raises questions about the transferability of findings to European landscapes and other Bombus taxa with different immune systems, microbiota, and foraging ecologies, and highlights a major knowledge gap for pollinator health strategies outside North America. More generally, field studies indicate that C. bombi prevalence and its association with fitness outcomes can vary across host populations and parasite contexts (Imhoof and Schmid‐Hempel 1999; Gillespie 2010; Cordes et al. 2012), reinforcing the need for broader taxonomic and geographic inference.

A second limitation is the prevalence of highly controlled laboratory or cage experiments using monofloral diets and artificial infection protocols. These designs are invaluable for isolating mechanisms but poorly reflect the complexity of natural foraging environments, where bees encounter mixed pollen diets, fluctuating resource availability, and heterogeneous parasite and pesticide pressure (Malfi et al. 2023; Husband et al. 2025). Under field conditions, bees rarely rely on a single pollen source, and interactions among multiple pollen types, as well as sublethal pesticide exposures, can modulate or mask the effects of medicinal resources. This is particularly relevant in simplified agricultural landscapes, where limited floral heterogeneity may prevent bees from balancing nutritionally suboptimal but potentially antiparasitic pollen with complementary resources. Empirical evidence from agricultural landscapes further supports that “field type” and local resource context shape bumble bee pollen diets and therefore likely constrain the achievable dose of any medicinal pollen (Piko et al. 2021).

Environmental context is often underreported or inadequately controlled. Landscape floral composition, pesticide residue profiles, and local microbiota can influence infection dynamics and dietary responses, yet many studies lack detailed habitat or exposure information (Cohen et al. 2021; Tiritelli et al. 2024; Rivest et al. 2024). In addition, global‐change drivers are rarely considered: rising atmospheric CO2 can alter pollen nutritional quality and stoichiometry, potentially exacerbating the trade‐off between parasite suppression and nutritional stress (Ziska et al. 2016; Bernauer et al. 2024; Filipiak 2024), while agrochemical exposure may impair immunity or gut microbial function, thereby constraining any “medicinal” effects of pollen under real‐world conditions. Given the strong genotype‐by‐genotype and mixed‐infection structure documented for C. bombi in natural systems (Schmid‐Hempel and Funk 2004; Tognazzo et al. 2012), context dependence likely also extends to parasite population composition and evolutionary history. Moreover, null or ambiguous results, including cases where sunflower pollen fails to confer benefits or even impairs performance, appear underrepresented, which may bias perceptions of efficacy (Stevenson et al. 2022). This concern is consistent with broader evidence for publication bias in ecology and evolution (Møller and Jennions 2001), and it cautions against overgeneralising from a literature that may preferentially retain “positive medicinal effects”.

Finally, critical data gaps remain for key crops and European species. For S. perfoliatum , pollen chemistry and health effects on bumble bees are virtually undocumented, despite growing use of this crop in bioenergy and agri‐environmental schemes (Mueller et al. 2020; Häfner et al. 2023; Wevera et al. 2019). Likewise, studies that explicitly compare Apis and Bombus responses to Asteraceae pollen under shared conditions are rare, limiting insights into cross‐taxon generality or trade‐offs. Without broader taxonomic and ecological coverage, there is a risk of overgeneralizing from narrow model systems, particularly when extrapolating laboratory efficacy to landscape‐scale pollinator management. Importantly, parasite evolution and rapid adaptive responses remain largely untested in this literature despite experimental evidence that trypanosomatid parasites can evolve under controlled selection (Marxer et al. 2016) and that host immune responses differ among parasite genotypes (Barribeau and Schmid‐Hempel 2013), implying that “medicinal” effects could be spatially variable or transient over longer timescales.

9. Future Directions for Mechanistic and Field‐Integrated Research

Addressing these limitations will require coordinated efforts that integrate mechanistic, nutritional, and ecological perspectives. A clear priority is to expand beyond B. impatiens and to develop mechanistic datasets for B. terrestris and other European Bombus taxa, which dominate wild and managed pollinator communities yet remain underrepresented in experimental work and colony‐level endpoints. Standardised feeding and infection assays should explicitly include multiple castes and sexes and report key contextual variables (e.g., parasite inoculum, exposure duration, pollen handling/storage), because responses to sunflower pollen and Crithidia can differ across workers, queens, and males and may not scale from one life stage to another (Fowler et al. 2020; Giacomini et al. 2023). Where possible, assays should move from monofloral to field‐realistic mixed diets to improve translatability, as pollen quality trade‐offs and nutrient regulation can strongly shape performance outcomes under realistic foraging constraints (Archer et al. 2021; Brochu et al. 2020).

