Salivary secretions from the honeybee mite, Varroa destructor: effects on insect haemocytes and preliminary biochemical characterization

The salivary gland transcriptome of Varroa destructor reveals suitable targets for RNAi-based mite control

The mite Varroa destructor Anderson and Trueman (Mesostigmata: Varroidae) has a dramatic impact on beekeeping and is one of the main causes of honey bee colony losses. This ectoparasite feeds on honey bees' liquid tissues, through a wound created on the host integument, determining weight loss and a reduction of lifespan, as well as the transmission of viral pathogens. However, despite its importance, the mite feeding strategy and the host regulation role by the salivary secretions have been poorly explored. Here, we contribute to fill this gap by identifying the salivary components of V. destructor, to study their functional importance for mite feeding and survival. The differential expression analysis identified 30 salivary gland genes encoding putatively secreted proteins, among which only 15 were found to be functionally annotated. These latter include proteins with putative anti-bacterial, anti-fungal, cytolytic, digestive and immunosuppressive function. The three most highly transcribed genes, coding for a chitin-binding domain protein, a Kazal domain serine protease inhibitor and a papain-like cysteine protease were selected to study their functional importance by reverse genetics. Knockdown (90%-99%) by RNA interference (RNAi) of the transcript of a chitin-binding domain protein, likely interfering with the immune reaction to facilitate mite feeding, was associated with a 40%-50% decrease of mite survival. This work expands our knowledge of the host regulation and nutritional exploitation strategies adopted by ectoparasites of arthropods and allows the identification of potential targets for RNAi, paving the way towards the development of new strategies for Varroa mite control.

Life-history stage determines the diet of ectoparasitic mites on their honey bee hosts

Ectoparasitic mites of the genera Varroa and Tropilaelaps have evolved to exclusively exploit honey bees as food sources during alternating dispersal and reproductive life history stages. Here we show that the primary food source utilized by Varroa destructor depends on the host life history stage. While feeding on adult bees, dispersing V. destructor feed on the abdominal membranes to access to the fat body as reported previously. However, when V. destructor feed on honey bee pupae during their reproductive stage, they primarily consume hemolymph, indicated by wound analysis, preferential transfer of biostains, and a proteomic comparison between parasite and host tissues. Biostaining and proteomic results were paralleled by corresponding findings in Tropilaelaps mercedesae, a mite that only feeds on brood and has a strongly reduced dispersal stage. Metabolomic profiling of V. destructor corroborates differences between the diet of the dispersing adults and reproductive foundresses. The proteome and metabolome differences between reproductive and dispersing V. destructor suggest that the hemolymph diet coincides with amino acid metabolism and protein synthesis in the foundresses while the metabolism of non-reproductive adults is tuned to lipid metabolism. Thus, we demonstrate within-host dietary specialization of ectoparasitic mites that coincides with life history of hosts and parasites.

Interaction of chlorothalonil and Varroa destructor on immature honey bees rearing in vitro

Under realistic environmental conditions, bees are often exposed to multiple stressors, especially Varroa destructor and pesticides. In this study, the effects of exposure to NOAEC of chlorothalonil during the larval stage, in the presence or absence of V. destructor, was examined in terms of survival, morphological and transcriptional changes. The interaction between chlorothalonil and V. destructor on the survival of honey bee was additive. V. destructor are the dominant factor in the interaction for survival and transcriptome alternation. The downregulation of the genes related to tissue growth and caste differentiation may directly link to the mortality of honey bees. Either chlorothalonil or V. destructor induces the irregular morphology of trophocytes and oenocytes in the fat body. In addition to irregular shapes, oenocytes in V. destructor alone and double-stressor treatment group showed altered nuclei and vacuoles in the cytoplasm. The interaction of V. destructor and chlorothalonil at the larval stage have potential adverse effects on the subsequent adult bees, with up-regulation of genes involved in lipid metabolism and detoxification/defense in fat body tissue. Our findings provide a comprehensive understanding of combinatorial effects between biotic and abiotic stressors on one of the most important pollinators, honey bees.

Resilin Distribution and Abundance in Apis mellifera across Biological Age Classes and Castes

The presence of resilin, an elastomeric protein, in insect vein joints provides the flexible, passive deformations that are crucial to flapping flight. This study investigated the resilin gene expression and autofluorescence dynamics among Apis mellifera (honey bee) worker age classes and drone honey bees. Resilin gene expression was determined via ddPCR on whole honey bees and resilin autofluorescence was measured in the 1m-cu, 2m-cu, Cu-V, and Cu2-V joints on the forewing and the Cu-V joint of the hindwing. Resilin gene expression varied significantly with age, with resilin activity being highest in the pupae. Autofluorescence of the 1m-cu and the Cu-V joints on the ventral forewing and the Cu-V joint on the ventral hindwing varied significantly between age classes on the left and right sides of the wing, with the newly emerged honey bees having the highest level of resilin autofluorescence compared to all other groups. The results of this study suggest that resilin gene expression and deposition on the wing is age-dependent and may inform us more about the physiology of aging in honey bees.

A comparative transcriptome analysis of the head of 1 and 9 days old worker honeybees (Apis mellifera)

The role of bees in the environment, economic, biodiversity and pharmaceutical industries is due to its social behavior, which is oriented from the brain and hypopharyngeal gland that is the center of royal jelly (RJ) production. Limited studies have been performed on the head gene expression profile at the RJ production stage. The aim of this study was to compare the gene expressions in 9 and 1-day-old (DO) honeybee workers in order to achieve better understanding about head gene expression pattern. After sequencing of RNAs, transcriptome and their networks were compared. The head expression profile undergoes various changes. 1662 gene transcripts had differential expressions which 1125 and 537 were up and down regulated, respectively, in 9_DO compared with 1_DO honey bees. The day 1th had more significant role in the expression of genes related to RJ production as major RJ protein 1, 2, 3, 5, 6 and 9 encoding genes, but their maximum secretion occurred at day 9th. All process related to hypopharyngeal glands activities as CYP450 gene, fatty acid synthase gene, vitamin B6 metabolism and some of genes involved in fatty acid elongation and degradation process had an upward trend from 1_DO and were age-dependent. By increasing the age, the activity of pathways related to immune system increased for keeping the health of bees against the chemical compound. The expression of aromatic amino acid genes involved in Phenylalanine, tyrosine and tryptophan biosynthesis pathway are essential for early stage of life. In 9_DO honeybees, the energy supplying, reducing stress, protein production and export pathways have a crucial role for support the body development and the social duties. It can be stated that the activity of honeybee head is focused on energy supply instead of storage, while actively trying to improve the level of cell dynamics for increasing the immunity and reducing stress. Results of current study identified key genes of certain behaviors of honeybee workers. Deeper considering of some pathways will be evaluated in future studies.

