The Effect of Oral Administration of dsRNA on Viral Replication and Mortality in Bombus terrestris

DsRNA-based pesticides: Considerations for efficiency and risk assessment

In view of the ongoing climate change and the ever-growing world population, novel agricultural solutions are required to ensure sustainable food supply. Microbials, natural substances, semiochemicals and double stranded RNAs (dsRNAs) are all considered potential low risk pesticides. DsRNAs function at the molecular level, targeting specific regions of specific genes of specific organisms, provided that they share a minimal sequence complementarity of approximately 20 nucleotides. Thus, dsRNAs may offer a great alternative to conventional chemicals in environmentally friendly pest control strategies. Any low-risk pesticide needs to be efficient and exhibit low toxicological potential and low environmental persistence. Having said that, in the current review, the mode of dsRNA action is explored and the parameters that need to be taken into consideration for the development of efficient dsRNA-based pesticides are highlighted. Moreover, since dsRNAs mode of action differs from those of synthetic pesticides, custom-made risk assessment schemes may be required and thus, critical issues related to the risk assessment of dsRNA pesticides are discussed here.

Can immune gene silencing via dsRNA feeding promote pathogenic viruses to control the globally invasive Argentine ant?

Pest control methods that can target pest species with limited environmental impacts are a conservation and economic priority. Species-specific pest control using RNA interference is a challenging but promising avenue in developing the next generation of pest management. We investigate the feasibility of manipulating a biological invader's immune system using double-stranded RNA (dsRNA) in order to increase susceptibility to naturally occurring pathogens. We used the invasive Argentine ant as a model, targeting the immunity-associated genes Spaetzle and Dicer-1 with dsRNA. We show that feeding with Spaetzle dsRNA can result in partial target gene silencing for up to 28 days in the laboratory and 5 days in the field. Dicer-1 dsRNA only resulted in partial gene knockdown after 2 days in the laboratory. Double-stranded RNA treatments were associated with significant gene expression disruptions across immune pathways in the laboratory and to a lower extent in the field. In total, 12 viruses and four bacteria were found in these ant populations. Some changes in viral loads in dsRNA-treated groups were observed. For example, Linepithema humile Polycipivirus 2 (LhuPCV2) loads increased after 2 days of treatment with Spaetzle and Dicer-1 dsRNA treatments in the laboratory. After treatment with the dsRNA in the field, after 5 days the virus Linepithema humile toti-like virus 1 (LhuTLV1) was significantly more abundant. However, immune pathway disruption did not result in a consistent increase in microbial infections, nor did it alter ant abundance in the field. Some viruses even declined in abundance after dsRNA treatment. Our study explored the feasibility of lowering a pest's immunity as a control tool. We demonstrate that it is possible to alter immune gene expression of pest species and pathogen loads, although in our specific system the affected pathogens did not appear to influence pest abundance. We provide suggestions on future directions for dsRNA-mediated immune disruption in pest species, including potential avenues to improve dsRNA delivery as well as the importance of pest and pathogen biology. Double-stranded RNA targeting immune function might be especially useful for pest control in systems in which viruses or other microorganisms are prevalent and have the potential to be pathogenic.

Occurrence of Honey Bee (Apis mellifera L.) Pathogens in Wild Pollinators in Northern Italy

Diseases contribute to the decline of pollinator populations, which may be aggravated by the interspecific transmission of honey bee pests and pathogens. Flowers increase the risk of transmission, as they expose the pollinators to infections during the foraging activity. In this study, both the prevalence and abundance of 21 honey bee pathogens (11 viruses, 4 bacteria, 3 fungi, and 3 trypanosomatids) were assessed in the flower-visiting entomofauna sampled from March to September 2021 in seven sites in the two North-Italian regions, Emilia-Romagna and Piedmont. A total of 1,028 specimens were collected, identified, and analysed. Of the twenty-one pathogens that were searched for, only thirteen were detected. Altogether, the prevalence of the positive individuals reached 63.9%, with Nosema ceranae, deformed wing virus (DWV), and chronic bee paralysis virus (CBPV) as the most prevalent pathogens. In general, the pathogen abundance averaged 5.15 * 10⁶ copies, with CBPV, N. ceranae, and black queen cell virus (BQCV) as the most abundant pathogens, with 8.63, 1.58, and 0.48 * 10⁷ copies, respectively. All the detected viruses were found to be replicative. The sequence analysis indicated that the same genetic variant was circulating in a specific site or region, suggesting that interspecific transmission events among honey bees and wild pollinators are possible. Frequently, N. ceranae and DWV were found to co-infect the same individual. The circulation of honey bee pathogens in wild pollinators was never investigated before in Italy. Our study resulted in the unprecedented detection of 72 wild pollinator species as potential hosts of honey bee pathogens. Those results encourage the implementation of monitoring actions aiming to improve our understanding of the environmental implications of such interspecific transmission events, which is pivotal to embracing a One Health approach to pollinators’ welfare.

Critical View on the Importance of Host Defense Strategies on Virus Distribution of Bee Viruses: What Can We Learn from SARS-CoV-2 Variants?

Bees, both wild and domesticated ones, are hosts to a plethora of viruses, with most of them infecting a wide range of bee species and genera. Although viral discovery and research on bee viruses date back over 50 years, the last decade is marked by a surge of new studies, new virus discoveries, and reports on viral transmission in and between bee species. This steep increase in research on bee viruses was mainly initiated by the global reports on honeybee colony losses and the worldwide wild bee decline, where viruses are regarded as one of the main drivers. While the knowledge gained on bee viruses has significantly progressed in a short amount of time, we believe that integration of host defense strategies and their effect on viral dynamics in the multi-host viral landscape are important aspects that are currently still missing. With the large epidemiological dataset generated over the last two years on the SARS-CoV-2 pandemic, the role of these defense mechanisms in shaping viral dynamics has become eminent. Integration of these dynamics in a multi-host system would not only greatly aid the understanding of viral dynamics as a driver of wild bee decline, but we believe bee pollinators and their viruses provide an ideal system to study the multi-host viruses and their epidemiology.

