Honey bees overwintering in a southern climate: longitudinal effects of nutrition and queen age on colony-level molecular physiology and performance

A longitudinal study of queen health in honey bees reveals tissue specific response to seasonal changes and pathogen pressure

The health of honey bee queens is crucial for colony success, particularly during stressful periods like overwintering. To accompany a previous longitudinal study of colony and worker health, we explored niche-specific gut microbiota, host gene expression, and pathogen prevalence in honey bee queens overwintering in a warm southern climate. We found differential gene expression and bacterial abundance with respect to various pathogens throughout the season. Biologically older queens had larger microbiotas, particularly enriched in Bombella and Bifidobacterium. Both Deformed Wing Virus A and B subtypes were highest in the fat body tissue in January, correlating with colony Varroa levels, and Deformed Wing Virus titers in workers. High viral titers in queens were associated with decreased vitellogenin expression, suggesting a potential trade-off between immune function and reproductive capacity. Additionally, we found a complex and dynamic relationship between these viral loads and immune gene expression, indicating a possible breakdown in the coordinated immune response as the season progressed. Our study also revealed a potential link between Nosema and Melissococcus plutonius infections in queens, demonstrating that seasonal opportunism is not confined to just workers. Overall, our findings highlight the intricate interplay between pathogens, metabolic state, and immune response in honey bee queens. Combined with worker and colony-level metrics from the same colonies, our findings illustrate the social aspect of queen health and resilience over the winter dearth.

Africanized honey bee colonies in Costa Rica: first evidence of its management, brood nest structure and factors associated with varroa mite infestation

Management, brood nest structure and factors associated with varroa mite infestation were studied in 60 apiaries of Africanized honey bees in the northwest region of the Central Valley of Costa Rica. Apiaries were monitored two times. The first monitoring was taken forward during the rainy season between May and November 2019. The second monitoring during the dry season between February and March 2020. Information about the beekeepers, apiaries and management was collected through a survey. Amount of open and capped brood, honey and pollen were measured in the field. The infestation rate of varroa (IRV) was quantified using standard laboratory methods. A determination of multi-residue pesticides in bee bread was made through GC-MS/MS and LC-MS/MS techniques. According to the results, most of the beekeepers produce honey (96.7%), participate in training activities (82.2%), and change the bee queens annually (70%). The first monitoring was characterized by a lower amount of capped brood and honey reserves compared to the second one. IRV was significantly higher in the first monitoring (6.0 ± 0.4) in comparison with the second one (3.0 ± 0.3) (U Mann-Whitney p < 0.001). The maximum value for the first monitoring exceeds 40%, while this value was close to 25% in the second monitoring. Mite infestation exposed significant differences in relation to the variables associated to the beekeeper's management, i.e., change of bee queen (p = 0.002) or when beekeepers monitor varroa mites (p = 0.004). Additionally, the IRV had inverse correlations (p < 0.01) with the number of comb sides with capped brood (Spearman's rho coefficient = - 0.190), and honey reserves (Spearman's rho coefficient = - 0.168). Furthermore, 23 of 60 bee bread samples presented one to five pesticide residues, being the most frequent antifungal agrochemicals.

Seasonal trends of the ABPV, KBV, and IAPV complex in Italian managed honey bee (Apis mellifera L.) colonies

Acute bee paralysis virus (ABPV), Kashmir bee virus (KBV), and Israeli acute paralysis virus (IAPV) usually persist as covert infections in honey bee colonies. They can cause rapid bee mortality in cases of severe infection, often associated with high Varroa destructor infestation, by which they are transmitted. In various countries, these viruses have been associated with colony collapse. Despite their potential danger, these viruses are often disregarded, and little information is available on their occurrence in many countries, including Italy. In 2021, 370 apiaries representing all of the Italian regions were investigated in four different months (June, September, November, and March) for the presence of ABPV, KBV, and IAPV. IAPV was not found in any of the apiaries investigated, whereas 16.45% and 0.67% of the samples tested positive for ABPV and KBV, respectively. Most ABPV cases occurred in late summer-autumn in both northern and southern regions. We observed a scattered pattern of KBV-positive colonies that did not allow any seasonal or regional trends to be discerned. Differences observed among regions and months were potentially related to the dynamics of varroa infestation, viral genetic variations, and different climatic conditions resulting in variations in bee behaviour. This study improves our understanding of the circulation of bee viruses and will contribute to better disease prevention and preservation of bee health.

A longitudinal field study of commercial honey bees shows that non-native probiotics do not rescue antibiotic treatment, and are generally not beneficial

Probiotics are widely used in agriculture including commercial beekeeping, but there is little evidence supporting their effectiveness. Antibiotic treatments can greatly distort the gut microbiome, reducing its protective abilities and facilitating the growth of antibiotic resistant pathogens. Commercial beekeepers regularly apply antibiotics to combat bacterial infections, often followed by an application of non-native probiotics advertised to ease the impact of antibiotic-induced gut dysbiosis. We tested whether probiotics affect the gut microbiome or disease prevalence, or rescue the negative effects of antibiotic induced gut dysbiosis. We found no difference in the gut microbiome or disease markers by probiotic application or antibiotic recovery associated with probiotic treatment. A colony-level application of the antibiotics oxytetracycline and tylosin produced an immediate decrease in gut microbiome size, and over the longer-term, very different and persistent dysbiotic effects on the composition and membership of the hindgut microbiome. Our results demonstrate the lack of probiotic effect or antibiotic rescue, detail the duration and character of dysbiotic states resulting from different antibiotics, and highlight the importance of the gut microbiome for honeybee health.