Bee Stressors from an Immunological Perspective and Strategies to Improve Bee Health

Honeybees are the most prevalent insect pollinator species; they pollinate a wide range of crops. Colony collapse disorder (CCD), which is caused by a variety of biotic and abiotic factors, incurs high economic/ecological loss. Despite extensive research to identify and study the various ecological stressors such as microbial infections, exposure to pesticides, loss of habitat, and improper beekeeping practices that are claimed to cause these declines, the deep understanding of the observed losses of these important insects is still missing. Honeybees have an innate immune system, which includes physical barriers and cellular and humeral responses to defend against pathogens and parasites. Exposure to various stressors may affect this system and the health of individual bees and colonies. This review summarizes and discusses the composition of the honeybee immune system and the consequences of exposure to stressors, individually or in combinations, on honeybee immune competence. In addition, we discuss the relationship between bee nutrition and immunity. Nutrition and phytochemicals were highlighted as the factors with a high impact on honeybee immunity.

Varroa mite and deformed wing virus infestations interactively make honey bees (Apis mellifera) more susceptible to insecticides

Varroa mite is one of the major adverse factors causing honey bee population decline. In this study, Varroa destructor-infested and uninfested honey bee colonies were established by selective applying miticide (Apivar® amitraz). Mite population was monitored monthly (April-October 2016), and deformed wing virus (DWV) loading was detected seasonally (April, July, and October). Four immunity- and two physiology-related gene expressions, natural mortality, and susceptibility to five insecticides were comparatively and seasonally examined in field-collected honey bee workers. Results showed that Apivar-treated bee colonies had minor or undetectable mite and DWV (using RT-qPCR) infestations in whole bee season, while untreated colonies had substantially higher mite and DWV infestations. In untreated colonies, Varroa mite population irregularly fluctuated over the bee season with higher mite counts in Jun (318 ± 89 mites dropped in 48 h) or August (302) than that (25 ± 4 or 34) in October, and mite population density was not dynamically or closely correlated with the seasonal shift of honey bee natural mortality (regression slope = -0.5212). Unlike mite, DWV titer in untreated colonies progressively increased over the bee season, and it was highly correlated (R² = 1) with the seasonal increase of honey bee natural mortality. Significantly lower gene expressions of dor, PPO, mfe, potentially PPOa and eat as well, in untreated colonies also indicated an association of increased DWV infestation with decreased physiological and immunity-related functions in late-season honey bees. Furthermore, bees with lower mite/DWV infestations exhibited generally consistently lower susceptibilities (contact and oral toxicities) to five representative insecticides than the bees without Apivar treatment. All of these data from this study consistently indicated an interaction of Varroa/viral infestations with insecticide susceptibilities in honey bees, potentially through impairing bee's physiology and immunity, emphasizing the importance of mite control in order to minimize honey bee decline.

Varroa destructor from the Laboratory to the Field: Control, Biocontrol and IPM Perspectives-A Review

Varroa destructor is a real challenger for beekeepers and scientists: fragile out of the hive, tenacious inside a bee colony. From all the research done on the topic, we have learned that a better understanding of this organism in its relationship with the bee but also for itself is necessary. Its biology relies mostly on semiochemicals for reproduction, nutrition, or orientation. Many treatments have been developed over the years based on hard or soft acaricides or even on biocontrol techniques. To date, no real sustainable solution exists to reduce the pressure of the mite without creating resistances or harming honeybees. Consequently, the development of alternative disruptive tools against the parasitic life cycle remains open. It requires the combination of both laboratory and field results through a holistic approach based on health biomarkers. Here, we advocate for a more integrative vision of V. destructor research, where in vitro and field studies are more systematically compared and compiled. Therefore, after a brief state-of-the-art about the mite's life cycle, we discuss what has been done and what can be done from the laboratory to the field against V. destructor through an integrative approach.

Effects of Insecticides and Microbiological Contaminants on Apis mellifera Health

Over the past two decades, there has been an alarming decline in the number of honey bee colonies. This phenomenon is called Colony Collapse Disorder (CCD). Bee products play a significant role in human life and have a huge impact on agriculture, therefore bees are an economically important species. Honey has found its healing application in various sectors of human life, as well as other bee products such as royal jelly, propolis, and bee pollen. There are many putative factors of CCD, such as air pollution, GMO, viruses, or predators (such as wasps and hornets). It is, however, believed that pesticides and microorganisms play a huge role in the mass extinction of bee colonies. Insecticides are chemicals that are dangerous to both humans and the environment. They can cause enormous damage to bees' nervous system and permanently weaken their immune system, making them vulnerable to other factors. Some of the insecticides that negatively affect bees are, for example, neonicotinoids, coumaphos, and chlorpyrifos. Microorganisms can cause various diseases in bees, weakening the health of the colony and often resulting in its extinction. Infection with microorganisms may result in the need to dispose of the entire hive to prevent the spread of pathogens to other hives. Many aspects of the impact of pesticides and microorganisms on bees are still unclear. The need to deepen knowledge in this matter is crucial, bearing in mind how important these animals are for human life.

Heat Tolerance, Energetics, and Thermal Treatments of Honeybees Parasitized With Varroa

Ongoing global change affects both wildlife and economically relevant species, which are now subjected to combined challenges from climate change and higher exposure to pathogens. Honeybee colonies worldwide are under threat by higher temperatures and the ectoparasitic mite Varroa destructor, hence we studied the impact of these combined challenges in the thermal biology and energetics of Apis mellifera. We estimated the heat tolerance and energy expenditure (CO2 production and VCO2) of honeybees acclimated to different temperatures (32 and 38°C) and subjected to different levels of parasitism (0, 1, and 2 mites). Heat tolerance was quantified employing thermal death time (TDT) curves describing how survival times vary as a function of temperature, which differed significantly between treatments. Warm-acclimated uninfected bees exhibited a higher thermal tolerance than their cold-acclimated counterparts, but parasitism by Varroa resulted in a substantial drop in tolerance rendering TDT curves of parasitized bees virtually indistinguishable. Accordingly, VCO2 increased dramatically in parasitized bees (46.5 and 67.1% with 1 and 2 Varroa, respectively), suggesting that Varroa impinges on substantial costs on energy expenditure which, in combination with lower fat reserves due to parasitism, should have synergistic effects on bees’ survival and performance. Results provide conclusive evidence of the detrimental impact of Varroa on heat tolerance that undermines potentially adaptive responses associated with thermal acclimation. Results also show that heat treatments are a realistic venue to control Varroa, and we discuss how TDT curves may be employed to optimize management strategies in this context.