Chemical Stimulants and Stressors Impact the Outcome of Virus Infection and Immune Gene Expression in Honey Bees (Apis mellifera)

Western honey bees (Apis mellifera) are ecologically, agriculturally, and economically important plant pollinators. High average annual losses of honey bee colonies in the US have been partially attributed to agrochemical exposure and virus infections. To examine the potential negative synergistic impacts of agrochemical exposure and virus infection, as well as the potential promise of phytochemicals to ameliorate the impact of pathogenic infections on honey bees, we infected bees with a panel of viruses (i.e., Flock House virus, deformed wing virus, or Sindbis virus) and exposed to one of three chemical compounds. Specifically, honey bees were fed sucrose syrup containing: (1) thyme oil, a phytochemical and putative immune stimulant, (2) fumagillin, a beekeeper applied fungicide, or (3) clothianidin, a grower-applied insecticide. We determined that virus abundance was lower in honey bees fed 0.16 ppb thyme oil augmented sucrose syrup, compared to bees fed sucrose syrup alone. Parallel analysis of honey bee gene expression revealed that honey bees fed thyme oil augmented sucrose syrup had higher expression of key RNAi genes (argonaute-2 and dicer-like), antimicrobial peptide expressing genes (abaecin and hymenoptaecin), and vitellogenin, a putative honey bee health and age indicator, compared to bees fed only sucrose syrup. Virus abundance was higher in bees fed fumagillin (25 ppm or 75 ppm) or 1 ppb clothianidin containing sucrose syrup relative to levels in bees fed only sucrose syrup. Whereas, honey bees fed 10 ppb clothianidin had lower virus levels, likely because consuming a near lethal dose of insecticide made them poor hosts for virus infection. The negative impact of fumagillin and clothianidin on honey bee health was indicated by the lower expression of argonaute-2, dicer-like, abaecin, and hymenoptaecin, and vitellogenin. Together, these results indicate that chemical stimulants and stressors impact the outcome of virus infection and immune gene expression in honey bees.

The Honey Bee Gene Bee Antiviral Protein-1 Is a Taxonomically Restricted Antiviral Immune Gene

Insects have evolved a wide range of strategies to combat invading pathogens, including viruses. Genes that encode proteins involved in immune responses often evolve under positive selection due to their co-evolution with pathogens. Insect antiviral defense includes the RNA interference (RNAi) mechanism, which is triggered by recognition of non-self, virally produced, double-stranded RNAs. Indeed, insect RNAi genes (e.g., dicer and argonaute-2) are under high selective pressure. Honey bees (Apis mellifera) are eusocial insects that respond to viral infections via both sequence specific RNAi and a non-sequence specific dsRNA triggered pathway, which is less well-characterized. A transcriptome-level study of virus-infected and/or dsRNA-treated honey bees revealed increased expression of a novel antiviral gene, GenBank: MF116383, and in vivo experiments confirmed its antiviral function. Due to in silico annotation and sequence similarity, MF116383 was originally annotated as a probable cyclin-dependent serine/threonine-protein kinase. In this study, we confirmed that MF116383 limits virus infection, and carried out further bioinformatic and phylogenetic analyses to better characterize this important gene-which we renamed bee antiviral protein-1 (bap1). Phylogenetic analysis revealed that bap1 is taxonomically restricted to Hymenoptera and Blatella germanica (the German cockroach) and that the majority of bap1 amino acids are evolving under neutral selection. This is in-line with the results from structural prediction tools that indicate Bap1 is a highly disordered protein, which likely has relaxed structural constraints. Assessment of honey bee gene expression using a weighted gene correlation network analysis revealed that bap1 expression was highly correlated with several immune genes-most notably argonaute-2. The coexpression of bap1 and argonaute-2 was confirmed in an independent dataset that accounted for the effect of virus abundance. Together, these data demonstrate that bap1 is a taxonomically restricted, rapidly evolving antiviral immune gene. Future work will determine the role of bap1 in limiting replication of other viruses and examine the signal cascade responsible for regulating the expression of bap1 and other honey bee antiviral defense genes, including coexpressed ago-2, and determine whether the virus limiting function of bap1 acts in parallel or in tandem with RNAi.

Pathogens Spillover from Honey Bees to Other Arthropods

Honey bees, and pollinators in general, play a major role in the health of ecosystems. There is a consensus about the steady decrease in pollinator populations, which raises global ecological concern. Several drivers are implicated in this threat. Among them, honey bee pathogens are transmitted to other arthropods populations, including wild and managed pollinators. The western honey bee, Apis mellifera, is quasi-globally spread. This successful species acted as and, in some cases, became a maintenance host for pathogens. This systematic review collects and summarizes spillover cases having in common Apis mellifera as the mainteinance host and some of its pathogens. The reports are grouped by final host species and condition, year, and geographic area of detection and the co-occurrence in the same host. A total of eighty-one articles in the time frame 1960-2021 were included. The reported spillover cases cover a wide range of hymenopteran host species, generally living in close contact with or sharing the same environmental resources as the honey bees. They also involve non-hymenopteran arthropods, like spiders and roaches, which are either likely or unlikely to live in close proximity to honey bees. Specific studies should consider host-dependent pathogen modifications and effects on involved host species. Both the plasticity of bee pathogens and the ecological consequences of spillover suggest a holistic approach to bee health and the implementation of a One Health approach.

Investigating Virus-Host Interactions in Cultured Primary Honey Bee Cells

Honey bee (Apis mellifera) health is impacted by viral infections at the colony, individual bee, and cellular levels. To investigate honey bee antiviral defense mechanisms at the cellular level we further developed the use of cultured primary cells, derived from either larvae or pupae, and demonstrated that these cells could be infected with a panel of viruses, including common honey bee infecting viruses (i.e., sacbrood virus (SBV) and deformed wing virus (DWV)) and an insect model virus, Flock House virus (FHV). Virus abundances were quantified over the course of infection. The production of infectious virions in cultured honey bee pupal cells was demonstrated by determining that naïve cells became infected after the transfer of deformed wing virus or Flock House virus from infected cell cultures. Initial characterization of the honey bee antiviral immune responses at the cellular level indicated that there were virus-specific responses, which included increased expression of bee antiviral protein-1 (GenBank: MF116383) in SBV-infected pupal cells and increased expression of argonaute-2 and dicer-like in FHV-infected hemocytes and pupal cells. Additional studies are required to further elucidate virus-specific honey bee antiviral defense mechanisms. The continued use of cultured primary honey bee cells for studies that involve multiple viruses will address this knowledge gap.

