RNAi-mediated mortality in southern green stinkbug Nezara viridula by oral delivery of dsRNA

The use of RNA interference for the management of arthropod pests in livestock farms

Pest management in farm animals is an important action to contain economic damage to livestock production and prevent transmission of severe diseases to the stock. The use of chemical insecticides is still the most common approach followed by farmers; however, avoiding possible toxic effects on animals is a fundamental task for pest control measures compatible with animal well-being. Moreover, legal constraints and insurgence of resistance by target species to the available insecticidal compounds are increasingly complicating farmers' operations. Alternatives to chemical pesticides have been explored with some promising results in the area of biological control or the use of natural products as sprays. The application of RNA interference techniques has enabled the production of new means of pest control in agriculture, and it is opening a promising avenue for controlling arthropod pests of livestock. Transcript depletion of specific target genes of the recipient organisms is based on the action of double-strand RNAs (dsRNA) capable of impairing the production of fundamental proteins. Their mode of action, based on the specific recognition of short genomic sequences, is expected to be highly selective towards non-target organisms potentially exposed; in addition, there are physical and chemical barriers to dsRNA uptake by mammalian cells that render these products practically innocuous for higher animals. Summarising existing literature on gene silencing for main taxa of arthropod pests of livestock (Acarina, Diptera, Blattoidea), this review explores the perspectives of practical applications of dsRNA-based pesticides against the main pests of farm animals. Knowledge gaps are summarised to stimulate additional research in this area.

RNA interference-mediated knockdown of genes involved in sugar transport and metabolism disrupts psyllid Bactericera cockerelli (Order: Hemiptera) gut physiology and results in high mortality

Introduction

The causal agent of zebra chip of potato and vein-greening diseases of tomato is "Candidatus Liberibacter solanacearum" (CLso), a fastidious bacterium transmitted by the potato psyllid. In the absence of disease-resistant cultivars, disease management has relied on minimizing vector population size to reduce CLso transmission, which requires frequent insecticide applications. There is growing interest in the use of RNA interference (RNAi) technology to supplant traditional insecticides with biopesticides. This requires knowledge of genes essential for insect livelihood whose knockdown leads to significant mortality or other phenotypes. Such candidate genes can be evaluated by reverse genetics approaches to further corroborate predicted gene function.

Methods

Here, five potato psyllid genes involved in sugar homeostasis in the potato psyllid gut, α-glucosidase1 (AGLU1), aquaporin2 (AQP2), facilitated trehalose transporter1 (TRET1), Trehalase1 (TRE1), and Trehalase2 (TRE2), were investigated as candidates for effective gene silencing. Potato psyllid dsRNAs were designed to optimize knockdown of gene targets. Third instar PoP nymphs were given a 48-hr ingestion-access period (IAP) on individual or groups of dsRNA in 20% sucrose. Mortality was recorded 0, 3, 5, 7, and 9 days post-IAP. Gene knockdown was analyzed 9 days post-IAP by quantitative real-time reverse-transcriptase polymerase chain reaction amplification.

Results

The individual or stacked dsRNA combinations resulted in 20-60% and 20-40% knockdown, respectively, while subsequent psyllid mortality ranged from 20-40% to >60% for single and stacked dsRNA combinations, respectively. Reverse genetics analysis showed that simultaneous knockdown of the five selected candidate genes with predicted functions in pathways involved in sugar-homeostasis, metabolism, and -transport yielded the highest mortality, when compared with single or combinations of targets.

Discussion

Results confirmed the functions afforded by psyllid gut genes responsible for osmotic homeostasis and sugar metabolism/transport are essential for livelihood, identifying them as potentially lucrative RNAi biopesticide targets and highlighted the translational relevance of targeting multiple nodes in a physiological pathway simultaneously.

A dsRNA delivery system based on the rosin-modified polyethylene glycol and chitosan induces gene silencing and mortality in Nilaparvata lugens

BACKGROUND

RNA interference (RNAi) technology has been considered as a promising pest control strategy due to its species selectivity. One of the popular RNAs is exogenous double strand RNA (dsRNA). However, dsRNA is easily degraded by nucleases and is difficult to pass through the insect body walls, and these factors restrict the application of RNAi technology in pest management. Here, the brown planthopper (BPH, Nilaparvata lugens), a major hemipteran pest of rice in Asia countries was used as a model insect, and a dsRNA topical delivery system was constructed.

