Ingested RNA interference for managing the populations of the Colorado potato beetle, Leptinotarsa decemlineata

Orally delivered dsRNA induces knockdown of target genes and mortality in the Asian long-horned beetle, Anoplophora glabripennis

The Asian long-horned beetle (ALB) Anoplophora glabripennis is a serious invasive forest pest in several countries, including the United States. Methods available to manage or eradicate this pest are extremely limited, but RNA interference (RNAi) technology is a potentially effective method to control ALB. In this study, we used sucrose feeding bioassay for oral delivery of double-strand RNA (dsRNA) to ALB larvae. ³² P-labeled dsRNA orally delivered to ALB larvae using the sucrose droplet feeding method was processed to small interfering RNA. Feeding neonate larvae with dsRNA targeting genes coding for the inhibitor of apoptosis (IAP), vacuolar sorting protein SNF7 (SNF7), and snakeskin (SSK) induced knockdown of target genes and mortality. Feeding 2 µg of dsRNA per day for 3 days did not induce a significant decrease in the expression of target genes or mortality. However, feeding 5 or 10 µg of dsRNA per day for 3 days induced a significant decrease in the expression of target genes and 50-90% mortality. Interestingly, feeding 2.5 µg each of dsIAP plus dsSNF7, dsIAP plus dsSSK, or dsSNF7 plus dsSSK per day for 3 days induced a significant decrease in the expression of both target genes and approximately 80% mortality. Our findings demonstrate that orally delivered dsRNA induces target gene knockdown and mortality in ALB neonate larvae and RNAi technology may have the potential for effective ALB control.

Development of RNAi methods to control the harlequin bug, Murgantia histrionica

The harlequin bug (HB), Murgantia histrionica, is a major pest of cabbage family plants throughout its range in the United States. RNA interference (RNAi) is a posttranscriptional gene silencing mechanism that is showing promise as a biopesticide due to the ability to target species-specific genes necessary for growth and/or survival with synthetic double-stranded RNA (dsRNA). In the present study, dsRNA stability assays revealed that nucleases present in the saliva of harlequin bugs did not rapidly degrade dsRNA. We tracked the movement and localization of radioactively labeled dsRNA in both mustard plant seedlings and harlequin bug nymphs that fed on treated host plants. Movement of ³² P-labeled-dsRNA from soil to plant and plant to insect was detected. The efficacy of RNAi in inducing mortality in harlequin bug adults and nymphs injected or fed with dsRNA targeting inhibitor of apoptosis (IAP), ATPase N2B (ATPase), serine/threonine-protein phosphatase PP1-β catalytic subunit (PP1), signal recognition particle 54 kDa protein (SRP), and G protein-coupled receptor 161-like (GPCR) genes was evaluated. Injection of dsRNA targeting candidate genes into adults caused between 40% and 75% mortality and induced significant knockdown of target gene expression. Feeding dsRNA targeting the IAP gene to nymphs by plant-mediated and droplet feeding methods induced knockdown of the target gene and caused 40-55% mortality. These findings suggest that RNAi may be a viable approach for managing this pest.

Dendrimer-coated carbon nanotubes deliver dsRNA and increase the efficacy of gene knockdown in the red flour beetle Tribolium castaneum

In this study, the use of dendrimer-coated carbon nanotubes (CNTs) as a delivery vehicle for dsRNA was assessed in Tribolium castaneum. Exposure to low dosages of polyamidoamine dendrimer carbon nanotubes (PAMAM-CNTs) did not affect T. castaneum larval mortality. Expression of key apoptotic factors, Dronc (Tc12580), Dredd (Tcn-like, Tc014026) and Buffy, (Tcinhib apop1), which can act as toxicity indicators, were not altered in T. castaneum larvae following injection of PAMAM-CNTs. The level of knockdown of two target genes, α-tubulin and mitochondrial RNA polymerase (mtpol), were significantly increased when larvae were injected with double-stranded RNA bound to CNTs (PAMAM-CNT-dsRNA), compared to those injected with target dsRNA alone. PAMAM-CNTs were visualised in cellular vacuoles and in the cell nucleus. Increase occurrence of a blistered wing phenotype was found in a subset of PAMAM-CNT-dsRNA_αtub injected larvae, relative to the level seen in larvae injected with naked dsRNA_αtub alone. These results suggest that the use of functionalised CNTs for dsRNA delivery could increase the efficacy of RNA interference in insect pest species.

De Novo Assembly of the Asian Citrus Psyllid Diaphorina citri (Hemiptera: Psyllidae) Transcriptome across Developmental Stages

Asian citrus psyllid Diaphorina citri Kuwayama is an important economic pest of citrus, as it transmits Candidatus Liberibacter asiaticus, the causative agent of huanglongbing. In this study, we used RNA-seq to identify novel genes and provide the first high-resolution view of the of D. citri transcriptome throughout development. The transcriptomes of D. citri during eight developmental stages, including the egg, five instars, and male and female adults were sequenced. In total, 115 million clean reads were obtained and assembled into 354,726 unigenes with an average length of 925.65 bp and an N50 length of 1733 bp. Clusters of Orthologous Groups, Gene Ontology, and Kyoto Encyclopedia of Genes and Genomes analyses were conducted to functionally annotate the genes. Differential expression analysis highlighted developmental stage-specific expression patterns. Furthermore, two trehalase genes were characterized with lower expression in adults compared to that in the other stages. The RNA interference (RNAi)-mediated suppression of the two trehalase genes resulted in significantly high D. citri mortality. This study enriched the genomic information regarding D. citri. Importantly, these data represent the most comprehensive transcriptomic resource currently available for D. citri and will facilitate functional genomics studies of this notorious pest.

Oral Ingestion of Bacterially Expressed dsRNA Can Silence Genes and Cause Mortality in a Highly Invasive, Tree-Killing Pest, the Emerald Ash Borer

RNA interference (RNAi) is a naturally occurring process inhibiting gene expression, and recent advances in our understanding of the mechanism have allowed its development as a tool against insect pests. A major challenge for deployment in the field is the development of convenient and efficient methods for production of double stranded RNA (dsRNA). We assessed the potential for deploying bacterially produced dsRNA as a bio-pesticide against an invasive forest pest, the emerald ash borer (EAB). EAB feeds on the cambial tissue of ash trees (Fraxinus spp.), causing rapid death. EAB has killed millions of trees in North America since its discovery in 2002, prompting the need for innovative management strategies. In our study, bacterial expression and synthesis of dsRNA were performed with E. coli strain HT115 using the L4440 expression vector. EAB-specific dsRNAs (shi and hsp) over-expressed in E. coli were toxic to neonate EAB after oral administration, successfully triggering gene silencing and subsequent mortality; however, a non-specific dsRNA control was not included. Our results suggest that ingestion of transformed E. coli expressing dsRNAs can induce an RNAi response in EAB. To our knowledge, this is the first example of an effective RNAi response induced by feeding dsRNA-expressing bacteria in a forest pest.

