Identification of the ryanodine receptor mutation I4743M and its contribution to diamide insecticide resistance in Spodoptera exigua (Lepidoptera: Noctuidae)

Loop-Mediated Isothermal Amplification for Detecting Gly-4891-Glu and Ile-4734 Multiple Mutations of Ryanodine Receptor in the Fall Armyworm, Spodoptera frugiperda

The key mutations, such as the Gly-4891-Glu substitution and the Ile-4734 multiple substitutions within the ryanodine receptors (RyR), are linked to diamide resistance in fall armyworm (FAW), Spodoptera frugiperda. In this study, we found that FAW remained sensitive to cyantraniliprole and chlorantraniliprole, while its sensitivity to flubendiamide was reduced. Moreover, a low level of heterozygous mutation at I4743 was observed. To facilitate the detection procedure of these mutations, a simple and efficient loop-mediated isothermal amplification (LAMP) protocol was developed for operation. The reaction for detecting the G4891E and I4743 single or multiple mutations was carried out at 68 °C for 85 min and 68 °C for 85 min or 68 °C for 65 min, respectively. These LAMP reactions can be easily observed via visualization of the color change from pink to yellow. This assay provides a simple, convenient, and effective means of detecting mutations in the RyR of FAW for pest management purposes.

Identification and detection of the V1848I indoxacarb resistance mutation in the beet armyworm, Spodoptera exigua

Indoxacarb is a pivotal insecticide used worldwide to manage Spodoptera exigua, a devastating agricultural pest. This active compound plays a crucial role in resistance management strategies due to its distinctive mode of action. A field population of S. exigua (SH23) from Shanghai, China, exhibited significantly reduced susceptibility to indoxacarb, with a resistance ratio of 113.84-fold in biological assays. Following two rounds of laboratory screening with indoxacarb, the resistance of the new strain (SH23-S2) escalated steeply to 876.15-fold. Genetic analyses of both the SH23 and SH23-S2 strains demonstrated autosomal inheritance and incompletely dominant resistance patterns. Synergist assays indicated a minor role of detoxification enzymes (glutathione s-transferases and cytochrome P450) of SH23-S2 strain in this resistance, implicating target-site resistance as the primary mechanism. To explore the impact of target-site resistance, segment 1-6 of domain IV (IVS1-6) of the sodium channel in S. exigua was cloned, and the sequences from susceptible and indoxacarb-resistant S. exigua were compared. The V1848I mutation, linked to indoxacarb resistance in Plutella xylostella, Tuta absoluta and Liriomyza trifolii, was identified and strongly associated with the indoxacarb-resistant phenotype in the S. exigua SH23-S2 strain, whereas the F1845Y mutation was not detected. Furthermore, a molecular test for the V1848I mutation in field populations was created using an allele-specific PCR (AS-PCR). The discovery of indoxacarb resistance mutation and the creation of diagnostic tool will enable the early detection of indoxacarb resistance, which will facilitate the implementation of targeted resistance management strategies, ultimately delaying the proliferation of resistance.

Design, Synthesis and Insecticidal Activity of Novel meta-Diamide Compounds Containing Pyridine Rings

In order to discover new meta-diamide compounds with good activity and novel structure, 15 related compounds were designed and synthesized by the bioisosterism principle with cyproflanilide as the lead compound. The insecticidal activities of these compounds against Plutella xylostella and Tetranychus cinnabarinus were tested, and the results of biological activity test showed that some compounds had more than 90 % insecticidal activity against Plutella xylostella at 1 mg/L and Tetranychus cinnabarinus at 100 mg/L. Especially, N-(2-bromo-6-(difluoromethoxy)-4-(perfluoro propan-2-yl)phenyl)-6-(isonicotinamido)picolinamide against Tetranychus cinnabarinus at 10 mg/L was 100 %, which was better than that of cyproflanilide. Molecular docking studies suggested that N-(2-bromo-6-(difluoromethoxy)-4-(perfluoropropan-2-yl)phenyl)-6-(4-cyano-2-methylbenzamido)picolinamide had a closely combined with the Plutella xylostella 3RHW (a glutamate-gated chloride channel). This study provides an avenue for designing and synthesizing a new generation of more effective pesticides.

Recent developments in fluorine-containing pesticides

To ensure ongoing sustainability, the modern agrochemical industry is faced with enormous challenges. These arise from provision of high-quality food to increasing water use and environmental impact as well as a growing world population. The loss of previous agrochemicals due to consumer perception, changing grower needs and ever-changing regulatory requirements is higher than the number of active ingredients that are being introduced into the crop protection market. Therefore, the development of novel agrochemicals is essential to provide improved efficacy and environmental profiles. In this context, the introduction of fluorine atoms and fluorine-containing motifs into a molecule is an important method to influence its physicochemical properties. These include, for example, small difluoro- and trifluoromethyl, or trifluoromethoxy groups at aryl or heterocyclic aryl moieties but also fragments like 2,2,2-trifluoroethoxycarbonyl, trifluoromethylsulfonyl, trifluoroacetyl, as well as the so far unusal rest like heptafluoro-iso-propyl. This review gives an overview of recent developments of fluorine-containing pesticides launched over the past 7 years and describes a selection of current fluorine-containing development candidates. © 2023 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Key role of the ryanodine receptor I4790K mutation in mediating diamide resistance in Plutella xylostella

The diamondback moth Plutella xylostella, a global insect pest of cruciferous vegetables, has evolved resistance to many classes of insecticides including diamides. Three point mutations (I4790M, I4790K, and G4946E) in the ryanodine receptor of P. xylostella (PxRyR) have been identified to associate with varying levels of resistance. In this study, we generated a knockin strain (I4790K-KI) of P. xylostella, using CRISPR/Cas9 to introduce the I4790K mutation into PxRyR of the susceptible IPP-S strain. Compared to IPP-S, the edited I4790K-KI strain exhibited high levels of resistance to both anthranilic diamides (chlorantraniliprole 1857-fold, cyantraniliprole 1433-fold) and the phthalic acid diamide flubendiamide (>2272-fold). Resistance to chlorantraniliprole in the I4790K-KI strain was inherited in an autosomal and recessive mode, and genetically linked with the I4790K knockin mutation. Computational modeling suggests the I4790K mutation reduces the binding of diamides to PxRyR by disrupting key hydrogen bonding interactions within the binding cavity. The approximate frequencies of the 4790M, 4790K, and 4946E alleles were assessed in ten geographical field populations of P. xylostella collected in China in 2021. The levels of chlorantraniliprole resistance (2.3- to 1444-fold) in these populations were significantly correlated with the frequencies (0.017-0.917) of the 4790K allele, but not with either 4790M (0-0.183) or 4946E (0.017-0.450) alleles. This demonstrates that the PxRyR I4790K mutation is currently the major contributing factor to chlorantraniliprole resistance in P. xylostella field populations within China. Our findings provide in vivo functional evidence for the causality of the I4790K mutation in PxRyR with high levels of diamide resistance in P. xylostella, and suggest that tracking the frequency of the I4790K allele is crucial for optimizing the monitoring and management of diamide resistance in this crop pest.

Characterization of the inheritance of field-evolved resistance to diamides in the fall armyworm (Spodoptera frugiperda) (Lepidoptera: Noctuidae) population from Puerto Rico

The fall armyworm (Spodoptera frugiperda) is one of the most destructive pests of corn. New infestations have been reported in the East Hemisphere, reaching India, China, Malaysia, and Australia, causing severe destruction to corn and other crops. In Puerto Rico, practical resistance to different mode of action compounds has been reported in cornfields. In this study, we characterized the inheritance of resistance to chlorantraniliprole and flubendiamide and identified the possible cross-resistance to cyantraniliprole and cyclaniliprole. The Puerto Rican (PR) strain showed high levels of resistance to flubendiamide (RR50 = 2,762-fold) and chlorantraniliprole (RR50 = 96-fold). The inheritance of resistance showed an autosomal inheritance for chlorantraniliprole and an X-linked inheritance for flubendiamide. The trend of the dominance of resistance demonstrated an incompletely recessive trait for H1 (♂ SUS × ♀ PR) × and an incompletely dominant trait for H2 (♀ SUS × ♂ PR) × for flubendiamide and chlorantraniliprole. The PR strain showed no significant presence of detoxification enzymes (using synergists: PBO, DEF, DEM, and VER) to chlorantraniliprole; however, for flubendiamide the SR = 2.7 (DEM), SR = 3.2 (DEF) and SR = 7.6 (VER) indicated the role of esterases, glutathione S- transferases and ABC transporters in the metabolism of flubendiamide. The PR strain showed high and low cross-resistance to cyantraniliprole (74-fold) and cyclaniliprole (11-fold), respectively. Incomplete recessiveness might lead to the survival of heterozygous individuals when the decay of diamide residue occurs in plant tissues. These results highlight the importance of adopting diverse pest management strategies, including insecticide rotating to manage FAW populations in Puerto Rico and other continents.

