Pollen Diet Research Articles

Pollen specialist bee species are accurately predicted from visitation, occurrence and phylogenetic data

An animal’s diet breadth is a central aspect of its life history, yet the factors determining why some species have narrow dietary breadths (specialists) and others have broad dietary breadths (generalists) remain poorly understood. This challenge is pronounced in herbivorous insects due to incomplete host plant data across many taxa and regions. Here, we develop and validate machine learning models to predict pollen diet breadth in bees, using a bee phylogeny and occurrence data for 682 bee species native to the United States, aiming to better understand key drivers. We found that pollen specialist bees made an average of 72.9% of their visits to host plants and could be predicted with high accuracy (mean 94%). Our models predicted generalist bee species, which made up a minority of the species in our dataset, with lower accuracy (mean 70%). The models tested on spatially and phylogenetically blocked data revealed that the most informative predictors of diet breadth are plant phylogenetic diversity, bee species’ geographic range, and regional abundance. Our findings also confirm that range size is predictive of diet breadth and that both male and female specialist bees mostly visit their host plants. Overall, our results suggest we can use visitation data to predict specialist bee species in regions and for taxonomic groups where diet breadth is unknown, though predicting generalists may be more challenging. These methods can thus enhance our understanding of plant-pollinator interactions, leading to improved conservation outcomes and a better understanding of the pollination services bees provide.

Extrafloral nectar from coffee-associated trees as alternative food for a predatory mite.

Plant diversity can enhance natural pest control in agriculture by providing resources and conditions that are not regularly available in conventional crops tonatural enemies of crop pests. Extrafloral nectar-producing plants, for example, might cause reduction of pest densities on neighboring plants because the nectar can increase the performance of natural enemies. Coffee agroforestry systems often contain extrafloral-nectar-producing Inga spp. trees that serve several purposes. Recent studies suggest that they attract and arrest a diversity of natural enemies that contribute to the control of coffee pests. Mites from the Phytoseiid family are key natural enemies of coffee pest mites, but no study has investigated whether Inga extrafloral nectar increases the performance of predatory mites in coffee ecosystems. Thus, here, we assessed whether the extrafloral nectar of Inga edulis Mart. (Fabaceae) can be considered a suitable nutritional resource for the predatory mite Amblyseius herbicolus (Chant), one of the most abundant phytoseiids in coffee crops. We found that feeding on extrafloral nectar allows for development and survival, but not reproduction, of A. herbicolus. Whereas individuals that fed on a diet of nectar during their immature development could subsequently only oviposit after having fed on a pollen diet, individuals that had developed on pollen stopped ovipositing when fed nectar. Our findings suggest that interplanted Inga trees can help to conserve populations of predatory mites in crop ecosystems through the provision of nectar and may boost biological control services. Future research should investigate the effects of extrafloral nectar-producing trees on coffee pest control by these predatory mites.

Mechanistic exploration of royal jelly production in caged honey bees (Apis mellifera)

This study investigates the impact of bee pollen nutrition on the royal jelly production of honey bees (Apis mellifera). Results demonstrate that pollen diet significantly impacts hypopharyngeal gland (HPG) development and the expression of genes associated with royal jelly biosynthesis. Bees fed Brassica napus pollen exhibited superior HPG development, and increased mrjp1 expression (encoding a key royal jelly protein). While the cyp450 6AS8 gene expression (encoding a key enzyme in 10-HDA biosynthesis) was increased by pollen consumption, no distinct expression patterns were observed among the different pollen types tested. An in vitro bee cage platform for royal jelly production has been established to further understand the mechanisms behind royal jelly production in bees. The experiment demonstrated a positive correlation between the number of worker bees and the total yield of royal jelly per cage. However, when the number of worker bees is low, the amount of royal jelly each individual worker bee needs to produce increases. In conclusion, these findings enhance our understanding of the role of bee pollen nutrition in royal jelly production. Furthermore, the results from this in vitro bee cage platform suggest that the number of worker bees is a critical factor in royal jelly production, and that bees may possess a controllable mechanism for regulating royal jelly secretion.