Changing the Recipe: Pathogen Directed Changes in Tick Saliva Components

Ticks are obligate hematophagous parasites and are important vectors of a wide variety of pathogens. These pathogens include spirochetes in the genus Borrelia that cause Lyme disease, rickettsial pathogens, and tick-borne encephalitis virus, among others. Due to their prolonged feeding period of up to two weeks, hard ticks must counteract vertebrate host defense reactions in order to survive and reproduce. To overcome host defense mechanisms, ticks have evolved a large number of pharmacologically active molecules that are secreted in their saliva, which inhibits or modulates host immune defenses and wound healing responses upon injection into the bite site. These bioactive molecules in tick saliva can create a privileged environment in the host’s skin that tick-borne pathogens take advantage of. In fact, evidence is accumulating that tick-transmitted pathogens manipulate tick saliva composition to enhance their own survival, transmission, and evasion of host defenses. We review what is known about specific and functionally characterized tick saliva molecules in the context of tick infection with the genus Borrelia, the intracellular pathogen Anaplasma phagocytophilum, and tick-borne encephalitis virus. Additionally, we review studies analyzing sialome-level responses to pathogen challenge.

A salivary chitinase of Varroa destructor influences host immunity and mite's survival

Varroa destructor is an ectoparasite of honey bees and an active disease vector, which represents one of the most severe threats for the beekeeping industry. This parasitic mite feeds on the host’s body fluids through a wound in the cuticle, which allows food uptake by the mother mite and its progeny, offering a potential route of entrance for infecting microorganisms. Mite feeding is associated with saliva injection, whose role is still largely unknown. Here we try to fill this gap by identifying putative host regulation factors present in the saliva of V. destructor and performing a functional analysis for one of them, a chitinase (Vd-CHIsal) phylogenetically related to chitinases present in parasitic and predatory arthropods, which shows a specific and very high level of expression in the mite’s salivary glands. Vd-CHIsal is essential for effective mite feeding and survival, since it is apparently involved both in maintaining the feeding wound open and in preventing host infection by opportunistic pathogens. Our results show the important role in the modulation of mite-honey bee interactions exerted by a host regulation factor shared by different evolutionary lineages of parasitic arthropods. We predict that the functional characterization of Varroa sialome will provide new background knowledge on parasitism evolution in arthropods and the opportunity to develop new bioinspired strategies for mite control based on the disruption of their complex interactions with a living food source.

Varroa destructor is a parasitic mite of honey bees and a major driver of honey bee colony losses. The feeding mites inject a salivary blend of poorly known molecules, which regulate host physiology. Here, we have identified in silico putative host regulation factors occurring in Varroa saliva and characterized the functional role of a highly expressed chitinase, which is conserved across different evolutionary lineages of parasitic arthropods. This enzyme influences host immune response and mite’s survival. An in-depth functional analysis of Varroa saliva will shed light on parasitism evolution in arthropods and will pave the way towards the development of new bioinspired strategies for mite control.

Interaction of Varroa destructor and Sublethal Clothianidin Doses during the Larval Stage on Subsequent Adult Honey Bee (Apis mellifera L.) Health, Cellular Immunity, Deformed Wing Virus Levels and Differential Gene Expression

Honeybees (Apis mellifera L.) are exposed to many parasites, but little is known about interactions with abiotic stressors on their health, particularly when affected as larvae. Larvae were exposed singly and in combination to the parasitic mite Varroa destructor and three sublethal doses of the neonicotinoid insecticide clothianidin to evaluate their effects on survivorship, weight, haemocyte counts, deformed wing virus (DWV) levels and gene expression of the adult bees that subsequently developed. Clothianidin significantly reduced bee weight at the highest dose and was associated with an increase in haemocyte counts at the lowest dose, whereas V. destructor parasitism increased DWV levels, reduced bee emergence, lowered weight and reduced haemocyte counts. An interaction between the two stressors was observed for weight at emergence. Among the differentially expressed genes (DEGs), V. destructor infestation resulted in broader down-regulatory effects related to immunity that was often shared with the combined stressors, while clothianidin resulted in a broader up-regulatory effect more related to central metabolic pathways that was often shared with the combined stressors. Parasites and abiotic stressors can have complex interactions, including additive effects on reduced weight, number of up-regulated DEGs and biological pathways associated with metabolism.

Varroa destructor: A Complex Parasite, Crippling Honey Bees Worldwide

The parasitic mite, Varroa destructor, has shaken the beekeeping and pollination industries since its spread from its native host, the Asian honey bee (Apis cerana), to the naïve European honey bee (Apis mellifera) used commercially for pollination and honey production around the globe. Varroa is the greatest threat to honey bee health. Worrying observations include increasing acaricide resistance in the varroa population and sinking economic treatment thresholds, suggesting that the mites or their vectored viruses are becoming more virulent. Highly infested weak colonies facilitate mite dispersal and disease transmission to stronger and healthier colonies. Here, we review recent developments in the biology, pathology, and management of varroa, and integrate older knowledge that is less well known.