Self-Powered Smart Beehive Monitoring and Control System (SBMaCS)

Beekeeping in Africa has been practiced for many years through successive generations and along inherited patterns. Beekeepers continue to face challenges in accessing consistent and business-driven markets for their bee products. In addition, the honeybee populations are decreasing due to colony collapse disorder (CCD), fire, loss of bees in swarming, honey buggers and other animals, moths, starvation, cold weather, and Varoa mites. The main issues are related to un-controlled temperature, humidity, and traditional management of beekeeping. These challenges result in low production of honey and colony losses. The control of the environmental conditions within and surrounding the beehives are not available to beekeepers due to the lack of monitoring systems. A Smart Beehive System using Internet of Things (IoT) technology would allow beekeepers to keep track of the amount of honey created in their hives and bee colonies even when they are far from their hives, through mobile phones, which would curtail the challenges currently faced by the beekeepers. However, there are challenges in the design of energy-efficient embedded electronic devices for IoT. A promising solution is to provide energy autonomy to the IoT nodes that will harvest residual energy from ambient sources, such as motion, vibrations, light, or heat. This paper proposes a Self-Powered Smart Beehive Monitoring and Control System (SBMaCS) using IoT to support remote follow-up and control, enhancing bee colonies' security and thus increasing the honey productivity. First, we develop the SBMaCS hardware prototype interconnecting various sensors, such as temperature sensor, humidity sensor, piezoelectric transducer-which will work as a weight sensor-motion sensor, and flame sensor. Second, we introduce energy harvesting models to self-power the SBMaCS by analyzing the (i) energy harvested from adult bees' vibrations, (ii) energy harvesting through the piezoelectric transducer, and (iii) radio frequency energy harvesting. Third, we develop a mobile phone application that interacts with the SBMaCS hardware to monitor and control the various parameters related to the beehives. Finally, the SBMaCS PCB layout is also designed. SBMaCS will help beekeepers to successfully monitor and control some important smart beekeeping activities wherever they are using their mobile phone application.

Non-Target Effects of dsRNA Molecules in Hemipteran Insects

Insect pest control by RNA interference (RNAi)-mediated gene expression knockdown can be undermined by many factors, including small sequence differences between double-stranded RNA (dsRNA) and the target gene. It can also be compromised by effects that are independent of the dsRNA sequence on non-target organisms (known as sequence-non-specific effects). This study investigated the species-specificity of RNAi in plant sap-feeding hemipteran pests. We first demonstrated sequence-non-specific suppression of aphid feeding by dsRNA at dietary concentrations ≥0.5 µg µL⁻¹. Then we quantified the expression of NUC (nuclease) genes in insects administered homologous dsRNA (with perfect sequence identity to the target species) or heterologous dsRNA (generated against a related gene of non-identical sequence in a different insect species). For the aphids Acyrthosiphon pisum and Myzus persicae, significantly reduced NUC expression was obtained with the homologous but not heterologous dsRNA at 0.2 µg µL⁻¹, despite high dsNUC sequence identity. Follow-up experiments demonstrated significantly reduced expression of NUC genes in the whitefly Bemisia tabaci and mealybug Planococcus maritimus administered homologous dsNUCs, but not heterologous aphid dsNUCs. Our demonstration of inefficient expression knockdown by heterologous dsRNA in these insects suggests that maximal dsRNA sequence identity is required for RNAi targeting of related pest species, and that heterologous dsRNAs at appropriate concentrations may not be a major risk to non-target sap-feeding hemipterans.

Characterization, expression patterns, and transcriptional responses of three core RNA interference pathway genes from Ostrinia nubilalis

RNA interference (RNAi) is commonly used in the laboratory to analyze gene function, and RNAi-based pest management strategies are now being employed. Unfortunately, RNAi is hindered by inefficient and highly-variable results when different insects are targeted, especially lepidopterans, such as the European corn borer (ECB), Ostrinia nubilalis (Lepidoptera: Crambidae). Previous efforts to achieve RNAi-mediated gene suppression in ECB revealed low RNAi efficiency with both double-stranded RNA (dsRNA) injection and ingestion. One mechanism that can affect RNAi efficiency in insects is the expression and function of core RNAi pathway genes, such as those encoding Argonaut 2 (Ago2), Dicer 2 (Dcr2), and a dsRNA binding protein (R2D2). To determine if deficiencies in these core RNAi pathway genes contribute to low RNAi efficiency in ECB, full-length complementary DNAs encoding OnAgo2, OnDcr2, and OnR2D2 were cloned, sequenced, and characterized. A comparison of domain architecture suggested that all three predicted proteins contained the necessary domains to function. However, a comparison of evolutionary distances revealed potentially important variations in the first RNase III domain of OnDcr2, the double-stranded RNA binding domains of OnR2D2, and both the PAZ and PIWI domains of OnAgo2, which may indicate functional differences in enzymatic activity between species. Expression analysis indicated that transcripts for all three genes were expressed in all developmental stages and tissues investigated. Interestingly, the introduction of non-target dsRNA into ECB second-instar larvae via microinjection did not affect OnAgo2, OnDcr2, or OnR2D2 expression. In contrast, ingestion of the same dsRNAs resulted in upregulation of OnDcr2 but downregulation of OnR2D2. The unexpected transcriptional responses of the core machinery and the divergence in amino-acid sequence between specific domains in each core RNAi protein may possibly contribute to low RNAi efficiency in ECB. Understanding the contributions of different RNAi pathway components is critical to adapting this technology for use in controlling lepidopteran pests that exhibit low RNAi efficiency.

Possible Spillover of Pathogens between Bee Communities Foraging on the Same Floral Resource

Simple Summary

Floral resource availability is one of the keys to preserving the health of bee communities. However, flowers also present a risk of pathogen transmission, as infected pollinators could deposit pathogens while foraging, exposing other pollinators to infection via the consumption of contaminated nectar or pollen. Here, we studied, over time, the prevalence of seven viruses in bee communities that share the same small surface of floral resource in order to assess the risk of virus spillover. In total, 2057 bee specimens from 30 species were caught, identified and checked for the presence of viruses. Specimens from the Halictidae family were the dominant wild bees. The prevalence of viruses was quite high: at least one virus was detected in 78% of the samples, and co-infections were frequent. The genetic diversity of the viruses was also investigated to look for the possible association of geographic origin or host with shared ancestry.