Wildflower plantings and honeybee competition impact nutritional quality of wild bee diets

Abstract Wildflower habitats planted along field borders are a widely promoted strategy for supporting bees in agricultural landscapes. However, honeybees (Apis mellifera), which are often stocked at high densities in crop lands can compete with wild bees for pollen and nectar, potentially limiting the successfulness of wildflower plantings in supporting diverse bee communities. Using weekly samples of five study sites in Northern California we assessed how plants in pollinator‐friendly seed mixes varied in their ability to provide bees with abundant and nutritious pollen under intense honeybee competition. We quantified pollen production, protein and lipid content, and end‐of‐day pollen availability for different plant species. We also sampled bee visits to flowers and assessed the composition of pollen on bee bodies. Using these data, we investigate how the nutritional quality of pollen in wildflower plantings and honeybee abundance impacted native bee pollen nutrition. Bees collected more nutritious pollen (i.e. pollen with more protein) from plantings with more nutritious plant species (i.e. sites with more high‐protein plants). However, as honeybee abundance increased, the nutritional quality of native bee diets declined. We also detected important interactions between honeybee abundance and the nutritional quality of flowers in plantings, such that, for some bee taxa, there was no impact of competition on pollen diet quality in high‐nutrition plantings. Synthesis and applications: Our study reveals that honeybee competition can reduce the nutritional quality of native bee diets. From an applied conservation perspective, we therefore recommend that honeybee introductions in natural areas be approached with extreme caution. However, our results also suggest that high‐protein flower plantings could mitigate negative effects of honeybee competition in managed landscapes. Where simultaneous support of managed and wild bees is a key management objective, we recommend including high‐protein plant species in plant mixes to support diverse bee populations.

Pollen contaminated with a triple-action fungicide induced oxidative stress and reduced longevity though with less impact on lifespan in honey bees from well fed colonies

Experiments were conducted to determine the effects of a triple-action fungicide on bees and whether improved nutrition can ameliorate eventual negative impacts. In cage tests, newly-emerged bees from well fed and from nutritionally-restricted honey bee colonies were fed for five days with pollen from sunflowers that had been sprayed or not with a commercial fungicide containing bixafen, prothioconazole and trifloxystrobin. Bees from well-fed colonies were significantly larger and consumed more uncontaminated pollen. They also exhibited increased glutathione peroxidase activity and higher concentrations of pyridine nucleotides, both of which are involved in antioxidase defense. However, pollen contaminated with fungicide led to an increase in lipoperoxidation, regardless of nutritional status. Bee longevity was reduced by both fungicide contamination of the pollen diet and poor nutritional condition. The fungicide adversely affected bees fed with contaminated pollen, though nutritional supplementation of the bee colonies that reared the bees partially compensated for these effects.

Phytochemical profiles of honey bees (Apis mellifera) and their larvae differ from the composition of their pollen diet.

Pollen and nectar consumed by honey bees contain plant secondary metabolites (PSMs) with vital roles in plant-insect interactions. While PSMs can be toxic to bees, they can also be health-promoting, e.g. by improving pesticide and pathogen tolerances. As xenobiotics, PSMs undergo post-ingestion chemical modifications that can affect their bioactivity and transmission to the brood. Despite the importance of understanding honey bee PSM metabolism and distribution for elucidating bioactivity mechanisms, these aspects remain largely unexplored. In this study, we used HPLC-MS/MS to profile 47 pollen PSMs in honey bees and larvae. Both adult bees and larvae had distinct PSM profiles that differed from their diet. This is likely due to post-ingestion metabolism and compound-dependent variations in PSM transmission to the brood via nurse bee jelly. Phenolic acids and flavonoid aglycones were most abundant in bees and larvae, whereas alkaloids, cyanogenic glycosides and diterpenoids had the lowest abundance despite being consumed in higher concentrations. This study documents larval exposure to a variety of PSMs for the first time, with concentrations increasing from early to late larval instars. Our findings provide novel insights into the post-ingestion fate of PSMs in honey bees, providing a foundation for further exploration of biotransformation pathways and PSM effects on honey bee health.

