A practical approach to the sampling, fixation, softening, and sectioning of whole honey bees for histologic evaluation

The Western honey bee (Apis mellifera) is economically important as the primary managed pollinator of many agricultural crops and for the production of various hive-related commodities. Honey bees are not classically or thoroughly covered in veterinary pathology training programs. Given their unique anatomic and biological differences from the other species more traditionally evaluated by veterinary pathologists, establishing routine and consistent methods for processing samples for histology ensures accurate diagnostic and research conclusions. We developed and tested several field protocols for the sampling of honey bees. We compared the tissue-quality outcomes for worker bees fixed, collected, and/or softened under the following protocols: 1) routine formalin fixation; 2) softening chitin via exposure to Nair for 2 d or 3) 5 d; 4) shortened times between formalin submersion and trimming of body segments to enhance penetration of formalin into internal tissues; 5) ethanol submersion of specimen prior to formalin fixation; 6) indirect dry ice exposure; and 7) prolonged -80°C storage. Routine formalin fixation, exposure to Nair for 2 d, indirect dry ice exposure, and trimming body segments within 2 h of formalin submersion resulted in the highest quality histologic tissue sections. The poorest quality sections resulted from softening of chitin by exposure to Nair for 5 d, submersion in ethanol for 3 d before formalin fixation, and prolonged storage at -80°C. Our results indicate that routine formalin fixation is adequate, and that immobilizing bees with indirect dry ice exposure aids in sample collection without negatively impacting the quality of histologic sections.

Poor Air Quality Is Linked to Stress in Honeybees and Can Be Compounded by the Presence of Disease

Climate change-related extreme weather events have manifested in the western United States as warmer and drier conditions with an increased risk of wildfires. Honeybees, essential for crop pollination in California, are at the center of these extreme weather events. We associated the maximum daily temperature and air quality index values with the performance of colonies placed in wildfire-prone areas and determined the impact of these abiotic stressors on gene expression and histopathology. Our results indicate that poor air quality was associated with higher maximum daily temperatures and a lower gene expression level of Prophenoloxidase (ProPO), which is tied to immune system strength; however, a higher gene expression level of Vitellogenin (Vg) is tied to oxidative stress. There was a positive relationship between Varroa mites and N. ceranae pathogen loads, and a negative correlation between Varroa mites and Heat Shock Protein 70 (HSP70) gene expression, suggesting the limited ability of mite-infested colonies to buffer against extreme temperatures. Histological analyses did not reveal overt signs of interaction between pathology and abiotic stressors, but N. ceranae infections were evident. Our study provides insights into interactions between abiotic stressors, their relation to common biotic stressors, and the expression of genes related to immunity and oxidative stress in bees.

Dietary phytochemicals alter hypopharyngeal gland size in honey bee (Apis mellifera L.) workers

Honey bees are the most efficient pollinators of several important fruits, nuts and vegetables and are indispensable for the profitable production of these crops. Health and performance of honey bee colonies have been declining for decades due to a combination of factors including poor nutrition, agrochemicals, pests and diseases. Bees depend on a diversity of plants for nutrition as pollen is the predominant protein and lipid source, and nectar, the source of carbohydrates for larval development. Additionally, pollen and nectar also contain small amounts of plant secondary metabolites or phytochemicals that are primarily plant defense compounds. Bees have coevolved to benefit from these compounds as seen by the improved longevity, pathogen tolerance and gut microbiome abundance in worker bees whose diets were supplemented with select phytochemicals. Here we investigate the impact of four phytochemicals, known to benefit bees, – caffeine, kaempferol, gallic acid and p-coumaric acid, on hypopharyngeal gland (HPG) size of nurse bees. Newly emerged bees were provided with 25 ppm of each of the four phytochemicals in 20% (w/v) sucrose solution and the size of HPGs were measured after a 10 d period. Bees that received p-coumaric acid or kaempferol showed a significant increase in HPG size. A significant decrease in HPG size was seen in bees receiving caffeine or gallic acid. The implication of our findings on worker bee ontogeny, transitioning from nurses to foragers and relevance to foraging related competencies are discussed. It is critical that bees have access to phytochemicals to ensure colony health and performance. Such access could be through natural habitats that provide a diversity of pollen and nectar sources or through dietary supplements for bee colonies.