Abstract

Viruses are known to contribute to bee population decline. Possible spillover is suspected from the co-occurrence of viruses in wild bees and honey bees. In order to study the risk of virus transmission between wild and managed bee species sharing the same floral resource, we tried to maximize the possible cross-infections using Phacelia tanacetifolia, which is highly attractive to honey bees and a broad range of wild bee species. Virus prevalence was compared over two years in Southern France. A total of 1137 wild bees from 29 wild bee species (based on COI barcoding) and 920 honey bees (Apis mellifera) were checked for the seven most common honey bee RNA viruses. Halictid bees were the most abundant. Co-infections were frequent, and Sacbrood virus (SBV), Black queen cell virus (BQCV), Acute bee paralysis virus (ABPV) and Israeli acute paralysis virus (IAPV) were widespread in the hymenopteran pollinator community. Conversely, Deformed wing virus (DWV) was detected at low levels in wild bees, whereas it was highly prevalent in honey bees (78.3% of the samples). Both wild bee and honey bee virus isolates were sequenced to look for possible host-specificity or geographical structuring. ABPV phylogeny suggested a specific cluster for Eucera bees, while isolates of DWV from bumble bees (Bombus spp.) clustered together with honey bee isolates, suggesting a possible spillover.

Identification and temporal expression profiles of cuticular proteins in the endoparasitoid wasp, Microplitis mediator

Recently, parasitoid wasp species Microplitis mediator has evoked increasing research attention due to its possible use in the control of Lepidoptera insects. Because insect development involves changes in cuticle composition, identification and expression analysis of M. mediator cuticular proteins may clarify the mechanisms involved in parasite development processes. We found 70 cuticular proteins from the M. mediator transcriptome and divided them into seven distinct families. Expression profiling indicated that most of these cuticular protein genes have expression peaks specific for one particular developmental stage of M. mediator. Eggs and pupae have the highest number of transcriptionally active cuticular protein genes (47 and 52 respectively). Only 12 of these genes maintained high expression activity during late larval development. Functional analysis of two larval proteins, MmCPR3 and MmCPR14, suggested their important role in the proper organization of the cuticle layers of larvae. During M. mediator larval development, normal cuticle formation can be supported by a limited number of cuticular proteins.

RNAi in Insects: A Revolution in Fundamental Research and Pest Control Applications

In this editorial for the Special Issue on 'RNAi in insect pest control', three important applications of RNA interference (RNAi) in insects are briefly discussed and linked to the different studies published in this Special Issue. The discovery of the RNAi mechanism revolutionized entomological research, as it presented researchers with a tool to knock down genes, which is easily applicable in a wide range of insect species. Furthermore, RNAi also provides crop protection with a novel and promising pest control mode-of-action. The sequence-dependent nature allows RNAi-based control strategies to be highly species selective and the active molecule, a natural biological molecule known as double-stranded RNA (dsRNA), has a short environmental persistence. However, more research is needed to investigate different cellular and physiological barriers, such as cellular uptake and dsRNA degradation in the digestive system in insects, in order to provide efficient control methods against a wide range of insect pest species. Finally, the RNAi pathway is an important part of the innate antiviral immune defence of insects, and could even lead to applications targeting viruses in beneficial insects such as honeybees in the future.

Targeting genes involved in nucleopolyhedrovirus DNA multiplication through RNA interference technology to induce resistance against the virus in silkworms

RNA interference (RNAi) has become an efficient tool for inducing resistance to viruses in many organisms. In this study, Escherichia coli cells were engineered to produce stable double-stranded RNA (dsRNA) against the nucleopolyhedrosis virus to elicit RNAi in silkworms. The immediate-early-1 (ie-1) and late expression factor-1 (lef-1) genes of the Bombyx mori nucleopolyhedrovirus (BmNPV) involved in viral DNA multiplication were cloned in the plasmid L4440 under the influence of the double T7 promoter and transformed to E. coli HT115 DE3 host cells. On induction with isopropyl β-D-thiogalactopyranoside, these cells efficiently produced dsRNA of the cloned genes. The B. mori larvae were fed with 50 µL of E. coli cells expressing ie-1 and lef-1 dsRNAs (each approximately 25 µg) to elicit RNAi. The semi-quantitative and quantitative PCR analysis of RNA from the midgut of the dsRNA-fed larvae revealed a significant reduction in the expression of the target genes involved in BmNPV multiplication, which restricted virus copy numbers to 100 compared with 1.9 × 10⁵ in the infected controls. Furthermore, the dsRNA-fed infected larvae showed > 50% increased survivability compared with the infected controls. The study revealed the successful use of bacteria as vectors for efficiently delivering dsRNA to elicit RNAi against BmNPV in silkworms.

Assessing the Risks of Topically Applied dsRNA-Based Products to Non-target Arthropods

RNA interference (RNAi) is a powerful technology that offers new opportunities for pest control through silencing of genes that are essential for the survival of arthropod pests. The approach relies on sequence-specificity of applied double-stranded (ds) RNA that can be designed to have a very narrow spectrum of both the target gene product (RNA) as well as the target organism, and thus allowing highly targeted pest control. Successful RNAi has been reported from a number of arthropod species belonging to various orders. Pest control may be achieved by applying dsRNA as foliar sprays. One of the main concerns related to the use of dsRNA is adverse environmental effects particularly on valued non-target species. Arthropods form an important part of the biodiversity in agricultural landscapes and contribute important ecosystem services. Consequently, environmental risk assessment (ERA) for potential impacts that plant protection products may have on valued non-target arthropods is legally required prior to their placement on the market. We describe how problem formulation can be used to set the context and to develop plausible pathways on how the application of dsRNA-based products could harm valued non-target arthropod species, such as those contributing to biological pest control. The current knowledge regarding the exposure to and the hazard posed by dsRNA in spray products for non-target arthropods is reviewed and suggestions are provided on how to select the most suitable test species and to conduct laboratory-based toxicity studies that provide robust, reliable and interpretable results to support the ERA.