Shared use of a mass‐flowering crop drives dietary niche overlap between managed honeybees and bumblebees

Abstract Resource competition between wild pollinators and managed honeybees (Apis mellifera) has the potential to detrimentally impact insect biodiversity as well as wild plant and crop pollination. As honeybees are central place foragers, their competitive impact on wild bees is expected to be structured by hive proximity, in conjunction with foraging decisions related to landscape‐level resource availability. Yet, how these factors structure dietary niche overlap between wild bees and managed honeybees remains unclear. We conducted a field experiment in seminatural grasslands, where honeybee foraging densities and niche overlap with buff‐tailed bumblebees (Bombus terrestris) were measured at four distances (<100, 500, 1000 and 2000 m) from experimentally placed apiaries, during and after the blooming period of mass‐flowering oilseed rape (Brassica napus). We measured honeybee foraging densities using standardized transect surveys, and quantified species diet composition and dietary niche overlap from pollen samples collected from both bee species. Honeybee foraging densities were highest near apiaries and declined sharply beyond 500 m from the apiaries. However, niche overlap was unrelated to hive proximity but positively related to the availability of oilseed rape. Furthermore, there were significant inter‐ and intraspecific differences in pollen diet composition and breadth throughout the season. Synthesis and applications: Niche overlap between honeybees and bumblebees in agricultural environments was due to resource sharing of mass‐flowering oilseed rape. When both honeybees and bumblebees predominantly forage on wild plants, they maintain distinct pollen diets, suggesting there is a low risk of resource competition between these generalist taxa. Conservation actions that promote floral resource availability and diversity in agricultural landscapes are crucial to maintain niche differentiation between managed honeybees and wild bees.

A bee's-eye view of landscape change: differences in diet of 2 Andrena species (Hymenoptera: Andrenidae) between 1943 and 2021.

Declines in pollinating insects have been linked to changes in land cover, affecting the availability of nesting sites and floral resources. Our study is the first analysis of changes in pollen load composition of 2 mining bees, Andrena barbilabris (Kirby) and Andrena flavipes (Panzer) (Hymenoptera: Andrenidae), at the same sites in central England, over 75 years. This provides a unique opportunity to remove spatial variation and review temporal changes in pollen diet within the context of landscape change. We analyzed modern-day pollen load composition for these species and compared it with historical data from the same sites. We then examined potential links between land-use change and the bees' diets. Both bees showed dietary flexibility and lower diet breadth for A. barbilabris, and the bees' foraging strategies appear to have changed. Andrena flavipes collected more pollen taxa in a single load, while A. barbilabris appeared to source pollen from greater distances. Landscape changes at the studied sites have affected the nutritional environment for these bees. Our findings are supported by an existing assessment of floral resources, which found floral diversity has decreased overall in both the habitats used by these bees. However, more research is needed on the nutritional content of pollens used by these bees, both now and historically, to estimate how pollen diversity has changed. The bee's-eye view underlines the importance of understanding how species respond to local changes so that effective conservation strategies can be developed.

Improving bee feed recipes to safeguard honeybee colonies during times of food scarcity.

In beekeeping, when natural nectar or pollen sources become limited, it is crucial to provide supplemental bee feed to maintain the viability of the bee colony. This study was conducted during the autumn food shortage season, during which bees were fed with different proportions of modified bee feed. We identified an optimal bee diet by evaluating honeybee longevity, food consumption, body weight, and gut microbe distribution, with natural pollen serving as a control diet. The results indicated that bees preferred a mixture of 65% defatted soy flour, 20% corn protein powder, 13% wheat germ flour, 2% yeast powder, and a 50% sucrose solution. This bee food recipe significantly increased the longevity, feed consumption, and body weight of bees. The group fed the natural pollen diet exhibited a greater abundance of essential intestinal bacteria. The bee diets used in this study contained higher protein levels and lower concentrations of unsaturated fatty acids and vitamins than did the diets stored within the colonies. Therefore, we propose that incorporating both bee feed and natural pollen in beekeeping practices will achieve more balanced nutritional intake.