Delivery Methods for RNAi in Mosquito Larvae

Mosquito-transmitted diseases pose a threat for a great portion of the world population. Chemical insecticides are the main tool for mosquito control. Heavy dependence on chemicals created several problems such as resistance development in many mosquito species, environmental effects, and human health issues. Other tools for mosquito control were developed and used in some parts of the world. Ribonucleic acid interference (RNAi) is a reverse genetic mechanism that was recently introduced as a new tool for pest control. Regarding mosquito, RNAi was used to study gene function and to discover genes that can be used as targets for control purposes. Several delivery methods are used to induce RNAi in mosquito larvae. Some methods such as injection and soaking are used routinely in RNAi research but have no application in the field. Other methods such as nanoparticles and microbes have some characteristics that make them good candidates for field application. In this report, we will focus on delivery methods for RNAi in mosquito larvae and will give examples for each method.

A novel plasmid-Escherichia coli system produces large batch dsRNAs for insect gene silencing

BACKGROUND

RNA interference (RNAi)-based pest management requires efficient delivery and large-batch production of double-stranded (ds)RNA. We previously developed a nanocarrier-mediated dsRNA delivery system that could penetrate an insect's body and efficiently silence gene expression. However, there is a great need to improve the plasmid-Escherichia coli system for the mass production of dsRNA. Here, for efficient dsRNA production, we removed the rnc gene encoding endoribonuclease RNase III in E. coli BL21(DE3) and matched with the RNAi expression vector containing a single T7 promoter.

RESULTS

The novel pET28-BL21(DE3) RNase III-system was successfully constructed to express vestigial (vg)-dsRNA against Harmonia axyridis. dsRNA was extracted and purified from cell cultures in four E. coil systems, and the yields of dsRNA in pET28-BL21(DE3) RNase III-, pET28-HT115(DE3), L4440-BL21(DE3) RNase III- and L4440-HT115(DE3) were 4.23, 2.75, 0.88 and 1.30 μg mL^-1 respectively. The dsRNA expression efficiency of our novel E. coil system was three times that of L4440-HT115(DE3), a widely used dsRNA production system. The RNAi efficiency of dsRNA produced by our system and by biochemical synthesis was comparable when injected into Harmonia axyridis.

CONCLUSION

Our system expressed dsRNA more efficiently than the widely used L4440-HT115(DE3) system, and the produced dsRNA showed a high gene-silencing effect. Notably, our pET28-BL21(DE3) RNase III-system provides a novel method for the mass production of dsRNA at low cost and high efficiency, which may promote gene function analysis and RNAi-based pest management. © 2020 Society of Chemical Industry.

Polymer-Coated Hydroxyapatite Nanocarrier for Double-Stranded RNA Delivery

Conventional synthetic insecticides have limited success due to insect resistance and negative effects on off-target biota and the environment. Although RNA interference (RNAi) is a tool that is becoming more widely utilized in pest control products, naked dsRNA has limited success in most taxa. Nanocarriers have shown promising results in enhancing the efficacy of this tool. In this study, we used a layer-by-layer electrostatic assembly where we synthesized poly(acrylic acid) (PAA)-coated hydroxyapatite (HA) nanoparticles (PAA-HA NPs) as inorganic nanocarriers, which were then coated with a layer of a cationic poly(amino acid), 10 kDa poly-l-arginine (PLR₁₀), to allow for binding of a layer of negatively charged dsRNA. Binding of PLR₁₀-PAA-HA NPs to dsRNA was found to increase as the mass ratio of NPs to dsRNA increased. In vitro studies with transgenic SF9 cells (from Spodoptera frugiperda) expressing the firefly luciferase gene showed a significant gene silencing (35% decrease) at a 5:1 NP-to-dsRNA ratio, while naked dsRNA was ineffective at gene silencing. There was a significant concentration-response relationship in knockdown; however, cytotoxicity was observed at higher concentrations. Confocal microscopy studies showed that dsRNA from PLR₁₀-PAA-HA NPs was not localized within endosomes, while naked dsRNA appeared to be entrapped within the endosomes. Overall, polymer-functionalized HA nanocarriers enabled dsRNA to elicit gene knockdown in cells, whereas naked dsRNA was not effective in causing gene knockdown.

Profiling of RNAi sensitivity after foliar dsRNA exposure in different European populations of Colorado potato beetle reveals a robust response with minor variability

In recent years, substantial effort was spent on the exploration and implementation of RNAi technology using double-stranded RNA (dsRNA) for pest management purposes. However, only few studies investigated the geographical variation in RNAi sensitivity present in field-collected populations of the targeted insect pest. In this baseline study, 2nd instar larvae of 14 different European populations of Colorado potato beetle (CPB), Leptinotarsa decemlineata, collected from nine different countries were exposed to a foliarly applied diagnostic dose of dsactin (dsact) to test for possible variations in RNAi response. Only minor variability in RNAi sensitivity was observed between populations. However, the time necessary to trigger a dsRNA-mediated phenotypic response varied significantly among populations, indicated by significant differences in mortality figures obtained five days after treatment. An inbred German laboratory reference strain D01 and a Spanish field strain E02 showed almost 100% mortality after foliar exposure to 30 ng dsactin (equal to 0.96 g/ha), whereas another Spanish strain E01 was least responsive and showed only 30% mortality. Calculated LD₅₀-values for foliarly applied dsact against strains D01 (most sensitive) and E01 (least sensitive) were 9.22 and 68.7 ng/leaf disc, respectively. The variability was not based on target gene sequence divergence or knock-down efficiency. Variability in expression of the core RNAi machinery genes dicer (dcr2a) and argonaute (ago2a) was observed but did not correlate with sensitivity. Interestingly, RT-qPCR data collected for all strains revealed a strong correlation between the expression level of dcr2a and ago2a (r 0.93) as well as ago2a and stauC (r 0.94), a recently described dsRNA binding protein in Coleopterans. Overall, this study demonstrates that sensitivity of CPB to sprayable RNAi slightly varies between strains but also shows that foliar RNAi as a control method works against all tested CPB populations collected across a broad geographic range in Europe. Thus, underpinning the potential of RNAi-based CPB control as a promising component in integrated pest management (IPM) and resistance management programs.

Length-dependent accumulation of double-stranded RNAs in plastids affects RNA interference efficiency in the Colorado potato beetle

Transplastomic potato plants expressing double-stranded RNA (dsRNA) targeted against essential genes of the Colorado potato beetle (CPB) can be lethal to larvae by triggering an RNA interference (RNAi) response. High accumulation levels of dsRNAs in plastids are crucial to confer an efficient RNAi response in the insects. However, whether length and sequence of the dsRNA determine the efficacy of RNAi and/or influence the level of dsRNA accumulation in plastids is not known. We compared the RNAi efficacy of different lengths of dsRNA targeted against the CPB β-Actin gene (ACT) by feeding in vitro-synthesized dsRNAs to larvae. We showed that, while the 60 bp dsRNA induced only a relatively low RNAi response in CPB, dsRNAs of 200 bp and longer caused high mortality and similar larval growth retardation. When the dsRNAs were expressed from the plastid (chloroplast) genome of potato plants, we found that their accumulation were negatively correlated with length. The level of dsRNA accumulation was positively associated with the observed mortality, suppression of larval growth, and suppression of target gene expression. Importantly, transplastomic potato plants expressing the 200 bp dsRNA were better protected from CPB than plants expressing the 297 bp dsRNA, the best-performing line in our previous study. Our results suggest that the length of dsRNAs is an important factor that influences their accumulation in plastids and thus determines the strength of the insecticidal RNAi effect. Our findings will aid the design of optimized dsRNA expression constructs for plant protection by plastid-mediated RNAi.