RESULTS

The carrier part of the delivery system was composed of rosin-modified polyethylene glycol and chitosan, termed ROPE@C. When the N/P ratio was greater than 1:1.25, the dsRNA/ROPE@C complex encouraged full binding of the dsRNA. The gel electrophoresis results showed that ROPE@C improved the stability of dsRNA in the presence of nucleases in gut and lumen contents for at least 6 h and in the temperature range from 4 °C to 37 °C. The dsNlCHSA/ROPE@C/alkyl polyglycoside (APG) nano-formulation directly penetrated the body walls reaching hemocoel within 6 h, and consequently, the relative expression of chitin synthetase A (CHSA) in BPH was reduced by 54.3% and the mortality rate was 65.8%.

CONCLUSION

We developed an appropriate delivery method for dsRNA application in BPH, which is helpful for a large-scale application of RNAi pesticides. © 2022 Society of Chemical Industry.

Bacteria-mediated RNAi for managing fall webworm, Hyphantria cunea: screening target genes and analyzing lethal effect

BACKGROUND

The fall webworm, Hyphantria cunea, an invasive forest pest found worldwide, causes serious ecological and economic damage. Currently, the application of chemical pesticides is the most widely used strategy for H. cunea management. However, long-term pesticide use leads to pest resistance, phytotoxicity, human poisoning, and environmental deterioration. RNA interference (RNAi) technology may provide an environmentally friendly and cost-effective option for H. cunea control. However, effective RNAi targets and application methods for H. cunea are lacking.

RESULTS

We screened and obtained two highly effective RNAi targets, vATPase A (V-type proton ATPase catalytic subunit A) and Rop (Ras opposite), from 23 candidate genes, using initial and repeat screening tests with the double-stranded RNA (dsRNA) injection method. RNAi against these two genes was effective in suppressing each target messenger RNA level and interfering with larval growth, leading to significant larval mortality and pupal abnormality. For massive production of dsRNA and practical application of RNAi technology in H. cunea, transformed bacteria expressing dsRNAs of these two genes were prepared using the L4440 expression vector and HT115 strain of Escherichia coli. Oral administration of bacterially expressed dsRNA targeting vATPase A and Rop genes showed high mortality and the same malformed phenotype as the injection treatment. To further investigate the lethal effects of targeting these two genes on larval development, transcriptome sequencing (RNA-seq) was performed on RNAi samples. The results demonstrated disorders in multiple metabolic pathways, and the expression levels of most genes related to insect cuticle metabolism were significantly different, which may directly threaten insect survival. In addition, some new findings were obtained via RNA-seq analysis; for example, the progesterone-mediated oocyte maturation and oocyte meiosis processes were significantly different after silencing vATPase A, and the insect olfactory protein-related genes were significantly downregulated after dsHcRop treatment.

CONCLUSION

vATPase A and Rop are two highly effective RNAi-mediated lethal genes in H. cunea that regulate insect growth via multiple metabolic pathways. Oral delivery of bacterially expressed dsRNA specific to vATPase A and Rop can potentially be used for RNAi-based H. cunea management. This is the first study to apply bacteria-mediated RNAi for the control of this invasive pest, which is a major step forward in the application of the RNAi technology in H. cunea. © 2022 Society of Chemical Industry.