Sublethal effects of imidacloprid-contaminated honey stores on colony performance, queens, and worker activities in fall and early winter colonies

Neonicotinoid-contaminated sugar stores can have both near term and long term effects on honey bees due to their persistence in honey stores. Effects of imidacloprid food stores contaminants were examined in subtropical colonies that experience reduced brood rearing and foraging during overwintering. Colonies were given treatment sugar syrup containing 0 ppb (control), 20 ppb (field relevant), or 100 ppb (above field relevant) imidacloprid over six weeks to simulate contaminated fall nectar. Colonies were evaluated immediately (post-treatment) and 10 weeks (mid-winter) after treatment to compare proximal and latent effects. Post-treatment 0 ppb and 20 ppb colonies had more workers than 100 ppb colonies while 0 ppb colonies more brood than 20 ppb or 100 ppb colonies. Mid-winter 0 ppb and 20 ppb colonies had more workers than 100 ppb colonies and 0 ppb colonies more brood than 100 ppb colonies. Colonies experienced seasonal declines in stored pollen but no treatment effects. Lower 100 ppb colony performance was associated with reduced effort rather than lifespan. RFID (Radio Frequency Identification) tracking revealed that workers had similar adult lifespans across treatments; however, 100 ppb workers engaged in activities outside the colony for less time than 0 ppb workers. Imidacloprid exposure affected queen but not worker nutritional physiology. Nurses retained well-developed hypopharyngeal glands (as indicated by head protein) across treatments. Mid-winter queens from 0 ppb colonies had marginally higher ovary protein than queens from 100 ppb colonies and more ovary lipids than queens from 20 ppb colonies. However, queen nutrient stores in non-reproductive tissues (fat bodies) did not differ across treatments. Queens from different treatments were attended by comparable numbers of retinue workers and had similar gland contents of four QMP (Queen Mandibular Pheromone) components essential to queen care. High levels of imidacloprid in sugar stores can negatively affect colony performance months after initial storage.

Indirect exposure to insect growth disruptors affects honey bee (Apis mellifera) reproductive behaviors and ovarian protein expression

Pesticide exposure and queen loss are considered to be major causes of honey bee colony mortality, yet little is known regarding the effects of regularly encountered agrochemicals on honey bee reproduction. Here, we present the results of a two-generational study using specialized cages to expose queens to commonly used insect growth disrupting pesticides (IGDs) via their retinue of worker bees. Under IGD exposure, we tracked queen performance and worker responses to queens, then the performance of the exposed queens' offspring was assessed to identify patterns that may contribute to the long-term health and stability of a social insect colony. The positive control, novaluron, resulted in deformed larvae hatching from eggs laid by exposed queens, and methoxyfenozide, diflubenzuron, and novaluron caused a slight decrease in daily egg laying rates, but this was not reflected in the total egg production over the course of the experiment. Curiously, eggs laid by queens exposed to pyriproxyfen exhibited increased hatching rates, and those larvae developed into worker progeny with increased responsiveness to their queens. Additionally, pyriproxyfen and novaluron exposure affected the queen ovarian protein expression, with the overwhelming majority of differentially expressed proteins coming from the pyriproxyfen exposure. We discuss these results and the potential implications for honey bee reproduction and colony health.

Honey bee retinue workers respond similarly to queens despite seasonal differences in Queen Mandibular Pheromone (QMP) signaling

Honey bee colonies maintain viable queens in part through communication with Queen Mandibular Pheromone (QMP), a mixture that signals the queen's presence and reproductive quality to workers. In turn, workers are thought to provide retinue queen care or replace queens partially based on QMP profiles. We examined the effects of seasonal dearth (overwintering in a warm subtropical location) on queen-worker interactions. Retinue worker responses to continuously ovipositing queens were considered in view of QMP signaling and queen reproductive quality. QMP signaling was estimated from QMP residues recovered from nest worker bodies, which is the primary mode of QMP transfer from the queen to the colony at large. QMP residues varied seasonally but not at all with queen reproductive quality (spermatheca sperm storage, ovary protein and lipid contents). 9-HDA and 9-ODA were lower in January than other months. HOB decreased from July to January, while HVA, a component associated with mated queens, increased sharply in January. Despite these seasonal signaling differences, retinue workers attended queens at similar levels through the months. In terms of reproductive quality, queens did not differ over the months in matedness (spermatheca sperm storage) or physiological age (protein carbonyl content), but varied in nutrient allocation to reproductive and non-reproductive tissues. Queen ovaries contained more protein in September than in November, and more lipid in July and September than in November and January. Queen fat bodies had more protein in July than September or November, but less lipid in July and September than November or January. Retinue worker responses did not vary with seasonal QMP changes, but reflected overall continuous brood rearing efforts and queen matedness throughout the year. The absence of seasonal differences in worker responses to QMP should be considered in the broader context of continuous reproductive efforts in warm subtropical colonies.

The survival and growth of honey bee (Hymenoptera: Apidae) colonies overwintered in cold storage: the effects of time and colony location

For over a decade, high percentages of honey bee colonies have been perishing during the winter creating economic hardship to beekeepers and growers of early-season crops requiring pollination. A way to reduce colony losses might be moving hives into cold storage facilities for the winter. We explored factors that could affect the size and survival of colonies overwintered in cold storage and then used for almond pollination. The factors were when hives were put into cold storage and their location prior to overwintering. We found that colonies summered in North Dakota, USA and moved to cold storage in October were larger after cold storage and almond pollination than those moved in November. Colony location prior to overwintering also affected size and survival. Colonies summered in southern Texas, USA and moved to cold storage in November were smaller after cold storage and almond pollination than those from North Dakota. The colonies also were smaller than those overwintered in Texas apiaries. Fat body metrics of bees entering cold storage differed between summer locations. North Dakota bees had higher lipid and lower protein concentrations than Texas bees. While in cold storage, fat bodies gained weight, protein concentrations increased, and lipids decreased. The decrease in lipid concentrations was correlated with the amount of brood reared while colonies were in cold storage. Our study indicates that in northern latitudes, overwintering survival might be affected by when colonies are put into cold storage and that colonies summered in southern latitudes should be overwintered there.

Cold storage as part of a Varroa management strategy: effects on honey bee colony performance, mite levels and stress biomarkers

Placing honey bee colonies in cold storage has been proposed as a way to induce a pause in brood production as part of a Varroa mite treatment plan. Here, we exposed colonies to combinations of with or without an October cold storage period and with or without a subsequent miticide application. We then measured the effects of those treatments on colony-level variables (i.e. colony size, Varroa infestation level, survivorship and hive weight and temperature) and pooled individual-level variables that are associated with nutritional and stress responses. Colonies were assessed before and after cold storage, and again post winter, for a total duration of about 5 months, and the experiment was repeated. Brood levels were significantly lower after cold storage, and hive temperatures indicated that most or all brood had emerged after about two weeks in cold storage. However, Varroa levels at the end of the experiments in February were not significantly different among treatment groups. Colonies kept outside (not subjected to cold storage) and treated with a miticide had higher survivorship on average than any other treatment group, but no other group comparisons were significant, and long-term impact of cold storage on adult bee populations and on colony thermoregulation was low. The bee forage environment was also very different between the 2 years of the study, as rainfall and bee forage availability were much higher the second year. Colonies were over 2.5 times larger on average the second year compared to the first, both in terms of adult bee mass and brood area, and expression levels of nutrition and stress response genes were also significantly higher the second year. The results indicate that limited cold storage would likely have little long-term impact on most colony and individual measures of health, but for such a strategy to succeed levels of stressors, such as Varroa, may also need to be low.