Risk of resistance and the metabolic resistance mechanism of Laodelphax striatellus (Fallén) to cyantraniliprole

Cyantraniliprole is a highly effective diamide insecticide used to control of Laodelphax striatellus (Fallén). This study aimed to assess the insecticide resistance risk of L. striatellus and its metabolic resistance mechanisms. After 25 continuous generations of selection, the resistance of L. striatellus to cyantraniliprole increased by 17.14-fold. The realistic heritability of resistance was 0.0751. After successive rearing for five generations without exposure to insecticides, the resistance ratio for the resistant strain of L. striatellus decreased by 3.47-fold, and the average resistance decline rate per generation was 0.0266. Cyantraniliprole-resistant strains did not exhibit cross-resistance to triflumezopyrim, pymetrozine, flonicamid, sulfoxaflor, dinotefuran, clothianidin, thiamethoxam, nitenpyram, or imidacloprid. Compared to those of the sensitive strain, the 2nd, 3rd, and 4th instars, nymphal stage durations, total preoviposition period, and average generation time of the resistant strain were markedly reduced. Furthermore, the activity of cytochrome P450 monooxygenase (P450) and carboxylesterase (CarE) were markedly increased. The upregulation of CYP419A1v2 expression was most evident among the P450 genes, with a 6.10-fold increase relative to that in the sensitive strain. The CarE gene LsCarE5 was significantly upregulated by 1.94-fold compared with that in the sensitive strain. With the continuous use of cyantraniliprole, L. striatellus may develop resistance to this insecticide. This resistance may be related to the increase in metabolic enzyme activities regulated by the overexpression of P450 and CarE genes.

3D-QSAR Combination with Molecular Dynamics Simulations to Effectively Design the Active Ryanodine Receptor Agonists against Spodoptera frugiperda

Computer-aided molecular modeling was applied to design a series of Spodoptera frugiperda RyR agonists. Comparative molecular field analysis (CoMFA) and comparative molecular similarity indices analysis (CoMSIA) were used to generate 3D-QSAR models. MD simulations in the complex with S. frugiperda native, mutant RyR, and mammalian RyR1 under physiological conditions were used to validate the detailed binding mechanism. Binding free energy calculation by molecular mechanics generalized surface area (MM-GBSA) explained the role of key amino acid residues in ligand-receptor binding. Therefore, 14 new compounds were effectively designed and synthesized, and a bioassay indicated that compounds A-2 and A-3 showed comparable activity to that of chloranthraniliprole with LC50 values of 0.27, 0.18, and 0.20 mg L-1, respectively, against S. frugiperda. Most target compounds also displayed good activity against Mythinma separata at 0.1 mg L-1. Molecular docking and MM-GBSA calculations demonstrated that A-3 had a better binding capacity with native and mutant S. frugiperda RyRs.

SeBLOS2 knockout via CRISPR/Cas9 leads to the loss of larval integument coloration in Spodoptera exigua (Lepidoptera: Noctuidae)

CRISPR/Cas9 technology is a precise and powerful tool for functionally exploring insect genes. The present study tested CRISPR/Cas9 as a way of undertaking effective gene mutagenesis in an important agricultural pest, the beet armyworm Spodoptera exigua. Based on a S. exigua transcriptome database, the entire complementary DNA sequence of SeBLOS2 encoding 140 amino acid residues was cloned. The gene was highly expressed in late larval stages (L3-L5). Using the CRISPR/Cas9 method, SeBLOS2 was knocked out by altering two sites in the coding region. This resulted in 70%-74% of the G0 generation (L4-L5) larvae displaying mosaic translucent integument. Four different mutations occurred at SeBLOS2-specific target sites, as demonstrated by further polymerase chain reaction-based genotypic analysis. Homozygote mutant L3 larvae were obtained in the G1 generation, with complete loss of white stripes and spots on their larval integument. These results demonstrate a crucial role of SeBLOS2 in integument pigmentation and suggest that the gene can act as a suitable nonlethal marker for functional research on genes in S. exigua and other Lepidopteran pests.

The knockout of the nicotinic acetylcholine receptor subunit gene α1 (nAChR α1) through CRISPR/Cas9 technology exposes its involvement in the resistance of Spodoptera exigua to insecticides

Insect nicotinic acetylcholine receptors (nAChRs) are the directed targets of many insecticides. However, there have been no reports on the molecular characterization of the nAChR gene family or the causal association between nAChR α1 and resistance to insecticides in S. exigua, which is a significant agricultural pest. In this study, we identified a total of 9 candidate nAChR subunits in S. exigua, namely nAChR α1-α7 and nAChR β1-β2. For functional validation roles of Seα1 in insecticide resistance of S. exigua, we introduced a ∼ 1041-bp deletion of the Seα1 gene in a homozygous mutant strain (Seα1-KO) by CRISPR/Cas9 genome editing system, resulting in a premature truncation of the Seα1 protein and the subsequent loss of functional transmembrane (TM) 3 and TM4 elements. Compared with WH-S strain (wild-type strain), the Seα1-KO strain exhibited 2.62-folds resistant to trifluoropyrimidine, 8.3-folds resistant to dimehypo, and 5.28-folds resistant to dinotefuran, but no significant change in susceptibility to emamectin benzoate, spinetoram, lambda-cyhalothrin, permethrin and chlorpyrifos. Thus, this study has laid a solid foundation for investigating the role of nAChRs in S. exigua, and provides evidence for the crucial involvement of the α1 subunit in the mechanism of trifluoropyrimidine, dimehypo, and dinotefuran in S. exigua. Moreover, it provides a reference for the value of Seα1 subunit and its homologues in other species as insecticide targets.

The ryanodine receptor mutation I4728M confers moderate-level resistance to diamide insecticides in Spodoptera litura

BACKGROUND

The common cutworm, Spodoptera litura (Fabricius), is one of the most widespread and destructive polyphagous pests in tropical and subtropical Asia. S. litura has evolved resistance to different insecticides, including diamide insecticides. Here, we identified a ryanodine receptor (RyR) mutation (I4728M) associated with target site resistance to diamides in a field-collected population of S. litura. The contribution of this mutation to diamide resistance was investigated through establishing a near-isogenic resistant strain of S. litura.

RESULTS

The ND21 population of S. litura, collected from Ningde, Fujian province of China in 2021, exhibited 130.6-fold resistance to chlorantraniliprole compared to the susceptible NJ-S strain. S. litura RyR mutation I4728M, corresponding to Plutella xylostella RyR I4790M, was identified in the ND21 population. SlRyR I4728M mutation of ND21 was introgressed into a susceptible background strain (NJ-S) with marker-assisted backcrossing. The introgressed strain named ND21-R, which was homozygous for the mutant 4728M allele, shared about 94% of the genetic background with the NJ-S strain. ND21-R strain showed moderate levels of resistance to two anthranilic diamides (19.1-fold to chlorantraniliprole, 19.7-fold to cyantraniliprole) and the phthalic diamide flubendiamide (23.4-fold). Genetic analysis showed that chlorantraniliprole resistance was autosomal, incompletely recessive and tightly linked with SlRyR I4728M mutation in the introgressed ND21-R strain of S. litura.

CONCLUSION

Identification of the I4728M mutation and its contribution to diamide resistance in S. litura will help develop allelic discrimination assays for resistance monitoring and guide resistance management practices for diamides in S. litura. © 2023 Society of Chemical Industry.

Insecticide resistance monitoring of Cnaphalocrocis medinalis (Lepidoptera: Pyralidae) and its mechanism to chlorantraniliprole

BACKGROUND

The rice leaffolder, Cnaphalocrocis medinalis (Guenée), has become an increasingly occurring pest in Asia in recent years. Chemical control remains the most efficient and primary tool for controlling this pest. In this study, we report the resistance status of C. medinalis in China to multiple insecticides including chlorantraniliprole and the main resistance mechanism.