Bee Viruses: Routes of Infection in Hymenoptera

Numerous studies have recently reported on the discovery of bee viruses in different arthropod species and their possible transmission routes, vastly increasing our understanding of these viruses and their distribution. Here, we review the current literature on the recent advances in understanding the transmission of viruses, both on the presence of bee viruses in Apis and non-Apis bee species and on the discovery of previously unknown bee viruses. The natural transmission of bee viruses will be discussed among different bee species and other insects. Finally, the research potential of in vivo (host organisms) and in vitro (cell lines) serial passages of bee viruses is discussed, from the perspective of the host-virus landscape changes and potential transmission routes for emerging bee virus infections.

The Heat Shock Response in the Western Honey Bee (Apis mellifera) is Antiviral

Honey bees (Apismellifera) are an agriculturally important pollinator species that live in easily managed social groups (i.e., colonies). Unfortunately, annual losses of honey bee colonies in many parts of the world have reached unsustainable levels. Multiple abiotic and biotic stressors, including viruses, are associated with individual honey bee and colony mortality. Honey bees have evolved several antiviral defense mechanisms including conserved immune pathways (e.g., Toll, Imd, JAK/STAT) and dsRNA-triggered responses including RNA interference and a non-sequence specific dsRNA-mediated response. In addition, transcriptome analyses of virus-infected honey bees implicate an antiviral role of stress response pathways, including the heat shock response. Herein, we demonstrate that the heat shock response is antiviral in honey bees. Specifically, heat-shocked honey bees (i.e., 42 °C for 4 h) had reduced levels of the model virus, Sindbis-GFP, compared with bees maintained at a constant temperature. Virus-infection and/or heat shock resulted in differential expression of six heat shock protein encoding genes and three immune genes, many of which are positively correlated. The heat shock protein encoding and immune gene transcriptional responses observed in virus-infected bees were not completely recapitulated by administration of double stranded RNA (dsRNA), a virus-associated molecular pattern, indicating that additional virus-host interactions are involved in triggering antiviral stress response pathways.

The Israeli acute paralysis virus IRES captures host ribosomes by mimicking a ribosomal state with hybrid tRNAs

Colony collapse disorder (CCD) is a multi-faceted syndrome decimating bee populations worldwide, and a group of viruses of the widely distributed Dicistroviridae family have been identified as a causing agent of CCD. This family of viruses employs non-coding RNA sequences, called internal ribosomal entry sites (IRESs), to precisely exploit the host machinery for viral protein production. Using single-particle cryo-electron microscopy (cryo-EM), we have characterized how the IRES of Israeli acute paralysis virus (IAPV) intergenic region captures and redirects translating ribosomes toward viral RNA messages. We reconstituted two in vitro reactions targeting a pre-translocation and a post-translocation state of the IAPV-IRES in the ribosome, allowing us to identify six structures using image processing classification methods. From these, we reconstructed the trajectory of IAPV-IRES from the early small subunit recruitment to the final post-translocated state in the ribosome. An early commitment of IRES/ribosome complexes for global pre-translocation mimicry explains the high efficiency observed for this IRES. Efforts directed toward fighting CCD by targeting the IAPV-IRES using RNA-interference technology are underway, and the structural framework presented here may assist in further refining these approaches.

Responses of two ladybird beetle species (Coleoptera: Coccinellidae) to dietary RNAi

BACKGROUND

One concern with the adoption of RNAi-based genetically engineered (GE) crops is the potential harm to valued non-target organisms. Species of Coccinellidae (Coleoptera) are important natural enemies and might be exposed to the insecticidal dsRNA produced by the plant. To assess their susceptibility to dietary RNAi, we fed Adalia bipunctata and Coccinella septempunctata with a dsRNA designed to target the vATPase A of the western corn rootworm, Diabrotica virgifera virgifera (Dvv dsRNA). Specific dsRNAs designed to target the vATPase A of the two ladybird beetle species served as positive controls.

RESULTS

Our results revealed that both species were sensitive to dietary RNAi when ingesting their own dsRNAs, with C. septempunctata being more sensitive than A. bipunctata. Dvv dsRNA also adversely affected the two ladybird beetles as indicated by a significantly (but marginally) prolonged developmental time for A. bipunctata and a significantly reduced survival rate for C. septempunctata. These results, however, were obtained at Dvv dsRNA concentrations that were orders of magnitude higher than expected to occur in the field. Gene expression analyses confirmed the bioactivity of the dsRNA treatments and the results from the feeding bioassays. These results are consistent with the bioinformatics analyses, which revealed a higher number of 21-nucleotide-long matches, a requirement for effective RNAi, of the Dvv dsRNA with the vATPase A of C. septempunctata (34 matches) than with that of A. bipunctata (six matches).

CONCLUSION

Feeding bioassays revealed that two ladybird species are responsive to dietary RNAi. The two species, however, differed in their sensitivity. © 2019 Society of Chemical Industry.

RNA Interference in Insects: Protecting Beneficials and Controlling Pests

Insects constitute the largest and most diverse group of animals on Earth with an equally diverse virome. The main antiviral immune system of these animals is the post-transcriptional gene-silencing mechanism known as RNA(i) interference. Furthermore, this process can be artificially triggered via delivery of gene-specific double-stranded RNA molecules, leading to specific endogenous gene silencing. This is called RNAi technology and has important applications in several fields. In this paper, we review RNAi mechanisms in insects as well as the potential of RNAi technology to contribute to species-specific insecticidal strategies. Regarding this aspect, we cover the range of strategies considered and investigated so far, as well as their limitations and the most promising approaches to overcome them. Additionally, we discuss patterns of viral infection, specifically persistent and acute insect viral infections. In the latter case, we focus on infections affecting economically relevant species. Within this scope, we review the use of insect-specific viruses as bio-insecticides. Last, we discuss RNAi-based strategies to protect beneficial insects from harmful viral infections and their potential practical application. As a whole, this manuscript stresses the impact of insect viruses and RNAi technology in human life, highlighting clear lines of investigation within an exciting and promising field of research.

Bee Viruses: Ecology, Pathogenicity, and Impacts

Bees-including solitary, social, wild, and managed species-are key pollinators of flowering plant species, including nearly three-quarters of global food crops. Their ecological importance, coupled with increased annual losses of managed honey bees and declines in populations of key wild species, has focused attention on the factors that adversely affect bee health, including viral pathogens. Genomic approaches have dramatically expanded understanding of the diversity of viruses that infect bees, the complexity of their transmission routes-including intergenus transmission-and the diversity of strategies bees have evolved to combat virus infections, with RNA-mediated responses playing a prominent role. Moreover, the impacts of viruses on their hosts are exacerbated by the other major stressors bee populations face, including parasites, poor nutrition, and exposure to chemicals. Unraveling the complex relationships between viruses and their bee hosts will lead to improved understanding of viral ecology and management strategies that support better bee health.