Double-stranded RNA targeting vATPase B reveals a potential target for pest management of Henosepilachna vigintioctopunctata

The development of genetic based techniques, specifically RNA interference (RNAi), has emerged as a powerful tool in novel pest management strategies for pestiferous coleoptera. The 28-spotted ladybird beetle, Henosepilachna vigintioctopunctata, is a dynamic foliar pest of solenaceous plants, primarily potato plants, and has quickly become one of the most important pests attacking many crops in Asian countries. In this study, we demonstrate the efficacy of dietary RNAi targeting vATPase B, which led to significant gene silencing. Downstream effects of vATPase B silencing appeared to be both time- and partial dose-dependent. Our results indicate that silencing of vATPase B caused a significant decrease in survival rate, as well as reduced the food stuffs consumption and inhibited the overall development of H. vigintioctopunctata. Furthermore, results demonstrate expression of insect melanism related genes, TH and DDC, was significantly up regulated under the dsvATPase B (RNAi molecule designed against vATPase B) treatment. The impact of oral dsvATPase B delivery on the survival of 1st, 3rd instars, and adults was investigated through bacterially expressed dsRNA. The effectiveness of RNAi-based gene silencing in H. vigintioctopunctata provides a powerful reverse genetic tool for the functional annotation of its genes. This study demonstrates that vATPase B may represent a candidate gene for RNAi-based control of H. vigintioctopunctata.

Control of western corn rootworm via RNAi traits in maize: lethal and sublethal effects of Sec23 dsRNA

BACKGROUND

RNA interference (RNAi) triggered by maize plants expressing RNA hairpins against specific western corn rootworm (WCR) transcripts have proven to be effective at controlling this pest. To provide robust crop protection, mRNA transcripts targeted by double-stranded RNA must be sensitive to knockdown and encode essential proteins.

RESULTS

Using WCR adult feeding assays, we identified Sec23 as a highly lethal RNAi target. Sec23 encodes a coatomer protein, a component of the coat protein (COPII) complex that mediates ER-Golgi transport. The lethality detected in WCR adults was also observed in early instar larvae, the life stage causing most of the crop damage, suggesting that WCR adults can serve as an alternative to larvae for dsRNA screening. Surprisingly, over 85% transcript inhibition resulted in less than 40% protein knockdown, suggesting that complete protein knockdown is not necessary for Sec23 RNAi-mediated mortality. The efficacy of Sec23 dsRNA for rootworm control was confirmed in planta; T₀ maize events carrying rootworm Sec23 hairpin transgenes showed high levels of root protection in greenhouse assays. A reduction in larval survival and weight were observed in the offspring of WCR females exposed to Sec23 dsRNA LC₂₅ in diet bioassays.

CONCLUSION

We describe Sec23 as RNAi target for in planta rootworm control. High mortality in exposed adult and larvae and moderate sublethal effects in the offspring of females exposed to Sec23 dsRNA LC₂₅ , suggest the potential for field application of this RNAi trait and the need to factor in responses to sublethal exposure into insect resistance management programs. © 2019 Society of Chemical Industry.

Improving RNAi efficiency for pest control in crop species

The application of RNAi promotes the development of novel approaches toward plant protection in a sustainable way. Genetically modified crops expressing dsRNA have been developed as commercial products with great potential in insect pest management. Alternatively, some nontransformative approaches, including foliar spray, irrigation and trunk injection, are favorable in actual utilization. In this review, we summarize the recent progress and successful cases of RNAi-based pest management strategy, explore essential implications and possibilities to improve RNAi efficiency by delivery of dsRNA through transformative and nontransformative approaches, and highlight the remaining challenges and important issues related to the application of this technology.

Alteration of insulin signaling to control insect pest by using transformed bacteria expressing dsRNA

BACKGROUND

Insulin/insulin-like growth factor signaling (IIS) is known to mediate larval growth and adult reproduction in the legume pod borer, Maruca vitrata (Lepidoptera: Crambidae). Four IIS components (InR, FOXO, Akt, and TOR) play crucial roles in the IIS pathway.

RESULTS

RNA interference (RNAi) against any of these four IIS component genes was effective in suppressing each target mRNA level by either hemocoelic injection or oral administration using gene-specific double-stranded RNAs (dsRNAs). These RNAi treatments interfered with larval growth, leading to small pupae or significant larval mortality. For massive production of dsRNA, transformed bacteria expressing dsRNAs of these four IIS components were prepared with L4440 expression vector and HT115 strain of Escherichia coli. The transformed bacteria killed the larvae in a dose-dependent manner by feeding administration. An ultra-sonication pretreatment was performed to impair bacterial membrane and increase dsRNA release from the bacteria in insect intestine. This pretreatment increased the insecticidal activity of these recombinant bacteria. To further increase dsRNA toxicity, its mixture with Bacillus thuringiensis (Bt) was prepared and showed significant increase of Bt insecticidal activity in the laboratory. The bacterial mixture also showed a high control efficacy (83.3%) in an adzuki bean (Vigna angularis) field infested by M. vitrata. Furthermore, such a dsRNA effect was specific for M. vitrata, but not for non-target insects.

CONCLUSION

The bacteria expressing dsRNA specific to IIS components can be used to develop dsRNA insecticide. © 2019 Society of Chemical Industry.

Delivery of gene-specific dsRNA by microinjection and feeding induces RNAi response in Sri Lanka weevil, Myllocerus undecimpustulatus undatus Marshall

BACKGROUND

RNA interference (RNAi) is widely used in entomological research for functional analysis of genes and is being considered as a new tool for insect pest management. Sri Lanka weevil (SLW) is a highly polyphagous pest of agronomically important plants, but currently only a few control methods are available for this insect.

RESULTS

In the present study, we evaluated the stability of candidate reference genes β-ACT, α-TUB, EF1-α, RPL12 and GAPDH, and identified EF1-α as the most reliable for gene expression normalization. Four target genes involved in different cellular processes, including Prosα2, RPS13, Snf7 and V-ATPase A were selected to evaluate whether RNAi response in SLW adults can be triggered by microinjection and oral feeding of their double-stranded RNAs (dsRNAs). Three days after injection of the dsRNAs for the target genes, their transcript levels were significantly reduced (up to 91.4%) when compared to the control. Additionally, weevils fed with the target dsRNAs showed significant decreases in gene transcript levels and significant mortality was observed in insects treated with Prosα2 and Snf7 dsRNAs (78.6 to 92.7%).

CONCLUSION

Our data demonstrate that microinjection and feeding of dsRNA produce a strong RNAi response in SLW, indicating that RNAi-based strategies could be explored to develop a selective and environmentally safe control method against SLW. © 2019 Society of Chemical Industry.

Downregulation of imidacloprid resistant genes alters the biological parameters in Colorado potato beetle, Leptinotarsa decemlineata Say (chrysomelidae: Coleoptera)

Colorado potato beetle, Leptinotarsa decemlineata Say (coleoptera: chrysomelidae), is the important pest of potato all over the world. This insect pest is resistant to more than 50 active compounds belonging to various chemical groups. Potential of RNA interference (RNAi) was explored to knock down transcript levels of imidacloprid resistant genes in Colorado potato beetle (CPB) under laboratory conditions. Three important genes belonging to cuticular protein (CP), cytochrome P450 monoxygenases (P450) and glutathione synthetase (GSS) families encoding imidacloprid resistance were targeted. Feeding bio-assays were conducted on various stages of imidacloprid resistant CPB lab population by applying HT115 expressing dsRNA on potato leaflets. Survival rate of insects exposed to CP-dsRNA decreased to 4.23%, 15.32% and 47.35% in 2nd, 3rd and 4th instar larvae respectively. Larval weight and pre-adult duration were also affected due to dsRNAs feeding. Synergism of RNAi with imidacloprid conducted on the 2nd instar larvae, exhibited 100% mortality of larvae when subjected to reduced doses of GSS and CP dsRNAs along with imidacloprid. Utilization of three different dsRNAs against imidacloprid resistant CPB population reveal that dsRNAs targeting CP, P450 and GSS enzymes could be useful tool in management of imidacloprid resistant CPB populations.