Recent trends and advances of RNA interference (RNAi) to improve agricultural crops and enhance their resilience to biotic and abiotic stresses

Over the last two decades, significant advances have been made using genetic engineering technology to modify genes from various exotic origins and introduce them into plants to induce favorable traits. RNA interference (RNAi) was discovered earlier as a natural process for controlling the expression of genes across all higher species. It aims to enhance precision and accuracy in pest/pathogen resistance, quality improvement, and manipulating the architecture of plants. However, it existed as a widely used technique recently. RNAi technologies could well be used to down-regulate any genes' expression without disrupting the expression of other genes. The use of RNA interference to silence genes in various organisms has become the preferred method for studying gene functions. The establishment of new approaches and applications for enhancing desirable characters is essential in crops by gene suppression and the refinement of knowledge of endogenous RNAi mechanisms in plants. RNAi technology in recent years has become an important and choicest method for controlling insects, pests, pathogens, and abiotic stresses like drought, salinity, and temperature. Although there are certain drawbacks in efficiency of this technology such as gene candidate selection, stability of trigger molecule, choice of target species and crops. Nevertheless, from past decade several target genes has been identified in numerous crops for their improvement towards biotic and abiotic stresses. The current review is aimed to emphasize the research done on crops under biotic and abiotic stress using RNAi technology. The review also highlights the gene regulatory pathways/gene silencing, RNA interference, RNAi knockdown, RNAi induced biotic and abiotic resistance and advancements in the understanding of RNAi technology and the functionality of various components of the RNAi machinery in crops for their improvement.

Current Scenario of Exogenously Induced RNAi for Lepidopteran Agricultural Pest Control: From dsRNA Design to Topical Application

Invasive insects cost the global economy around USD 70 billion per year. Moreover, increasing agricultural insect pests raise concerns about global food security constraining and infestation rising after climate changes. Current agricultural pest management largely relies on plant breeding-with or without transgenes-and chemical pesticides. Both approaches face serious technological obsolescence in the field due to plant resistance breakdown or development of insecticide resistance. The need for new modes of action (MoA) for managing crop health is growing each year, driven by market demands to reduce economic losses and by consumer demand for phytosanitary measures. The disabling of pest genes through sequence-specific expression silencing is a promising tool in the development of environmentally-friendly and safe biopesticides. The specificity conferred by long dsRNA-base solutions helps minimize effects on off-target genes in the insect pest genome and the target gene in non-target organisms (NTOs). In this review, we summarize the status of gene silencing by RNA interference (RNAi) for agricultural control. More specifically, we focus on the engineering, development and application of gene silencing to control Lepidoptera through non-transforming dsRNA technologies. Despite some delivery and stability drawbacks of topical applications, we reviewed works showing convincing proof-of-concept results that point to innovative solutions. Considerations about the regulation of the ongoing research on dsRNA-based pesticides to produce commercialized products for exogenous application are discussed. Academic and industry initiatives have revealed a worthy effort to control Lepidoptera pests with this new mode of action, which provides more sustainable and reliable technologies for field management. New data on the genomics of this taxon may contribute to a future customized target gene portfolio. As a case study, we illustrate how dsRNA and associated methodologies could be applied to control an important lepidopteran coffee pest.

RNAi-induced knockdown of white gene in the southern green stink bug (Nezara viridula L.)

The southern green stink bug (SGSB) Nezara viridula L. is one of the most common stink bug species in the United States and can cause significant yield loss in a variety of crops. A suitable marker for the assessment of gene-editing tools in SGSB has yet to be characterized. The white gene, first documented in Drosophila, has been a useful target to assess the efficiency of introduced mutations in many species as it controls pigmentation processes and mutants display readily identifiable phenotypes. In this study we used the RNAi technique to investigate functions and phenotypes associated with the white ortholog in the SGSB and to validate white as a marker for genetic transformation in this species. This study revealed that white may be a suitable marker for germline transformation in the SGSB as white transcript knockdown was not lethal, did not impair embryo development and provided a distinguishable phenotype. Our results demonstrated that the white ortholog in SGSB is involved in the pathway for ommochrome synthesis and suggested additional functions of this gene such as in the integument composition, management of hemolymph compounds and riboflavin mobilization.