Survey Results of Honey Bee Colony Losses in Winter in China (2009-2021)

There is growing concern that massive loss of honey bees can cause serious negative effects on biodiversity and ecosystems. Surveys of colony losses have been performed worldwide to monitor the dynamic changes and health status of honey bee colonies. Here, we present the results of surveys regarding winter colony losses from 21 provinces in China from 2009 to 2021, with a total of 1,744,324 colonies managed by 13,704 beekeepers. The total colony losses were low (9.84%; 95% Confidence Interval (CI): 9.60-10.08%) but varied among years, provinces, and scales of apiaries. As little is known about the overwintering mortality of Apis cerana, in this study, we surveyed and compared the loss rates between Apis mellifera and A. cerana in China. We found colonies of A. mellifera suffered significantly lower losses than A. cerana in China. Larger apiaries resulted in higher losses in A. mellifera, whereas the opposite was observed in A. cerana. Furthermore, we used generalized linear mixed-effects models (GLMMs) to evaluate the effects of potential risk factors on winter colony losses and found that the operation size, species, migration, migration×species interaction, and queen problems were significantly related to the loss rates. New queens can increase their colony overwintering survival. Migratory beekeepers and large operations reported lower loss rates.

Honeybees are buffered against undernourishment during larval stages

The negative impact of juvenile undernourishment on adult behavior has been well reported for vertebrates, but relatively little is known about invertebrates. In honeybees, nutrition has long been known to affect task performance and timing of behavioral transitions. Whether and how a dietary restriction during larval development affects the task performance of adult honeybees is largely unknown. We raised honeybees in-vitro, varying the amount of a standardized diet (150 µl, 160 µl, 180 µl in total). Emerging adults were marked and inserted into established colonies. Behavioral performance of nurse bees and foragers was investigated and physiological factors known to be involved in the regulation of social organization were quantified. Surprisingly, adult honeybees raised under different feeding regimes did not differ in any of the behaviors observed. No differences were observed in physiological parameters apart from weight. Honeybees were lighter when undernourished (150 µl), while they were heavier under the overfed treatment (180 µl) compared to the control group raised under a normal diet (160 µl). These data suggest that dietary restrictions during larval development do not affect task performance or physiology in this social insect despite producing clear effects on adult weight. We speculate that possible effects of larval undernourishment might be compensated during the early period of adult life.

Viral species differentially influence macronutrient preferences based on honey bee genotype

Food quantity and macronutrients contribute to honey bee health and colony survival by mediating immune responses. We determined if this held true for bees injected with chronic bee paralysis virus (CBPV) and deformed wing virus (DWV), two common honey bee ssRNA viruses. Pollen-substitute diet and syrup consumption rates and macronutrient preferences of two Varroa-resistant stocks (Pol-Line and Russian bees) were compared to Varroa-susceptible Italian bees. Bee stocks varied in consumption, where Italian bees consumed more than Pol-Line and Russian bees. However, the protein: lipid (P:L) ratios of diet consumed by the Italian and Russian bees was greater than that of the Pol-Line bees. Treatment had different effects on consumption based on the virus injected. CBPV was positively correlated with syrup consumption, while DWV was not correlated with consumption. P:L ratios of consumed diet were significantly impacted by the interaction of bee stock and treatment, with the trends differing between CBPV and DWV. Variation in macronutrient preferences based on viral species may indicate differences in energetic costs associated with immune responses to infections impacting different systems. Further, virus species interacted with bee genotype, indicating different mechanisms of viral resistance or tolerance among honey bee genotypes.

Overwintering in North American domesticated honeybees (Apis mellifera) causes mitochondrial reprogramming while enhancing cellular immunity

Many factors negatively impact domesticated honeybee (Apis mellifera) health causing a global decrease in their population year after year with major losses occurring during winter, and the cause remains thus far unknown. Here, we monitored for 12 months North American colonies of honeybees enduring important temperature variations throughout the year, to assess the metabolism and immune system of honeybees of summer and winter individuals. Our results show that in flight muscle, mitochondrial respiration via complex I during winter is drastically reduced compared to summer. However, the capacity for succinate and glycerol-3-phosphate (G3P) oxidation by mitochondria is increased during winter, resulting in higher mitochondrial oxygen consumption when complex I substrates, succinate and G3P were assessed altogether. Pyruvate kinase, lactate dehydrogenase, aspartate aminotransferase, citrate synthase and malate dehydrogenase tend to have reduced activity levels in winter unlike hexokinase, NADH dehydrogenase and pyruvate dehydrogenase. Transcript abundance of highly important immunity proteins like Vitellogenin and Defensin-1 were also increased in winter bees, and a stronger phagocytic response as well as a better hemocyte viability was observed during winter. Thus, a reorganization of substrate utilization favoring succinate and G3P while negatively affecting complex I of the ETS is occurring during winter. We suggest that this might be due to complex I transitioning to a dormant conformation through post-translational modification. Winter bees also have an increased response for antibacterial elimination in honeybees. Overall, this study highlights previously unknown cellular mechanisms between summer and winter honeybees that further our knowledge about this important species.

Early Queen Development in Honey Bees: Social Context and Queen Breeder Source Affect Gut Microbiota and Associated Metabolism

The highly social honey bee has dense populations but a significantly reduced repertoire of immune genes relative to solitary species, suggesting a greater reliance on social immunity. Here we investigate immune gene expression and gut microbial succession in queens during colony introduction. Recently mated queens were placed into an active colony or a storage hive for multiple queens: a queen-bank. Feeding intensity, social context, and metabolic demand differ greatly between the two environments. After 3 weeks, we examined gene expression associated with oxidative stress and immunity and performed high-throughput sequencing of the queen gut microbiome across four alimentary tract niches. Microbiota and gene expression in the queen hindgut differed by time, queen breeder source, and metabolic environment. In the ileum, upregulation of most immune and oxidative stress genes occurred regardless of treatment conditions, suggesting postmating effects on gut gene expression. Counterintuitively, queens exposed to the more social colony environment contained significantly less bacterial diversity indicative of social immune factors shaping the queens microbiome. Queen bank queens resembled much older queens with decreased Alpha 2.1, greater abundance of Lactobacillus firm5 and Bifidobacterium in the hindgut, and significantly larger ileum microbiotas, dominated by blooms of Snodgrassella alvi. Combined with earlier findings, we conclude that the queen gut microbiota experiences an extended period of microbial succession associated with queen breeder source, postmating development, and colony assimilation.