Suppressing tawny crazy ant (Nylanderia fulva) by RNAi technology

The tawny crazy ant (Nylanderia fulva) is a new invasive pest in the United States. At present, its management mainly relies on the use of synthetic insecticides, which are generally ineffective at producing lasting control of the pest, necessitating alternative environmentally friendly measures. In this study, we evaluated the feasibility of gene silencing to control this ant species. Six housekeeping genes encoding actin (NfActin), coatomer subunit β (NfCOPβ), arginine kinase (NfArgK), and V-type proton ATPase subunits A (NfvATPaseA), B (NfvATPaseB) and E (NfvATPaseE) were cloned. Phylogenetic analysis revealed high sequence similarity to homologs from other ant species, particularly the Florida carpenter ant (Camponotus floridanus). To silence these genes, vector L4440 was used to generate six specific RNAi constructs for bacterial expression. Heat-inactivated, dsRNA-expressing Escherichia coli were incorporated into artificial diet. Worker ants exhibited reduced endogenous gene expression after feeding on such diet for 9 d. However, only ingestion of dsRNAs of NfCOPβ (a gene involved in protein trafficking) and NfArgK (a cellular energy reserve regulatory gene in invertebrates) caused modest but significantly higher ant mortality than the control. These results suggest that bacterially expressed dsRNA can be orally delivered to ant cells as a mean to target its vulnerabilities. Improved efficacy is necessary for the RNAi-based approach to be useful in tawny crazy ant management.

First report on CRISPR/Cas9-targeted mutagenesis in the Colorado potato beetle, Leptinotarsa decemlineata

Leptinotarsa decemlineata (Say), commonly known as the Colorado potato beetle (CPB), is an agricultural important pest for potatoes and other solanaceous plants. The CRISPR/Cas system is an efficient genome editing technology, which could be exploited to study the biology of CPB and possibly also lead to the development of better environmentally friendly pest management strategies. However, the use of CRISPR/Cas9 has been limited to only a few model insects. Here, for the first time, a CRISPR/Cas9 protocol for mutagenesis studies in CPB was developed. A gene with a clear phenotype such as the vestigial gene (vest), known to be involved in wing development in other insect species, was selected as a good indicator for the knockout study. First, vest was functionally characterized in CPB by using RNAi technology for knockdown studies. Once the expected deformed wing phenotypes were observed, a CRISPR/Cas9 work flow was established for mutagenesis in CPB. By co-injecting the Cas9 protein and a vest-guide RNA into 539 CPB eggs of <1 h old, sixty-two successfully developed to adults, among which mutation in the vest loci was confirmed in 5 of the 18 wingless CPBs (29% phenotypic mutation efficiency). The mutation in vest resulted in a clear phenotype in the CPBs, which developed to adulthood with no hindwing and elytron formed. Altogether, this study provides for the first time a useful methodology involving the use of the CRISPR/Cas9 system for mutagenesis studies in one of the most important pest insects.

Molecular characterization of RNase III protein of Asaia sp. for developing a robust RNAi-based paratransgensis tool to affect the sexual life-cycle of Plasmodium or Anopheles fitness

Background

According to scientific recommendations, paratransgenesis is one of the solutions for improving the effectiveness of the Global Malaria Eradication Programme. In paratransgenesis, symbiont microorganisms are used for distorting or blocking the parasite life-cycle, affecting the fitness and longevity of vectors or reducing the vectorial competence. It has been revealed recently that bacteria could be used as potent tools for double stranded RNA production and delivery to insects. Moreover, findings showed that RNase III mutant bacteria are more competent for this aim. Asaia spp. have been introduced as potent paratransgenesis candidates for combating malaria and, based on their specific features for this goal, could be considered as effective dsRNA production and delivery tools to Anopheles spp. Therefore, we decided to characterize the rnc gene and its related protein to provide the basic required information for creating an RNase III mutant Asaia bacterium.

Methods

Asaia bacteria were isolated from field-collected Anopheles stephensi mosquitoes. The rnc gene and its surrounding sequences were characterized by rapid amplification of genomic ends. RNase III recombinant protein was expressed in E. coli BL21 and biological activity of the purified recombinant protein was assayed. Furthermore, Asaia RNaseIII amino acid sequence was analyzed by in silico approaches such as homology modeling and docking to determine its structural properties.

Results

In this study, the structure of rnc gene and its related operon from Asaia sp. was determined. In addition, by performing superimposition and docking with specific substrate, the structural features of Asaia RNaseIII protein such as critical residues which are involved and essential for proper folding of active site, binding of magnesium ions and double stranded RNA molecule to protein and cleaving of dsRNA molecules, were determined.

Conclusions

In this study, the basic and essential data for creating an RNase III mutant Asaia sp. strain, which is the first step of developing an efficient RNAi-based paratransgenesis tool, were acquired. Asaia sp. have been found in different medically-important vectors and these data are potentially very helpful for researchers studying paratransgenesis and vector-borne diseases and are interested in applying the RNAi technology in the field.

Mechanisms, Applications, and Challenges of Insect RNA Interference

The RNA interference (RNAi) triggered by short/small interfering RNA (siRNA) was discovered in nematodes and found to function in most living organisms. RNAi has been widely used as a research tool to study gene functions and has shown great potential for the development of novel pest management strategies. RNAi is highly efficient and systemic in coleopterans but highly variable or inefficient in many other insects. Differences in double-stranded RNA (dsRNA) degradation, cellular uptake, inter- and intracellular transports, processing of dsRNA to siRNA, and RNA-induced silencing complex formation influence RNAi efficiency. The basic dsRNA delivery methods include microinjection, feeding, and soaking. To improve dsRNA delivery, various new technologies, including cationic liposome-assisted, nanoparticle-enabled, symbiont-mediated, and plant-mediated deliveries, have been developed. Major challenges to widespread use of RNAi in insect pest management include variable RNAi efficiency among insects, lack of reliable dsRNA delivery methods, off-target and nontarget effects, and potential development of resistance in insect populations.