A second priority is systematic screening of Asteraceae species and sunflower cultivars for antiparasitic efficacy and fitness trade‐offs, rather than treating “sunflower pollen” as a uniform intervention. Cultivar‐ and environment‐driven variation in pollen secondary chemistry (including phenolamides) and nutritional profiles is likely to contribute to heterogeneous outcomes across studies and should therefore be quantified alongside infection and performance endpoints (Palmer‐Young et al. 2023). For S. perfoliatum , pollen chemistry and health effects remain a key European knowledge gap, despite increasing use in agricultural contexts and flower‐resource planning; targeted pollen chemical characterisation paired with bioassays is therefore a high‐value starting point (Mueller et al. 2020; Häfner et al. 2023). To keep mechanistic interpretation conservative, screening should include designs that can separate mechanical from chemical contributions (e.g., spine‐manipulation, extract‐removal, microscopy/histology) and explicitly test whether effects depend on diet mixture or dose (Figueroa et al. 2023).

Third, diet–pathogen–pesticide interactions should be incorporated into risk assessment frameworks and into the interpretation of “medicinal pollen” effects under Anthropocene conditions. Sublethal pesticide exposure can modulate immunity and gut microbial function, creating plausible scenarios where antiparasitic pollen effects are weakened, reversed, or expressed only under specific landscape resource conditions (Goulson et al. 2015; Malfi et al. 2023; Rivest et al. 2024). Field evidence indicates that the floral context (monoculture vs. heterogeneous landscapes/flower‐field types) measurably shapes bumble bee pollen diets, reinforcing the need to evaluate “medicinal” resources as part of a wider resource mosaic rather than as a single‐crop solution (Piko et al. 2021). In parallel, global‐change drivers should be treated explicitly as boundary conditions: elevated atmospheric CO2 can reduce pollen protein concentration in some systems and can alter pollen chemistry in species‐specific ways, potentially shifting the balance between infection reduction and nutritional stress (Ziska et al. 2016; Bernauer et al. 2024).

Finally, there is an urgent need for field experiments that tightly couple mechanistic measurements with colony‐level and landscape‐level outcomes. One promising design is to compare colonies placed at sites dominated by sunflower, Silphium, or control vegetation, equipped with pollen traps, continuous weight monitoring, and periodic health assessments, including pathogen load, strain/genotype context where feasible, immune markers, microbiome composition, and reproductive output. Because C. bombi is genetically diverse and frequently occurs as mixed‐genotype infections, field‐integrated designs should acknowledge evolutionary potential and geographic variability rather than assuming stable efficacy across parasite populations (Schmid‐Hempel and Funk 2004; Tognazzo et al. 2012). Experimental evolution work further supports that trypanosomatids can respond to selection under controlled regimes, strengthening the rationale for long‐term and multi‐site validation rather than single‐season “snapshots” (Marxer et al. 2016). Advanced analytical tools, including untargeted metabolomics, high‐resolution microscopy, and next‐generation sequencing of gut microbiota, should be integrated into these field designs to map from pollen chemistry and gut processes to infection dynamics and colony performance (Koch et al. 2022; Figueroa et al. 2023; Giacomini et al. 2023). Only by linking these levels—while explicitly documenting landscape resource context and stressor co‐exposure—can we evaluate whether medicinal pollen is a robust, scalable component of pollinator‐supportive cropping systems (Goulson et al. 2015; Piko et al. 2021).

10. Conclusion

Over the past decade, sunflower pollen has emerged as a promising natural agent to mitigate pathogen pressure in bumble bees, particularly against C. bombi. A growing body of laboratory evidence—primarily from North American studies using B. impatiens —suggests that echinate pollen morphology, phenolamide‐rich chemistry, and associated effects on digestion and microbiota can combine to suppress infection. At the same time, emerging work indicates that these antiparasitic benefits are not cost free: nutritional constraints, immunological trade‐offs, and caste‐specific responses can reduce colony performance when sunflower dominates the diet (Giacomini et al. 2021, 2023; Fowler et al. 2020).