The Movement of Western Honey Bees (Apis mellifera L.) Among United States and Territories: History, Benefits, Risks, and Mitigation Strategies

Beekeeping is a cornerstone activity that has led to the human-mediated, global spread of western honey bees (Apis mellifera L.) outside their native range of Europe, western Asia, and Africa. The exportation/importation of honey bees (i.e., transfer of honey bees or germplasm between countries) is regulated at the national level in many countries. Honey bees were first imported into the United States in the early 1600’s. Today, honey bee movement (i.e., transport of honey bees among states and territories) is regulated within the United States at the state, territory, and federal levels. At the federal level, honey bees present in the country (in any state or territory) can be moved among states and territories without federal restriction, with the exception of movement to Hawaii. In contrast, regulations at the state and territory levels vary substantially, ranging from no additional regulations beyond those stipulated at the federal level, to strict regulations for the introduction of live colonies, packaged bees, or queens. This variability can lead to inconsistencies in the application of regulations regarding the movement of honey bees among states and territories. In November 2020, we convened a technical working group (TWG), composed of academic and USDA personnel, to review and summarize the (1) history of honey bee importation into/movement within the United States, (2) current regulations regarding honey bee movement and case studies on the application of those regulations, (3) benefits associated with moving honey bees within the United States, (4) risks associated with moving honey bees within the United States, and (5) risk mitigation strategies. This review will be helpful for developing standardized best practices for the safe movement of honey bees between the 48 contiguous states and other states/territories within the United States.

Floral Microbes Suppress Growth of Monilinia laxa with Minimal Effects on Honey Bee Feeding

Management of Monilinia laxa, the causal agent of brown rot blossom blight in almond (Prunus dulcis), relies heavily on the use of chemical fungicides during bloom. However, chemical fungicides can have nontarget effects on beneficial arthropods, including pollinators, and select for resistance in the pathogen of concern. Almond yield is heavily reliant on successful pollination by healthy honey bees (Apis mellifera); thus, identifying sustainable, effective, and pollinator-friendly control methods for blossom blight during bloom is desirable. Flower-inhabiting microbes could provide a natural, sustainable form of biocontrol for M. laxa, while potentially minimizing costly nontarget effects on almond pollinators and the services they provide. As pollinators are sensitive to floral microbes and their associated taste and scent cues, assessing effects of prospective biocontrol species on pollinator attraction is also necessary. Here, our objective was to isolate and identify potential biocontrol microbes from an array of agricultural and natural flowering hosts and test their efficacy in suppressing M. laxa growth in culture. Out of an initial 287 bacterial and fungal isolates identified, 56 were screened using a dual culture plate assay. Most strains reduced M. laxa growth in vitro. Ten particularly effective candidate microbes were further screened for their effect on honey bee feeding. Of the 10, nine were found to both strongly suppress M. laxa growth in culture and not reduce honey bee feeding. These promising results suggest a number of strong candidates for augmentative microbial biocontrol of brown rot blossom blight in almond with potentially minimal effects on honey bee pollination.

Proceedings of the 2019 American Bee Research Conference

The 2019 American Bee Research Conference (ABRC) was held January 10–12, 2019 in conjunction with the annual convention of the American Honey Producers Association in Tempe, AZ. Over the three-day conference, a total of 45 oral presentations and 13 poster presentations were given, representing work done from over 27 institutions and 34 different research groups from throughout the United States and Canada. This proceedings contains and overview of the conference and the submitted abstracts for presentations given at the 2018 American Bee Research Conference.