RNAi in Piezodorus guildinii (Hemiptera: Pentatomidae): Transcriptome Assembly for the Development of Pest Control Strategies

Red-banded stink bug Piezodorus guildinii (P. guildinii) has been described as the most damaging stink bug regarding soybean crops, leading to seed injury, low germination percentages, and foliar retention, at low population densities. In recent years, RNA interference (RNAi), a conserved eukaryote silencing mechanism has been explored to develop species-selective pesticides. In this work, we evaluated RNAi in P. guildinii to develop new pest-control strategies. For this, we assembled and annotated a P. guildinii transcriptome from a pool of all developmental stages. Analysis of this transcriptome led to the identification of 56 genes related to the silencing process encompassing siRNA, miRNA, and piRNA pathways. To evaluate the functionality of RNAi machinery, P. guildinii adults were injected with 28 ng/mg of body weight of double stranded RNA (dsRNA) targeting vATPase A. A mortality of 35 and 51.6% was observed after 7 and 14 days, respectively, and a downregulation of vATPase A gene of 84% 72 h post-injection. In addition, Dicer-2 and Argonaute-2 genes, core RNAi proteins, were upregulated 1.8-fold 48 h after injection. These findings showed for the first time that RNAi is functional in P. guildinii and the silencing of essential genes has a significant effect in adult viability. Taken together, the work reported here shows that RNAi could be an interesting approach for the development of red-banded stink bug control strategies.

Dynamics in Pest Status of Phytophagous Stink Bugs in the Neotropics

In this review article, we present and discuss the main factors influencing the change in pest status of phytophagous stink bugs (Hemiptera: Heteroptera: Pentatomidae) in the Neotropics. We have surveyed the published records over the past 50 years and divided this timeframe into decades. This was done to rank in time the relative abundance (percentage) of the following species, known pests of commodities, in the Neotropical Region: the Neotropical brown stink bug, Euschistus heros (F.); the green-bellied stink bugs, Diceraeus melacanthus Dallas and D. furcatus (F.); the redbanded stink bug, Piezodorus guildinii (Westwood); the southern green stink bug, Nezara viridula (L.); and the brown-winged stink bug, Edessa meditabunda (F.). The analysis showed that E. heros, D. melacanthus, and D. furcatus, formerly minor pests, in the last decade (2010s) became major pests. The once most important pest species, N. viridula and P. guildinii, decreased their pest status in the last decade. Edessa meditabunda, which never achieved high populations, showed a tendency to increase in abundance in the last two decades (2000s and 2010s). Major factors believed to influence the dynamics of pest populations of stink bugs in the Neotropics include cropping systems (no-tillage replacing conventional soil plowing, and crop rotation); genetically modified (GM) plants (mostly plants expressing insecticidal crystalline proteins derived from Bacillus thuringiensis Berliner - Bt); change in availability of host and associated plants in the new landscape scenario; increased usage of chemicals (insecticides, fungicides, and herbicides); and change in the role of natural enemies in modern day agriculture.

First Evidence of Feeding-Induced RNAi in Banana Weevil via Exogenous Application of dsRNA

Banana weevil (Cosmopolites sordidus) is the most devastating pest of banana and plantain worldwide, yet current control measures are neither effective, sustainable, nor environmentally sound, and no resistant farmer-preferred cultivars are known to date. In this paper, we examined the ability to induce RNA interference (RNAi) in the banana weevil via feeding. We first developed an agar- and banana corm (rhizome) flour-based artificial diet in a multi-well plate setup that allowed the banana weevils to complete their life cycle from egg through the larval instars to the pupal stage in an average period of 53 days. Adults emerged about 20 days later. The artificial diet allowed the tunneling and burrowing habits of the larvae and successful metamorphosis up to adult eclosion. Adding dsRNA for laccase2 to the artificial diet resulted in albino phenotypes, confirming gene-silencing. Finally, C. sordidus was fed with dsRNA against a selection of essential target genes: snf7, rps13, mad1, vha-a, vha-d, and lgl for a period of 45 days. 100% mortality within 9-16 days was realized with dssnf7, dsrps13, and dsmad1 at 200 ng/mL artificial diet, and this corresponded to a strong reduction in gene expression. Feeding the dsRNA targeting the two vha genes resulted in 100% mortality after about 3-4 weeks, while treatment with dslgl resulted in no mortality above the dsgfp-control and the water-control. Our results have implications for the development of RNAi approaches for managing important crop pests, in that banana weevils can be controlled based on the silencing of essential target genes as snf7, rps13, and mad1. They also highlight the need for research into the development of RNAi for banana protection, eventually the engineering of host-induced gene-silencing (HIGS) cultivars, given the high RNAi efficacy and its species-specific mode of action, adding the RNAi approach to the armory of integrated pest management (IPM).