Field data from European landscapes, including the pollen‐trap results presented here, show that B. terrestris often avoids sunflower pollen entirely and instead relies heavily on Solanaceae pollen, with Silphium playing a secondary but more consistent role than sunflower. Honey bees, in turn, collect only small fractions of either sunflower or Silphium pollen. This direct contrast between experimental efficacy and realised field exposure highlights a central constraint of the medicinal‐pollen concept: sunflower pollen is unlikely to function as a standalone therapeutic resource in most real‐world contexts. Instead, it may act as a context‐dependent supplement within diverse floral assemblages, with Silphium and other Asteraceae potentially providing complementary, and in some systems more relevant, resources.

Overall, the concept of medicinal pollen remains compelling but requires a more critical, field‐anchored framework before it can be translated into pollinator health strategies. Future work must explicitly link pollen chemistry, gut mechanisms, and infection outcomes to field‐realistic foraging patterns, nutritional landscapes, and pesticide regimes, and must broaden beyond a single model species. Only by integrating mechanistic insight with behavioural ecology and landscape context can laboratory findings be meaningfully applied to pollinator conservation and management. As pollinator declines continue, any proposed solution must be grounded in science that reflects the ecological and taxonomic diversity of real‐world systems rather than idealised laboratory scenarios.

Author Contributions

Richard Odemer: conceptualization (lead), data curation (lead), formal analysis (lead), investigation (lead), methodology (lead), project administration (lead), resources (lead), software (lead), validation (lead), visualization (lead), writing – original draft (lead), writing – review and editing (lead).

Funding

This work was supported by the Federal Ministry of Agriculture, Food and Regional Identity (BMLEH), Germany, through the Agency for Renewable Resources (FNR), based on a decision of the Parliament of the Federal Republic of Germany (Grant 22012118; project FInAL). The funders had no role in study design, analysis, interpretation, or the decision to publish, and the views expressed are those of the author only. This work was supported by Fachagentur Nachwachsende Rohstoffe, 22012118.

Ethics Statement

This article synthesises previously published studies and involves no new research with human participants, human data or tissue, or experimental work with animals.

Consent

The author has nothing to report.

Conflicts of Interest

The author declares no conflicts of interest.

Supporting information

Data S1: ece373107‐sup‐0001‐DataS1.docx.

ECE3-16-e73107-s001.docx (322KB, docx)

Acknowledgements

Pollen analyses for the A. mellifera and B. terrestris samples presented in this review were conducted by the LAVES Institute for Apiculture in Celle. I thank Fredrik Mühlberger and Marie Geiger for their assistance during field sampling of pollen in the FInAL landscape laboratory in the Elm region (Lower Saxony). I also kindly acknowledge the cooperation of the farmers participating in the FInAL landscape lab. Finally, I thank Salena Husband for critical reading of the manuscript and the three anonymous reviewers for their constructive and insightful comments, which helped to improve the clarity, balance, and critical scope of this review. Open Access funding enabled and organized by Projekt DEAL.

Use of AI‐assisted technology in the writing process: During the preparation of this work, the author used DeepL Write and ChatGPT to improve the structure, phrasing, and clarity of selected text passages originally written by the author. These tools were employed to enhance language quality and manuscript organisation. Following their use, the author carefully reviewed and edited all content to ensure accuracy, scientific integrity, and consistency with the intended meaning. The author takes full responsibility for the content of the publication.

Odemer, R. 2026. “Sunflower Pollen and Bumble Bee Health: Mechanisms, Modifiers and Trade‐Offs.” Ecology and Evolution 16, no. 2: e73107. 10.1002/ece3.73107.

Data Availability Statement

All data required to reproduce the analyses in this study are available in the Supporting Information and archived openly at the Open Science Framework (OSF): https://doi.org/10.17605/OSF.IO/ZYGHV.