RESULTS

Significant variations among field populations of C. medinalis in their resistance to 10 insecticides were observed during 2019-2022. Most of the tested field populations have developed low-to-moderate levels of resistance to abamectin (RR = 2.4-22.2), emamectin benzoate (RR = 1.9-40.3) and spinetoram (RR = 4.2-24.8). Some field populations have developed low resistance to chlorpyrifos (RR = 0.9-6.8). Indoxacarb, metaflumizone, methoxenozide and Bacillus thuringiensis (Bt) potency against all tested populations remained similar. For diamides, significantly higher levels of resistance to chlorantraniliprole (RR = 64.9-113.7) were observed in 2022, whereas all tested field populations in 2019-2021 exhibited susceptible or moderate resistance level to chlorantraniliprole (RR = 1.3-22.1). Cross-resistance between chlorantraniliprole and tetraniliprole was significant. Analysis of ryanodine receptor (RyR) mutations showed that mutation of I4712M was present in resistant populations of C. medinalis with different levels of chlorantraniliprole resistance and was the main mechanism conferring diamide resistance. Mutation of Y4621D also was detected in one tested population. Resistance management strategies for the control of C. medinalis are discussed.

CONCLUSION

C. medinalis has developed high level of resistance to chlorantraniliprole. RyR mutations were deemed as the mechanism. © 2023 Society of Chemical Industry.

Exaptation of I4760M mutation in ryanodine receptor of Spodoptera exigua (Lepidoptera: Noctuidae): Lessons from museum and field samples

Since 2007, diamide insecticides have been widely used in Korea to control various types of lepidopteran pests including Spodoptera exigua. For nearly a decade, diamide resistance in field populations of S. exigua across 18 localities has been monitored using bioassays. Despite their short history of use, resistance to diamide insecticides has emerged. Based on the LC50 values, some field populations showed a higher level of resistance to chlorantraniliprole, a diamide insecticide, compared to that of the susceptible strain, although regional and temporal variations were observed. To investigate resistance at a molecular level, we examined three mutations (Y4701C, I4790M, and G4946E) in the ryanodine receptor (RyR), which is the primary mechanism underlying diamide insecticide resistance. DNA sequencing showed that only the I4790M mutation was found in most field populations. As resistance levels varied significantly despite the uniform presence of the I4790M mutation, we considered the presence of another resistance factor. Further, the I4790M mutation was also found in S. exigua specimens collected prior to the commercialization of diamide insecticides in Korea as well as in other countries, such as the USA. This finding led us to hypothesize that the I4790M mutation were predisposed in field populations owing to selection factors other than diamide use. For further clarification, we conducted whole-genome sequencing of S. exigua (449.83 Mb) and re-sequencing of 18 individual whole genomes. However, no additional non-synonymous mutations were detected in the RyR-coding region. Therefore, we concluded that the high level of diamide insecticide resistance in Korean S. exigua is not caused by mutations at the target site, RyR, but is attributed to other factors that need to be investigated in future studies.

Investigating the Regulatory Mechanism of the Sesquiterpenol Nerolidol from a Plant on Juvenile Hormone-Related Genes in the Insect Spodoptera exigua

Various plant species contain terpene secondary metabolites, which disrupt insect growth and development by affecting the activity of juvenile hormone-degrading enzymes, and the juvenile hormone (JH) titers maintained in insects. Nerolidol, a natural sesquiterpenol belonging to the terpenoid group, exhibits structural similarities to insect JHs. However, the impact of nerolidol on insect growth and development, as well as its underlying molecular mechanism, remains unclear. Here, the effects of nerolidol on Spodoptera exigua were investigated under treatment at various sub-lethal doses (4.0 mg/mL, 1.0 mg/mL, 0.25 mg/mL). We found that a higher dose (4.0 mg/mL) of nerolidol significantly impaired the normal growth, development, and population reproduction of S. exigua, although a relatively lower dose (0.25 mg/mL) of nerolidol had no significant effect on this growth and development. Combined transcriptome sequencing and gene family analysis further revealed that four juvenile hormone esterase (JHE)-family genes that are involved in juvenile hormone degradation were significantly altered in S. exigua larvae after nerolidol treatment (4.0 mg/mL). Interestingly, the juvenile hormone esterase-like (JHEL) gene Sexi006721, a critical element responsive to nerolidol stress, was closely linked with the significant augmentation of JHE activity and JH titer in S. exigua (R2 = 0.94, p < 0.01). Taken together, we speculate that nerolidol can function as an analog of JH by modulating the expression of the enzyme genes responsible for degrading JH, resulting in JH disorders and ultimately disrupting the development of insect larvae. This study ultimately provides a theoretical basis for the sustainable control of S. exigua in the field whilst proposing a new perspective for the development of novel biological pesticides.

CRISPR/Cas9-mediated methoprene-tolerant 1 knockout results in precocious metamorphosis of beet armyworm (Spodoptera exigua) only at the late larval stage

Juvenile hormone (JH) controls almost every aspect of an insect, especially metamorphosis. Since RNA interference works on transcripts and is often insufficient in Lepidoptera, how JH affects larval development in these insects is not well studied. Using the CRISPR/Cas9 technique, we knocked out Spodoptera exigua methoprene-tolerant 1 (SeMet1) gene of beet armyworm by modifying two sites in the coding region. However, SeMet1 knockout did not affect egg hatch rate or larval development at L1-L3 stages. In contrast to the consistent five larval instars of the control group, L4 SeMet1 mutants began to show signs of precocious metamorphosis, that is, small patches of pupal cuticle. Most L4 and all L5 SeMet1 mutants died for failing to shed their mosaic cuticles. RNA-seq indicated that most genes encoding pupal cuticle proteins and chitinase genes were altered in SeMet1 mutant L4 larvae. SeKr-h1, a key transcription factor in JH action was significantly down-regulated in L3-L5 larvae, while SeBR-C, a pupal indicator was only upregulated in L4-L5 larvae. These results suggested that S. exigua larvae may initially develop independently of JH, and involve SeMet1 in transducing JH signalling, leading to controlled larval metamorphosis at the late larval stage. We believe our findings will enhance better understanding of JH regulation of larval development.

Comprehensive Overview of Diamide Derivatives Acting as Ryanodine Receptor Activators

The world's hunger is continuously rising due to conflicts, climate change, pandemics (such as the recent COVID-19), and crop pests and diseases. It is widely accepted that zero hunger is impossible without using agrochemicals to control crop pests and diseases. Diamide insecticides are one of the widely used green insecticides developed in recent years and play important roles in controlling lepidopteran pests. Currently, eight diamine insecticides have been commercialized, which target the insect ryanodine receptors. This review summarizes the development and optimization processes of diamide derivatives acting as ryanodine receptor activators. The review also discusses pest resistance to diamide derivatives and possible solutions to overcome the limitations posed by the resistance. Thus, with reference to structural biology, this study provides an impetus for designing and developing diamide insecticides with improved insecticidal activities.

Equivalent intensity but differential dominance of sodium channel blocker insecticide resistance conferred by F1845Y and V1848I mutations of the voltage-gated sodium channel in Plutella xylostella

Two point mutations (F1845Y and V1848I) in the voltage-gated sodium channel gene of Plutella xylostella are involved in the target-site resistance to sodium channel blocker insecticides (SCBIs). The contribution of the individual mutations to the SCBI resistance and the associated inheritance modes is as yet unclear. Through 2 rounds of single-pair crossing and marker-assisted selection, 2 P. xylostella strains (1845Y and 1848I) bearing homozygous F1845Y or V1848I mutant alleles were successfully established from a field-collected population, and the contribution of each mutation to SCBI resistance, as well as associated inheritance patterns, was determined. When compared with the susceptible SZPS strain, each of the mutations individually conferred equally high-level resistance to indoxacarb (378 and 313 fold) and metaflumizone (734 and 674 fold), respectively. However, dominance levels of resistance to SCBIs were significantly different between the 2 resistant strains. Resistance of the 1845Y strain to indoxacarb and metaflumizone was inherited as an autosomal and incompletely dominant trait (D values ranged from 0.43 to 0.76). In contrast, that of the 1848I strain followed an autosomal but incompletely recessive to semidominant mode (D values: -0.24 to 0.09). Our findings enriched the current understanding of inheritance and mechanisms of SCBI resistance in P. xylostella, and will help develop resistance management programs for P. xylostella and other economic pests.

Calmodulin Modulation of Insect Ryanodine Receptors

Ryanodine receptor (RyR) is a giant calcium release channel located on the membrane of the endoplasmic reticulum (ER). Here, we report the regulation of RyRs from two major agricultural pests, diamondback moth and fall armyworm, by insect calmodulin (CaM). The recombinantly expressed full-length insect RyR could be pulled down by insect CaM in the presence of Ca²⁺, but the efficiency is lower compared to rabbit RyR1 and insect RyR with the CaM-binding domain (CaMBD) replaced by rabbit RyR1 sequence. Interestingly, the enhanced binding of CaM in the mutant insect RyR resulted in an increased sensitivity to the diamide insecticide chlorantraniliprole (CHL), suggesting that this CaM-CaMBD interface could be targeted by potential synergists acting as molecular glue. The thermodynamics of the binding between insect CaM and CaMBD was characterized by isothermal titration calorimetry, and the key residues responsible for the insect-specific regulation were identified through mutagenesis studies.