Establishing RNAi for basic research and pest control and identification of the most efficient target genes for pest control: a brief guide

RNA interference (RNAi) has emerged as a powerful tool for knocking-down gene function in diverse taxa including arthropods for both basic biological research and application in pest control. The conservation of the RNAi mechanism in eukaryotes suggested that it should-in principle-be applicable to most arthropods. However, practical hurdles have been limiting the application in many taxa. For instance, species differ considerably with respect to efficiency of dsRNA uptake from the hemolymph or the gut. Here, we review some of the most frequently encountered technical obstacles when establishing RNAi and suggest a robust procedure for establishing this technique in insect species with special reference to pests. Finally, we present an approach to identify the most effective target genes for the potential control of agricultural and public health pests by RNAi.

Innate and adaptive resistance to RNAi: a major challenge and hurdle to the development of double stranded RNA-based pesticides

RNA interference (RNAi) is a post-transcriptional gene silencing process where short interfering RNAs degrade targeted mRNA. Exploration of gene function through reverse genetics is the major achievement of RNAi discovery. Besides, RNAi can be used as a potential strategy for the control of insect pests. This has led to the idea of developing RNAi-based pesticides. Differential RNAi efficiency in the different insect orders is the biggest biological obstacle in developing RNAi-based pesticides. dsRNA stability, the sensitivity of core RNAi machinery, uptake of dsRNA and amplification and spreading of the RNAi signal are the key factors responsible for RNAi efficiency in insects. This review discusses the physiological and adaptive factors responsible for reduced RNAi in insects that pose a major challenge in developing dsRNA- based pesticides.

Suppression of Bta11975, an α-glucosidase, by RNA interference reduces transmission of tomato chlorosis virus by Bemisia tabaci

BACKGROUND

Tomato chlorosis virus (ToCV) is mainly vectored by Bemisia tabaci in China, which has a worldwide distribution, and greatly reduces the yields of tomato and other vegetables. At present, control of ToCV has been focused mainly by the use of insecticides to control whitefly populations. Transcriptome sequencing showed high expression of the B. tabaci Bta11975 gene, an α-glucosidase (AGLU) during ToCV acquisition by whitefly Mediterranean (MED) species. To investigate the role of Bta11975 gene in ToCV acquisition and transmission by B. tabaci MED, we used RNA interference (RNAi) to reduce the expression of the Bta11975 gene.

RESULTS

The relative expression of the Bta11975 gene was correlated with the ToCV content in B. tabaci. The AGLU is highly expressed in primary salivary gland and gut. After the Bta11975 gene was silenced, the gene expression of B. tabaci was reduced and B. tabaci mortality was increased. Besides, ToCV acquisition by B. tabaci at 48 and 72 h AAP was reduced, and ToCV transmission was significantly reduced by 25 or 50 of B. tabaci.

CONCLUSIONS

These results indicate that suppression of expression of the Bta11975 gene in B. tabaci MED by RNAi can reduce acquisition and transmission of ToCV by B. tabaci MED.

Identification and characterization of an Apis cerana cerana nucleoside diphosphate kinase (AccNDPK) associated with oxidative stress

Nucleoside diphosphate kinases (NDPKs) are widespread nucleotide-metabolizing enzymes that are involved in a variety of biological processes, including responses to oxidative stress. Although studies have been conducted on NDPKs in mammals and some plants, there is scant research on insect NDPKs, especially in honey bees. In the present study, we isolated AccNDPK from Apis cerana cerana. Sequence analysis showed that AccNDPK has high homology with many NDPKs and contains a highly conserved NDPK active site motif. Based on phylogenetic analysis, AccNDPK has a relatively recent evolutionary relationship with NDPKs in other hymenopteran insects. AccNDPK was found to be highly expressed in newly emerged honey bees and muscle tissues, and RT-qPCR analysis and bacteriostatic assays showed that the expression level of AccNDPK is affected by abnormal temperature, UV light, H₂O₂, heavy metals, and various pesticides. After AccNDPK knockdown, antioxidant-related genes, including AccCAT, AccCYP4G11, AccGSTS4, AccTpx1 and AccMsrA, were upregulated, whereas AccGSTD, AccGST1, AccHSP22.6 and AccTrx1 were downregulated. Furthermore, catalase (CAT), superoxide dismutase (SOD), and peroxidase (POD) activities were significantly increased, and the tolerance of bees to oxidative stress caused by cyhalothrin was reduced by silencing of AccNDPK. Given these findings, we speculate that AccNDPK plays an important role in the oxidative stress response of A. cerana cerana.