Unity in defence: honeybee workers exhibit conserved molecular responses to diverse pathogens

BACKGROUND

Organisms typically face infection by diverse pathogens, and hosts are thought to have developed specific responses to each type of pathogen they encounter. The advent of transcriptomics now makes it possible to test this hypothesis and compare host gene expression responses to multiple pathogens at a genome-wide scale. Here, we performed a meta-analysis of multiple published and new transcriptomes using a newly developed bioinformatics approach that filters genes based on their expression profile across datasets. Thereby, we identified common and unique molecular responses of a model host species, the honey bee (Apis mellifera), to its major pathogens and parasites: the Microsporidia Nosema apis and Nosema ceranae, RNA viruses, and the ectoparasitic mite Varroa destructor, which transmits viruses.

RESULTS

We identified a common suite of genes and conserved molecular pathways that respond to all investigated pathogens, a result that suggests a commonality in response mechanisms to diverse pathogens. We found that genes differentially expressed after infection exhibit a higher evolutionary rate than non-differentially expressed genes. Using our new bioinformatics approach, we unveiled additional pathogen-specific responses of honey bees; we found that apoptosis appeared to be an important response following microsporidian infection, while genes from the immune signalling pathways, Toll and Imd, were differentially expressed after Varroa/virus infection. Finally, we applied our bioinformatics approach and generated a gene co-expression network to identify highly connected (hub) genes that may represent important mediators and regulators of anti-pathogen responses.

CONCLUSIONS

Our meta-analysis generated a comprehensive overview of the host metabolic and other biological processes that mediate interactions between insects and their pathogens. We identified key host genes and pathways that respond to phylogenetically diverse pathogens, representing an important source for future functional studies as well as offering new routes to identify or generate pathogen resilient honey bee stocks. The statistical and bioinformatics approaches that were developed for this study are broadly applicable to synthesize information across transcriptomic datasets. These approaches will likely have utility in addressing a variety of biological questions.

Improving the future of honey bee breeding programs by employing recent scientific advances

A recent abundance of studies investigating causes of honey bee (Apis mellifera) colony losses has led to enhanced recommendations in management practices with particular emphasis on breeding for resistant bee stocks. Here we review the latest advances in research which could improve the future of breeding programs. We discuss diversity in colonies particularly in breeding programs, giving special emphasis to recent improvement in cryopreservation of honey bee germplasm. We also review factors that affect the health and reproductive quality of queens and drones. We briefly discuss how techniques developed by scientists are finding more regular usage with breeders in the assessment of reproductive caste health and quality and in determining best management practices for breeding programs.

Genome-wide analysis of signatures of selection in populations of African honey bees (Apis mellifera) using new web-based tools

Background

With the development of inexpensive, high-throughput sequencing technologies, it has become feasible to examine questions related to population genetics and molecular evolution of non-model species in their ecological contexts on a genome-wide scale. Here, we employed a newly developed suite of integrated, web-based programs to examine population dynamics and signatures of selection across the genome using several well-established tests, including F_ST, pN/pS, and McDonald-Kreitman. We applied these techniques to study populations of honey bees (Apis mellifera) in East Africa. In Kenya, there are several described A. mellifera subspecies, which are thought to be localized to distinct ecological regions.

Results

We performed whole genome sequencing of 11 worker honey bees from apiaries distributed throughout Kenya and identified 3.6 million putative single-nucleotide polymorphisms. The dense coverage allowed us to apply several computational procedures to study population structure and the evolutionary relationships among the populations, and to detect signs of adaptive evolution across the genome. While there is considerable gene flow among the sampled populations, there are clear distinctions between populations from the northern desert region and those from the temperate, savannah region. We identified several genes showing population genetic patterns consistent with positive selection within African bee populations, and between these populations and European A. mellifera or Asian Apis florea.

Conclusions

These results lay the groundwork for future studies of adaptive ecological evolution in honey bees, and demonstrate the use of new, freely available web-based tools and workflows (http://usegalaxy.org/r/kenyanbee) that can be applied to any model system with genomic information.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1712-0) contains supplementary material, which is available to authorized users.