Management of Pest Insects and Plant Diseases by Non-Transformative RNAi

Since the discovery of RNA interference (RNAi), scientists have made significant progress towards the development of this unique technology for crop protection. The RNAi mechanism works at the mRNA level by exploiting a sequence-dependent mode of action with high target specificity due to the design of complementary dsRNA molecules, allowing growers to target pests more precisely compared to conventional agrochemicals. The delivery of RNAi through transgenic plants is now a reality with some products currently in the market. Conversely, it is also expected that more RNA-based products reach the market as non-transformative alternatives. For instance, topically applied dsRNA/siRNA (SIGS - Spray Induced Gene Silencing) has attracted attention due to its feasibility and low cost compared to transgenic plants. Once on the leaf surface, dsRNAs can move directly to target pest cells (e.g., insects or pathogens) or can be taken up indirectly by plant cells to then be transferred into the pest cells. Water-soluble formulations containing pesticidal dsRNA provide alternatives, especially in some cases where plant transformation is not possible or takes years and cost millions to be developed (e.g., perennial crops). The ever-growing understanding of the RNAi mechanism and its limitations has allowed scientists to develop non-transgenic approaches such as trunk injection, soaking, and irrigation. While the technology has been considered promising for pest management, some issues such as RNAi efficiency, dsRNA degradation, environmental risk assessments, and resistance evolution still need to be addressed. Here, our main goal is to review some possible strategies for non-transgenic delivery systems, addressing important issues related to the use of this technology.

Honey Bee and Bumble Bee Antiviral Defense

Bees are important plant pollinators in both natural and agricultural ecosystems. Managed and wild bees have experienced high average annual colony losses, population declines, and local extinctions in many geographic regions. Multiple factors, including virus infections, impact bee health and longevity. The majority of bee-infecting viruses are positive-sense single-stranded RNA viruses. Bee-infecting viruses often cause asymptomatic infections but may also cause paralysis, deformity or death. The severity of infection is governed by bee host immune responses and influenced by additional biotic and abiotic factors. Herein, we highlight studies that have contributed to the current understanding of antiviral defense in bees, including the Western honey bee (Apis mellifera), the Eastern honey bee (Apis cerana) and bumble bee species (Bombus spp.). Bee antiviral defense mechanisms include RNA interference (RNAi), endocytosis, melanization, encapsulation, autophagy and conserved immune pathways including Jak/STAT (Janus kinase/signal transducer and activator of transcription), JNK (c-Jun N-terminal kinase), MAPK (mitogen-activated protein kinases) and the NF-κB mediated Toll and Imd (immune deficiency) pathways. Studies in Dipteran insects, including the model organism Drosophila melanogaster and pathogen-transmitting mosquitos, provide the framework for understanding bee antiviral defense. However, there are notable differences such as the more prominent role of a non-sequence specific, dsRNA-triggered, virus limiting response in honey bees and bumble bees. This virus-limiting response in bees is akin to pathways in a range of organisms including other invertebrates (i.e., oysters, shrimp and sand flies), as well as the mammalian interferon response. Current and future research aimed at elucidating bee antiviral defense mechanisms may lead to development of strategies that mitigate bee losses, while expanding our understanding of insect antiviral defense and the potential evolutionary relationship between sociality and immune function.

Leishmania, microbiota and sand fly immunity

In this review, we explore the state-of-the-art of sand fly relationships with microbiota, viruses and Leishmania, with particular emphasis on the vector immune responses. Insect-borne diseases are a major public health problem in the world. Phlebotomine sand flies are proven vectors of several aetiological agents including viruses, bacteria and the trypanosomatid Leishmania, which are responsible for diseases such as viral encephalitis, bartonellosis and leishmaniasis, respectively. All metazoans in nature coexist intimately with a community of commensal microorganisms known as microbiota. The microbiota has a fundamental role in the induction, maturation and function of the host immune system, which can modulate host protection from pathogens and infectious diseases. We briefly review viruses of public health importance present in sand flies and revisit studies done on bacterial and fungal gut contents of these vectors. We bring this information into the context of sand fly development and immune responses. We highlight the immunity mechanisms that the insect utilizes to survive the potential threats involved in these interactions and discuss the recently discovered complex interactions among microbiota, sand fly, Leishmania and virus. Additionally, some of the alternative control strategies that could benefit from the current knowledge are considered.

RNA interference technology in crop protection against arthropod pests, pathogens and nematodes

Scientists have made significant progress in understanding and unraveling several aspects of double-stranded RNA (dsRNA)-mediated gene silencing during the last two decades. Now that the RNA interference (RNAi) mechanism is well understood, it is time to consider how to apply the acquired knowledge to agriculture and crop protection. Some RNAi-based products are already available for farmers and more are expected to reach the market soon. Tailor-made dsRNA as an active ingredient for biopesticide formulations is considered a raw material that can be used for diverse purposes, from pest control and bee protection against viruses to pesticide resistance management. The RNAi mechanism works at the messenger RNA (mRNA) level, exploiting a sequence-dependent mode of action, which makes it unique in potency and selectivity compared with conventional agrochemicals. Furthermore, the use of RNAi in crop protection can be achieved by employing plant-incorporated protectants through plant transformation, but also by non-transformative strategies such as the use of formulations of sprayable RNAs as direct control agents, resistance factor repressors or developmental disruptors. In this review, RNAi is presented in an agricultural context (discussing products that have been launched on the market or will soon be available), and we go beyond the classical presentation of successful examples of RNAi in pest-insect control and comprehensively explore its potential for the control of plant pathogens, nematodes and mites, and to fight against diseases and parasites in beneficial insects. Moreover, we also discuss its use as a repressor for the management of pesticide-resistant weeds and insects. Finally, this review reports on the advances in non-transformative dsRNA delivery and the production costs of dsRNA, and discusses environmental considerations. © 2017 Society of Chemical Industry.