Haemolymph removal by Varroa mite destabilizes the dynamical interaction between immune effectors and virus in bees, as predicted by Volterra's model

The association between the deformed wing virus and the parasitic mite Varroa destructor has been identified as a major cause of worldwide honeybee colony losses. The mite acts as a vector of the viral pathogen and can trigger its replication in infected bees. However, the mechanistic details underlying this tripartite interaction are still poorly defined, and, particularly, the causes of viral proliferation in mite-infested bees. Here, we develop and test a novel hypothesis that mite feeding destabilizes viral immune control through the removal of both virus and immune effectors, triggering uncontrolled viral replication. Our hypothesis is grounded on the predator-prey theory developed by Volterra, which predicts prey proliferation when both predators and preys are constantly removed from the system. Consistent with this hypothesis, we show that the experimental removal of increasing volumes of haemolymph from individual bees results in increasing viral densities. By contrast, we do not find consistent support for alternative proposed mechanisms of viral expansion via mite immune suppression or within-host viral evolution. Our results suggest that haemolymph removal plays an important role in the enhanced pathogen virulence observed in the presence of feeding Varroa mites. Overall, these results provide a new model for the mechanisms driving pathogen-parasite interactions in bees, which ultimately underpin honeybee health decline and colony losses.

Interaction of field realistic doses of clothianidin and Varroa destructor parasitism on adult honey bee (Apis mellifera L.) health and neural gene expression, and antagonistic effects on differentially expressed genes

While many studies have examined the effects of neonicotinoid insecticides and the parasitic mite Varroa destructor on honey bees (Apis mellifera), more information on the combined effects of such stressors on gene expression, including neural related genes, and their impact on biological pathways is needed. This study analyzed the effects of field realistic concentrations of the neonicotinoid clothianidin on adult bees infested and not infested with V. destructor over 21 consecutive days and then determined bee survivorship, weight, deformed wing virus (DWV) levels and gene expression. V. destructor parasitism with or without clothianidin exposure was significantly associated with decreased survivorship, weight loss and higher DWV levels, while clothianidin exposure was only associated with higher levels of DWV. Expression analysis of the neural genes AmNlg-1, BlCh and AmAChE-2 showed that V. destructor caused a significant down-regulation of all of them, whereas clothianidin caused a significant down-regulation of only AmNrx-1 and BlCh. An interaction was only detected for AmNrx-1 expression. RNAseq analysis showed that clothianidin exposure resulted in 6.5 times more up-regulated differentially expressed genes (DEGs) than V. destructor alone and 123 times more than clothianidin combined with V. destructor. Similar results were obtained with down-regulated DEGs, except for a higher number of DEGs shared between V. destructor and the combined stressors. KEGG (Kyoto Encyclopedia of Genes and Genomes) biological pathway analysis of the DEGs showed that the stressor linked to the highest number of KEGG pathways was clothianidin, followed by V. destructor, and then considerably fewer number of KEGG pathways with the combined stressors. The reduced numbers of DEGs and KEGG pathways associated with the DEGs for the combined stressors compared to the stressors alone indicates that the interaction of the stressors is not additive or synergistic, but antagonistic. The possible implications of the antagonistic effect on the number of DEGs are discussed.

Towards Precision Nutrition: A Novel Concept Linking Phytochemicals, Immune Response and Honey Bee Health

The high annual losses of managed honey bees (Apis mellifera) has attracted intensive attention, and scientists have dedicated much effort trying to identify the stresses affecting bees. There are, however, no simple answers; rather, research suggests multifactorial effects. Several works have been reported highlighting the relationship between bees' immunosuppression and the effects of malnutrition, parasites, pathogens, agrochemical and beekeeping pesticides exposure, forage dearth and cold stress. Here we analyze a possible connection between immunity-related signaling pathways that could be involved in the response to the stress resulted from Varroa-virus association and cold stress during winter. The analysis was made understanding the honey bee as a superorganism, where individuals are integrated and interacting within the colony, going from social to individual immune responses. We propose the term "Precision Nutrition" as a way to think and study bees' nutrition in the search for key molecules which would be able to strengthen colonies' responses to any or all of those stresses combined.

Insights into the feeding behaviors and biomechanics of Varroa destructor mites on honey bee pupae using electropenetrography and histology

Feeding behaviors and biomechanics of female Varroa destructor mites are revealed from AC-DC electropenetrography (EPG) recordings of mites feeding from Apis mellifera honey bee pupae and histology of mite internal ingestion apparatus. EPG signals characteristic of arthropod suction feeding (ingestion) were identified for mites that fed on pupae during overnight recordings. Ingestion by these mites was confirmed afterwards by observing internally fluorescent microbeads previously injected into their hosts. Micrographs of internal ingestion apparatus illustrate the connection between a gnathosomal tube and a pharyngeal lumen, which is surrounded by alternating dilator and constrictor muscles. Inspection of EPG signals showed the muscularized mite pharyngeal pump operates at a mean repetition rate of 4.5 cycles/s to ingest host fluids. Separate feeding events observed for mites numbered between 23 and 33 over approximately 16 h of recording, with each event lasting ~10 s. Feeding events were each separated by ~2 min. Consecutive feeding events separated by either locomotion or prolonged periods of quiescence were grouped into feeding bouts, which ranged in number from one to six. Statistical analyses of EPG data revealed that feeding events were prolonged for mites having lower pharyngeal pump frequencies, and mites having prolonged feeding events went unfed for significantly more time between feeding events. These results suggest that mites may adjust behaviors to meet limitations of their feeding apparatus to acquire similar amounts of food. Data reported here help to provide a more robust view of Varroa mite feeding than those previously reported and are both reminiscent of, as well as distinct from, some other acarines and fluid-feeding insects.

Varroa destructor parasitism has a greater effect on proteome changes than the deformed wing virus and activates TGF-β signaling pathways

Honeybee workers undergo metamorphosis in capped cells for approximately 13 days before adult emergence. During the same period, Varroa mites prick the defenseless host many times. We sought to identify proteome differences between emerging Varroa-parasitized and parasite-free honeybees showing the presence or absence of clinical signs of deformed wing virus (DWV) in the capped cells. A label-free proteomic analysis utilizing nanoLC coupled with an Orbitrap Fusion Tribrid mass spectrometer provided a quantitative comparison of 2316 protein hits. Redundancy analysis (RDA) showed that the combination of Varroa parasitism and DWV clinical signs caused proteome changes that occurred in the same direction as those of Varroa alone and were approximately two-fold higher. Furthermore, proteome changes associated with DWV signs alone were positioned above Varroa in the RDA. Multiple markers indicate that Varroa activates TGF-β-induced pathways to suppress wound healing and the immune response and that the collective action of stressors intensifies these effects. Furthermore, we indicate JAK/STAT hyperactivation, p53-BCL-6 feedback loop disruption, Wnt pathway activation, Wnt/Hippo crosstalk disruption, and NF-κB and JAK/STAT signaling conflict in the Varroa–honeybee–DWV interaction. These results illustrate the higher effect of Varroa than of DWV at the time of emergence. Markers for future research are provided.