Identification and characterization of multiple dsRNases from a lepidopteran insect, the tobacco cutworm, Spodoptera litura (Lepidoptera: Noctuidae)

RNA interference (RNAi) efficiency varies among insects. RNAi is highly efficient and systemic in coleopteran insects but quite variable and inefficient in lepidopteran insects. Degradation of double-stranded RNA (dsRNA) by double-stranded ribonucleases (dsRNases) is thought to contribute to the variability in RNAi efficiency observed among insects. One or two dsRNases involved in dsRNA digestion have been identified in a few insects. To understand the contribution of dsRNases to reduced RNAi efficiency in lepidopteran insects, we searched the transcriptome of Spodoptera litura and identified six genes coding for DNA/RNA non-specific endonucleases. Phylogenetic analysis revealed the evolutionary expansion of dsRNase genes in insects. The mRNA levels of three midgut-specific dsRNases increased during the larval stage, and the highest dsRNA-degrading activity was detected in third-instar larvae. Proteins produced via the expression of three midgut-specific dsRNases, and the widely expressed dsRNase3, in a baculovirus system showed dsRNase activity for four out of five dsRNases tested. In addition, the increase in dsRNA-degrading activity and upregulation of dsRNase1 and 2 in larvae fed on cabbage leaves suggests that the diet of S. litura can influence dsRNase expression, dsRNA stability, and thus probably RNAi efficiency. This is the first report that multiple dsRNases function together in an RNAi-recalcitrant insect. The data included in this paper suggest that multiple dsRNases coded by the S. litura genome might contribute to the lower and variable RNAi efficiency reported in this and other lepidopteran insects.

Feeding Delivery of dsHvSnf7 Is a Promising Method for Management of the Pest Henosepilachna vigintioctopunctata (Coleoptera: Coccinellidae)

RNA interference (RNAi) techniques have emerged as powerful tools in the development of novel management strategies for the control of insect pests, such as Henosepilachna vigintioctopunctata, which is a major solanaceous pest in Asia. Our results showed that levels of HvSnf7 expression were greater in larval midguts than in other tissues. Silencing of HvSnf7 led to greater H. vigintioctopunctata mortality rates and appeared to be time- and partially dose-dependent. Bacterially expressed dsHvSnf7 that was applied to detached plant leaves caused 98, 88, and 60% mortality in 1st and 3rd instars, and adults after 10, 12, and 14 d, respectively; when applied to living plants, bacterially expressed dsHvSnf7 led to mortality in 1st and 3rd instars, with no effect on adults. Bacterially expressed dsHvSnf7 led to improved plant protection against H. vigintioctopunctata. Ultrastructural changes caused by HvSnf7-RNAi in larval midguts showed extensive loss of cellular contents that indicate loss of membrane integrity. This study indicate that HvSnf7 potentially can be used as RNAi target gene for controlling of H. vigintioctopunctata.

Bacteria-Mediated RNA Interference for Management of Plagiodera versicolora (Coleoptera: Chrysomelidae)

RNA interference (RNAi) has emerged as a novel and feasible strategy for pest management. Methods for cost-effective production and stable delivery of double-stranded RNA (dsRNA) to the target insects are crucial for the wide application of RNAi for pest control. In this study, we tested the expression of dsRNA in RNaseIII-deficient Escherichia coli HT115 which was then fed to Plagiodera versicolora larvae, an insect pest of Salicaceae plants worldwide. By targeting six potential genes, including actin (ACT), signal recognition particle protein 54k (SRP54), heat shock protein 70 (HSC70), shibire (SHI), cactus (CACT), and soluble N-ethylmaleimide-sensitive fusion attachment proteins (SNAP), we found that feeding bacteria-expressed dsRNA successfully triggered the silencing of the five target genes tested and the suppression of ACT and SRP54 genes caused significant mortality. Our results suggest that the oral delivery of bacteria-expressed dsRNA is a potential alternative for the control of P. versicolora, and that ACT and SRP54 genes are the potent targets.

Genome-wide identification and analysis of genes associated with RNA interference in Bemisia tabaci

BACKGROUND

As a method of RNA-mediated gene silencing, RNA interference (RNAi) is a useful reverse genetic tool with which to study gene function, and holds great promise for pest management. Bemisia tabaci is a cosmopolitan pest that causes extensive damage to crops. The mechanism underlying RNAi efficiency in B. tabaci is not well known. We identified and analyzed candidate genes in the RNAi pathway to understand the RNAi mechanism and provide a basis for the application of RNAi in pest management.

RESULTS

We identified 33 genes putatively involved in the RNAi pathway from the B. tabaci Q genome. Phylogenetic and structural analyses confirmed the characteristics of these genes. Furthermore, quantitative real-time reverse transcription polymerase chain reaction (qRT-PCR) and transcriptomic analysis profiled gene expression patterns during different developmental stages. Gene expression levels estimated by qRT-PCR and RNA-seq analyses were significantly correlated. Moreover, gene functions were verified by RNAi. When accompanied by knockdown of AGO2, Dicer2 and Sid1, the efficiency of CYP6DB3 RNAi decreased correspondingly.

CONCLUSION

In this study, we annotated and validated genes involved in B. tabaci RNAi. A better understanding of the building blocks of the RNAi process in B. tabaci facilitates integration of this novel biotechnology into the management of this emerging pest, either directly or indirectly. © 2019 Society of Chemical Industry.

Selection of lethal genes for ingestion RNA interference against western flower thrips, Frankliniella occidentalis, via leaf disc-mediated dsRNA delivery

The western flower thrips, Frankliniella occidentalis, is a major pest that damages a wide variety of crops and vegetables. Following extensive use of insecticides, it has developed high levels of resistance to almost all groups of insecticides due to its high reproduction rate and short generation time. Therefore, an alternative pest control strategy, such as RNA interference (RNAi)-based control, is essential. To establish an ingestion RNAi-based control, a total of 57 genes involved in various biological processes were selected, and their double-stranded RNAs (dsRNA) were delivered to an insecticide-susceptible strain of F. occidentalis via the leaf disc-feeding method using a bioassay chamber optimized by 3D printing. The mortality of dsRNA-ingested thrips was examined every 24 h until 120 h post-treatment. Of the 57 genes screened, dsRNAs of the Toll-like receptor 6, apolipophorin, coatomer protein subunit epsilon and sorting and assembly machinery component were most lethal when ingested by thrips. The dsRNA-fed thrips showed substantially reduced transcription levels of target genes, demonstrating that the observed mortality was likely due to RNAi. When these genes were tested for ingestion RNAi against an insecticide-resistant strain of F. occidentalis, bioassay results were similar. In conclusion, this study provides the first evidence that ingestion RNAi can be lethal to F. occidentalis, a mesophyll sucking pest, and further suggests that transgenic plants expressing hairpin RNA of these essential genes can be employed to control insecticide-resistant thrips.

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.

Transcriptome and gene expression analysis of three developmental stages of the coffee berry borer, Hypothenemus hampei

Coffee production is a global industry valued at approximately 173 billion US dollars. One of the main challenges facing coffee production is the management of the coffee berry borer (CBB), Hypothenemus hampei, which is considered the primary arthropod pest of coffee worldwide. Current control strategies are inefficient for CBB management. Although biotechnological alternatives, including RNA interference (RNAi), have been proposed in recent years to control insect pests, characterizing the genetics of the target pest is essential for the successful application of these emerging technologies. In this study, we employed RNA-seq to obtain the transcriptome of three developmental stages of the CBB (larva, female and male) to increase our understanding of the CBB life cycle in relation to molecular features. The CBB transcriptome was sequenced using Illumina Hiseq and assembled de novo. Differential gene expression analysis was performed across the developmental stages. The final assembly produced 29,434 unigenes, of which 4,664 transcripts were differentially expressed. Genes linked to crucial physiological functions, such as digestion and detoxification, were determined to be tightly regulated between the reproductive and nonreproductive stages of CBB. The data obtained in this study help to elucidate the critical roles that several genes play as regulatory elements in CBB development.