Detection of ryanodine receptor G4911E and I4754M mutation sites and analysis of binding modes of diamide insecticides with RyR on Galeruca daurica (Coleoptera: Chrysomelidae)

In recent years, the leaf beetle Galeruca daurica has broken out in the northern grasslands of Inner Mongolia, its management still mainly depends on chemical control using traditional insecticides or with novel action. The study was aim to identify mutation locus associated with resistance to diamide insecticides in field population of G. daurica, to provide a reference for rational selection of insecticides and to avoid the rapid resistance development to diamide insecticides. We cloned the full length of the ryanodine receptor gene of G. daurica (GdRyR), constructed 3D model and transmembrane regions by homologous modeling based on deduced amino acid sequence. Two potential mutation loci (Gly4911Glu and Ile4754Met) and allelic mutation frequencies were detected in individuals of G. daurica. In addition, their binding patterns to two diamide insecticides (chlorantraniliprole, cyantraniliprole) were analyzed separately using a molecular docking method. The full-length cDNA sequence of GdRyR (GenBank accession number: OP828593) was obtained by splicing and assembling, which is 15,399 bp in length and encodes 5,133 amino acids. The amino acid similarity of GdRyR with that of other Coleopteran insects were 86.70%-91.33%, which possessed the typical structural characteristics. An individual resistance allelic mutation frequency test on fifty field leaf beetles has identified 12% and 32% heterozygous individuals at two potential mutation loci Gly4911Glu and Ile4754Met, respectively. The affinity of the I4754M mutant model of GdRyR for chlorantraniliprole and cyantraniliprole was not significantly different from that of the wild type, and all had non-covalent interactions such as hydrogen bonding, hydrophobic interactions and π-cation interactions. However, the G4911E mutant model showed reduced affinity and reduced mode of action with two diamide insecticides, thus affecting the binding stability of the ryanodine receptor to the diamide insecticides. In conclusion, the G4911E mutation in GdRyR may be a potential mechanism for the development of resistance to diamide insecticides on G. daurica and should be a key concern for resistance risk assessment and reasonable applications of diamide insecticides for control in future. Moreover, this study could provide a reference for ryanodine receptor structure-based insecticides design.

3D-QSAR Directed Discovery of Novel Halogenated Phenyl 3-Trifluoroethoxypyrazole Containing Ultrahigh Active Insecticidal Anthranilic Diamides

Pests are one of the major factors causing crop damage and food security problems worldwide. Based on our previous studies on the discovery of insecticidal leads targeting the ryanodine receptors (RyRs), a three-dimensional quantitative structure-activity relationship (3D-QSAR) model was established to design and synthesize a series of anthranilic diamides containing a halogenated phenyl 3-trifluoroethoxypyrazole moiety. The preliminary bioassays disclosed that IIb, IIIb, and IIIf against Mythimna separata showed comparable activity to chloranthraniliprole (LC₅₀: 0.16, 0.16, 0.14, and 0.13 mg·L^-1, respectively). More than half of the target compounds displayed good activity against Plutella xylostella, where IIIf was the most active compound, 25 times more active than chloranthraniliprole (LC₅₀: 6.0 × 10^-6 versus 1.5 × 10^-4 mg·L^-1). For Spodoptera frugiperda, IIIf displayed slightly inferior potency to chlorantraniliprole (LC₅₀: 0.47 versus 0.31 mg·L^-1). For RyR mutants of S. frugiperda (G4891E, I4734M), compound IIIf could show higher affinity than chlorantraniliprole according to the binding mode and energy in molecular docking experiments. Calcium imaging technique, molecular docking, density functional theory calculations, and electrostatic potential studies validated that the RyR was the target of the most active candidate IIIf, which deserves further development.

Functional Characterization of the Ryanodine Receptor Gene in Diaphorina citri

The Asian citrus psyllid Diaphorina citri (Hemiptera: Liviidae) is a major citrus pest spread around the world. It is also a vector of the bacterium 'Candidatus Liberibacter asiaticus', considered the cause of the fatal citrus disease huanglongbing (HLB). Insect ryanodine receptors (RyRs) are the primary target sites of diamide insecticides. In this study, full-length RyR cDNA from D. citri (named DcRyR) was isolated and identified. The 15,393 bp long open reading frame of DcRyR encoded a 5130 amino acid protein with a calculated molecular weight of 580,830 kDa. This protein had a high sequence identity (76-79%) with other insect homologs and a low sequence identity (43-46%) with mammals. An MIR domain, two RIH domains, three SPRY domains, four RyR repeat domains, an RIH-associated domain at the N-terminus, two consensus calcium-binding EF-hands, and six transmembrane domains were among the characteristics that DcRyR shared with insect and vertebrate RyRs. In expression analysis, the DcRyR gene displayed transcript abundance in all tissues and developmental stages as well as gene-differential and stage-specific patterns. In addition, diagnostic PCR experiments revealed that DcRyR had three potential alternative splice variants and that splicing events might have contributed to the various functions of DcRyR. However, diamide resistance-related amino acid residue mutations I4790M/K and G4946E were not found in DcRyR. These results can serve as the basis for further investigation into the target-based diamide pesticide resistance of D. citri.

Recent progress in the structural study of ion channels as insecticide targets

Ion channels, many expressed in insect neural and muscular systems, have drawn huge attention as primary targets of insecticides. With the recent technical breakthroughs in structural biology, especially in cryo-electron microscopy (cryo-EM), many new high-resolution structures of ion channel targets, apo or in complex with insecticides, have been solved, shedding light on the molecular mechanism of action of the insecticides and resistance mutations. These structures also provide accurate templates for structure-based insecticide screening and rational design. This review summarizes the recent progress in the structural studies of 5 ion channel families: the ryanodine receptor (RyR), the nicotinic acetylcholine receptor (nAChR), the voltage-gated sodium channel (VGSC), the transient receptor potential (TRP) channel, and the ligand-gated chloride channel (LGCC). We address the selectivity of the channel-targeting insecticides by examining the conservation of key coordinating residues revealed by the structures. The possible resistance mechanisms are proposed based on the locations of the identified resistance mutations on the 3D structures of the target channels and their impacts on the binding of insecticides. Finally, we discuss how to develop "green" insecticides with a novel mode of action based on these high-resolution structures to overcome the resistance.

Toxicological and transcriptomic effects in Mythimna separata (Lepidoptera: Noctuidae) exposed to chlorantraniliprole and functional characterization of glutathione S-transferases

BACKGROUND

Chlorantraniliprole (CAP) is an efficient anthranilic diamide insecticide against economically important pests such as the oriental armyworm, Mythimna separata (Lepidoptera: Noctuidae). Resistance to CAP may develop due to enhanced enzymatic detoxification. The glutathione S-transferase (GST) superfamily in M. separata has not been systematically characterized. The aim of this study was therefore to explore the effects of lethal and sublethal doses of CAP on M. separata larvae, screen differentially expressed genes (DEGs) responding to CAP exposure, identify and characterize the GST superfamily, and analyze the metabolism of CAP by recombinant GSTs.

RESULTS

The toxicity bioassay showed that CAP was active against M. separata third-instar larvae. LC50 was 17.615, 3.127, and 1.336 mg/L after 24, 48, and 72 h, respectively. Poisoned larvae showed contracted somites and disrupted midgut. Total GST activity in larvae was significantly elevated 24 h after CAP exposure. RNA-sequencing generated 43 055 unigenes with an average length of 1010 bp, and 567 up-regulated and 692 down-regulated DEGs responding to CAP treatment were screened. Thirty-five GST genes were identified from unigenes, including 31 cytosolic, three microsomal, and one unclassified. The expression profile of GST genes was analyzed using samples from different developmental stages, adult tissues, and CAP treatments. Metabolic assays indicated that CAP was depleted by recombinant MseGSTe2 and MseGSTs6.

CONCLUSIONS

This study provides insight into the toxicological and transcriptomic effects in M. separata larvae exposed to CAP. The identification and functional characterization of the GST superfamily will improve our understanding of CAP detoxification by GSTs. © 2022 Society of Chemical Industry.