References

  1. Adler, L. S. , Fowler A. E., Malfi R. L., et al. 2020. “Assessing Chemical Mechanisms Underlying the Effects of Sunflower Pollen on a Gut Pathogen in Bumble Bees.” Journal of Chemical Ecology 46: 649–658. 10.1007/s10886-020-01168-4. [DOI] [PubMed] [Google Scholar]
  2. Adler, M. , Ajhuacho‐Villalobos M., Flores‐Prado L., Benitez‐Vieyra S., Collao‐Alvarado K., and Pinto C. F.. 2025. “Dominance of Capsicum minutiflorum (Solanaceae) Pollen in Stingless Bee Hives: An Insight into Protein Composition and Foraging Behavior by Four Meliponini Species of the Bolivian‐Tucumano Forest.” Journal of Hymenoptera Research 98: 689–708. 10.3897/jhr.98.138703. [DOI] [Google Scholar]
  3. Archer, C. R. , Fähnle J., Pretzner M., et al. 2021. “Complex Relationship Between Amino Acids, Fitness and Food Intake in Bombus terrestris .” Amino Acids 53: 1545–1558. 10.1007/s00726-021-03075-8. [DOI] [PMC free article] [PubMed] [Google Scholar]
  4. Auer, W. , and Koelzer K.. 2021. “Silphium perfoliatum.” In: PalDat—A palynological database. Accessed 2025‐09‐27. https://www.paldat.org/pub/Silphium_perfoliatum/305920.
  5. Barberis, M. , Calabrese D., Galloni M., and Nepi M.. 2023. “Secondary Metabolites in Nectar‐Mediated Plant–Pollinator Relationships.” Plants 12: 550. 10.3390/plants12030550. [DOI] [PMC free article] [PubMed] [Google Scholar]
  6. Barribeau, S. M. , and Schmid‐Hempel P.. 2013. “Qualitatively Different Immune Response of the Bumblebee Host, Bombus terrestris, to Infection by Different Genotypes of the Trypanosome Gut Parasite, Crithidia bombi .” Infection, Genetics and Evolution 20: 249–256. 10.1016/j.meegid.2013.09.014. [DOI] [PubMed] [Google Scholar]
  7. Bergonzoli, S. , Romano E., Beni C., Latterini F., Lo Scalzo R., and Scarfone A.. 2022. “Nectar Dynamics and Pollinators' Preference in Sunflower.” Insects 13: 717. 10.3390/insects13080717. [DOI] [PMC free article] [PubMed] [Google Scholar]
  8. Bernauer, O. M. , Jain A., de Bivort B., et al. 2024. “Elevated Atmospheric CO2 Has Small, Species‐Specific Effects on Pollen Chemistry and Plant Growth Across Flowering Plant Species.” Scientific Reports 14: 13760. 10.1038/s41598-024-63967-z. [DOI] [PMC free article] [PubMed] [Google Scholar]
  9. Brochu, K. K. , van Dyke M. T., Milano N. J., et al. 2020. “Pollen Defenses Negatively Impact Foraging and Fitness in a Generalist Bee (Bombus impatiens: Apidae).” Scientific Reports 10: 3112. 10.1038/s41598-020-58274-2. [DOI] [PMC free article] [PubMed] [Google Scholar]
  10. Buchmann, S. L. , and Cane J. H.. 1989. “Bees Assess Pollen Returns While Sonicating Solanum Flowers.” Oecologia 81: 289–294. 10.1007/BF00377073. [DOI] [PubMed] [Google Scholar]
  11. Cameron, S. A. , and Sadd B. M.. 2020. “Global Trends in Bumble Bee Health.” Annual Review of Entomology 65: 209–232. 10.1146/annurev-ento-011118-111847. [DOI] [PubMed] [Google Scholar]
  12. Castelli, L. , Branchiccela B., Garrido M., et al. 2020. “Impact of Nutritional Stress on Honeybee Gut Microbiota, Immunity, and Nosema ceranae Infection.” Microbial Ecology 80: 908–919. 10.1007/s00248-020-01538-1. [DOI] [PubMed] [Google Scholar]
  13. Cohen, H. , Smith G. P., Sardiñas H., et al. 2021. “Mass‐Flowering Monoculture Attracts Bees, Amplifying Parasite Prevalence.” Proceedings of the Royal Society B: Biological Sciences 288: 20211369. 10.1098/rspb.2021.1369. [DOI] [PMC free article] [PubMed] [Google Scholar]