Construction of Corynebacterium glutamicum cells as containers encapsulating dsRNA overexpressed for agricultural pest control

Double-stranded RNA (dsRNA) inducing RNA interference (RNAi) is expected to be applicable to management of agricultural pests. In this study, we selected a ladybird beetle, Henosepilachna vigintioctopunctata, as a model target pest that devours vegetable leaves, and examined the effects of feeding the pest sterilized microbes highly accumulating a target dsRNA for RNAi induction. We constructed an efficient production system for diap1*-dsRNA, which suppresses expression of the essential gene diap1 (encoding death-associated inhibitor of apoptosis protein 1) in H. vigintioctopunctata, using an industrial strain of Corynebacterium glutamicum as the host microbe. The diap1*-dsRNA was overproduced in C. glutamicum by convergent transcription using a strong promoter derived from corynephage BFK20, and the amount of dsRNA accumulated in fermented cells reached about 75 mg per liter of culture. In addition, we developed a convenient method for stabilizing the dsRNA within the microbes by simply sterilizing the diap1*-dsRNA-expressing C. glutamicum cells with ethanol. When the sterilized microbes containing diap1*-dsRNA were fed to larvae of H. vigintioctopunctata, diap1 expression in the pest was suppressed, and the leaf-feeding activity of the larvae declined. These results suggest that this system is capable of producing stabilized dsRNA for RNAi at low cost, and it will open a way to practical application of dsRNA as an environmentally-friendly agricultural insecticide.

De Novo Transcriptome Analysis Reveals Abundant Gonad-specific Genes in the Ovary and Testis of Henosepilachna vigintioctopunctata

Henosepilachna vigintioctopunctata (Coleoptera: Coccinellidae) is a major pest affecting Solanaceae plants in Asian countries. In this study, we sequenced the ovary and testis transcriptomes of H. vigintioctopunctata to identify gonad-related genes. Comparison of the unigene sequences in ovary and testis libraries identified 1,421 and 5,315 ovary- and testis-specific genes, respectively. Among the ovary-specific genes, we selected the RC2-like and PSHS-like genes to investigate the effects of gene silencing on the mortality, percentage infertility, pre-oviposition period, fecundity, daily number of eggs laid, and hatching rate in female adults. Although the percentage mortality and infertility of females did not differ significantly among dsRNA treatments, fecundity was significantly reduced in the dsRC2-like and dsPSHS-like treatment groups. Moreover, the pre-oviposition period was markedly prolonged in response to dsPSHS-like treatment. This is the first reported RNA sequencing of H. vigintioctopunctata. The transcriptome sequences and gene expression profiles of the ovary and testis libraries will provide useful information for the identification of gonad-related genes in H. vigintioctopunctata and facilitate further research on the reproductive biology of this species. Moreover, the gonad-specific genes identified may represent candidate target genes for inhibiting the population growth of H. vigintioctopunctata.

Screening of reference genes using real-time quantitative PCR for gene expression studies in Neoseiulus barkeri Hughes (Acari: Phytoseiidae)

A stable reference gene is a key prerequisite for accurate assessment of gene expression. At present, the real-time reverse transcriptase quantitative polymerase chain reaction has been widely used in the analysis of gene expression in a variety of organisms. Neoseiulus barkeri Hughes (Acari: Phytoseiidae) is a major predator of mites on many important economically crops. Until now, however, there are no reports evaluating the stability of reference genes in this species. In view of this, we used GeNorm, NormFinder, BestKeeper, and RefFinder software tools to evaluate the expression stability of 11 candidate reference genes in developmental stages and under various abiotic stresses. According to our results, β-ACT and Hsp40 were the top two stable reference genes in developmental stages. The Hsp60 and Hsp90 were the most stable reference genes in various acaricides stress. For alterations in temperature, Hsp40 and α-TUB were the most suitable reference genes. About UV stress, EF1α and α-TUB were the best choice, and for the different prey stress, β-ACT and α-TUB were best suited. In normal conditions, the β-ACT and α-TUB were the two of the highest stable reference genes to respond to all kinds of stresses. The current study provided a valuable foundation for the further analysis of gene expression in N. barkeri.

Colorado potato beetle chymotrypsin genes are differentially regulated in larval midgut in response to the plant defense inducer hexanoic acid or the Bacillus thuringiensis Cry3Aa toxin

When Colorado potato beetle larvae ingested potato plants treated with the plant defense inducer compound hexanoic acid, midgut chymotrypsin enzyme activity increased, and the corresponding chymotrypsin genes were differentially expressed, evidence of the larval digestive proteolytic system's plasticity. We previously reported increased susceptibility to Cry3Aa toxin in larvae fed hexanoic acid treated plants. Here we show that the most expressed chymotrypsin gene in larvae fed hexanoic acid treated plants, CTR6, was dramatically downregulated in Cry3Aa intoxicated larvae. lde-miR-965-5p and lde-miR-9a-5p microRNAs, predicted to target CTR6, might be involved in regulating the response to hexanoic acid but not to Cry3Aa toxin.

Nuclease activity decreases the RNAi response in the sweetpotato weevil Cylas puncticollis

RNA interference (RNAi) refers to the process of suppression of gene expression in eukaryotes, which has a great potential for the control of pest and diseases. Unfortunately, the efficacy of this technology is limited or at best variable in some insects. In the African sweet potato weevil (SPW) Cylas puncticollis, a devastating pest that affects the sweet potato production in Sub-Saharan Africa (SSA), the RNAi response was highly efficient when dsRNA was delivered by injection, but it showed a reduced response by oral feeding. In the present study, the role of nucleases in the reduced RNAi efficiency in C. puncticollis is investigated. Several putative dsRNases were first identified in the transcriptome of the SPW through homology search and were subsequently further characterized. Two of these dsRNases were specifically expressed in the gut tissue of the insect and we could demonstrate through RNAi experiments that these affected dsRNA stability in the gut. Furthermore, RNAi-of-RNAi studies, using snf7 as a reporter gene, demonstrated that silencing one of these nucleases, Cp-dsRNase-3, clearly increases RNAi efficacy. After silencing this nuclease, significantly higher mortality was observed in dssnf7-treated insects 14 days post-feeding as compared to control treatments, and the gene downregulation was confirmed at the transcript level via qPCR analysis. Taken together, our results demonstrate that the RNAi efficiency is certainly impaired by nuclease activity in the gut environment of the SPW Cylas puncticollis.

Development of CS-TPP-dsRNA nanoparticles to enhance RNAi efficiency in the yellow fever mosquito, Aedes aegypti

Mosquito-borne diseases are a major threat to human health and are responsible for millions of deaths globally each year. Vector control is one of the most important approaches used in reducing the incidence of these diseases. However, increasing mosquito resistance to chemical insecticides presents challenges to this approach. Therefore, new strategies are necessary to develop the next generation vector control methods. Because of the target specificity of dsRNA, RNAi-based control measures are an attractive alternative to current insecticides used to control disease vectors. In this study, Chitosan (CS) was cross-linked to sodium tripolyphosphate (TPP) to produce nano-sized polyelectrolyte complexes with dsRNA. CS-TPP-dsRNA nanoparticles were prepared by ionic gelation method. The encapsulation efficiency, protection of dsRNA from nucleases, cellular uptake, in vivo biodistribution, larval mortality and gene knockdown efficiency of CS-TPP-dsRNA nanoparticles were determined. The results showed that at a 5:1 weight ratio of CS-TPP to dsRNA, nanoparticles of less than 200 nm mean diameter and a positive surface charge were formed. Confocal microscopy revealed the distribution of the fed CS-TPP-dsRNA nanoparticles in midgut, fat body and epidermis of yellow fever mosquito, Aedes aegypti larvae. Bioassays showed significant mortality of larvae fed on CS-TPP-dsRNA nanoparticles. These assays also showed knockdown of a target gene in CS-TPP-dsRNA nanoparticle fed larvae. These data suggest that CS-TPP nanoparticles may be used for delivery of dsRNA to mosquito larvae.