Improving Insecticidal Activity of Chlorantraniliprole by Replacing the Chloropyridinyl Moiety with a Substituted Cyanophenyl Group

Insect ryanodine receptors (RyRs) are molecular targets of the anthranilic diamide insecticides. In the present study, a new series of anthranilic diamides containing a cyanophenyl pyrazole moiety were rationally designed by active-fragment assembly and computer-aided design using the 3D structure of Plutella xylostella RyRs as a receptor and chlorantraniliprole as a ligand. Most of the titled compounds showed good toxicity against Mythimna separate, P. xylostella, and Spodoptera frugiperda. Compounds CN06, CN11, and CN16 with corresponding LC₅₀ values of 0.15, 0.29, and 0.52 mg·L^-1, respectively, against M. separate showed comparable activity to that of chlorantraniliprole (0.13 mg·L^-1). Surprisingly, CN06, CN11, and CN16 with corresponding LC₅₀ values of 1.6 × 10^-5, 3.0 × 10^-5, and 2.8 × 10^-5 mg·L^-1, respectively, against P. xylostella were at least 5-fold more active than chlorantraniliprole (1.5 × 10^-4 mg·L^-1). In the case of S. frugiperda, CN06, CN11, and CN16 had good potency but lower than chlorantraniliprole in terms of LC₅₀ values (0.58, 0.54, and 0.56 mg·L^-1 versus 0.31 mg·L^-1). Molecular docking of CN06 and chlorantraniliprole to P. xylostella RyRs validated the molecular design, and the calcium imaging technique further proved the potential target of CN06 as RyRs. Compounds CN06, CN11, and CN16 could be more effective than chlorantraniliprole in targeting the resistant RyR mutants of S. frugiperda (G4891E, I4734M) through the binding mode and energy obtained by molecular docking. Density functional theory calculations (DFT) and electrostatic potential (ESP) studies gave the structure-activity relationship. Compounds CN06, CN11, and CN16 could be used as potent insecticide leads for further optimization.

High frequency of ryanodine receptor and cytochrome P450 CYP9A186 mutations in insecticide-resistant field populations of Spodoptera exigua from China

The beet armyworm, Spodoptera exigua is a global agricultural pest that is polyphagous, highly dispersive, and often difficult to control due to resistance to many insecticides. Previous studies showed that a target site mutation in the S. exigua ryanodine receptor (SeRyR) corresponding to I4743M contributes approximately 20-fold resistance to chlorantraniliprole, whereas a mutation in the cytochrome P450 enzyme CYP9A186 corresponding to F116V confers 200-fold to emamectin benzoate through enhanced metabolic detoxification. Here, high frequencies of mutations were found among six China S. exigua field populations collected from 2016 to 2019 resulting in SeRyR I4743M and CYP9A186 F116V substitutions, with some populations having high levels of resistance to chlorantraniliprole and emamectin benzoate, respectively. Whereas we found a significant correlation between emamectin benzoate resistance level and the allele frequency of CYP9A186 F116V, no significant correlation was found between chlorantraniliprole resistance level and SeRyR I4743M allele frequency in the six field populations. These results suggest that CYP9A186 F116V is a major resistance mechanism for emamectin benzoate in the tested field populations, whereas it is likely that resistance mechanisms other than SeRyR I4743M are responsible for resistance to chlorantraniliprole in the six China field populations. Because of the growing resistance to these two insecticides by S. exigua in China, the use of insecticidal compounds with different modes of action and/or other integrated pest management strategies are needed to further delay the evolution of insecticide resistance and effectively manage S. exigua in China.

Large-Scale Monitoring of the Frequency of Ryanodine Receptor Target-Site Mutations Conferring Diamide Resistance in Brazilian Field Populations of Fall Armyworm, Spodoptera frugiperda (Lepidoptera: Noctuidae)

Fall armyworm (FAW), Spodoptera frugiperda, is an important lepidopteran pest in the Americas, and recently invaded the Eastern Hemisphere. In Brazil, FAW is considered the most destructive pest of corn and cotton. FAW has evolved resistance to many insecticides and Bacillus thuringiensis (Bt) proteins. Here, a large-scale monitoring was performed between 2019 and 2021 to assess diamide insecticide susceptibility in more than 65 FAW populations sampled in corn and cotton. We did not detect a significant shift in FAW susceptibility to flubendiamide, but a few populations were less affected by a discriminating rate. F2 screen results of 31 selected FAW populations across regions confirmed that the frequency of diamide resistance alleles remained rather stable. Two laboratory-selected strains exhibited high resistance ratios against flubendiamide, and cross-resistance to anthranilic diamides. Reciprocal crosses indicated that resistance is autosomal and (incompletely) recessive in both strains. F1 backcrosses suggested monogenic resistance, supported by the identification of an I4734M/K target-site mutation in the ryanodine receptor (RyR). Subsequent genotyping of field-collected samples employing a TaqMan-based allelic discrimination assay, revealed a low frequency of RyR I4790M/K mutations significantly correlated with phenotypic diamide resistance. Our findings will help to sustainably employ diamides in FAW resistance management strategies across crops.

Chemosensory proteins confer adaptation to the ryanoid anthranilic diamide insecticide cyantraniliprole in Aphis gossypii glover

Chemosensory proteins (CSPs) are a class of small transporter proteins expressed only in arthropods with various functions beyond chemoreception. Previous studies have been reported that CSPs are involved in the insecticide resistance. In this study, we found that AgoCSP1, AgoCSP4, and AgoCSP5 were constitutively overexpressed in an insecticide-resistant strain of Aphis gossypii and showed higher expression in broad body tissue (including fat bodies) than in the midgut but without tissue specificity. However, the function of these three upregulated AgoCSPs remains unknown. Here, we investigated the function of AgoCSPs in resistance to the diamide insecticide cyantraniliprole. Suppression of AgoCSP1, AgoCSP4 and AgoCSP5 transcription by RNAi significantly increased the sensitivity of resistant aphids to cyantraniliprole. Molecular docking and competitive binding assays indicated that these AgoCSPs bind moderate with cyantraniliprole. Transgenic Drosophila melanogaster expressing these AgoCSPs in the broad body or midgut showed higher tolerance to cyantraniliprole than control flies with the same genetic background; AgoCSP4 was more effective in broad body tissue, and AgoCSP1 and AgoCSP5 were more effective in the midgut, indicating that broad body and midgut tissues may be involved in the insecticide resistance mediated by the AgoCSPs examined. The present results strongly indicate that AgoCSPs participate in xenobiotic detoxification by sequestering and masking toxic insecticide molecules, providing insights into new factors involved in resistance development in A. gossypii.

Structural Insight Into Ryanodine Receptor Channelopathies

The ryanodine receptors (RyRs) are large cation-selective ligand-gated channels that are expressed in the sarcoplasmic reticulum (SR) membrane. They mediate the controlled release of Ca²⁺ from SR and play an important role in many cellular processes. The mutations in RyRs are associated with several skeletal muscle and cardiac conditions, including malignant hyperthermia (MH), central core disease (CCD), catecholaminergic polymorphic ventricular tachycardia (CPVT), and arrhythmogenic right ventricular dysplasia (ARVD). Recent breakthroughs in structural biology including cryo-electron microscopy (EM) and X-ray crystallography allowed the determination of a number of near-atomic structures of RyRs, including wildtype and mutant structures as well as the structures in complex with different modulating molecules. This allows us to comprehend the physiological gating and regulatory mechanisms of RyRs and the underlying pathological mechanisms of the disease-causing mutations. In this review, based on the insights gained from the available high-resolution structures of RyRs, we address several questions: 1) what are the gating mechanisms of different RyR isoforms; 2) how RyRs are regulated by multiple channel modulators, including ions, small molecules, and regulatory proteins; 3) how do disease-causing mutations affect the structure and function of RyRs; 4) how can these structural information aid in the diagnosis of the related diseases and the development of pharmacological therapies.

Systematic identification and characterization of differentially expressed microRNAs under tetraniliprole exposure in the fall armyworm, Spodoptera frugiperda

The fall armyworm, Spodoptera frugiperda, is a worldwide agricultural pest and causes huge losses of crop production each year. Tetraniliprole is a novel diamide insecticide with high efficacy against even the insecticide resistant pests of Lepidoptera, Coleoptera, and Diptera. MicroRNAs (miRNAs) are small noncoding RNAs that regulate gene expression at the posttranscriptional level and play an important regulatory role in the insecticide resistance in insects. However, the effects of miRNAs on the tetraniliprole tolerance in S. frugiperda are poorly understood. In the present research, the miRNAs response to tetraniliprole application in S. frugiperda were systematically investigated by high-throughput sequencing. A total of thirty differentially expressed miRNAs were identified under tetraniliprole treatment in S. frugiperda. The functions of the target genes of these differentially expressed miRNAs were further predicted by Gene Ontology terms and Kyoto Encyclopedia of Genes and Genomes database pathway, and the most significantly enriched pathway was MAPK signaling pathway. The expression changes of six differentially expressed miRNAs were validated by quantitative real-time polymerase chain reaction. Furthermore, miR-278-5p had the highest expression in the hemolymph and malpighian tubule and the lowest expression in the gut. Oversupply of miR-278-5p significantly increased the mortality of S. frugiperda following exposure to tetraniliprole. These results will provide the basis for understanding the regulatory roles of miRNAs regarding to tetraniliprole tolerance in S. frugiperda.