Insight Into the Salivary Secretome of Varroa destructor and Salivary Toxicity to Apis cerana

The Varroa destructor (Acari Varroidae) mite is a serious threat to honey bee due to hemolymph feeding and virus transmission. Mite salivary proteins are involved in these interactions. However, the salivary secretome has not been previously characterized. In this paper, the saliva of V. destructor was found to be toxic to the worker larvae of Apis cerana (Hymenoptera Apidae) in the absence of deformed wing virus (DWV) and to stimulate the development of deformed wings in Apis mellifera (Hymenoptera Apidae) adults in the presence of DWV. The salivary secretome was analyzed by nano-liquid chromatography coupled to tandem mass spectrometry (nano-LC-MS/MS). A search of the resulting data against peptide databases using the software Mascot yielded 356, 53, and 9 matched proteins from V. destructor, A. mellifera, and DWV, respectively. The saliva contained Varroa mite proteins identified as important for potential virulence to A. cerana larvae, for the inhibition of harmful microorganisms, for the utilization of bee nutrients, and for antioxidant, oxidation-reduction and detoxification functions as well as A. mellifera proteins identified as nutrients important for mite reproduction. The saliva proteins also contained viral proteins from one virus, DWV. These results provide a strong foundation for understanding the interactions among the Varroa mite, honeybee, and DWV.

Varroa destructor feeds primarily on honey bee fat body tissue and not hemolymph

The parasitic mite Varroa destructor is the greatest single driver of the global honey bee health decline. Better understanding of the association of this parasite and its host is critical to developing sustainable management practices. Our work shows that this parasite is not consuming hemolymph, as has been the accepted view, but damages host bees by consuming fat body, a tissue roughly analogous to the mammalian liver. Both hemolymph and fat body in honey bees were marked with fluorescent biostains. The fluorescence profile in the guts of mites allowed to feed on these bees was very different from that of the hemolymph of the host bee but consistently matched the fluorescence profile unique to the fat body. Via transmission electron microscopy, we observed externally digested fat body tissue in the wounds of parasitized bees. Mites in their reproductive phase were then fed a diet composed of one or both tissues. Mites fed hemolymph showed fitness metrics no different from the starved control. Mites fed fat body survived longer and produced more eggs than those fed hemolymph, suggesting that fat body is integral to their diet when feeding on brood as well. Collectively, these findings strongly suggest that Varroa are exploiting the fat body as their primary source of sustenance: a tissue integral to proper immune function, pesticide detoxification, overwinter survival, and several other essential processes in healthy bees. These findings underscore a need to revisit our understanding of this parasite and its impacts, both direct and indirect, on honey bee health.

Quantitative proteomics reveals divergent responses in Apis mellifera worker and drone pupae to parasitization by Varroa destructor

Varroa destructor is a haemophagous ectoparasite of honeybees and is considered a major causal agent of colony losses in Europe and North America. Although originating in Eastern Asia where it parasitizes Apis cerana, it has shifted hosts to the western honeybee Apis mellifera on which it has a greater deleterious effect on the individual and colony level. To investigate this important host-parasite interaction and to determine whether Varroa causes different effects on different castes we conducted a label free quantitative proteomic analysis of Varroa-parasitized and non-parasitized drone and worker Apis mellifera pupae. 1195 proteins were identified in total, of which 202 and 250 were differentially abundant in parasitized drone and worker pupae, respectively. Both parasitized drone and worker pupae displayed reduced abundance in proteins associated with the cuticle, lipid transport and innate immunity. Proteins involved in metabolic processes were more abundant in both parasitized castes although the response in workers was more pronounced. A number of caste specific responses were observed including differential abundance of numerous cytoskeletal and muscle proteins, which were of higher abundance in parasitized drones in comparison to parasitized workers. Proteins involved in fatty acid and carbohydrate metabolism were more abundant in parasitized workers as were a large number of ribosomal proteins highlighting either potentially divergent responses to Varroa or a different strategy by the mite when parasitizing the different castes. This data improves our understanding of this interaction and may provide a basis for future studies into improvements to therapy and control of Varroasis.

Varroa destructor parasitism reduces hemocyte concentrations and prophenol oxidase gene expression in bees from two populations

Circulating hemocytes are responsible for defensive and healing mechanisms in the honey bee, Apis mellifera. Parasitism by the mite Varroa destructor and injection of V. destructor homogenate in buffer, but not buffer injection, showed similar reductions in total hemocyte concentrations in both Africanized and European adult honey bees. This indicated that compounds in V. destructor homogenate can have similar effects as V. destructor parasitism and that the response is not solely due to wounding. Samples from honey bees with different hemocyte concentrations were compared for the expression patterns of hemolectin (AmHml), prophenol oxidase (AmPpo), and class C scavenger receptor (AmSRC-C). Of the genes tested, only the expression of AmPpo correlated well with hemocyte counts for all the treatments, indicating that melanization is associated with those responses. Thus, the expression of AmPpo might be a suitable biomarker for hemocyte counts as part of cellular defenses against injection of buffer or mite compounds and V. destructor parasitism and perhaps other conditions involving healing and immunity.

A Saliva Protein of Varroa Mites Contributes to the Toxicity toward Apis cerana and the DWV Elevation in A. mellifera

Varroa destructor mites express strong avoidance of the Apis cerana worker brood in the field. The molecular mechanism for this phenomenon remains unknown. We identified a Varroa toxic protein (VTP), which exhibited toxic activity toward A. cerana worker larvae, in the saliva of these mites, and expressed VTP in an Escherichia coli system. We further demonstrated that recombinant VTP killed A. cerana worker larvae and pupae in the absence of deformed-wing virus (DWV) but was not toxic to A. cerana worker adults and drones. The recombinant VTP was safe for A. mellifera individuals, but resulted in elevated DWV titers and the subsequent development of deformed-wing adults. RNAi-mediated suppression of vtp gene expression in the mites partially protected A. cerana larvae. We propose a modified mechanism for Varroa mite avoidance of worker brood, due to mutual destruction stress, including the worker larvae blocking Varroa mite reproduction and Varroa mites killing worker larvae by the saliva toxin. The discovery of VTP should provide a better understanding of Varroa pathogenesis, facilitate host-parasite mechanism research and allow the development of effective methods to control these harmful mites.