RNAi-knockdown of the Locusta migratoria nuclear export factor protein results in insect mortality and alterations in gut microbiome

BACKGROUND

The migratory locust Locusta migratoria is one of the most important agricultural pests worldwide. The nuclear export factor 1 (NXF1) protein plays a crucial role in mediating mRNA transport from the nucleus to the cytoplasm. This study evaluates whether NXF1 could be a potential target for RNAi-mediated pest control of L. migratoria.

RESULTS

We cloned and characterized the nuclear export factor lm-nxf1 of L. migratoria. Lm-nxf1 was expressed in all tissues examined, including head, fat body, hemolymph, trunk, leg and midgut, with high expression observed in the hemolymph and fat body. Injection of lm-nxf1 dsRNA into hemolymph resulted in inhibition of mRNA export in hemocytes, which were used as a target for observing mRNA export. Total hemocyte levels were reduced by ca. 97% in lm-nxf1-dsRNA-treated locusts, and high insect mortality occurred with LT₅₀ = 7.75 day as compared with 18.15 day for gfp-dsRNA-treated controls. Further, the locust intestine became atrophy, and the opportunistic pathogens Enterobacter aerogenes, Klebsiella pneumoniae and Enterobacter asburiae were specifically detected in midgut after lm-nxf1 dsRNA treatment.

CONCLUSIONS

The results reveal that knockdown of the lm-nxf1 gene affects the survival of L. migratoria, indicating that lm-nxf1 is a potential target for RNAi-mediated pest control. © 2018 Society of Chemical Industry.

RNA interference and validation of reference genes for gene expression analyses using qPCR in southern pine beetle, Dendroctonus frontalis

RNA interference (RNAi) is a highly specific gene-silencing mechanism that can cause rapid insect mortality when essential genes are targeted. RNAi is being developed as a tool for integrated pest management of some crop pests. Here we focus on an aggressive forest pest that kills extensive tracts of pine forests, the southern pine beetle (SPB), Dendroctonus frontalis. We sought to identify reference genes for quantitative real-time PCR (qPCR) and validate RNAi responses in SPB by mortality and gene silencing analysis. Using an adult beetle feeding bioassay for oral ingestion of dsRNA, we measured the expression and demonstrated knockdown of target genes as well as insect mortality after ingestion of target genes. Our study validates reference genes for expression analyses and demonstrates highly effective RNAi responses in SPB, with RNAi response to some target dsRNAs causing 100% beetle mortality after ingestion.

RNA-sequencing of a citrus bud-feeder, Podagricomela weisei (Coleoptera: Chrysomelidae), reveals xenobiotic metabolism/core RNAi machinery-associated genes and conserved miRNAs

The citrus leaf-mining beetle, Podagricomela weisei Heikertinger, is an important citrus pest that ingests the mesophyll and new shoots. The mechanism underlying the xenobiotic metabolism of P. weisei is not well understood, in part because of a lack of available genomic and transcriptomic data, which has hampered the development of novel pest management approaches [e.g., RNA interference (RNAi)]. In this study, we completed the deep sequencing of the P. weisei transcriptome to identify factors potentially involved in xenobiotic metabolism and the core RNAi machinery. The sequencing of the P. weisei transcriptome generated >27 million clean reads, ultimately yielding 90,410 unigenes with an N50 of 1065 bp. The unigenes were used as queries to search the Nr database, which revealed that 21,847 unigenes were homologous to known genes in various species. Transcripts encoding genes involved in xenobiotic metabolism were identified, including genes encoding cytochrome P450 monooxygenase (P450, 47 unigenes), glutathione S-transferase (GST, 12 unigenes), esterase (EST, 25 unigenes), and the ATP-binding cassette transporter (ABC transporter, 32 unigenes). A parallel sequencing of small RNAs detected 30 conserved miRNAs, with the most abundant being Pwe-miR-1-3p, with an expression level reaching 517,996 reads in the prepared library, followed by Pwe-miR-8-3p (149,402 reads). Genes encoding components of the miRNA, siRNA, and piRNA pathways were also identified, and the results indicated that P. weisei possesses only one of each gene in all three pathways. In summary, this is the first detailed analysis of the transcriptome and small RNAs of P. weisei. The datasets presented herein may form the basis for future molecular characterizations of P. weisei as well as the development of enhanced pest control strategies.

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.

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.

How do oral insecticidal compounds cross the insect midgut epithelium?

The use of oral insecticidal molecules (small molecules, peptides, dsRNA) via spray or plant mediated applications represents an efficient way to manage damaging insect species. With the exception of Bt toxins that target the midgut epithelium itself, most of these compounds have targets that lie within the hemocoel (body) of the insect. Because of this, one of the greatest factors in determining the effectiveness of an oral insecticidal compound is its ability to traverse the gut epithelium and enter the hemolymph. However, for many types of insecticidal compounds, neither the pathway taken across the gut nor the specific genes which influence uptake are fully characterized. Here, we review how different types of insecticidal compounds enter or cross the midgut epithelium through passive (diffusion) or active (transporter based, endocytosis) routes. A deeper understanding of how insecticidal molecules cross the gut will help to best utilize current insecticides and also provide for more rational design of future ones.

Transcriptomic analysis reveals similarities in genetic activation of detoxification mechanisms resulting from imidacloprid and chlorothalonil exposure

The Colorado potato beetle, Leptinotarsa decemlineata (Say), is an agricultural pest of commercial potatoes in parts of North America, Europe, and Asia. Plant protection strategies within this geographic range employ a variety of pesticides to combat not only the insect, but also plant pathogens. Previous research has shown that field populations of Leptinotarsa decemlineata have a chronological history of resistance development to a suite of insecticides, including the Group 4A neonicotinoids. The aim of this study is to contextualize the transcriptomic response of Leptinotarsa decemlineata when exposed to the neonicotinoid insecticide imidacloprid, or the fungicides boscalid or chlorothalonil, in order to determine whether these compounds induce similar detoxification mechanisms. We found that chlorothalonil and imidacloprid induced similar patterns of transcript expression, including the up-regulation of a cytochrome p450 and a UDP-glucuronosyltransferase transcript, which belong to protein families associated with xenobiotic metabolism. Further, transcriptomic responses varied among individuals within the same treatment group, suggesting individual insects’ responses vary within a population and may cope with chemical stressors in a variety of manners.