Effect of Adding Date Palm Pollen and Physical Diet Form on Productive Performance and Egg Quality of Female Quail

This experiment was conducted to estimate the main effect of adding Data palm pollen (DPP) and the form of diet on the productive performance of female quail compliant with: Hen day egg production (HD), Egg weight (EW), Egg mass (EM), Feed intake (FI) and Feed conversion ratio (FCR) and eggs quality of quail compliant with: Shape index (ShI %), shell weight (%), yolk weight (%), albumen weight (%), Yolk index (YI%), Albumen Index (AI%) and Hough unit (HU). For eight weeks, from February 14 to April 13, 2023. Two hindered fifty-two female quail with age about forty-seven days were divided into six treatments, each including forty-two birds with three replicates (four ten quail of each). The treatments were T1: 0% DPP with mash feed form, T2: 0% DPP with pellet feed form, T3: 0.5% DPP with mash feed form, T4: 0.5% DPP with pellet feed form, T5: 1% DPP with mash feed form and T6: 1% DPP with pellet feed form. The quail were fed a productive diet with 2794 kcal/kg energy and 18.39% crude protein, and similar rearing conditions. Results showed there were significant effects in the percentage of HD, EW., EM, and (FCR) in treatments with DPP and pellet feed form when compared with the other treatments, as well as an improvement in egg quality in treatments with DPP and pellet feed form when compared with the other treatments.

Exploring the impact of protein diets on the honey bee (Apis mellifera L): a study on intestinal tissue health and royal jelly protein level

The growth and development of a honey bee colony fundamentally rely on proper nutrition, leading beekeepers to nourish their hives to maintain the health and productivity of their colonies. Nevertheless, the substances utilized in honey bee nutrition vary widely, necessitating thorough research to evaluate their effects on the performance and health of honey bees. Herein, the supplemental diets commonly used by beekeepers for feeding bee colonies including soybean (Glycine max), maize pollen grains (Zea mays L), and a mixture of soybean and pollen grains, in addition to sugar solution as control group are evaluated. This study was conducted on both intestinal tissues as the main place of digestion and absorption and royal jelly (RJ) proteins as a final product that reflects physiological reactions within the insect’s body. Soybean has been observed to have detrimental effects on both the health of the intestinal tissue, causing atrophy of the epithelial cells and weakening of the stomach muscles. Additionally, a decrease in the secretory vesicles (SV) and the gelatinous layer (GL) within the intestine was noted. However, when honey bee colonies were fed on pollen, it helped maintain the molecular balance and high molecular weight proteins in RJ. Bee pollen diet also resulted in an increased production of SV. Conversely, feeding honey bees a mixture of soybean and pollen had catastrophic effects on both the intestinal tissue and RJ proteins. It also led to coagulation in the peritrophic membrane (PMb), which is crucial for protecting the intestinal walls and performing other physiological functions. This, in turn, impacted the absorption of proteins, the levels of SV, and GL. Consequently, it is suggested to incorporate bee pollen as a dietary supplement in the feeding plan of honey bees.

Bombus terrestris Prefer Mixed-Pollen Diets for a Better Colony Performance: A Laboratory Study.

Pollen is a major source of proteins and lipids for bumblebees. The nutritional content of pollen may differ from source plants, ultimately affecting colony development. This study investigated the foraging preferences of Bombus terrestris in regard to four pollen species, i.e., oilseed rape, wild apricot, sunflower, and buckwheat, under laboratory conditions. The results show that B. terrestris diversified their preference for pollens; the bumblebees mostly preferred wild apricot pollen, whereas sunflower pollen was the least preferred. The colonies fed on a mixed four-pollen diet, with a protein-lipid ratio of 4.55-4.86, exhibited better development in terms of the number of offspring, individual body size and colony weight. The colonies fed with buckwheat and sunflower pollens produced a significantly lower number of workers and failed to produce queen and male offspring. Moreover, wild apricot pollen had the richest protein content (23.9 g/100 g) of the four pollen species, whereas oilseed rape pollen had the highest lipid content (6.7 g/100 g), as revealed by the P:L ratios of wild apricot, sunflower, buckwheat, and oilseed rape, which were 6.76, 5.52, 3.50, and 3.37, respectively. Generally, B. terrestris showed feeding preferences regarding different pollens and a mixture of pollens, which ultimately resulted in differences in colony development. The findings of this study provide important baseline information to researchers and developers of nutritive pollen diets for bumblebees.