  14. Cordes, N. , Huang W.‐F., Strange J. P., et al. 2012. “Interspecific Geographic Distribution and Variation of the Pathogens Nosema bombi and Crithidia Species in United States Bumble Bee Populations.” Journal of Invertebrate Pathology 109, no. 2: 209–216. 10.1016/j.jip.2011.11.005. [DOI] [PubMed] [Google Scholar]
  15. Ferguson, B. , Prasifka J. R., Carroll M. J., Corby‐Harris V., and DeGrandi‐Hoffman G.. 2024. “Honey Bee (Apis mellifera L.) Foraging Rewards in Sunflowers: Effect of Floral Traits on Visitation and Variation in Pollen Quality Over Two Consecutive Years.” Journal of Apicultural Research 63, no. 4: 622–630. 10.1080/00218839.2024.2364948. [DOI] [Google Scholar]
  16. Figueroa, L. L. , Fowler A., Lopez S., et al. 2023. “Sunflower Spines and Beyond: Mechanisms and Breadth of Pollen That Reduce Gut Pathogen Infection in the Common Eastern Bumble Bee.” Functional Ecology 37: 1757–1769. 10.1111/1365-2435.14320. [DOI] [Google Scholar]
  17. Filipiak, M. , Shields M. W., Cairns S. M., Grainger M. N. C., and Wratten S. D.. 2022. “The Conserved and High K‐To‐Na Ratio in Sunflower Pollen: Possible Implications for Bee Health and Plant–Bee Interactions.” Frontiers in Plant Science 13: 1042348. 10.3389/fpls.2022.1042348. [DOI] [PMC free article] [PubMed] [Google Scholar]
  18. Filipiak, M. 2024. “Plants Other Than Animal‐Pollinated Herbs Provide Wild Bees with Vital Nutrients.” Global Ecology and Conservation 52: e02984. 10.1016/j.gecco.2024.e02984. [DOI] [Google Scholar]
  19. Fitch, G. , Figueroa L. L., Koch H., Stevenson P. C., and Adler L. S.. 2022. “Understanding Effects of Floral Products on Bee Parasites: Mechanisms, Synergism, and Ecological Complexity.” International Journal for Parasitology: Parasites and Wildlife, Volume 17: 244–256. 10.1016/j.ijppaw.2022.02.011. [DOI] [PMC free article] [PubMed] [Google Scholar]
  20. Fowler, A. E. , Stone E. C., Irwin R. E., and Adler L. S.. 2020. “Sunflower Pollen Reduces a Gut Pathogen in Worker and Queen but Not Male Bumble Bees.” Ecological Entomology 45: 1318–1326. 10.1111/een.12915. [DOI] [Google Scholar]
  21. Gekière, A. , Semay I., Gérard M., Michez D., Gerbaux P., and Vanderplanck M.. 2022. “Poison or Potion: Effects of Sunflower Phenolamides on Bumble Bees and Their Gut Parasite.” Biology 11: 545. 10.3390/biology11040545. [DOI] [PMC free article] [PubMed] [Google Scholar]
  22. Giacomini, J. J. , Adler L. S., Reading B. J., and Irwin R. E.. 2023. “Differential Bumble Bee Gene Expression Associated With Pathogen Infection and Pollen Diet.” BMC Genomics 24: 157. 10.1186/s12864-023-09143-5. [DOI] [PMC free article] [PubMed] [Google Scholar]
  23. Giacomini, J. J. , Connon S. J., Marulanda D., Adler L. S., and Irwin R. E.. 2021. “The Costs and Benefits of Sunflower Pollen Diet on Bumble Bee Colony Disease and Health.” Ecosphere 12, no. 7: e03663. 10.1002/ecs2.3663. [DOI] [Google Scholar]
  24. Giacomini, J. J. , Leslie J., Tarpy D. R., Palmer‐Young E. C., Irwin R. E., and Adler L. S.. 2018. “Medicinal Value of Sunflower Pollen Against Bee Pathogens.” Scientific Reports 8: 14394. 10.1038/s41598-018-32681-y. [DOI] [PMC free article] [PubMed] [Google Scholar]
  25. Giacomini, J. J. , Moore N., Adler L. S., and Irwin R. E.. 2022. “Sunflower Pollen Induces Rapid Excretion in Bumble Bees: Implications for Host–Pathogen Interactions.” Journal of Insect Physiology 137: 104356. 10.1016/j.jinsphys.2022.104356. [DOI] [PubMed] [Google Scholar]
  26. Gillespie, S. 2010. “Factors Affecting Parasite Prevalence Among Wild Bumblebees.” Ecological Entomology 35: 737–747. 10.1111/j.1365-2311.2010.01234.x2. [DOI] [Google Scholar]