Spodoptera exigua (Hubner) (Lepidoptera: Noctuidae) Fitness and Resistance Stability to Diamide and Pyrethroid Insecticides in the United States

Simple Summary

Spodoptera exigua is a polyphagous pest, commonly known as beet armyworm. This pest is distributed worldwide and causes yield reduction in a variety of crops. Chemical control using synthetic insecticides is the primary strategy to manage beet armyworm. In the United States, beet armyworm resistance to both chlorantraniliprole and bifenthrin insecticides was first reported in 2020. Here we examined beet armyworm fitness and stability of resistance to chlorantraniliprole and pyrethroid insecticides, since knowledge of the stability of resistance is a crucial aspect when recommending rotation of insecticides with different mode of action. Our results have indicated no decrease in bifenthrin resistance for at least a three-year period (i.e., 27 generations) when insecticide exposure was suspended. However, susceptibility to chlorantraniliprole dropped approximately 160-fold through this three-year period. Our results indicate that beet armyworm resistance to bifenthrin is stable, but unstable to chlorantraniliprole. Unstable resistance can be successfully managed at field level by switching off the selection pressure with replacement of the insecticide other than a pyrethroid.

Abstract

In the United States, beet armyworm resistance to both chlorantraniliprole and bifenthrin insecticides was first reported in 2020. Here we examined beet armyworm fitness and stability of resistance to chlorantraniliprole and pyrethroid insecticides since knowledge of the stability of resistance is a crucial aspect when recommending rotation of insecticides with different mode of action. Concentration-mortality bioassays were performed with field and laboratory susceptible populations. The F2, F13, and F27 generations of the field-derived population, maintained in the laboratory without insecticide, were exposed to commercial formulations of bifenthrin and chlorantraniliprole using the leaf-dip bioassay method (IRAC n. 007). Insects from F27 had the fitness components (survival, body weight, development time) documented and compared by LSM in each insecticide concentration tested. The resistance ratio to chlorantraniliprole reached 629, 80, 15-fold at F2, F13, and F27, respectively. These results contrast with an over 1000-fold resistance ratio to bifenthrin in all generations. The field-derived population had fitness reduced by chlorantraniliprole, but not by bifenthrin. In summary, the resistance of beet armyworm to bifenthrin was stable with no shift in fitness. In contrast, resistance to chlorantraniliprole was not stable through the generations kept in the laboratory without selection pressure, likely due to fitness cost.

Diamide insecticide resistance in transgenic Drosophila and Sf9-cells expressing a full-length diamondback moth ryanodine receptor carrying an I4790M mutation

BACKGROUND

Resistance to diamide insecticides in Lepidoptera is known to be caused primarily by amino acid changes on the ryanodine receptor (RyR). Recently, two new target site mutations, G4946V and I4790M, have emerged in populations of diamondback moth, Plutella xylostella, as well as in other lepidopteran species, and both mutations have been shown empirically to decrease diamide efficacy. Here, we quantify the impact of the I4790M mutation on diamide activation of the receptor, as compared to alterations at the G4946 locus.

RESULTS

I4790M when introduced into P. xylostella RyR expressed in an insect-derived Sf9 cell line was found to mediate just a ten-fold reduction in chlorantraniliprole efficacy (compared to 104- and 146-fold reductions for the G4946E and G4946V variants, respectively), whilst in the field its presence is associated with a ≥150-fold reduction. I4790M-mediated resistance to flubendiamide was estimated to be >24-fold. When the entire coding sequence of P. xylostella RyR was integrated into Drosophila melanogaster, the I4790M variant conferred ~4.4-fold resistance to chlorantraniliprole and 22-fold resistance to flubendiamide in the 3rd instar larvae, confirming that it imparts only a moderate level of resistance to diamide insecticides. Although the I4790M substitution appears to bear no fitness costs in terms of the flies' reproductive capacity, when assessed in a noncompetitive environment, it does, however, have potentially major impacts on mobility at both the larval and adult stages.

CONCLUSIONS

I4790M imparts only a moderate level of resistance to diamide insecticides and potentially confers significant fitness costs to the insect.

A mechanism-based approach unveils metabolic routes potentially mediating chlorantraniliprole synergism in honey bees, Apis mellifera L., by azole fungicides

BACKGROUND

Almond production in California is an intensively managed agroecosystem dependent on managed pollination by honey bees, Apis mellifera L. A recent laboratory study reported synergism in honey bees between chlorantraniliprole, a common diamide insecticide used in almond orchards, and the fungicide propiconazole. Indeed, there is an emerging body of evidence that honey bee cytochrome P450 monooxygenases of the CYP9Q subfamily are involved in the detoxification of insecticides across a diverse range of chemical classes. The objective of the present study was to unveil the molecular background of the described synergism and to explore the potential role of CYP9Q enzymes in diamide detoxification.

RESULTS

Our study confirmed the previously reported synergistic potential of propiconazole on chlorantraniliprole in acute contact toxicity bioassays, whereas no synergism was observed for flubendiamide. Fluorescence-based biochemical assays revealed an interaction of chlorantraniliprole, but not flubendiamide, with functionally expressed CYP9Q2 and CYP9Q3. These findings were validated by an increased chlorantraniliprole tolerance of transgenic Drosophila lines expressing CYP9Q2/3, and an analytically confirmed oxidative metabolism of chlorantraniliprole by recombinantly expressed enzymes. Furthermore, we showed that several triazole fungicides used in almond orchards, including propiconazole, were strong nanomolar inhibitors of functionally expressed honey bee CYP9Q2 and CYP9Q3, whereas other fungicides such as iprodione and cyprodinil did not inhibit these enzymes.

CONCLUSION

Honey bee CYP9Q enzymes are involved in chlorantraniliprole metabolism and inhibited by triazole fungicides possibly leading to synergism in acute contact toxicity bioassays. Our mechanistic approach has the potential to inform tier I honey bee pesticide risk assessment.

A review of physiological resistance to insecticide stress in Nilaparvata lugens

Insecticides are widely used in agriculture as effective means to control pests. However, pests have not been completely mitigated with the increased use of insecticides. Instead, many side effects have arisen, especially the '3Rs' (resistance, resurgence, and residue). The brown planthopper, Nilaparvata lugens, is one of the most threatening rice pests. The main insecticides for controlling N. lugens belong to organochlorine, organophosphorus, carbamate, neonicotinoid and pyrethroid groups. However, metabolic enzymes, including cytochrome P450s, esterases, glutathione-S-transferases, and ATP-binding cassette transporters, effectively promote the detoxification of insecticides. Besides, mutations of neurological target sites, such as acetylcholinesterase, nicotinic acetylcholine, γ-aminobutyric acid receptor, and ryanodine receptor, result in insensitivity to insecticides. Here, we review the physiological metabolic resistance in N. lugens under insecticide stress to provide a theoretical basis for identifying and developing more effective and harmless insecticides.

Extraction and determination of flubendiamide insecticide in food samples: A review

Flubendiamide (FBD) is the first commercially available phthalic acid diamide that targets ryanodine receptors (RyRs) in insects, which play a major role in lepidoptera control. However, excessive use of FBD can influence the quality of treated products leading to toxic effects on human health. The availability of rapid and convenient methods for evaluating FBD amount in the environment is necessary. Therefore, analytical methods were developed for the determination of residues of FBD and its metabolite desiodo in different food matrices like tomato, cabbage, pigeon pea, apple, chilli and rice. The current review carries forward methods for FBD residues analysis in foods by using several chromatographic techniques including sample preparation steps. The comparison between the different methods employed for quantitative and qualitative analysis of food quality and safety is also discussed. Liquid chromatography (LC) is the predominant analytical method for assessing the quality of foods treated with FBD. Studies related to LC coupled multichannel detector (Ultraviolet (UV), Mass spectrometry (MS)) are also applied to detect pesticide residues. Extraction and clean up steps are essential to obtain reliable results. Moreover, this review reports the allowed limits of residues for the safety of consuming products treated with FBD.