Recently identified bee viruses and their impact on bee pollinators

Bees are agriculturally and ecologically important plant pollinators. Recent high annual losses of honey bee colonies, and reduced populations of native and wild bees in some geographic locations, may impact the availability of affordable food crops and the diversity and abundance of native and wild plant species. Multiple factors including viral infections affect pollinator health. The majority of well-characterized bee viruses are picorna-like RNA viruses, which may be maintained as covert infections or cause symptomatic infections or death. Next generation sequencing technologies have been utilized to identify additional bee-infecting viruses including the Lake Sinai viruses and Rhabdoviruses. In addition, sequence data is instrumental for defining specific viral strains and characterizing associated pathogenicity, such as the recent characterization of Deformed wing virus master variants (DWV-A, DWV-B, and DWV-C) and their impact on bee health.

Insights into RNAi-based antiviral immunity in Lepidoptera: acute and persistent infections in Bombyx mori and Trichoplusia ni cell lines

The control of viral infections in insects is a current issue of major concern and RNA interference (RNAi) is considered the main antiviral immune response in this group of animals. Here we demonstrate that overexpression of key RNAi factors can help to protect insect cells against viral infections. In particular, we show that overexpression of Dicer2 and Argonaute2 in lepidopteran cells leads to improved defense against the acute infection of the Cricket Paralysis Virus (CrPV). We also demonstrate an important role of RNAi in the control of persistent viral infections, as the one caused by the Macula-like Latent Virus (MLV). Specifically, a direct interaction between Argonaute2 and virus-specific small RNAs is shown. Yet, while knocking down Dicer2 and Argonaute2 resulted in higher transcript levels of the persistently infecting MLV in the lepidopteran cells under investigation, overexpression of these proteins could not further reduce these levels. Taken together, our data provide deep insight into the RNAi-based interactions between insects and their viruses. In addition, our results suggest the potential use of an RNAi gain-of-function approach as an alternative strategy to obtain reduced viral-induced mortality in Lepidoptera, an insect order that encompasses multiple species of relevant economic value.

Identification of Secreted Proteins Involved in Nonspecific dsRNA-Mediated Lutzomyia longipalpis LL5 Cell Antiviral Response

Hematophagous insects transmit infectious diseases. Sand flies are vectors of leishmaniasis, but can also transmit viruses. We have been studying immune responses of Lutzomyia longipalpis, the main vector of visceral leishmaniasis in the Americas. We identified a non-specific antiviral response in L. longipalpis LL5 embryonic cells when treated with non-specific double-stranded RNAs (dsRNAs). This response is reminiscent of interferon response in mammals. We are investigating putative effectors for this antiviral response. Secreted molecules have been implicated in immune responses, including interferon-related responses. We conducted a mass spectrometry analysis of conditioned medium from LL5 cells 24 and 48 h after dsRNA or mock treatment. We identified 304 proteins. At 24 h, 19 proteins had an abundance equal or greater than 2-fold change, while the levels of 17 proteins were reduced when compared to control cells. At the 48 h time point, these numbers were 33 and 71, respectively. The two most abundant secreted peptides at 24 h in the dsRNA-transfected group were phospholipid scramblase, an interferon-inducible protein that mediates antiviral activity, and forskolin-binding protein (FKBP), a member of the immunophilin family, which mediates the effect of immunosuppressive drugs. The transcription profile of most candidates did not follow the pattern of secreted protein abundance.

Systemic Israeli acute paralysis virus (IAPV) infection in bumblebees (Bombus terrestris) through feeding and injection

Israeli acute paralysis virus (IAPV) can cause a systemic infection, resulting in mortality in both Apis and Bombus spp. bees. However, little is known about the virus infection dynamics within bee tissues. Here, we established systemic IAPV infections in reared bumblebee Bombus terrestris workers through feeding and injection and investigated the mortality, tissue tropism and viral localization. Injection of approximately 500 IAPV (IAPV^inj stock) particles resulted in acute infection, viral loads within tissues that were relatively stable from bee to bee, and a distinctive tissue tropism, making this method suitable for studying systemic IAPV infection in bumblebees. Feeding with approximately 1 × 10⁶ particles of the same virus stock did not result in systemic infection. A high-concentration stock of IAPV (IAPV^fed stock) allowed us to feed bumblebees with approximately 1 × 10⁹ viral particles, which induced both chronic and acute infection. We also observed a higher variability in viral titers within tissues and less clear tissue tropism during systemic infection, making feeding with IAPV^fed stock less optimal for studying IAPV systemic infection. Strikingly, both infection methods and stocks with different viral loads gave a similar viral localization pattern in the brain and midgut of bumblebees with an acute infection. The implications of these findings in the study of the local immunity in bees and barriers to oral transmission are discussed. Our data provide useful information on the establishment of a systemic viral infection in bees.

Virus and dsRNA-triggered transcriptional responses reveal key components of honey bee antiviral defense

Recent high annual losses of honey bee colonies are associated with many factors, including RNA virus infections. Honey bee antiviral responses include RNA interference and immune pathway activation, but their relative roles in antiviral defense are not well understood. To better characterize the mechanism(s) of honey bee antiviral defense, bees were infected with a model virus in the presence or absence of dsRNA, a virus associated molecular pattern. Regardless of sequence specificity, dsRNA reduced virus abundance. We utilized next generation sequencing to examine transcriptional responses triggered by virus and dsRNA at three time-points post-infection. Hundreds of genes exhibited differential expression in response to co-treatment of dsRNA and virus. Virus-infected bees had greater expression of genes involved in RNAi, Toll, Imd, and JAK-STAT pathways, but the majority of differentially expressed genes are not well characterized. To confirm the virus limiting role of two genes, including the well-characterized gene, dicer, and a probable uncharacterized cyclin dependent kinase in honey bees, we utilized RNAi to reduce their expression in vivo and determined that virus abundance increased, supporting their involvement in antiviral defense. Together, these results further our understanding of honey bee antiviral defense, particularly the role of a non-sequence specific dsRNA-mediated antiviral pathway.