dsRNA-Mediated Pest Management of Tuta absoluta Is Compatible with Its Biological Control Agent Nesidiocoris tenuis

Simple Summary

The zoophytophagous mirid bug Nesidiocoris tenuis is an efficient predator of the tomato leafminer, Tuta absoluta. RNA interference (RNAi) targeting the alphaCOP (αCOP) (Coatomer subunit alpha protein) gene of N. tenuis (Nt-αCOP) was proven to be functional in N. tenuis, causing downregulation of gene expression, mortality and sub-lethal effects. In contrast, when N. tenuis were fed with dsRNA (dsTa-αCOP) targeting the ortholog αCOP gene of T. absoluta, no lethal nor sub-lethal effects were observed. These results indicate the compatibility of this biocontrol agent along with RNAi-mediated management in order to suppress T. absoluta efficiently in tomato crop.

Abstract

RNAi-mediated insect pest management has recently shown promising results against the most serious pest of tomato, the tomato leafminer, Tuta absoluta. This study aimed to investigate whether dsRNA (dsTa-αCOP) designed to target the T. absoluta-αCOP gene could cause adverse effects to its biocontrol agent, the mirid predator, Nesidiocoris tenuis. Oral exposure of N. tenuis to dsRNA (dsNt-αCOP) designed to target N. tenuis-αCOP resulted in a 61%, 67% and 55% reduction in its transcript level in comparison to the sucrose, dsGFP and dsTa-αCOP treatments, respectively. In addition, significantly higher mortality of 57% was recorded in dsNt-αCOP-treated N. tenuis when compared to the sucrose (7%), dsGFP (10%) and dsTa-αCOP (10%) treatments. Moreover, the predation rate of ~33–39 Ephestia kuehniella eggs per N. tenuis adult dramatically reduced to almost half in the surviving dsNt-αCOP-treated N. tenuis. This worst-case exposure scenario confirmed for the first time that the RNAi machinery is functional in this species and that the risk of exposure through the oral route is possible. In contrast, dsTa-αCOP did not cause any sub-lethal effects to N. tenuis upon oral exposure. Oral exposure of T. absoluta to dsTa-αCOP resulted in 50% mortality. In the context of a biosafety risk assessment of RNAi-mediated insect management, investigating the effects on non-target organisms is essential in order to include this method as part of an integrated pest management strategy. Based on our laboratory assays, RNAi-mediated control is compatible with the biological control of T. absoluta by its natural enemy N. tenuis, adding the RNAi approach in the armoire of integrated pest management of T. absoluta.

Silencing of Double-Stranded Ribonuclease Improves Oral RNAi Efficacy in Southern Green Stinkbug Nezaraviridula

Variability in RNA-interference (RNAi) efficacy among different insect orders poses a big hurdle in the development of RNAi-based pest control strategies. The activity of double-stranded ribonucleases (dsRNases) in the digestive canal of insects can be one of the critical factors affecting oral RNAi efficacy. Here, the involvement of these dsRNases in the southern green stinkbug Nezaraviridula was investigated. First, the full sequence of the only dsRNase (NvdsRNase) in the transcriptome of N. viridula was obtained, followed by an oral feeding bioassay to evaluate the effect of NvdsRNase-silencing on oral RNAi efficacy. The NvdsRNase was first silenced in nymphs by NvdsRNase-dsRNA injections, followed by exposure to an artificial diet containing a lethal αCop-specific dsRNA. A significantly higher mortality was observed in the NvdsRNase-silenced nymphs when placed on the dsαCop-containing diet (65%) than in the dsGFP injected and dsαCop fed control (46.67%). Additionally, an ex vivo dsRNA degradation assay showed a higher stability of dsRNA in the saliva and midgut juice of NvdsRNase-silenced adults. These results provide evidence for the involvement of NvdsRNase in the reduction of oral RNAi efficacy in N. viridula. This information will be useful in further improving potential RNAi-based strategies to control this pest.