A scaffold-level genome assembly of a minute pirate bug, Orius laevigatus (Hemiptera: Anthocoridae), and a comparative analysis of insecticide resistance-related gene families with hemipteran crop pests

BACKGROUND

Orius laevigatus, a minute pirate bug, is a highly effective beneficial predator of crop pests including aphids, spider mites and thrips in integrated pest management (IPM) programmes. No genomic information is currently available for O. laevigatus, as is the case for the majority of beneficial predators which feed on crop pests. In contrast, genomic information for crop pests is far more readily available. The lack of publicly available genomes for beneficial predators to date has limited our ability to perform comparative analyses of genes encoding potential insecticide resistance mechanisms between crop pests and their predators. These mechanisms include several gene/protein families including cytochrome P450s (P450s), ATP binding cassette transporters (ABCs), glutathione S-transferases (GSTs), UDP-glucosyltransferases (UGTs) and carboxyl/cholinesterases (CCEs).

METHODS AND FINDINGS

In this study, a high-quality scaffold level de novo genome assembly for O. laevigatus has been generated using a hybrid approach with PacBio long-read and Illumina short-read data. The final assembly achieved a scaffold N50 of 125,649 bp and a total genome size of 150.98 Mb. The genome assembly achieved a level of completeness of 93.6% using a set of 1658 core insect genes present as full-length genes. Genome annotation identified 15,102 protein-coding genes - 87% of which were assigned a putative function. Comparative analyses revealed gene expansions of sigma class GSTs and CYP3 P450s. Conversely the UGT gene family showed limited expansion. Differences were seen in the distributions of resistance-associated gene families at the subfamily level between O. laevigatus and some of its targeted crop pests. A target site mutation in ryanodine receptors (I4790M, PxRyR) which has strong links to diamide resistance in crop pests and had previously only been identified in lepidopteran species was found to also be present in hemipteran species, including O. laevigatus.

CONCLUSION AND SIGNIFICANCE

This assembly is the first published genome for the Anthocoridae family and will serve as a useful resource for further research into target-site selectivity issues and potential resistance mechanisms in beneficial predators. Furthermore, the expansion of gene families often linked to insecticide resistance may be an indicator of the capacity of this predator to detoxify selective insecticides. These findings could be exploited by targeted pesticide screens and functional studies to increase effectiveness of IPM strategies, which aim to increase crop yields by sustainably, environmentally-friendly and effectively control pests without impacting beneficial predator populations.

Metabolic-based insecticide resistance mechanism and ecofriendly approaches for controlling of beet armyworm Spodoptera exigua: a review

The beet army worm, Spodoptera exigua, is a widely distributed polyphagous pest of economically important crops worldwide. The management of this pest insect continues to face many challenges. Despite synthetic chemicals posing a serious threat to the environment, these remain the conventional approach for controlling S. exigua in the field. An over-reliance on chemical control has not only led to selection for resistance to insecticides and to a reduction of natural enemies, but has also polluted various components of ecosystem. Given these increasing pressures on the ecosystem, there is a need to implement integrated pest management (IPM) approaches exploiting a wider range of tools (biotechnological approaches, microbial control, biological control, cultural control, and use of host plant resistance) for an alternative to chemical control. The IPM approach can not only reduce the hazard of chemical residues in the environment and associated health problems, but may also provide best strategies to control insect pests. This review synthesizes published information on insecticide resistance of S. exigua and explores alternative IPM approaches to control S. exigua.

Double ryanodine receptor mutations confer higher diamide resistance in rice stem borer, Chilo suppressalis

BACKGROUND

The striped rice stem borer, Chilo suppressalis (Lepidoptera: Pyraidae), is one of the most serious rice pests in China. Chlorantraniliprole was used extensively for C. suppressalis control over the past ten years, and some field populations have developed high resistance. In this study, we report the chlorantraniliprole resistance status of C. suppressalis in China and the resistance mechanism.

RESULTS

Significant geographical variations of chlorantraniliprole susceptibility were observed among 28 C. suppressalis field populations in 2019-2020. The LC₅₀ values varied from 2907.874 mg L^-1 (XS19) to 1.524 mg L^-1 (QW19). Most tested field populations collected from Zhejiang, Jiangxi, Hunan and Anhui provinces in 2020 showed a high level of resistance to chlorantraniliprole (RR = 311.9-2060.1), whereas Jiangsu and Sichuan province populations remained susceptible. Analysis of RyR mutations showed that mutations of I4758M, Y4667D, Y4667C and Y4891F were present in resistant populations of C. suppressalis with different levels of chlorantraniliprole resistance. The frequency of the Y4667C mutation was correlated with chlorantraniliprole resistance in YY19 (RR = 702.6) and YY20 (RR = 1426.8) populations, with the homozygous mutation frequencies of 15.6% and 29.4%, respectively. High contributions of the I4758M and Y4667C double mutation to diamide resistance was demonstrated with CRISPR/Cas9-modified D. melanogaster. Flies bearing the Y4667C mutation (I4758M and Y4667C double mutation in C. suppressalis) exhibited high resistance to chlorantraniliprole (RR = 172.1), and moderate resistance to cyantraniliprole (RR = 79.2) and tetra chlorantraniliprole (RR = 43.6), which were higher than that of single mutations.

CONCLUSIONS

Chlorantraniliprole resistance in C. suppressalis is intensifying in China. RyR double mutations (i.e. I4758M and Y4667C) confer higher diamide resistance than single mutations.

Varying contributions of three ryanodine receptor point mutations to diamide insecticide resistance in Plutella xylostella

BACKGROUND

Although decoding the molecular mechanisms underlying insecticide resistance has often proven difficult, recent progress has revealed that specific mutations in the ryanodine receptor (RyR) of the diamondback moth, Plutella xylostella, can confer resistance to diamide insecticides. The extent to which specific RyR mutations contribute to the diamide resistance phenotype, the associated genetic traits and fitness costs remain limited.

RESULTS

Three field-evolved PxRyR mutations (G4946E, I4790 M, and I4790 K) were respectively introgressed into a common susceptible background strain (IPP-S) of P. xylostella with marker-assisted backcrossing. The mutations alone can result in moderate to high levels of resistance to five commercial diamides (flubendiamide, chlorantraniliprole, cyantraniliprole, tetraniliprole, and cyclaniliprole), and the resistance intensity mediated by the three mutations was hierarchical in order of I4790 K (1199- to >2778-fold) > G4946E (39- to 739-fold) > I4790 M (16- to 57-fold). Flubendiamide resistance was autosomal and incompletely recessive, and was significantly linked with the introgressed mutations in the three constructed strains. In addition, the resistance levels to flubendiamide of hybrid progeny from any two resistant strains fell in between the status of their parents. Furthermore, by comparing the net replacement rate, the fitness of 4946E, 4790 M and 4790 K strains were 0.77, 0.93 and 0.92 relative to the IPP-S strain, respectively.

CONCLUSION

Three independent PxRyR mutations confer varying degrees of resistance to diamides in P. xylostella. Among the three mutations, I4790 K confers highest levels of resistance (> 1000-fold) to all five commercial diamides. The findings can guide resistance management practices for diamides in P. xylostella and other arthropods.

Ryanodine receptor as insecticide target

Ryanodine receptor (RyR) is one of the primary targets of commercial insecticides. The diamide insecticide family, including flubendiamide, chlorantraniliprole, cyantraniliprole, etc, targets insect RyRs and can be used to control a wide range of destructive agricultural pests. The diamide insecticides are highly selective against lepidopteran and coleopteran pests with relatively low toxicity for non-target species, such as mammals, fishes, and beneficial insects. However, recently mutations identified on insect RyRs have emerged and caused resistance in several major agricultural pests throughout different continents. This review paper summarizes the recent findings on structure and function of insect RyRs as insecticide target. Specifically, we examine the structures of RyRs from target and non-target species, which reveals the molecular basis for insecticide action and selectivity. We also examine the structural and functional changes of RyR caused by the resistance mutations. Finally, we examine the progress in RyR structure-based insecticide design, and discuss how this might help the development of new generation of green insecticides.

A Comparative Perspective on Functionally-Related, Intracellular Calcium Channels: The Insect Ryanodine and Inositol 1,4,5-Trisphosphate Receptors

Calcium (Ca²⁺) homeostasis is vital for insect development and metabolism, and the endoplasmic reticulum (ER) is a major intracellular reservoir for Ca²⁺. The inositol 1,4,5- triphosphate receptor (IP₃R) and ryanodine receptor (RyR) are large homotetrameric channels associated with the ER and serve as two major actors in ER-derived Ca²⁺ supply. Most of the knowledge on these receptors derives from mammalian systems that possess three genes for each receptor. These studies have inspired work on synonymous receptors in insects, which encode a single IP₃R and RyR. In the current review, we focus on a fundamental, common question: “why do insect cells possess two Ca²⁺ channel receptors in the ER?”. Through a comparative approach, this review covers the discovery of RyRs and IP₃Rs, examines their structures/functions, the pathways that they interact with, and their potential as target sites in pest control. Although insects RyRs and IP₃Rs share structural similarities, they are phylogenetically distinct, have their own structural organization, regulatory mechanisms, and expression patterns, which explains their functional distinction. Nevertheless, both have great potential as target sites in pest control, with RyRs currently being targeted by commercial insecticide, the diamides.