Differential Gene Expression Associated with Honey Bee Grooming Behavior in Response to Varroa Mites

Honey bee (Apis mellifera) grooming behavior is an important mechanism of resistance against the parasitic mite Varroa destructor. This research was conducted to study associations between grooming behavior and the expression of selected immune, neural, detoxification, developmental and health-related genes. Individual bees tested in a laboratory assay for various levels of grooming behavior in response to V. destructor were also analyzed for gene expression. Intense groomers (IG) were most efficient in that they needed significantly less time to start grooming and fewer grooming attempts to successfully remove mites from their bodies than did light groomers (LG). In addition, the relative abundance of the neurexin-1 mRNA, was significantly higher in IG than in LG, no groomers (NG) or control (bees without mite). The abundance of poly U binding factor kd 68 and cytochrome p450 mRNAs were significantly higher in IG than in control bees. The abundance of hymenoptaecin mRNA was significantly higher in IG than in NG, but it was not different from that of control bees. The abundance of vitellogenin mRNA was not changed by grooming activity. However, the abundance of blue cheese mRNA was significantly reduced in IG compared to LG or NG, but not to control bees. Efficient removal of mites by IG correlated with different gene expression patterns in bees. These results suggest that the level of grooming behavior may be related to the expression pattern of vital honey bee genes. Neurexin-1, in particular, might be useful as a bio-marker for behavioral traits in bees.

Effect of Varroa destructor, Wounding and Varroa Homogenate on Gene Expression in Brood and Adult Honey Bees

Honey bee (Apis mellifera) gene expression related to immunity for hymenoptaecin (AmHym) and defensin-1 (AmDef-1), longevity for vitellogenin (AmVit2) and stem cell proliferation for poly U binding factor 68 kDa (AmPuf68) was compared following Varroa destructor parasitism, buffer injection and injection of V. destructor compounds in its homogenate. In adults, V. destructor parasitism decreased expression of all four genes, while buffer injection decreased expression of AmHym, AmPuf68 and AmVit2, and homogenate injection decreased expression of AmPuf68 and AmVit2 but increased expression of AmDef-1 relative to their respective controls. The effect of V. destructor parasitism in adults relative to the controls was not significantly different from buffer injection for AmHym and AmVit2 expression, and it was not significantly different from homogenate injection for AmPuf68 and AmVit2. In brood, V. destructor parasitism, buffer injection and homogenate injection decreased AmVit2 expression, whereas AmHym expression was decreased by V. destructor parasitism but increased by buffer and homogenate injection relative to the controls. The effect of varroa parasitism in brood was not significantly different from buffer or homogenate injection for AmPuf68 and AmVit2. Expression levels of the four genes did not correlate with detectable viral levels in either brood or adults. The results of this study indicate that the relative effects of V. destructor parasitism on honey bee gene expression are also shared with other types of stresses. Therefore, some of the effects of V. destructor on honey bees may be mostly due to wounding and injection of foreign compounds into the hemolymph of the bee during parasitism. Although both brood and adults are naturally parasitized by V. destructor, their gene expression responded differently, probably the result of different mechanisms of host responses during development.

The Salivary Protein Repertoire of the Polyphagous Spider Mite Tetranychus urticae: A Quest for Effectors

The two-spotted spider mite Tetranychus urticae is an extremely polyphagous crop pest. Alongside an unparalleled detoxification potential for plant secondary metabolites, it has recently been shown that spider mites can attenuate or even suppress plant defenses. Salivary constituents, notably effectors, have been proposed to play an important role in manipulating plant defenses and might determine the outcome of plant-mite interactions. Here, the proteomic composition of saliva from T. urticae lines adapted to various host plants-bean, maize, soy, and tomato-was analyzed using a custom-developed feeding assay coupled with nano-LC tandem mass spectrometry. About 90 putative T. urticae salivary proteins were identified. Many are of unknown function, and in numerous cases belonging to multimembered gene families. RNAseq expression analysis revealed that many genes coding for these salivary proteins were highly expressed in the proterosoma, the mite body region that includes the salivary glands. A subset of genes encoding putative salivary proteins was selected for whole-mount in situ hybridization, and were found to be expressed in the anterior and dorsal podocephalic glands. Strikingly, host plant dependent expression was evident for putative salivary proteins, and was further studied in detail by micro-array based genome-wide expression profiling. This meta-analysis revealed for the first time the salivary protein repertoire of a phytophagous chelicerate. The availability of this salivary proteome will assist in unraveling the molecular interface between phytophagous mites and their host plants, and may ultimately facilitate the development of mite-resistant crops. Furthermore, the technique used in this study is a time- and resource-efficient method to examine the salivary protein composition of other small arthropods for which saliva or salivary glands cannot be isolated easily.

A mutualistic symbiosis between a parasitic mite and a pathogenic virus undermines honey bee immunity and health

Honey bee colony losses are triggered by interacting stress factors consistently associated with high loads of parasites and/or pathogens. A wealth of biotic and abiotic stressors are involved in the induction of this complex multifactorial syndrome, with the parasitic mite Varroa destructor and the associated deformed wing virus (DWV) apparently playing key roles. The mechanistic basis underpinning this association and the evolutionary implications remain largely obscure. Here we narrow this research gap by demonstrating that DWV, vectored by the Varroa mite, adversely affects humoral and cellular immune responses by interfering with NF-κB signaling. This immunosuppressive effect of the viral pathogen enhances reproduction of the parasitic mite. Our experimental data uncover an unrecognized mutualistic symbiosis between Varroa and DWV, which perpetuates a loop of reciprocal stimulation with escalating negative effects on honey bee immunity and health. These results largely account for the remarkable importance of this mite-virus interaction in the induction of honey bee colony losses. The discovery of this mutualistic association and the elucidation of the underlying regulatory mechanisms sets the stage for a more insightful analysis of how synergistic stress factors contribute to colony collapse, and for the development of new strategies to alleviate this problem.