IMPORTANCE In modern agriculture, honey bee queen failure is repeatedly cited as one of the major reasons for yearly colony loss. Here we discovered that the honey bee queen gut microbiota alters according to early social environment and is strongly tied to the identity of the queen breeder. Like human examples, this early life variation appears to set the trajectory for ecological succession associated with social assimilation and queen productivity. The high metabolic demand of natural colony assimilation is associated with less bacterial diversity, a smaller hindgut microbiome, and a downregulation of genes that control pathogens and oxidative stress. Queens placed in less social environments with low metabolic demand (queen banks) developed a gut microbiota that resembled much older queens that produce fewer eggs. The queens key reproductive role in the colony may rely in part on a gut microbiome shaped by social immunity and the early queen rearing environment.

Social microbiota and social gland gene expression of worker honey bees by age and climate

Winter forage dearth is a major contributor to honey bee colony loss and can influence disease susceptibility. Honey bees possess a secretory head gland that interfaces with the social environment on many levels. During winter or forage dearth, colonies produce a long-lived (diutinus) worker phenotype that survives until environmental conditions improve. We used a known-age worker cohort to investigate microbiome integrity and social gene expression of workers in early and late winter. We provide additional context by contrasting host-microbial interactions from warm outdoor and cold indoor environments. Our results provide novel evidence that social immune gene expression is associated with worker longevity, and highlight the midgut as a target of opportunistic disease during winter. Host microbial interactions suggest opportunistic disease progression and resistance in long-lived workers, but susceptibility to opportunistic disease in younger workers that emerged during the winter, including increases in Enterobacteriaceae, fungal load and non-core bacterial abundance. The results are consistent with increased social immunity, including host associations with the social microbiota, and a social immune response by long-lived workers to combat microbial opportunism. The cost/benefit ratio associated with limited expression of the diutinus phenotype may be a strong determinant of colony survival during winter forage dearth.

Colony-Level Viral Load Influences Collective Foraging in Honey Bees

Nutrition is an important component of social insect colony health especially in the face of stressors such as parasitism and viral infections. Honey bees are known to preferentially select nectar and pollen based on macronutrient and phytochemical contents and in response to pathogen loads. However, given that honey bees live in colonies, collective foraging decisions may be impacted directly by forager infection status but also by colony health. This field experiment was conducted to determine if honey bee viral infections are correlated with pollen and nectar foraging and if these associations are impacted more by colony or forager infection. By comparing regressions with and without forager and colony variables and through structural equation models, we were able to determine the relative contributions of colony and forager virus loads on forager decisions. We found that foragers had higher numbers and levels of BQCV and CBPV but lower levels of DWV viruses than their respective colonies. Overall, individuals appeared to forage based a combination of their own and colony health but with greater weight given to colony metrics. Colony parasitism by Varroa mites, positively correlated with both forager and colony DWV-B levels, was negatively associated with nectar weight. Further, colony DWV-B levels were negatively associated with individually foraged pollen protein: lipid ratios but positively correlated with nectar weight and sugar content. This study shows that both colony and forager health can simultaneously mediate individual foraging decisions and that the importance of viral infections and parasite levels varies with foraging metrics. Overall, this work highlights the continued need to explore the interactions of disease, nutrition, and genetics in social interactions and structures.

Age and Season Effect the Timing of Adult Worker Honeybee Infection by Nosema ceranae

The microsporidia Nosema ceranae is an intracellular parasite of honeybees’ midgut, highly prevalent in Apis mellifera colonies for which important epidemiological information is still unknown. Our research aimed at understanding how age and season influence the onset of infection in honeybees and its development in the colony environment. Adult worker honeybees of less than 24h were marked and introduced into 6 different colonies in assays carried out in spring and autumn. Bees of known age were individually analyzed by PCR for Nosema spp. infection and those resulting positive were studied to determine the load by Real Time-qPCR. The age of onset and development of infection in each season was studied on a total of 2401 bees and the probability and the load of infection for both periods was established with two statistical models. First N. ceranae infected honeybees were detected at day 5 post emergence (p.e.; spring) and at day 4 p.e. (autumn) and in-hive prevalence increased from that point onwards, reaching the highest mean infection on day 18 p.e. (spring). The probability of infection increased significantly with age in both periods although the age variable better correlated in spring. The N. ceranae load tended to increase with age in both periods, although the age-load relationship was clearer in spring than in autumn. Therefore, age and season play an important role on the probability and the development of N. ceranae infection in honeybees, bringing important information to understand how it spreads within a colony.

Effects of different artificial diets on commercial honey bee colony performance, health biomarkers, and gut microbiota

Background

Honey bee colonies managed for agricultural pollination are highly dependent on human inputs, especially for disease control and supplemental nutrition. Hives are routinely fed artificial “pollen substitute” diets to compensate for insufficient nutritional forage in the environment. The aim of this study was to investigate the effects of different artificial diets in a northern California, US commercial beekeeping operation from August through February. This time period represents an extended forage dearth when supplemental nutrition is used to stimulate late winter colony growth prior to almond pollination in the early spring. A total of 144 honey bee colonies were divided into 8 feeding groups that were replicated at three apiary sites. Feeding groups received commercial diets (Global, Ultra Bee, Bulk Soft, MegaBee, AP23, Healthy Bees), a beekeeper-formulated diet (Homebrew), or a sugar negative control. Diets were analyzed for macronutrient and amino acid content then evaluated with respect to honey bee colony population size, average bee weight, nutrition-related gene expression, gut microbiota abundance, and pathogen levels.

Results

Replicated at three apiary sites, two pollen-containing diets (Global and Homebrew) produced the largest colonies and the heaviest bees per colony. Two diets (Bulk Soft and AP23) that did not contain pollen led to significantly larger colonies than a sugar negative control diet. Diet macronutrient content was not correlated with colony size or health biomarkers. The sum of dietary essential amino acid deficiencies relative to leucine content were correlated with average bee weight in November and colony size used for almond pollination in February. Nutrition-related gene expression, gut microbiota, and pathogen levels were influenced by apiary site, which overrode some diet effects. Regarding microbiota, diet had a significant impact on the abundance of Bifidobacterium and Gilliamella and trended towards effects on other prominent bee gut taxa.

Conclusions

Multiple colony and individual bee measures are necessary to test diet efficacy since honey bee nutritional responses are complex to evaluate. Balancing essential amino acid content relative to leucine instead of tryptophan may improve diet protein efficiency ratios. Optimization of bee diets could improve feed sustainability and agricultural pollination efficiency by supporting larger, healthier honey bee colonies.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12917-022-03151-5.