Identification of target genes for RNAi-mediated control of the Twospotted Spider Mite

RNA interference (RNAi) is being developed for the management of pests that destroy crops. The twospotted Spider Mite (TSSM), Tetranychus urticae is a worldwide pest due to its unique physiological and behavioral characteristics including extraordinary ability to detoxify a wide range of pesticides and feed on many host plants. In this study, we conducted experiments to identify target genes that could be used for the development of RNAi-based methods to control TSSM. Leaf disc feeding assays revealed that knockdown in the expression genes coding for proteins involved in the biosynthesis and action of juvenile hormone (JH) and action of ecdysteroids [Methoprene-tolerant (Met), retinoid X receptor β, farnesoic acid O-methyltransferase, and CREB-binding protein] caused 35-56% mortality. Transgenic tobacco plants expressing hairpin dsRNA targeting Met gene were generated and tested. About 48% mortality was observed in TSSM raised on transgenic tobacco plants expressing dsMet. These studies not only broaden our knowledge on understanding hormone action in TSSM but also identified target genes that could be used in RNAi-mediated control of TSSM.

Diverse Factors Affecting Efficiency of RNAi in Honey Bee Viruses

Infection and transmission of honey bee viruses pose a serious threat to the pollination services of crops and wild plants, which plays a vital role in agricultural economy and ecology. RNA interference (RNAi) is an effective defense mechanism against commonly occurring viral infections of animals and plants. However, recent studies indicate that the effects of RNAi on the honey bee can induce additional impacts and might not always be effective in suppressing the virus. Moreover, the RNAi responses differed in relation to the developmental stage of the insect and the target tissue used, even though the same method of delivery was used. These results indicate that further analysis and field experiments should be performed to characterize the varying effectiveness of RNAi-based methods for treating honey bee viral infections. In this review, we provide an overview of the current knowledge and the recent progress in RNAi-based anti-viral treatments for honey bees, focusing in particular highlight the role of the dsRNA-delivery method used and its effect on RNAi efficiency and demonstrate the potential practical value of this tool for controlling the virus. We conclude studying the gene function and disease control of honey bee by RNAi technology requires a complex consideration from physiology, genetics to environment.

Agricultural fungicides inadvertently influence the fitness of Colorado potato beetles, Leptinotarsa decemlineata, and their susceptibility to insecticides

The Colorado potato beetle (CPB), Leptinotarsa decemlineata (Say), is an agricultural pest of solanaceous crops which has developed insecticide resistance at an alarming rate. Up to this point, little consideration has been given to unintended, or inadvertent effects that non-insecticide xenobiotics may have on insecticide susceptibility in L. decemlineata. Fungicides, such as chlorothalonil and boscalid, are often used to control fungal pathogens in potato fields and are applied at regular intervals when L. decemlineata populations are present in the crop. In order to determine whether fungicide use may be associated with elevated levels of insecticide resistance in L. decemlineata, we examined phenotypic responses in L. decemlineata to the fungicides chlorothalonil and boscalid. Using enzymatic and transcript abundance investigations, we also examined modes of molecular detoxification in response to both insecticide (imidacloprid) and fungicide (boscalid and chlorothalonil) application to more specifically determine if fungicides and insecticides induce similar metabolic detoxification mechanisms. Both chlorothalonil and boscalid exposure induced a phenotypic, enzymatic and transcript response in L. decemlineata which correlates with known mechanisms of insecticide resistance.

Improved insect-proofing: expressing double-stranded RNA in chloroplasts

RNA interference (RNAi) was discovered almost 20 years ago and has been exploited worldwide to silence genes in plants and animals. A decade later, it was found that transforming plants with an RNAi construct targeting an insect gene could protect the plant against feeding by that insect. Production of double-stranded RNA (dsRNA) in a plant to affect the viability of a herbivorous animal is termed trans-kingdom RNAi (TK-RNAi). Since this pioneering work, there have been many further examples of successful TK-RNAi, but also reports of failed attempts and unrepeatable experiments. Recently, three laboratories have shown that producing dsRNA in a plant's chloroplast, rather than in its cellular cytoplasm, is a very effective way of delivering TK-RNAi. Our review examines this potentially game-changing approach and compares it with other transgenic insect-proofing schemes. © 2018 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Double-stranded RNA binding protein, Staufen, is required for the initiation of RNAi in coleopteran insects

RNA interference (RNAi) is being used to develop methods to control pests and disease vectors. RNAi is robust and systemic in coleopteran insects but is quite variable in other insects. The determinants of efficient RNAi in coleopterans, as well as its potential mechanisms of resistance, are not known. RNAi screen identified a double-stranded RNA binding protein (StaufenC) as a major player in RNAi. StaufenC homologs have been identified in only coleopteran insects. Experiments in two coleopteran insects, Leptinotarsa decemlineata and Tribolium castaneum, showed the requirement of StaufenC for RNAi, especially for processing of double-stranded RNA (dsRNA) to small interfering RNA. RNAi-resistant cells were selected by exposing L. decemlineata, Lepd-SL1 cells to the inhibitor of apoptosis 1 dsRNA for multiple generations. The resistant cells showed lower levels of StaufenC expression compared with its expression in susceptible cells. These studies showed that coleopteran-specific StaufenC is required for RNAi and is a potential target for RNAi resistance. The data included in this article will help improve RNAi in noncoleopteran insects and manage RNAi resistance in coleopteran insects.

RNAi for Resistance Against Biotic Stresses in Crop Plants

RNA interference (RNAi)-based gene silencing has become one of the most successful strategies in not only identifying gene function but also in improving agronomical traits of crops by silencing genes of different pathogens/pests and also plant genes for improvement of desired trait. The conserved nature of RNAi pathway across different organisms increases its applicability in various basic and applied fields. Here we attempt to summarize the knowledge generated on the fundamental mechanisms of RNAi over the years, with emphasis on insects and plant-parasitic nematodes (PPNs). This chapter also reviews the rich history of RNAi research, gene regulation by small RNAs across different organisms, and application potential of RNAi for generating transgenic plants resistant to major pests_. But, there are some limitations too which restrict wider applications of this technology to its full potential. Further refinement of this technology in terms of resolving these shortcomings constitutes one of the thrust areas in present RNAi research. Nevertheless, its application especially in breeding agricultural crops resistant against biotic stresses will certainly offer the possible solutions for some of the breeding objectives which are otherwise unattainable.

Engineered Flock House Virus for Targeted Gene Suppression Through RNAi in Fruit Flies (Drosophila melanogaster) in Vitro and in Vivo

RNA interference (RNAi) is a powerful tool to study functional genomics in insects and the potential of using RNAi to suppress crop pests has made outstanding progress. However, the delivery of dsRNA is a challenging step in the development of RNAi bioassays. In this study, we investigated the ability of engineered Flock House virus (FHV) to induce targeted gene suppression through RNAi under in vitro and in vivo condition. As proxy for fruit flies of agricultural importance, we worked with S2 cells as derived from Drosophila melanogaster embryos, and with adult stages of D. melanogaster. We found that the expression level for all of the targeted genes were reduced by more than 70% in both the in vitro and in vivo bioassays. Furthermore, the cell viability and median survival time bioassays demonstrated that the recombinant FHV expressing target gene sequences caused a significantly higher mortality (60-73% and 100%) than the wild type virus (24 and 71%), in both S2 cells and adult insects, respectively. This is the first report showing that a single stranded RNA insect virus such as FHV, can be engineered as an effective in vitro and in vivo RNAi delivery system. Since FHV infects many insect species, the described method could be exploited to improve the efficiency of dsRNA delivery for RNAi-related studies in both FHV susceptible insect cell lines and live insects that are recalcitrant to the uptake of naked dsRNA.