Understanding the biology and control of the poultry red mite Dermanyssus gallinae: a review

Dermanyssus gallinae, the poultry red mite (PRM), is a blood-feeding ectoparasite capable of causing pathology in birds, amongst other animals. It is an increasingly important pathogen in egg layers and is responsible for substantial economic losses to the poultry industry worldwide. Even though PRM poses a serious problem, very little is known about the basic biology of the mite. Here we review the current body of literature describing red mite biology and discuss how this has been, or could be, used to develop methods to control PRM infestations. We focus primarily on the PRM digestive system, salivary glands, nervous system and exoskeleton and also explore areas of PRM biology which have to date received little or no study but have the potential to offer new control targets.

Lower virus infections in Varroa destructor-infested and uninfested brood and adult honey bees (Apis mellifera) of a low mite population growth colony compared to a high mite population growth colony

A comparison was made of the prevalence and relative quantification of deformed wing virus (DWV), Israeli acute paralysis virus (IAPV), black queen cell virus (BQCV), Kashmir bee virus (KBV), acute bee paralysis virus (ABPV) and sac brood virus (SBV) in brood and adult honey bees (Apis mellifera) from colonies selected for high (HMP) and low (LMP) Varroa destructor mite population growth. Two viruses, ABPV and SBV, were never detected. For adults without mite infestation, DWV, IAPV, BQCV and KBV were detected in the HMP colony; however, only BQCV was detected in the LMP colony but at similar levels as in the HMP colony. With mite infestation, the four viruses were detected in adults of the HMP colony but all at higher amounts than in the LMP colony. For brood without mite infestation, DWV and IAPV were detected in the HMP colony, but no viruses were detected in the LMP colony. With mite infestation of brood, the four viruses were detected in the HMP colony, but only DWV and IAPV were detected and at lower amounts in the LMP colony. An epidemiological explanation for these results is that pre-experiment differences in virus presence and levels existed between the HMP and LMP colonies. It is also possible that low V. destructor population growth in the LMP colony resulted in the bees being less exposed to the mite and thus less likely to have virus infections. LMP and HMP bees may have also differed in susceptibility to virus infection.

Host regulation and nutritional exploitation by parasitic wasps

The physiological alterations observed in naturally parasitized hosts are characterized by a number of reproductive and developmental changes. Some of these changes are also associated with alterations in host physiology that benefit the nutrition and development of wasp offspring. Here we review the breadth of host-parasitoid nutritional interactions, and discuss current understanding of underlying mechanisms. We also discuss priorities for future studies that could enhance understanding of basic questions about the parasitoid lifestyle and provide insights of value for insect control.

A virulent strain of deformed wing virus (DWV) of honeybees (Apis mellifera) prevails after Varroa destructor-mediated, or in vitro, transmission

The globally distributed ectoparasite Varroa destructor is a vector for viral pathogens of the Western honeybee (Apis mellifera), in particular the Iflavirus Deformed Wing Virus (DWV). In the absence of Varroa low levels DWV occur, generally causing asymptomatic infections. Conversely, Varroa-infested colonies show markedly elevated virus levels, increased overwintering colony losses, with impairment of pupal development and symptomatic workers. To determine whether changes in the virus population were due Varroa amplifying and introducing virulent virus strains and/or suppressing the host immune responses, we exposed Varroa-naïve larvae to oral and Varroa-transmitted DWV. We monitored virus levels and diversity in developing pupae and associated Varroa, the resulting RNAi response and transcriptome changes in the host. Exposed pupae were stratified by Varroa association (presence/absence) and virus levels (low/high) into three groups. Varroa-free pupae all exhibited low levels of a highly diverse DWV population, with those exposed per os (group NV) exhibiting changes in the population composition. Varroa-associated pupae exhibited either low levels of a diverse DWV population (group VL) or high levels of a near-clonal virulent variant of DWV (group VH). These groups and unexposed controls (C) could be also discriminated by principal component analysis of the transcriptome changes observed, which included several genes involved in development and the immune response. All Varroa tested contained a diverse replicating DWV population implying the virulent variant present in group VH, and predominating in RNA-seq analysis of temporally and geographically separate Varroa-infested colonies, was selected upon transmission from Varroa, a conclusion supported by direct injection of pupae in vitro with mixed virus populations. Identification of a virulent variant of DWV, the role of Varroa in its transmission and the resulting host transcriptome changes furthers our understanding of this important viral pathogen of honeybees.

Honeybees are the most important managed pollinating insect, contributing billions of dollars to annual global agricultural production. Over the last century a parasitic mite, Varroa, has spread worldwide, with significant impacts on honeybee colony health as a consequence of its transmission of a cocktail of viruses while feeding on honeybee ‘blood’. The most important virus for colony health is deformed wing virus (DWV), high levels of which cause developmental deformities and premature ageing resulting in high overwintering colony losses. In experiments on individual Varroa-exposed pupae we demonstrate that a single type of virulent DWV is amplified 1,000–10,000 times in the recipient pupae, despite the mite containing a high diversity of replicating DWV strains. We could recapitulate this by direct injection of pupae with mixed virus populations, showing the virulent strain is advantaged by the route of transmission. In parallel, we detected changes in the immune response and developmental gene expression of the honeybee and propose that these contribute to the characteristic pathogenesis of DWV. Identification of a virulent strain of DWV has implications for therapeutic or prophylactic interventions to improve honeybee colony health, as well as contributing to our understanding of the biology of this important honeybee viral pathogen.

Esterases of Varroa destructor (Acari: Varroidae), parasitic mite of the honeybee

Varroa destructor is an ectoparasite that causes serious damage to the population of the honeybee. Increasing resistance of the parasite to acaricides is related, among others, to metabolic adaptations of its esterases to facilitate decomposition of the chemicals used. Esterases are a large heterogeneous group of enzymes that metabolize a number of endogenous and exogenous substrates with ester binding. The aim of the present study was to determine the activity of esterases in the body extracts (BE) and excretion/secretion products (E/SP) of the mite. The enzymes contained in the E/SP should originate mainly from the salivary glands and the alimentary system and they may play a particularly important role in the first line of defence of the mite against acaricides. Activity of cholinesterases (ChEs) [acetylcholinesterase (AChE) and butyrylcholinesterase], carboxylesterases (CEs) and phosphatases [alkaline phosphatase (AP) and acid phosphatase (AcP)] was investigated. The activity of all the enzymes except AChE was higher in the E/SP than in the BE. ChEs from the BE and from the E/SP reacted differently on eserine, a ChE inhibitor. Eserine inhibited both enzymes from the BE, increased decomposition of acetylcholine, but did not influence hydrolysis of butyrylcholine by the E/SP. Activity of the CEs from the BE in relation to the esters of carboxylic acids can be presented in the following series: C10 > C12 > C14 > C8 > C2 > C4 = C16, while activity of the CEs from the E/SP was: C4 > C8 > C2 > C14 > C10 > C12 > C16. The inhibitor of CEs, triphenyl phosphate, reduced the activity of esterases C2–C8 and C14–C16; however, it acted in the opposite way to CEs C10 and C12. The activity of both phosphatases was higher in the E/SP than in the BE (AcP about twofold and AP about 2.6-fold); the activities of AP and AcP in the same material were similar. Given the role of esterases in resistance to pesticides, further studies are necessary to obtain complete biochemical characteristics of the enzymes currently present in V. destructor.