Warmer winters are associated with lower levels of the cryoprotectant glycerol, a slower decrease in vitellogenin expression and reduced virus infections in winter honeybees

Within the context of climate change, winter temperatures at high latitudes are predicted to rise faster than summer temperatures. This phenomenon is expected to negatively affect the diapause performance and survival of insects, since they largely rely on low temperatures to lower their metabolism and preserve energy. However, some insects like honeybees, remain relatively active during the winter and elevate their metabolic rate to produce endothermic heat when temperatures drop. Warming winters are thus expected to improve overwintering performance of honeybees. In order to verify this hypothesis, for two consecutive years, we exposed honeybee colonies to either a mild or cold winter. We then monitored the influence of wintering conditions on several parameters of honeybee overwintering physiology, such as levels of the cryoprotectant glycerol, expression levels of immune and antioxidant genes, and genes encoding multifunctional proteins, including vitellogenin, which promotes bee longevity. Winter conditions had no effect on the expression of antioxidant genes, and genes related to immunity were not consistently affected. However, mild winters were consistently associated with a lower investment in glycerol synthesis and a higher expression of fat body genes, especially apidaecin and vitellogenin. Finally, while we found that viral loads generally decreased through the winter, this trend was more pronounced under mild winter conditions. In conclusion, and without considering how warming temperatures might affect other aspects of honeybee biology before overwintering, our data suggest that warming temperatures will likely benefit honeybee vitality by notably reducing their viral loads over the winter.

Honey bee (Apis mellifera jemenitica) colony performance and queen fecundity in response to different nutritional practices

Honey bee colony nutritional dynamics depend on the availability of floral resources throughout a countryside with varying forage circumstances. Few studies quantify the queen fecundity and colony performance about certain management approaches on a broad scale. The present study was conducted to investigate the queen bee fecundity and various colony performance parameters in response to different nutritional practices, i.e., Group-I, supplied with sucrose solution (1:1; w/v), Group-II, provided with locally available commercial pollen substitute, Group-III, supplied with both sucrose solution + locally available commercial pollen substitute, and Group-IV without any sugar solution and pollen substitute. Our results demonstrated that eggs laid by queen bees were significantly higher (1241.83 ± 46.24) in Group-III than in other groups over the time of observations. Similarly, a significant difference was noticed in the mean sealed worker brood area and honey store area between the different groups of management practices. Both, the max mean sealed worker brood area (2153.53 ± 29.18 cm²) and max mean honey store area (1713.33 ± 12.06 cm²) were observed in Group-III while, the mini mean sealed worker brood area (1066.53 ± 20.18 cm²) and mini mean honey store area (1058.86 ± 4.07 cm²) were observed in Group-IV. In contrast, a non-significant difference was observed in pollen stores between Group-II and Group-III (p > 0.005). Current findings add to our understanding of the mechanisms that underpin large-scale controlled colony performance when the natural pollens resources are insufficient.

Host Genotype and Tissue Type Determine DWV Infection Intensity

Varroa mite-vectored viruses such as Deformed wing virus (DWV) are of great concern for honey bee health as they can cause disease in individuals and increase colony mortality. Two genotypes of DWV (A and B) are prevalent in the United States and may have differential virulence and pathogenicity. Honey bee genetic stocks bred to resist Varroa mites also exhibit differential infection responses to the Varroa mite-vectored viruses. The goal of this project was to determine if interactions between host genotype could influence the overall infection levels and dissemination of DWV within honey bees. To do this, we injected DWV isolated from symptomatic adult bees into mite-free, newly emerged adult bees from five genetic stocks with varying levels of resistance to Varroa mites. We measured DWV-A and DWV-B dissemination among tissues chosen based on relevance to general health outcomes for 10 days. Injury from sham injections did not increase DWV-A levels but did increase DWV-B infections. DWV injection increased both DWV-A and DWV-B levels over time with significant host stock interactions. While we did not observe any differences in viral dissemination among host stocks, we found differences in virus genotype dissemination to different body parts. DWV-A exhibited the highest initial levels in heads and legs while the highest initial levels of DWV-B were found in heads and abdomens. These interactions underscore the need to evaluate viral genotype and tissue specificity in conjunction with host genotype, particularly when the host has been selected for traits relative to virus-vector and virus resistance.

Molecular evolution of bumble bee vitellogenin and vitellogenin-like genes

Vitellogenin (Vg), a storage protein, has been significantly studied for its egg yolk precursor role in oviparous animals. Recent studies found that vitellogenin and its Vg-like homologs were fundamentally involved in many other biological processes in social insects such as female caste differences and oxidative stress resilience. In this study, we conducted the first large-scale molecular evolutionary analyses of vitellogenin coding genes (Vg) and Vg-like genes of bumble bees, a primitively eusocial insect belonging to the genus Bombus. We obtained sequences for each of the four genes (Vg, Vg-like-A, Vg-like-B, and Vg-like-C) from 27 bumble bee genomes (nine were newly sequenced in this study), and sequences from the two closest clades of Bombus, including five Apis species and five Tetragonula species. Our molecular evolutionary analyses show that in bumble bee, the conventional Vg experienced strong positive selection, while the Vg-like genes showed overall relaxation of purifying selection. In Apis and Tetragonula; however, all four genes were found under purifying selection. Furthermore, the conventional Vg showed signs of strong positive selection in most subgenera in Bombus, apart from the obligate parasitic subgenus Psithyrus which has no caste differentiation. Together, these results indicate that the conventional Vg, a key pleiotropic gene in social insects, is the most rapidly evolving copy, potentially due to its multiple known social functions for both worker and queen castes. This study shows that concerted evolution and purifying selection shaped the evolution of the Vg gene family following their ancient gene duplication and may be the leading forces behind the evolution of new potential protein function enabling functional social pleiotropy.

Bee Health and Productivity in Apis mellifera, a Consequence of Multiple Factors

Managed honeybees play an important role as pollinators. The health and nutritional condition of honeybee colonies (Apis mellifera L.) depends for an important part on management practices, and it is influenced by multiple factors. This study aims to identify the stressors that lead to the loss of honeybee health and its consequences on the colony's productivity. Different aspects related to management practices, productivity, clinical observations related to diseases, presence of sanitary gaps in the apiaries, colony strength, weather and infestation rates by Varroa sp. mites were measured. The information was collected during two monitoring in 53 apiaries in the Province of Santa Fe, Argentina. The results show correlations among many of the management practices, health condition and yield. The most important factors affecting the productivity of the studied honeybee colonies were nuclei preparation, the number of combs in the brood chamber, change of bee queen, disinfection of beekeeping material, among other less significant ones. Although honey production is important in the region, the colony strength was deficient and inadequate during both monitoring. Due to its dependence on management by the beekeeper, it is suggested that a holistic approach could improve bee health, increasing the productivity of honeybees.