Effectiveness of orally-delivered double-stranded RNA on gene silencing in the stinkbug Plautia stali

Development of a reliable method for RNA interference (RNAi) by orally-delivered double-stranded RNA (dsRNA) is potentially promising for crop protection. Considering that RNAi efficiency considerably varies among different insect species, it is important to seek for the practical conditions under which dsRNA-mediated RNAi effectively works against each pest insect. Here we investigated RNAi efficiency in the brown-winged green stinkbug Plautia stali, which is notorious for infesting various fruits and crop plants. Microinjection of dsRNA into P. stali revealed high RNAi efficiency-injection of only 30 ng dsRNA into last-instar nymphs was sufficient to knockdown target genes as manifested by their phenotypes, and injection of 300 ng dsRNA suppressed the gene expression levels by 80% to 99.9%. Knockdown experiments by dsRNA injection showed that multicopper oxidase 2 (MCO2), vacuolar ATPase (vATPase), inhibitor of apoptosis (IAP), and vacuolar-sorting protein Snf7 are essential for survival of P. stali, as has been demonstrated in other insects. By contrast, P. stali exhibited very low RNAi efficiency when dsRNA was orally administered. When 1000 ng/μL of dsRNA solution was orally provided to first-instar nymphs, no obvious phenotypes were observed. Consistent with this, RT-qPCR showed that the gene expression levels were not affected. A higher concentration of dsRNA (5000 ng/μL) induced mortality in some cohorts, and the gene expression levels were reduced to nearly 50%. Simultaneous oral administration of dsRNA against potential RNAi blocker genes did not improve the RNAi efficiency of the target genes. In conclusion, P. stali shows high sensitivity to RNAi with injected dsRNA but, unlike the allied pest stinkbugs Halyomorpha halys and Nezara viridula, very low sensitivity to RNAi with orally-delivered dsRNA, which highlights the varied sensitivity to RNAi across different species and limits the applicability of the molecular tool for controlling this specific insect pest.

Parental RNA interference as a tool to study genes involved in rostrum development in the Neotropical brown stink bug, Euschistus heros

In insects, the identity of body segments is controlled by homeotic genes and the knockdown of these genes during embryogenesis can lead to an abnormal development and/or atypical phenotypes. The main goal of this study was to investigate the involvement of labial (lab), deformed (dfd), sex comb reduced (scr), extradenticle (exd) and proboscipedia (pb) in rostrum development in the Neotropical brown stink bug Euschistus heros, using parental RNAi (pRNAi). To achieve this objective, 10-days-old adult females were first microinjected with double-stranded RNAs (dsRNA) targeting these five genes. Then, the number of eggs laid per female, the percentage of hatched nymphs with normal or abnormal phenotype and target gene silencing were evaluated. Except for the dsDfd-treatment, the number of eggs laid per female per day was not affected by the different dsRNA-treatments compared to the control (dsGFP). However, treatment with either dsLab, dsDfd, dsScr or dsExd caused a strong reduction in egg hatching. The dsExd-treatment caused no apparent change in phenotype in the nymphs while hatched nymphs from the dsDfd, dsScr and dsPb-treatment showed abnormalities in the rostrum. Particularly for the dsPb-treatment, 91% of the offspring displayed a bifurcated rostrum with a leg-like structure. Overall, these results indicate that these five genes are involved in E. heros embryonic development and that the knockdown of dfd, scr and pb leads to an abnormal development of the rostrum. Additionally, this study demonstrates the efficiency of pRNAi in studying genes involved in embryogenesis in E. heros, with clear phenotypes and a strong target gene silencing in the next generation, after treatment of the parent female adult with gene-specific dsRNA.