  27. Goulson, D. , Nicholls E., Botías C., and Rotheray E. L.. 2015. “Bee Declines Driven by Combined Stress from Parasites, Pesticides, and Lack of Flowers.” Science 347: 1435. 10.1126/science.1255957. [DOI] [PubMed] [Google Scholar]
  28. Häfner, B. , Dauber J., Schittenhelm S., et al. 2023. “The Perennial Biogas Crops Cup Plant (Silphium perfoliatum L.) and Field Grass Pose Better Autumn and Overwintering Habitats for Arthropods Than Silage Maize (Zea mays L.).” GCB Bioenergy 15: 346–364. 10.1111/gcbb.13027. [DOI] [Google Scholar]
  29. Halbritter, H. , Heigl H., and Svojtka N.. 2020. “Helianthus annuus,” In: PalDat—A palynological database. Accessed 2025‐09‐27. https://www.paldat.org/pub/Helianthus_annuus/304619.
  30. Husband, S. , Cankar K., Catrice O., Chabert S., and Erler S.. 2025. “A Guide to Sunflowers: Floral Resource Nutrition for Bee Health and Key Pollination Syndromes.” Frontiers in Plant Science 16: 1552335. 10.3389/fpls.2025.1552335. [DOI] [PMC free article] [PubMed] [Google Scholar]
  31. Imhoof, B. , and Schmid‐Hempel P.. 1999. “Colony Success of Bombus terrestris Is Related to the Prevalence of Crithidia bombi .” Ecological Entomology 24: 343–348. 10.1046/j.1365-2311.1999.00207.x. [DOI] [Google Scholar]
  32. Knoerr, S. A. , Rivest S., Hotchkiss M. Z., and Forrest J. R. K.. 2024. “Impacts of Asteraceae Pollen Spines on Bumble Bee Survival and Larval Growth.” Arthropod‐Plant Interactions 18: 417–424. 10.1007/s11829-024-10058-5. [DOI] [Google Scholar]
  33. Koch, H. , Welcome V., Kendal‐Smith A., et al. 2022. “Host and Gut Microbiome Modulate the Antiparasitic Activity of Nectar Metabolites in a Bumblebee Pollinator.” Philosophical Transactions of the Royal Society B 377: 20210162. 10.1098/rstb.2021.0162. [DOI] [PMC free article] [PubMed] [Google Scholar]
  34. Kowalski, R. 2003. “Evaluation of Phenolic Acid Content in Silphium perfoliatum L. Leaves, Inflorescences and Rhizomes.” Electronic Journal of Polish Agricultural Universities—Horticulture. [Google Scholar]
  35. Kowalski, R. , and Kędzia B.. 2007. “Antibacterial Activity of Silphium perfoliatum Extracts.” Pharmaceutical Biology 45, no. 6: 494–500. 10.1080/13880200701389409. [DOI] [Google Scholar]
  36. Kowalski, R. , and Wolski T.. 2005. “The Chemical Composition of Essential Oils of Silphium perfoliatum L.” Flavour and Fragrance Journal 20: 306–310. 10.1002/ffj.1418. [DOI] [Google Scholar]
  37. LoCascio, G. M. , Aguirre L., Irwin R. E., and Adler L. S.. 2019. “Pollen From Multiple Sunflower Cultivars and Species Reduces a Common Bumblebee Gut Pathogen.” Royal Society Open Science 6: 190279. 10.1098/rsos.190279. [DOI] [PMC free article] [PubMed] [Google Scholar]
  38. LoCascio, G. M. , Pasquale R., Amponsah E., Irwin R. E., and Adler L. S.. 2019. “Effect of Timing and Exposure of Sunflower Pollen on a Common Gut Pathogen of Bumble Bees.” Ecological Entomology 44: 702–710. 10.1111/een.12751. [DOI] [Google Scholar]
  39. Malfi, R. L. , McFrederick Q. S., Lozano G., Irwin R. E., and Adler L. S.. 2023. “Sunflower Plantings Reduce a Common Gut Pathogen and Increase Queen Production in Common Eastern Bumblebee Colonies.” Proceedings of the Royal Society B: Biological Sciences 290: 20230055. [DOI] [PMC free article] [PubMed] [Google Scholar]
  40. Marxer, M. , Barribeau S. M., and Schmid‐Hempel P.. 2016. “Experimental Evolution of a Trypanosome Parasite of Bumblebees and Its Implications for Infection Success and Host Immune Response.” Evolutionary Biology 43: 160–170. 10.1007/s11692-015-9366-z. [DOI] [Google Scholar]