Viral Delivery of dsRNA for Control of Insect Agricultural Pests and Vectors of Human Disease: Prospects and Challenges

RNAi is applied as a new and safe method for pest control in agriculture but efficiency and specificity of delivery of dsRNA trigger remains a critical issue. Various agents have been proposed to augment dsRNA delivery, such as engineered micro-organisms and synthetic nanoparticles, but the use of viruses has received relatively little attention. Here we present a critical view of the potential of the use of recombinant viruses for efficient and specific delivery of dsRNA. First of all, it requires the availability of plasmid-based reverse genetics systems for virus production, of which an overview is presented. For RNA viruses, their application seems to be straightforward since dsRNA is produced as an intermediate molecule during viral replication, but DNA viruses also have potential through the production of RNA hairpins after transcription. However, application of recombinant virus for dsRNA delivery may not be straightforward in many cases, since viruses can encode RNAi suppressors, and virus-induced silencing effects can be determined by the properties of the encoded RNAi suppressor. An alternative is virus-like particles that retain the efficiency and specificity determinants of natural virions but have encapsidated non-replicating RNA. Finally, the use of viruses raises important safety issues which need to be addressed before application can proceed.

Differential expression pattern of Vago in bumblebee (Bombus terrestris), induced by virulent and avirulent virus infections

Viruses are one of the main drivers of the decline of domesticated and wild bees but the mechanisms of antiviral immunity in pollinators are poorly understood. Recent work has suggested that next to the small interfering RNA (siRNA) pathway other immune-related pathways play a role in the defense of the bee hosts against viral infection. In addition, Vago plays a role in the cross-talk between the innate immune pathways in Culex mosquito cells. Here we describe the Vago orthologue in bumblebees of Bombus terrestris, and investigated its role upon the infection of two different bee viruses, the virulent Israeli acute paralysis virus (IAPV) and the avirulent slow bee paralysis virus (SBPV). Our results showed that BtVago was downregulated upon the infection of IAPV that killed all bumblebees, but not with SBPV where the workers survived the virus infection. Thus, for the first time, Vago/Vago-like expression appears to be associated with the virulence of virus and may act as a modulator of antiviral immunity.

Israeli acute paralysis virus associated paralysis symptoms, viral tissue distribution and Dicer-2 induction in bumblebee workers (Bombus terrestris)

Although it is known that Israeli acute paralysis virus (IAPV) can cause bee mortality, the symptoms of paralysis and the distribution of the virus in different body tissues and their potential to respond with an increase of the siRNA antiviral immune system have not been studied. In this project we worked with Bombus terrestris, which is one of the most numerous bumblebee species in Europe and an important pollinator for wild flowers and many crops in agriculture. Besides the classic symptoms of paralysis and trembling prior to death, we report a new IAPV-related symptom, crippled/immobilized forelegs. Reverse-transcriptase quantitative PCR showed that IAPV accumulates in different body tissues (midgut, fat body, brain and ovary). The highest levels of IAPV were observed in the fat body. With fluorescence in situ hybridization (FISH) we detected IAPV in the Kenyon cells of mushroom bodies and neuropils from both antennal and optic lobes of the brain in IAPV-infected workers. Finally, we observed an induction of Dicer-2, a core gene of the RNAi antiviral immune response, in the IAPV-infected tissues of B. terrestris workers. According to our results, tissue tropism and the induction strength of Dicer-2 could not be correlated with virus-related paralysis symptoms.

In vivo study of Dicer-2-mediated immune response of the small interfering RNA pathway upon systemic infections of virulent and avirulent viruses in Bombus terrestris

Recent studies suggest a potent role of the small interfering RNA (siRNA) pathway in the control of bee viruses and its usefulness to tackle these viral diseases. However, the involvement of the siRNA pathway in the defense against different bee viruses is still poorly understood. Therefore, in this report, we comprehensively analyzed the response of the siRNA pathway in bumblebees of Bombus terrestris to systemic infections of the virulent Israeli acute paralysis virus (IAPV) and the avirulent slow bee paralysis virus (SBPV). Our results showed that IAPV and SBPV infections induced the expression of Dicer-2. IAPV infections also triggered the production of predominantly 22 nt-long virus-derived siRNAs (vsiRNAs). Intriguingly, these 22 nt-long vsiRNAs showed a high proportion of antigenomic IAPV sequences. Conversely, these predominantly 22 nt-long vsiRNAs of SBPV were not detected in SBPV infected bees. Furthermore, an "RNAi-of-RNAi" experiment on Dicer-2 did not result in altered genome copy numbers of IAPV (n = 17-18) and also not of SBPV (n = 11-12). Based on these results, we can speculate about the importance of the siRNA pathway in bumblebees for the antiviral response. During infection of IAPV, this pathway is probably recruited but it might be insufficient to control viral infection in our experimental setup. The host can control SBPV infection, but aside from the induction of Dicer-2 expression, no further evidence of the antiviral activity of the siRNA pathway was observed. This report may also enhance the current understanding of the siRNA pathway in the innate immunity of non-model insects upon different viral infections.

RNAi and Antiviral Defense in the Honey Bee

Honey bees play an important agricultural and ecological role as pollinators of numerous agricultural crops and other plant species. Therefore, investigating the factors associated with high annual losses of honey bee colonies in the US is an important and active area of research. Pathogen incidence and abundance correlate with Colony Collapse Disorder- (CCD-) affected colonies in the US and colony losses in the US and in some European countries. Honey bees are readily infected by single-stranded positive sense RNA viruses. Largely dependent on the host immune response, virus infections can either remain asymptomatic or result in deformities, paralysis, or death of adults or larvae. RNA interference (RNAi) is an important antiviral defense mechanism in insects, including honey bees. Herein, we review the role of RNAi in honey bee antiviral defense and highlight some parallels between insect and mammalian immune systems. A more thorough understanding of the role of pathogens on honey bee health and the immune mechanisms bees utilize to combat infectious agents may lead to the development of strategies that enhance honey bee health and result in the discovery of additional mechanisms of immunity in metazoans.

Special Issue: Honey Bee Viruses

Pollination of flowering plants is an important ecosystem service provided by wild insect pollinators and managed honey bees. Hence, losses and declines of pollinating insect species threaten human food security and are of major concern not only for apiculture or agriculture but for human society in general. Honey bee colony losses and bumblebee declines have attracted intensive research interest over the last decade and although the problem is far from being solved we now know that viruses are among the key players of many of these bee losses and bumblebee declines. With this special issue on bee viruses we, therefore, aimed to collect high quality original papers reflecting the current state of bee virus research. To this end, we focused on newly discovered viruses (Lake Sinai viruses, bee macula-like virus), or a so far neglected virus species (Apis mellifera filamentous virus), and cutting edge technologies (mass spectrometry, RNAi approach) applied in the field.