Deforestation narrows pollen diet diversity of generalist orchid bees

Orchid bee species are important pollinators in the Neotropics. While male orchid bees are known to have a close interaction with odor-rewarding flowers, orchid bee females are often pollen generalists. In the current study, we investigated differences in the pollen diet diversity of orchid bees in various vegetation types and across sites with varying levels of forest cover by means of an analysis of the frass pellets of the orchid bee, Euglossa cordata (Linnaeus, 1758), and multiple species based on a literature review. The pollen grains found in the inner part of brood cells were used to assess the plant composition making up the diet of E. cordata in various areas of the state of São Paulo in Brazil. We found that this plant composition differed among vegetation types, and that diet specialization was lower in forested areas. Forest cover appeared to be the factor mostly associated with pollen richness in nests of E. cordata. Number of brood cells per nest were also associated with forest cover. The model used to assess the effect of forest cover on plant richness as part of brood diet of E. cordata was tested with the inclusion of published data and a newly generated data for E. annectans. This new data set allowed to understand the influence of forest in the pollen diet of multiple species in a more extensive geographic scale. Forest cover appeared to be important for a diversified pollen diet in several orchid bee species.Implications for insect conservationSince a diversified pollen diet offered to larva bees is associated with bee survival and health, our results indicate that orchid bees relying on areas with higher forest cover have a more resilient population. Such findings emphasize the importance of conservation of forests, especially in the Neotropical region where many pollinator species are adapted to continuous forest environments.

Expression of honey bee (Apis mellifera) sterol homeostasis genes in food jelly producing glands of workers.

Adult workers of Western honey bees (Apis mellifera L.) acquire sterols from their pollen diet. These food sterols are transported by the hemolymph to peripheral tissues such as the mandibular and the hypopharyngeal glands in the worker bees' heads that secrete food jelly which is fed to developing larvae. As sterols are obligatory components of biological membranes and essential precursors for molting hormone synthesis in insects, they are indispensable to normal larval development. Thus, the study of sterol delivery to larvae is important for a full understanding of honey bee larval nutrition and development. Whereas hypopharyngeal glands only require sterols for their membrane integrity, mandibular glands add sterols, primarily 24-methylenecholesterol, to its secretion. For this, sterols must be transported through the glandular epithelial cells. We have analyzed for the first time in A. mellifera the expression of genes which are involved in intracellular movement of sterols. Mandibular and hypopharyngeal glands were dissected from newly emerged bees, 6-day-old nurse bees that feed larvae and 26-day-old forager bees. The expression of seven genes involved in intracellular sterol metabolism was measured with quantitative real-time PCR. Relative transcript abundance of sterol metabolism genes was significantly influenced by the age of workers and specific genes but not by gland type. Newly emerged bees had significantly more transcripts for six out of seven genes than older bees indicating that the bulk of the proteins needed for sterol metabolism are produced directly after emergence.

Pollen diet diversity across bee lineages varies with lifestyle rather than colony size.

The shift to a pollen diet and the evolution of more highly organized societies, i.e., eusocial, were key milestones in bee diversification over their evolutionary history, culminating in a high dependence on feeding broods with a large variety of floral resources. Here, we hypothesized that obligatory eusocial bees have a wider diet diversity than their relatives with solitary lifestyles, and this would be related to colony size. To test both hypotheses, we surveyed diet breadth data (palynological analysis) based on the Shannon-Wiener index (H') for 85 bee taxa. We also obtained colony size for 47 eusocial bee species. These data were examined using phylogenetic comparative methods. The results support the generalist strategy as a derived trait for the bee taxa evaluated here. The dietary diversity of eusocial bees (H': 2.1, on average) was 67.5% higher than that of noneusocial bees (H': 1.21, on average). There was, however, no relationship between diet breadth and colony size, indicating that smaller colonies can harvest a pollen variety as diverse as larger colonies. Taken together, these results provide new insights into the impact of lifestyle on the diversity of collected pollen. Furthermore, this work sheds light on an advantage of living in more highly structured societies irrespective of the size of the colony.