Genomic analysis of the interactions between social environment and social communication systems in honey bees (Apis mellifera)

Social context is often a primary regulator of social behavior, but genes that affect or are affected by social context have rarely been investigated. In social insects, caste specific pheromones are key modulators of social behavior, e.g., in honey bees the queen mandibular gland (MG) pheromone mediates reproductive dominance, its absence prompting ovary activation and queen pheromone production in workers. Here, we investigate the effect of social environment on genome-wide expression patterns in the MG, to determine how social context modulates expression of genes that, in turn alter social environment. We used microarrays to examine the MGs of virgin and mated queens, and queenright (QR) and queenless (QL) workers with or without activated ovaries. Approximately 2554 transcripts were significantly differentially expressed among these groups, with caste and social context being the main regulators of gene expression patterns, while physiological state (ovary activation) only minimally affecting gene expression. Thus, social context strongly regulates expression of genes, which, in turn, shape social environment. Among these, 25 genes that are putatively involved in caste selective production of the fatty-acid derived MG pheromone were differentially expressed in queens and workers. These genes whose functions correspond with enzymatic or transport processes emphasize the occurrence of disparate pheromone biosynthetic pathways for queens and workers, adding another dimension regarding the regulation of these important pheromones. Gene ontology analysis also revealed genes of different functional categories whose expression was impacted by caste or by the social environment, suggesting that the MG has broader functions than pheromone biosynthesis.

Differential effects of insemination volume and substance on reproductive changes in honey bee queens (Apis mellifera L.)

Mating causes dramatic changes in female insects at the behavioural, physiological and molecular level. The factors driving these changes (e.g. seminal proteins, seminal volume) and the molecular pathways by which these factors are operating have been characterized only in a handful of insect species. In the present study, we use instrumental insemination of honey bee queens to examine the role of the insemination substance and volume in triggering post-mating changes. We also examine differences in gene expression patterns in the fat bodies of queens with highly activated ovaries to determine if events during copulation can cause long-term changes in gene expression. We found that the instrumental insemination procedure alone caused cessation of mating flights and triggered ovary activation, with high-volume inseminated queens having the greatest ovary activation. Hierarchical clustering grouped queens primarily by insemination substance and then insemination volume, suggesting that while volume may trigger short-term physiological changes (i.e. ovary activation) substance plays a greater role in regulating long-term transcriptional changes. The results of gene ontology analysis and comparison with previous studies suggest that both insemination substance and volume trigger molecular post-mating changes by altering overlapping gene pathways involved in honey bee reproduction. We also discuss the effects on two genes (vitellogenin and transferrin) involved in reproduction and defence responses.

Effects of honey bee (Apis mellifera L.) queen insemination volume on worker behavior and physiology

Honey bee colonies consist of tens of thousands of workers and a single reproductive queen that produces a pheromone blend which maintains colony organization. Previous studies indicated that the insemination quantity and volume alter queen mandibular pheromone profiles. In our 11-month long field study we show that workers are more attracted to high-volume versus low-volume inseminated queens, however, there were no significant differences between treatments in the number of queen cells built by workers in preparation for supersedure. Workers exposed to low-volume inseminated queens initiated production of queen-like esters in their Dufour's glands, but there were no significant difference in the amount of methyl farnesoate and juvenile hormone in worker hemolymph. Lastly, queen overwintering survival was unexpectedly lower in high-volume inseminated queens. Our results suggest that the queen insemination volume could ultimately affect colony health and productivity.

Genome-wide analysis of brain transcriptional changes in honey bee (Apis mellifera L.) queens exposed to carbon dioxide and physical manipulation

Mating is a complex process causing many behavioural and physiological changes, but the factors triggering them and the underlying molecular processes are not well characterized. In the present study we examine the effects of CO(2) (a commonly used anaesthetic in instrumental insemination that causes changes similar to those occurring after mating) and physical manipulation (which may mimic certain aspects of copulation) on the behavioural, physiological and brain transcriptional changes in honey bee queens. We show that while CO(2) causes cessation of mating flights and ovary activation, physical manipulation has additional effects on ovary activation and brain transcriptional changes. Comparisons with previous studies of honey bees and female Drosophila indicate that common molecular mechanisms may be responsible for regulating reproductive changes across different mating regimes and insect orders.