  41. McCormick, E. C. , and Adler L. S.. 2026. “Sunflower Pollen Diets Reduce Infection in Closely Related Bumble Bee Species.” Ecological Entomology: 1–9. 10.1111/een.70054. [DOI] [Google Scholar]
  42. Møller, A. P. , and Jennions M. D.. 2001. “Testing and Adjusting for Publication Bias.” Trends in Ecology & Evolution 16: 580–586. 10.1016/S0169-5347(01)02235-2. [DOI] [Google Scholar]
  43. Mueller, A. L. , Biertümpfel A., Friedritz L., Power E. F., Wright G. A., and Dauber J.. 2020. “Floral Resources Provided by the New Energy Crop Silphium perfoliatum L. (Asteraceae).” Journal of Apicultural Research 59, no. 2: 232–245. 10.1080/00218839.2019.1668140. [DOI] [Google Scholar]
  44. Muturi, M. N. K. , Bargul J. L., and Lattorff H. M. G.. 2022. “Influence of the Type of Pollen Diet on the Survival, Body Weight, and Immune Response in the African Honeybee.” Journal of Apicultural Science 66, no. 1: 29–43. 10.2478/JAS-2022-0003. [DOI] [Google Scholar]
  45. Nakagawa, S. , Lagisz M., Jennions M. D., et al. 2022. “Methods for Testing Publication Bias in Ecological and Evolutionary Meta‐Analyses.” Methods in Ecology and Evolution 13: 4–21. 10.1111/2041-210X.13724. [DOI] [Google Scholar]
  46. Nicolson, S. W. , and Human H.. 2013. “Chemical Composition of the ‘Low Quality’ Pollen of Sunflower (Helianthus annuus, Asteraceae).” Apidologie 44: 144–152. 10.1007/s13592-012-0166-5. [DOI] [Google Scholar]
  47. Odemer, R. , Geiger M., and Geiger L.. 2025. “Development of a Modular 3D‐Printed Pollen Trap for Bumble Bee Monitoring.” Journal of Applied Entomology 150, no. 1: 101–106. 10.1111/jen.70011. [DOI] [Google Scholar]
  48. Palmer‐Young, E. C. , Malfi R. L., Zhou Y., et al. 2023. “Sunflower‐Associated Reductions in Varroa Mite Infestation of Honey Bee Colonies.” Journal of Economic Entomology 116, no. 1: 68–77. 10.1093/jee/toac196. [DOI] [PubMed] [Google Scholar]
  49. Palmer‐Young, E. C. , Tozkar C. Ö., Schwarz R. S., et al. 2017. “Nectar and Pollen Phytochemicals Stimulate Honey Bee (Hymenoptera: Apidae) Immunity to Viral Infection.” Journal of Economic Entomology 110, no. 5: 1959–1972. 10.1093/jee/tox193. [DOI] [PubMed] [Google Scholar]
  50. Piko, J. , Keller A., Geppert C., et al. 2021. “Effects of Three Flower Field Types on Bumblebees and Their Pollen Diets.” Basic and Applied Ecology, Volume 52: 95–108. 10.1016/j.baae.2021.02.005. [DOI] [Google Scholar]
  51. Pluta, T. , Czechofsky K., Hass A., et al. 2024. “Organic Farming and Annual Flower Strips Reduce Parasite Prevalence in Bumble Bees.” Journal of Applied Ecology 61: 1486–1499. 10.1111/1365-2664.14723. [DOI] [Google Scholar]
  52. Rivest, S. , Muralidhar M., and Forrest J. R. K.. 2024. “Pollen Chemical and Mechanical Defences Restrict Host‐Plant Use by Bees.” Proceedings of the Royal Society B 291: 20232298. 10.1098/rspb.2023.2298. [DOI] [PMC free article] [PubMed] [Google Scholar]
  53. Schmid‐Hempel, P. , and Funk C. R.. 2004. “The Distribution of Crithidia bombi Among Host Populations of Bombus terrestris .” Parasitology 129: 147–158. 10.1017/S0031182004005542. [DOI] [PubMed] [Google Scholar]
  54. Schmidt, L. S. , Schmidt J. O., Rao R., Wang W., and Xu L.. 1995. “Feeding Preference and Survival of Young Worker Honey Bees Fed Rape, Sesame, and Sunflower Pollen.” Journal of Economic Entomology 88, no. 6: 1591–1595. [Google Scholar]
  55. Stabler, D. , Paoli P. P., Nicolson S. W., and Wright G. A.. 2015. “Nutrient Balancing of the Adult Worker Bumblebee (Bombus terrestris) Depends on the Dietary Source of Essential Amino Acids.” Journal of Experimental Biology 218, no. 5: 793–802. 10.1242/jeb.1142493. [DOI] [PMC free article] [PubMed] [Google Scholar]