Honey bee (Apis mellifera) preference towards micronutrients and their impact on bee colonies

Honey bees are important pollinators and take micronutrients from different natural floral resources and turbid water to adequately meet their nutritional requirements. But the role of micronutrients for honey bee health is not well understood. Here, the present study was conducted to determine honey bees' micronutrients preference in summer and winter seasons. Also, the impact of micronutrients on foraging behaviour and brood increase was studied in different honey bee colonies. The results elucidated that honey bees exhibited a strong preference for a salt solution compared to deionized water during the summer and winter seasons. However, there was a notable switch in salt preference between seasons. Overall, honey bees showed significantly more foraging activity, more pollen collection, and increased brood area after sodium consumption compared to other minerals in the summer season. Further, pollen collection and brood area were significantly higher after the use of potassium in the winter season. Thus, the food preference of honey bees is strongly linked with the seasons and the availability of the floral resources. Our data suggested that honey bees may seek specific nutrients during variation of the seasonal conditions.

Overwintering Honey Bee Colonies: Effect of Worker Age and Climate on the Hindgut Microbiota

Honey bee overwintering health is essential to meet the demands of spring pollination. Managed honey bee colonies are overwintered in a variety of climates, and increasing rates of winter colony loss have prompted investigations into overwintering management, including indoor climate controlled overwintering. Central to colony health, the worker hindgut gut microbiota has been largely ignored in this context. We sequenced the hindgut microbiota of overwintering workers from both a warm southern climate and controlled indoor cold climate. Congruently, we sampled a cohort of known chronological age to estimate worker longevity in southern climates, and assess age-associated changes in the core hindgut microbiota. We found that worker longevity over winter in southern climates was much lower than that recorded for northern climates. Workers showed decreased bacterial and fungal load with age, but the relative structure of the core hindgut microbiome remained stable. Compared to cold indoor wintering, collective microbiota changes in the southern outdoor climate suggest compromised host physiology. Fungal abundance increased by two orders of magnitude in southern climate hindguts and was positively correlated with non-core, likely opportunistic bacteria. Our results contribute to understanding overwintering honey bee biology and microbial ecology and provide insight into overwintering strategies.

Metatranscriptome Analysis of Sympatric Bee Species Identifies Bee Virus Variants and a New Virus, Andrena-Associated Bee Virus-1

Bees are important plant pollinators in agricultural and natural ecosystems. High average annual losses of honey bee (Apis mellifera) colonies in some parts of the world, and regional population declines of some mining bee species (Andrena spp.), are attributed to multiple factors including habitat loss, lack of quality forage, insecticide exposure, and pathogens, including viruses. While research has primarily focused on viruses in honey bees, many of these viruses have a broad host range. It is therefore important to apply a community level approach in studying the epidemiology of bee viruses. We utilized high-throughput sequencing to evaluate viral diversity and viral sharing in sympatric, co-foraging bees in the context of habitat type. Variants of four common viruses (i.e., black queen cell virus, deformed wing virus, Lake Sinai virus 2, and Lake Sinai virus NE) were identified in honey bee and mining bee samples, and the high degree of nucleotide identity in the virus consensus sequences obtained from both taxa indicates virus sharing. We discovered a unique bipartite + ssRNA Tombo-like virus, Andrena-associated bee virus-1 (AnBV-1). AnBV-1 infects mining bees, honey bees, and primary honey bee pupal cells maintained in culture. AnBV-1 prevalence and abundance was greater in mining bees than in honey bees. Statistical modeling that examined the roles of ecological factors, including floral diversity and abundance, indicated that AnBV-1 infection prevalence in honey bees was greater in habitats with low floral diversity and abundance, and that interspecific virus transmission is strongly modulated by the floral community in the habitat. These results suggest that land management strategies that aim to enhance floral diversity and abundance may reduce AnBV-1 spread between co-foraging bees.

Impacts of Different Winter Storage Conditions on the Physiology of Diutinus Honey Bees (Hymenoptera: Apidae)

Global decline in insect pollinators, especially bees, have resulted in extensive research into understanding the various causative factors and formulating mitigative strategies. For commercial beekeepers in the United States, overwintering honey bee colony losses are significant, requiring tactics to overwinter bees in conditions designed to minimize such losses. This is especially important as overwintered honey bees are responsible for colony expansion each spring, and overwintered bees must survive in sufficient numbers to nurse the spring brood and forage until the new 'replacement' workers become fully functional. In this study, we examined the physiology of overwintered (diutinus) bees following various overwintering storage conditions. Important physiological markers, i.e., head proteins and abdominal lipid contents were higher in honey bees that overwintered in controlled indoor storage facilities, compared with bees held outdoors through the winter months. Our findings provide new insights into the physiology of honey bees overwintered in indoor and outdoor environments and have implications for improved beekeeping management.

Winter honeybee (Apis mellifera) populations show greater potential to induce immune responses than summer populations after immune stimuli

In the temperate climates of central Europe and North America, two distinct honeybee (Apis mellifera) populations are found in colonies: short-living summer bees emerge in spring and survive until summer, whereas long-living winter bees emerge in late August and overwinter. Besides the difference in their life spans, each of these populations fulfils a different role in the colonies and individual bees have distinct physiological and immunological adaptations depending on their roles. For instance, winter worker bees have higher vitellogenin levels and larger reserves of nutrients in the fat body than summer bees. The differences between the immune systems of both populations are well described at the constitutive level; however, our knowledge of its inducibility is still very limited. In this study, we focus on the response of 10-day-old honeybee workers to immune challenges triggered in vivo by injecting heat-killed bacteria, with particular focus on honeybees that emerge and live under hive conditions. Responses to bacterial injections differed between summer and winter bees. Winter bees exhibited a more intense response, including higher expression of antimicrobial genes and antimicrobial activity, as well as a significant decrease in vitellogenin gene expression and its concentration in the hemolymph. The intense immune response observed in winter honeybees may contribute to our understanding of the relationships between colony fitness and infection with pathogens, as well as its association with successful overwintering.