How seasonality, semi-natural habitat cover and compositional landscape heterogeneity affect pollen collection and development of Apis mellifera colonies in Mediterranean agro-sylvo-pastoral systems

ContextThe reduction in the abundance and diversity of flowering plant species, mainly due to landscape alteration and consequent habitat loss, negatively affects honey bee populations and is the main driver of their decline. To safeguard pollination service and food security, we need to know how landscape characteristics affect the availability of pollen sources and consequently colony performance across seasons.ObjectivesWe therefore here explore how seasonality, percentage of semi-natural habitat cover and compositional landscape heterogeneity affect the abundance, richness, diversity and composition of pollen collected by honey bees and the development of their colonies in a Mediterranean area where agro-sylvo-pastoral systems are widely spread.MethodsFor the purpose of our study, pollen samples were collected over a year from colonies of Apis mellifera placed in four sampling sites selected across a gradient of landscape heterogeneity and where semi-natural habitats were predominant at various levels within an agro-sylvo-pastoral zone. Simultaneously, the extension of the sealed brood of the same colonies was periodically evaluated.ResultsWe found that the amount of pollen collected by honey bees was positively correlated with the percentage of semi-natural habitat cover and that increasing habitat diversity at the landscape level had a significant positive effect on the richness and diversity of pollen collected by honey bees. In addition, season significantly affected the amount, diversity and composition of pollen collected by honey bees. Furthermore, bee colony growth was positively correlated with richness, diversity and amount of pollen collected by honey bees.ConclusionOur study highlights the importance of an abundant and diverse pollen diet for honey bee colony development, which in agro-sylvo-pastoral systems seems to be ensured by a heterogeneous landscape dominated by different types of semi-natural habitats.

Canola bee pollen is an effective artificial diet additive for improving larval development of predatory coccinellids: a lesson from Harmonia axyridis.

Pollen is a common plant-derived food source for predatory ladybird beetles under field conditions, yet the potential for pollen to improve the quality of artificial diets remains largely unexplored. In this study, we developed three pollen diets by incorporating varying proportions of canola bee pollen (7.5%, 15.0% and 22.5% with 2.5%, 5.0%, and 7.5% of water, respectively) into a conventional diet. The feeding efficiency of Harmonia axyridis, an omnivorous predator, was evaluated and compared on three pollen diets, a conventional nonpollen diet and pea aphids. The larvae fed a medium or high pollen diet exhibited significantly higher survival in the 4th instar, pupa and adult stages than those fed a nonpollen diet. These larvae also developed into significantly heavier adults, and their survival rates in adulthood were comparable to those fed pea aphids. Specifically, we revealed the underlying mechanisms through which a high pollen diet enhances pupal development. Consumption of high pollen diet versus nonpollen diet resulted not only in a significant decrease in pupal glycogen content, but also an increase in adult lipid content. Both diet treatments induced similar changes in carbohydrate and glycogen content compared to the aphid diet while exhibiting different alterations in pupal protein content and adult lipid content. Furthermore, the transcriptome analysis revealed that the nutrient metabolism, immune response, and cuticle development pathways were predominantly enriched among the differentially expressed genes (DEGs). Canola bee pollen offers diverse advantages in terms of rearing H. axyridis larvae with an artificial diet, which will advance the development of effective diets for predaceous coccinellids. © 2024 Society of Chemical Industry.