  56. Stephen, K. W. , Chau K. D., and Rehan S. M.. 2024. “Dietary Foundations for Pollinators: Nutritional Profiling of Plants for Bee Health.” Frontiers in Sustainable Food Systems 8: 1411410. 10.3389/fsufs.2024.1411410. [DOI] [Google Scholar]
  57. Stevenson, P. C. , Koch H., Nicolson S. W., and Brown M. J. F.. 2022. “Natural Processes Influencing Pollinator Health.” Philosophical Transactions of the Royal Society B 377: 20210154. 10.1098/rstb.2021.0154. [DOI] [PMC free article] [PubMed] [Google Scholar]
  58. Tiritelli, R. , Flaminio S., Zavatta L., et al. 2024. “Ecological and Social Factors Influence Interspecific Pathogens Occurrence Among Bees.” Scientific Reports 14: 5136. 10.1038/s41598-024-55718-x. [DOI] [PMC free article] [PubMed] [Google Scholar]
  59. Tognazzo, M. , Schmid‐Hempel R., and Schmid‐Hempel P.. 2012. “Probing Mixed‐Genotype Infections II: High Multiplicity in Natural Infections of the Trypanosomatid, Crithidia bombi, in Its Host, Bombus spp.” PLoS One 7, no. 11: e49137. 10.1371/journal.pone.0049137. [DOI] [PMC free article] [PubMed] [Google Scholar]
  60. Vaudo, A. D. , Patch H. M., Mortensen D. A., Tooker J. F., and Grozinger C. M.. 2016. “Macronutrient Ratios in Pollen Shape Bumble Bee (Bombus impatiens) Foraging Strategies and Floral Preferences.” Proceedings. National Academy of Sciences. United States of America 113, no. 28: E4035–E4042. 10.1073/pnas.1606101113. [DOI] [PMC free article] [PubMed] [Google Scholar]
  61. Vaudo, A. D. , Tooker J. F., Patch H. M., et al. 2020. “Pollen Protein: Lipid Macronutrient Ratios May Guide Broad Patterns of Bee Species Floral Preferences.” Insects 11: 132. 10.3390/insects11020132. [DOI] [PMC free article] [PubMed] [Google Scholar]
  62. Weiner, C. N. , Hilpert A., Werner L., Linsenmair K. E., and Blüthgen N.. 2010. “Pollen Amino Acids and Flower Specialisation in Solitary Bees.” Apidologie 41: 476–487. 10.1051/apido/2009083. [DOI] [Google Scholar]
  63. Wevera, C. , Höller M., Becker L., et al. 2019. “Towards High‐Biomass Yielding Bioenergy Crop Silphium perfoliatum L.: Phenotypic and Genotypic Evaluation of Five Cultivated Populations.” Biomass and Bioenergy 124: 102–113. 10.1016/j.biombioe.2019.03.016. [DOI] [Google Scholar]
  64. Wolf, S. , and Moritz R. F. A.. 2008. “Foraging Distance in Bombus terrestris L. (Hymenoptera: Apidae).” Apidologie 39: 419–427. 10.1051/apido:2008020. [DOI] [Google Scholar]
  65. Yost, R. T. , Fowler A. E., and Adler L. S.. 2023. “Gut Transplants from Bees Fed an Antipathogenic Pollen Diet Do Not Confer Pathogen Resistance to Recipients.” Microbial Ecology 86: 3133–3137. 10.1007/s00248-023-02309-4. [DOI] [PubMed] [Google Scholar]
  66. Ziska, L. H. , Pettis J. S., Edwards J., et al. 2016. “Rising Atmospheric CO2 Is Reducing the Protein Concentration of a Floral Pollen Source Essential for North American Bees.” Proceedings of the Biological Sciences 283, no. 1828: 20160414. 10.1098/rspb.2016.0414. [DOI] [PMC free article] [PubMed] [Google Scholar]

Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

Data S1: ece373107‐sup‐0001‐DataS1.docx.

ECE3-16-e73107-s001.docx (322KB, docx)

Data Availability Statement

All data required to reproduce the analyses in this study are available in the Supporting Information and archived openly at the Open Science Framework (OSF): https://doi.org/10.17605/OSF.IO/ZYGHV.

Articles from Ecology and Evolution are provided here courtesy of Wiley

RESOURCES