Longitudinal monitoring of honey bee colonies reveals dynamic nature of virus abundance and indicates a negative impact of Lake Sinai virus 2 on colony health

Honey bees (Apis mellifera) are important pollinators of plants, including those that produce nut, fruit, and vegetable crops. Therefore, high annual losses of managed honey bee colonies in the United States and many other countries threaten global agriculture. Honey bee colony deaths have been associated with multiple abiotic and biotic factors, including pathogens, but the impact of virus infections on honey bee colony population size and survival are not well understood. To further investigate seasonal patterns of pathogen presence and abundance and the impact of viruses on honey bee colony health, commercially managed colonies involved in the 2016 California almond pollination event were monitored for one year. At each sample date, colony health and pathogen burden were assessed. Data from this 50-colony cohort study illustrate the dynamic nature of honey bee colony health and the temporal patterns of virus infection. Black queen cell virus, deformed wing virus, sacbrood virus, and the Lake Sinai viruses were the most readily detected viruses in honey bee samples obtained throughout the year. Analyses of virus prevalence and abundance revealed pathogen-specific trends including the overall increase in deformed wing virus abundance from summer to fall, while the levels of Lake Sinai virus 2 (LSV2) decreased over the same time period. Though virus prevalence and abundance varied in individual colonies, analyses of the overall trends reveal correlation with sample date. Total virus abundance increased from November 2015 (post-honey harvest) to the end of the almond pollination event in March 2016, which coincides with spring increase in colony population size. Peak total virus abundance occurred in late fall (August and October 2016), which correlated with the time period when the majority of colonies died. Honey bee colonies with larger populations harbored less LSV2 than weaker colonies with smaller populations, suggesting an inverse relationship between colony health and LSV2 abundance. Together, data from this and other longitudinal studies at the colony level are forming a better understanding of the impact of viruses on honey bee colony losses.

A National Survey of Managed Honey Bee Colony Winter Losses (Apis mellifera) in China (2013–2017)

Surveys of managed honey bee colony losses worldwide have become fundamental for engineering a sustainable and systematic approach to protect honey bees. Though China is a member of the world’s apiculture superpowers, the investigation of honey bee colony losses from Chinese government was not formally launched until recently. In this study, we investigated the colony winter losses of the western honey bee (Apis mellifera) of four consecutive years in 2013–2017 from 19 provinces in China, with a total of 2387 responding Chinese beekeepers (195 hobby beekeepers, 1789 side-line beekeepers, 403 commercial beekeepers) providing the records of overwintering mortality of honey bee colonies. The calculated colony losses were 8.7%, a relatively low mortality below the world average. There still exist considerable variations in total losses among provinces (ranging from 0.9% to 22.0%), years (ranging from 8.1% to 10.6%) and scales of apiaries (ranging from 7.5% to 10.0%). Furthermore, we deeply analyzed and estimated the effects of potential risk factors on the colonies’ winter losses, and speculated that the queen problems, the operation sizes and proportion of new queens are leading causes of the high honey bee colony mortality in China. More research and advanced technical methods are still required for correlation analysis and verification in future surveys of managed honey bee colony winter losses.

Probing the Honey Bee Diet-Microbiota-Host Axis Using Pollen Restriction and Organic Acid Feeding

Microbial metabolites are considered important drivers of diet-based microbiota influence on the host, however, mechanistic models are confounded by interactions between diet, microbiota function, and host physiology. The honey bee harbors a simple microbiota that produces organic acids as fermentation products of dietary nectar and pollen, making it a model for gut microbiota research. Herein, we demonstrate that bacterial abundance in the honey bee gut is partially associated with the anterior rectum epithelium. We used dietary pollen restriction and organic acid feeding treatments to obtain information about the role of undigested pollen as a microbiota growth substrate and the impact of bacterial fermentation products on honey bee enteroendocrine signaling. Pollen restriction markedly reduced total and specific bacterial 16S rRNA abundance in the anterior rectum but not in the ileum. Anterior rectum expression levels of bacterial fermentative enzyme gene transcripts (acetate kinase, lactate dehydrogenase, and hydroxybutyryl-CoA dehydrogenase) were reduced in association with diet-induced microbiota shifts. To evaluate the effects of fermentative metabolites on host enteroendocrine function, pollen-restricted bees were fed an equimolar mixture of organic acid sodium salts (acetate, lactate, butyrate, formate, and succinate). Organic acid feeding significantly impacted hindgut enteroendocrine signaling gene expression, rescuing some effects of pollen restriction. This was specifically manifested by tissue-dependent expression patterns of neuropeptide F and allatostatin pathways, which are implicated in energy metabolism and feeding behaviors. Our findings provide new insights into the diet-microbiota-host axis in honey bees and may inform future efforts to improve bee health through diet-based microbiota manipulations.

Microalgae as a promising and sustainable nutrition source for managed honey bees

Managed honey bee colony losses are attributed to a number of interacting stressors, but many lines of evidence point to malnutrition as a primary factor. Commercial beekeepers have become increasingly reliant on artificial pollen substitute diets to nourish colonies during periods of forage scarcity and to bolster colony size before pollination services. These artificial diets may be deficient in essential macronutrients (proteins, lipids, prebiotic fibers), micronutrients (vitamins, minerals), and antioxidants. Therefore, improving the efficacy of pollen substitutes can be considered vital to modern beekeeping. Microalgae are prolific sources of plant-based nutrition with many species exhibiting biochemical profiles that are comparable to natural pollen. This emerging feed source has been employed in a variety of organisms, including limited applications in honey bees. Herein, I introduce the nutritional value and functional properties of microalgae, extrapolating to central aspects of honey bee physiology and health. To conclude, I discuss the potential of microalgae-based feeds to sustainably provision managed colonies on an agricultural scale.

Landscape factors influencing honey bee colony behavior in Southern California commercial apiaries

Colony brood levels, frames of bees (adult bee mass) and internal hive temperature were monitored for 60 colonies for each of two years as they were moved from agricultural, tree crop and mountain landscapes in southern California to blueberry and almond pollination sites. Hive weight was also continuously monitored for 20 of those hives for 6 weeks for both years, during commercial pollination. Pesticide residues in wax, honey and beebread samples were analyzed by composite apiary samples. While colonies in mountain sites had more adult bees and brood than those in agricultural sites in August, by October brood levels were higher in colonies from agricultural sites. Though hives from different original landscapes differed in size in October, hive assessments revealed no differences between the groups after co-wintering when graded for commercial almond pollination. Beebread from hives in agricultural sites had greater agrochemical diversity and in general higher pesticide hazard quotients than those from mountain sites, but those hives also had higher and more constant temperatures from September until January than hives from mountain sites. Hives placed in commercial almond pollination gained on average 287 g per d, compared to an average loss of 68 g per d for colonies in commercial blueberry pollination, although weight data indicated greater foraging effort by colonies in blueberries, possibly due to the proximity and abundance of almond pollen during bloom. Temperature monitoring was effective at distinguishing hive groups and had the best overall value in terms of equipment, installation, colony disturbance and information yield.