Effects of the insect growth regulator, methoprene, on Onthophagus taurus (Coleoptera: Scarabaeidae)

A bioassay was conducted to determine the impact of methoprene, an insect growth regulator (IGR), on fecundity, larval survival, and size of progeny for Onthophagus taurus Schreber. Adult O. taurus dung beetles were offered methoprene-treated manure in three to five replications each at concentrations of 0.08, 0.45, and 4.5 ppm, respectively. An additional group of adult beetles was immersed in a methoprene-water solution and allowed to reproduce in containers with untreated manure. Data from all treatment groups were compared with untreated control groups. Methoprene did not seem to hinder brood production at 0.45 ppm. Survival of O. taurus was not affected by methoprene-treated manure at 0.08 ppm or when parent beetles were immersed in methoprene-water solution. However, progeny survival was significantly reduced on manure treated with methoprene at 4.5 ppm. Mean pronotal width of O. taurus progeny was significantly smaller in beetles fed methoprene-treated manure (4.5 ppm). The low dose of 0.08 ppm did not affect pronotal widths nor did topical application of methoprene to adults affect pronotal widths in resulting offspring. Although some adverse effects of methoprene were observed at higher concentrations, use of methoprene at concentrations of 0.08 ppm as part of a horn fly control program likely would not greatly affect populations of O. taurus, the most common paracoprid dung beetle in North Carolina.

Experimental evaluation of Musca domestica (Diptera: Muscidae) as a vector of Newcastle disease virus

House flies, Musca domestica L. (Diptera: Muscidae), were examined for their ability to harbor and transmit Newcastle disease virus (family Paramyxoviridae, genus Avulavirus, NDV) by using a mesogenic NDV strain. Laboratory-reared flies were experimentally exposed to NDV (Roakin strain) by allowing flies to imbibe an inoculum consisting of chicken embryo-propagated virus. NDV was detected in dissected crops and intestinal tissues from exposed flies for up to 96 and 24 h postexposure, respectively; no virus was detected in crops and intestines of sham-exposed flies. The potential of the house fly to directly transmit NDV to live chickens was examined by placing 14-d-old chickens in contact with NDV-exposed house flies 2 h after flies consumed NDV inoculum. NDV-exposed house flies contained approximately 10(4) 50% infectious doses (ID50) per fly, but no transmission of NDV was observed in chickens placed in contact with exposed flies at densities as high as 25 flies per bird. Subsequent dose-response studies demonstrated that oral exposure, the most likely route for fly-to-chicken transmission, required an NDV (Roakin) dose > or =10(6) ID50. These results indicate that house flies are capable of harboring NDV (Roakin) but that they are poor vectors of the virus because they carry an insufficient virus titer to cause infection.

Experimental evaluation of Musca domestica (Diptera: Muscidae) as a vector of Newcastle disease virus

House flies, Musca domestica L. (Diptera: Muscidae), were examined for their ability to harbor and transmit Newcastle disease virus (family Paramyxoviridae, genus Avulavirus, NDV) by using a mesogenic NDV strain. Laboratory-reared flies were experimentally exposed to NDV (Roakin strain) by allowing flies to imbibe an inoculum consisting of chicken embryo-propagated virus. NDV was detected in dissected crops and intestinal tissues from exposed flies for up to 96 and 24 h postexposure, respectively; no virus was detected in crops and intestines of sham-exposed flies. The potential of the house fly to directly transmit NDV to live chickens was examined by placing 14-d-old chickens in contact with NDV-exposed house flies 2 h after flies consumed NDV inoculum. NDV-exposed house flies contained approximately 10(4) 50% infectious doses (ID50) per fly, but no transmission of NDV was observed in chickens placed in contact with exposed flies at densities as high as 25 flies per bird. Subsequent dose-response studies demonstrated that oral exposure, the most likely route for fly-to-chicken transmission, required an NDV (Roakin) dose > or =10(6) ID50. These results indicate that house flies are capable of harboring NDV (Roakin) but that they are poor vectors of the virus because they carry an insufficient virus titer to cause infection.