The microsporidian parasites Nosema ceranae and Nosema apis are widespread in honeybee (Apis mellifera) colonies across Scotland

Nosema ceranae is spreading into areas where Nosema apis already exists. N. ceranae has been reported to cause an asymptomatic infection that may lead, ultimately, to colony collapse. It is thought that there may be a temperature barrier to its infiltration into countries in colder climates. In this study, 71 colonies from Scottish Beekeeper's Association members have been screened for the presence of N. apis and N. ceranae across Scotland. We find that only 11 of the 71 colonies tested positive for spores by microscopy. However, 70.4 % of colonies screened by PCR revealed the presence of both N. ceranae and N. apis, with only 4.2 or 7 % having either strain alone and 18.3 % being Nosema free. A range of geographically separated colonies testing positive for N. ceranae were sequenced to confirm their identity. All nine sequences confirmed the presence of N. ceranae and indicated the presence of a single new variant. Furthermore, two of the spore-containing colonies had only N. ceranae present, and these exhibited the presence of smaller spores that could be distinguished from N. apis by the analysis of average spore size. Differential quantification of the PCR product revealed N. ceranae to be the dominant species in all seven samples tested. In conclusion, N. ceranae is widespread in Scotland where it exists in combination with the endemic N. apis. A single variant, identical to that found in France (DQ374655) except for the addition of a single nucleotide polymorphism, is present in Scotland.

Correlation of proteome-wide changes with social immunity behaviors provides insight into resistance to the parasitic mite, Varroa destructor, in the honey bee (Apis mellifera)

BACKGROUND

Disease is a major factor driving the evolution of many organisms. In honey bees, selection for social behavioral responses is the primary adaptive process facilitating disease resistance. One such process, hygienic behavior, enables bees to resist multiple diseases, including the damaging parasitic mite Varroa destructor. The genetic elements and biochemical factors that drive the expression of these adaptations are currently unknown. Proteomics provides a tool to identify proteins that control behavioral processes, and these proteins can be used as biomarkers to aid identification of disease tolerant colonies.

RESULTS

We sampled a large cohort of commercial queen lineages, recording overall mite infestation, hygiene, and the specific hygienic response to V. destructor. We performed proteome-wide correlation analyses in larval integument and adult antennae, identifying several proteins highly predictive of behavior and reduced hive infestation. In the larva, response to wounding was identified as a key adaptive process leading to reduced infestation, and chitin biosynthesis and immune responses appear to represent important disease resistant adaptations. The speed of hygienic behavior may be underpinned by changes in the antenna proteome, and chemosensory and neurological processes could also provide specificity for detection of V. destructor in antennae.

CONCLUSIONS

Our results provide, for the first time, some insight into how complex behavioural adaptations manifest in the proteome of honey bees. The most important biochemical correlations provide clues as to the underlying molecular mechanisms of social and innate immunity of honey bees. Such changes are indicative of potential divergence in processes controlling the hive-worker maturation.

Synergistic parasite-pathogen interactions mediated by host immunity can drive the collapse of honeybee colonies

The health of the honeybee and, indirectly, global crop production are threatened by several biotic and abiotic factors, which play a poorly defined role in the induction of widespread colony losses. Recent descriptive studies suggest that colony losses are often related to the interaction between pathogens and other stress factors, including parasites. Through an integrated analysis of the population and molecular changes associated with the collapse of honeybee colonies infested by the parasitic mite Varroa destructor, we show that this parasite can de-stabilise the within-host dynamics of Deformed wing virus (DWV), transforming a cryptic and vertically transmitted virus into a rapidly replicating killer, which attains lethal levels late in the season. The de-stabilisation of DWV infection is associated with an immunosuppression syndrome, characterized by a strong down-regulation of the transcription factor NF-κB. The centrality of NF-κB in host responses to a range of environmental challenges suggests that this transcription factor can act as a common currency underlying colony collapse that may be triggered by different causes. Our results offer an integrated account for the multifactorial origin of honeybee losses and a new framework for assessing, and possibly mitigating, the impact of environmental challenges on honeybee health.

Blood feeding in juvenile Paragnathia formica (Isopoda: Gnathiidae): biochemical characterization of trypsin inhibitors, detection of anticoagulants, and molecular identification of fish hosts

The 3 post-marsupial juvenile stages of the gnathiid isopod, Paragnathia formica, are haematophagous ectoparasites of fishes that may, in heavy infestations, cause host mortality. Protein digestion in fed stage 3 juveniles is accomplished by cysteine proteinases, but what bioactive compounds attenuate host haemostatic, inflammatory and immunological responses during feeding is unknown. Trypsin inhibitory activity and anticoagulant activity were detected in crude extracts of unfed P. formica stage 1 juveniles; fractionation of stage 1 crude extracts by ion exchange chromatography resulted in 3 preparations each displaying these bioactivities. Further characterization revealed anti-thrombin activity in 2 of these preparations, whilst the third displayed the strongest anticoagulant activity that targeted a factor of the intrinsic coagulation pathway. Three trypsin inhibitors (18 kDa, 21 kDa, and 22 kDa) were also detected using reverse zymography. In parallel, homogenates of fed stage 2 and 3 juveniles were used to identify their fish hosts by amplifying the 16S mitochondrial rDNA and 18S genomic rDNA vertebrate gene regions. Blood from at least 4 fish families had been ingested by separate individuals during feeding. This study demonstrates that trypsin inhibitors and anticoagulants are present in P. formica juveniles which could suppress host haemostatic, inflammatory and immunological responses during feeding, and that juveniles are not host specific.