The genome of the blind bee louse fly reveals deep convergences with its social host and illuminates Drosophila origins

Social insects' nests harbor intruders known as inquilines,1 which are usually related to their hosts.2,3 However, distant non-social inquilines may also show convergences with their hosts,4,5 although the underlying genomic changes remain unclear. We analyzed the genome of the wingless and blind bee louse fly Braula coeca, an inquiline kleptoparasite of the western honey bee, Apis mellifera.6,7 Using large phylogenomic data, we confirmed recent accounts that the bee louse fly is a drosophilid8,9 and showed that it had likely evolved from a sap-breeder ancestor associated with honeydew and scale insects' wax. Unlike many parasites, the bee louse fly genome did not show significant erosion or strict reliance on an endosymbiont, likely due to a relatively recent age of inquilinism. However, we observed a horizontal transfer of a transposon and a striking parallel evolution in a set of gene families between the honey bee and the bee louse fly. Convergences included genes potentially involved in metabolism and immunity and the loss of nearly all bitter-tasting gustatory receptors, in agreement with life in a protective nest and a diet of honey, pollen, and beeswax. Vision and odorant receptor genes also exhibited rapid losses. Only genes whose orthologs in the closely related Drosophila melanogaster respond to honey bee pheromone components or floral aroma were retained, whereas the losses included orthologous receptors responsive to the anti-ovarian honey bee queen pheromones. Hence, deep genomic convergences can underlie major phenotypic transitions during the evolution of inquilinism between non-social parasites and their social hosts.

Pollen Diet Diversity does not Affect Gut Bacterial Communities or Melanization in a Social and Solitary Bee Species.

Pollinators face many stressors, including reduced floral diversity. A low-diversity diet can impair organisms' ability to cope with additional stressors, such as pathogens, by altering the gut microbiome and/or immune function, but these effects are understudied for most pollinators. We investigated the impact of pollen diet diversity on two ecologically and economically important generalist pollinators, the social bumble bee (Bombus impatiens) and the solitary alfalfa leafcutter bee (Megachile rotundata). We experimentally tested the effect of one-, two-, or three-species pollen diets on gut bacterial communities in both species, and the melanization immune response in B. impatiens. Pollen diets included dandelion (Taraxacum officinale), staghorn sumac (Rhus typhina), and hawthorn (Crataegus sp.) alone, each pair-wise combination, or a mix of all three species. We fed bees their diet for 7 days and then dissected out guts and sequenced 16S rRNA gene amplicons to characterize gut bacterial communities. To assess melanization in B. impatiens, we inserted microfilament implants into the bee abdomen and measured melanin deposition on the implant. We found that pollen diet did not influence gut bacterial communities in M. rotundata. In B. impatiens, pollen diet composition, but not diversity, affected gut bacterial richness in older, but not newly-emerged bees. Pollen diet did not affect the melanization response in B. impatiens. Our results suggest that even a monofloral, low-quality pollen diet such as dandelion can support diverse gut bacterial communities in captive-reared adults of these bee species. These findings shed light on the effects of reduced diet diversity on bee health.

Age-dependent hypopharyngeal gland size and protein content of stingless bee workers, Tetragonula pagdeni.

Eusocial insects, such as stingless bees (Meliponini), depend on division of labour, overlapping generations, and collaborative brood care to ensure the functionality and success of their colony. Female workers transition through a range of age-specific tasks during their lifespan (i.e., age-polyethism) and play a central role in the success of a colony. These age-specific tasks (e.g., brood care or foraging) often closely coincide with key physiological changes necessary to ensure optimal performance. However, our understanding of how nutrition, age, and polyethism may affect the development of such physiological traits in stingless bees remains limited. Here we show that pollen consumption and age-polyethism govern hypopharyngeal gland (HPG) acini size and protein content in Tetragonula pagdeni. By conducting a controlled laboratory experiment we monitored the effect of pollen consumption on worker bee survival as well as assessed how a pollen diet and age affected their HPG acini width and protein content. Further, we sampled nurses and foragers from field colonies to measure the effect of age-polyethism on HPG acini width. We found that pollen consumption enhanced survival and led to increased HPG acini width and protein content and that HPG acini were as expected largest in nurse bees. Our findings highlight the beneficial effects of an adequate diet for physiological development and health in stingless bees and reveal that age-polyethism is the key factor governing HPG size in worker bees. As HPGs are imperative for collaborative brood care-an essential component of eusociality-the data provide a foundation for future studies to investigate the impact of potential environmental stressors on a critical physiological trait in stingless bees which may serve as a proxy to understand the effects at the colony level.