Austrian COLOSS Survey of Honey Bee Colony Winter Losses 2018/19 and Analysis of Hive Management Practices

We conducted a citizen science survey on overwinter honey bee colony losses in Austria. A total of 1534 beekeepers with 33,651 colonies reported valid loss rates. The total winter loss rate for Austria was 15.2% (95% confidence interval: 14.4–16.1%). Young queens showed a positive effect on colony survival and queen-related losses. Observed queen problems during the season increased the probability of losing colonies to unsolvable queen problems. A notable number of bees with crippled wings during the foraging season resulted in high losses and could serve as an alarm signal for beekeepers. Migratory beekeepers and large operations had lower loss rates than smaller ones. Additionally, we investigated the impact of several hive management practices. Most of them had no significant effect on winter mortality, but purchasing wax from outside the own operation was associated with higher loss rates. Colonies that reported foraging on maize and late catch crop fields or collecting melezitose exhibited higher loss rates. The most common Varroa destructor control methods were a combination of long-term formic acid treatment in summer and oxalic acid trickling in winter. Biotechnical methods in summer had a favourable effect on colony survival.

Defining Pollinator Health: A Holistic Approach Based on Ecological, Genetic, and Physiological Factors

Evidence for global bee population declines has catalyzed a rapidly evolving area of research that aims to identify the causal factors and to effectively assess the status of pollinator populations. The term pollinator health emerged through efforts to understand causes of bee decline and colony losses, but it lacks a formal definition. In this review, we propose a definition for pollinator health and synthesize the available literature on the application of standardized biomarkers to assess health at the individual, colony, and population levels. We focus on biomarkers in honey bees, a model species, but extrapolate the potential application of these approaches to monitor the health status of wild bee populations. Biomarker-guided health measures can inform beekeeper management decisions, wild bee conservation efforts, and environmental policies. We conclude by addressing challenges to pollinator health from a One Health perspective that emphasizes the interplay between environmental quality and human, animal, and bee health.

Exposure to sublethal concentrations of methoxyfenozide disrupts honey bee colony activity and thermoregulation

Methoxyfenozide is an insect growth regulator (IGR) commonly used in agriculture to simultaneously control pests and preserve beneficial insect populations; however, its impact on honey bees in not fully understood. We conducted field and laboratory experiments to investigate bee health in response to field-relevant concentrations of this pesticide. Significant effects were observed in honey bee colony flight activity and thermoregulation after being exposed over 9 weeks to supplemental protein patty containing methoxyfenozide. Compared to bee colonies in the control group, colonies fed pollen patty with 200 ppb methoxyfenozide (as measured by residue analysis) had: 1) a significantly reduced rate of weight loss due to forager departure in the morning; and 2) higher temperature variability during the winter. Colonies in the 100 ppb (as measured by residue analysis) treatment group had values between the 200 ppb group and control for both response variables. The dusk break point, which is the time associated with the end of forager return, differed among all treatment groups but may have been confounded with direction the hives were facing. Bee colony metrics of adult bee mass and brood surface area, and measurements of bee head weight, newly-emerged bee weight, and hypopharyngeal gland size were not significantly affected by methoxyfenozide exposure, suggesting that there may be significant effects on honey bee colony behavior and health in the field that are difficult to detect using standard methods for assessing bee colonies and individuals. The second experiment was continued into the following spring, using the same treatment groups as in the fall. Fewer differences were observed among groups in the spring than the fall, possibly because of abundant spring forage and consequent reduced treatment patty consumption. Residue analyses showed that: 1) observed methoxyfenozide concentrations in treatment patty were about 18–60% lower than the calculated concentrations; 2) no residues were observed in wax in any treatment; and 3) methoxyfenozide was detected in bee bread only in the 200 ppb treatment group, at about 1–2.5% of the observed patty concentration.

Honey bee colony performance and health are enhanced by apiary proximity to US Conservation Reserve Program (CRP) lands

Honey bee colony performance and health are intimately linked to the foraging environment. Recent evidence suggests that the US Conservation Reserve Program (CRP) has a positive impact on environmental suitability for supporting honey bee apiaries. However, relatively little is known about the influence of habitat conservation efforts on honey bee colony health. Identifying specific factors that influence bee health at the colony level incorporates longitudinal monitoring of physiology across diverse environments. Using a pooled-sampling method to overcome individual variation, we monitored colony-level molecular biomarkers during critical pre- and post-winter time points. Major categories of colony health (nutrition, oxidative stress resistance, and immunity) were impacted by apiary site. In general, apiaries within foraging distance of CRP lands showed improved performance and higher gene expression of vitellogenin (vg), a nutritionally regulated protein with central storage and regulatory functions. Mirroring vg levels, gene transcripts encoding antioxidant enzymes and immune-related proteins were typically higher in colonies exposed to CRP environments. Our study highlights the potential of CRP lands to improve pollinator health and the utility of colony-level molecular diagnostics to assess environmental suitability for honey bees.

Intensity of Nosema ceranae infection is associated with specific honey bee gut bacteria and weakly associated with gut microbiome structure

The honey bee, Apis mellifera, pollinates a wide variety of essential crops in numerous ecosystems around the world but faces many modern challenges. Among these, the microsporidian pathogen Nosema ceranae is one of the primary detriments to honey bee health. Nosema infects the honey bee gut, which harbors a highly specific, coevolved microbiota heavily involved in bee immune function and nutrition. Here, we extend previous work investigating interactions between the honey bee gut microbiome and N. ceranae by studying experimentally infected bees that were returned to their colonies and sampled 5, 10, and 21 days post-infection. We measured Nosema load with quantitative PCR and characterized microbiota with 16S rRNA gene amplicon sequencing. We found significant colony level variation in infection levels, and subtle differences between the microbiota of colonies with high infection levels versus those with low infection levels. Two exact sequence variants of Gilliamella, a core gut symbiont that has previously been associated with gut dysbiosis, were significantly more abundant in bees from colonies with high Nosema loads versus those with low Nosema loads. These bacteria deserve further study to determine if they facilitate more intense infection by Nosema ceranae.