Functional validation of key cytochrome P450 monooxygenase and UDP-glycosyltransferase genes conferring cyantraniliprole resistance in Aphis gossypii Glover

Cytochrome P450 monooxygenases (P450s) and UDP-glycosyltransferases (UGTs) are major detoxifying enzymes that metabolize plant toxins and insecticides. In the present study, the synergists of piperonyl butoxide, sulfinpyrazone and 5-nitrouracil significantly increased cyantraniliprole and α-cypermethrin toxicity against the resistant strain. The transcripts of UGT341A4, UGT344B4, UGT344D6, UGT344J2 and UGT344M2 increased significantly in the CyR strain compared with the susceptible strain. Among these upregulated genes (including P450s), CYP6CY7 and UGT344B4 were highly expressed in the midgut. Transgenic expression of the P450 and UGT genes in broad body tissues in Drosophila melanogaster indicated that the expression of CYP380C6, CYP4CJ1, UGT341A4, UGT344B4 and UGT344M2 is sufficient to confer cyantraniliprole resistance, and CYP380C6, CYP6CY7, CYP6CY21, UGT341A4 and UGT344M2 are related to α-cypermethrin cross-resistance. The midgut-specific overexpression of CYP380C6, CYP6CY7, CYP6CY21, CYP4CJ1, UGT341A4, UGT344B4 and UGT344M2 significantly increased insensitivity to cyantraniliprole, and CYP380C6, CYP6CY7, CYP6CY21, UGT344B4 and UGT344M2 confer α-cypermethrin cross-resistance. The expression of CYP380C6, CYP4CJ1, UGT341A4 and UGT344M2 in broad tissues or in midgut has similar effects on insensitivity to insecticides; however, CYP6CY7, CYP6CY21 and UGT344B4 are more effective in the midgut. This result indicates that broad body tissues and midgut tissue are involved in insecticide resistance mediated by the candidate P450s and UGTs examined.

Resistance Risk Assessment of the Ryanoid Anthranilic Diamide Insecticide Cyantraniliprole in Aphis gossypii Glover

Cyantraniliprole targets the ryanodine receptor and shows cross-spectrum activity against a broad range of chewing and sucking pests. In this study, a cyantraniliprole-resistant cotton aphid strain (CyR) developed resistance 17.30-fold higher than that of a susceptible (SS) strain. Bioassay results indicated that CyR developed increased cross-resistance to cyfluthrin, α-cypermethrin, imidacloprid, and acephate. In CyR, piperonyl butoxide synergistically increased the toxicity of cyantraniliprole, α-cypermethrin, and cyfluthrin. The cytochrome P450 activities in the CyR strain were significantly higher than those in the SS strain. The mRNA expression of CYP6CY7, CYP6CY12, CYP6CY21, CYP6CZ1, CYP6DA1, and CYP6DC1 in the CYP3 clade, and CYP380C6, CYP380C12, CYP380C44, CYP4CJ1, and CYP4CJ5 in the CYP4 clade, was significantly higher in CyR than in SS. The depletion of the most abundant CYP380C6 transcript by RNAi also significantly increased the sensitivity of CyR to cyantraniliprole. Transgenic expression of CYP380C6, CYP6CY7, CYP6CY21, and CYP4CJ1 in Drosophila melanogaster suggested that the expression of CYP380C6 and CYP4CJ1 was sufficient to confer cyantraniliprole resistance, with CYP380C6 being the most effective, and that CYP380C6, CYP6CY7, and CYP6CY21 were related to α-cypermethrin cross-resistance. These results indicate the involvement of P450 genes in cyantraniliprole resistance and pyrethroid cross-resistance and provide an overall view of the metabolic factors involved in resistance development.

Detection of ryanodine receptor target-site mutations in diamide insecticide-resistant Spodoptera frugiperda in China

Spodoptera frugiperda (Lepidoptera: Noctuidae) is a widely distributed pest of corn. Since it invaded China in 2018, it has caused serious damage to local corn production. Chlorantraniliprole, an anthranilic diamide insecticide, has been widely used to control lepidopteran pests. Tetrachloropyramid is a new allosteric modulator insecticide developed based on chlorantraniliprole, so it has a similar mechanism and insecticidal effect. In this study, we investigated resistance levels to chlorantraniliprole and tetrachloropyramid in S. frugiperda from 13 populations in China. Among the populations tested, the relative highest resistance to chlorantraniliprole occurred in the Guangzhou population, and the most susceptible to chlorantraniliprole was found in the Wuhan population. The lethal dosage LD₅₀ value of the Guangzhou population against chlorantraniliprole was 27.8-fold higher than that of the Wuhan population. Minimal differences were observed among S. frugiperda populations in terms of sensitivity to tetrachloropyramid. Heterozygous mutations at the I4734 site of the ryanodine receptor (RyR) were found, while no mutations were found in the G4891 site. The mutations were detected in only two of the 786 individuals analyzed, one from the Qinzhou population and other from the Anshun population (frequency below 2% in both cases). There were no significant differences in the expression levels of RyR between Guangzhou and Wuhan populations. In summary, our results indicate that: (i) S. frugiperda has low resistance levels to diamide insecticides in China; and (ii) the differences in relative resistance among the 13 populations analyzed are not caused by the mutations in RyR or the expression of RyR.

Fitness Costs of Chlorantraniliprole Resistance Related to the SeNPF Overexpression in the Spodoptera exigua (Lepidoptera: Noctuidae)

Spodopteraexigua, a multifeeding insect pest, has developed a high level of resistance to chlorantraniliprole, which is a benzoylurea insecticide that targets the ryanodine receptors (RyRs). Herein, the resistant strain (SE-Sel) and sensitive strain (SE-Sus) were obtained by bidirectional screening for six generations. The potential oviposited eggs and oviposition rate of the SE-Sel strain were dramatically lower than those of the SE-Sus strain; on the contrary, the weights of prepupae and preadult were significantly increased. As a post-mating response, the higher number of non-oviposited eggs in the SE-Sel strain was caused by a lower mating rate. In addition, the expression levels of vitellogenin (SeVg) and its receptor (SeVgR) in the SE-Sel strain were consistently lower than those in the SE-Sus strain. An RyR^I4743M mutation, contributing to the resistance to chlorantraniliprole, was located in the S3 transmembrane segments and might have affected the release of calcium ions; it led to the upregulated expression of the neuropeptide SeNPF and its receptor SeNPFR, and the mating and oviposition rate were significantly recovered when the SeNPF was knocked down though RNA interference (RNAi) in the male adult of the SE-Sel strain. Moreover, the expression of the juvenile hormone-binding proteins SeJHBWDS3 and SeJHBAN in the male adult of the SE-Sel strain was significantly decreased, which proved the existence of a fitness cost from another angle. Therefore, these results indicate that the fitness cost accompanied by chlorantraniliprole resistance in S. exigua may be related to the decrease in mating desire due to SeNPF overexpression.

Monitoring and mechanisms of insecticide resistance in Spodoptera exigua (Lepidoptera: Noctuidae), with special reference to diamides

The beet armyworm, Spodoptera exigua, is a major lepidopteran pest of global importance in cultivation of numerous crops including cotton, maize, soybean, onion, cabbage, and ornamentals. It has evolved resistance to different insecticides. However, the current status of insecticide resistance in S. exigua has not been well examined in China. In this study, concentration-mortality responses of S. exigua to seven insecticides, including chlorantraniliprole, tetraniliprole, methoxyfenozide, indoxacarb, chlorfenapyr, emamectin benzoate and beta-cypermethrin were evaluated. The results showed that most of the tested populations had developed moderate to high resistance to chlorantraniliprole, with resistance ratios ranging from 6.3 to 2477.3-fold. Our results also showed that chlorantraniliprole have cross-resistance with tetraniliprole in S. exigua. The AY19 population collected from Anyang in Henan Province in 2019 exhibited a high resistance level to beta-cypermethrin (RR = 277.5). Methoxyfenozide and chlorfenapyr were highly effective against all of the tested populations with resistance ratios (RR) ranging from 0.1 to 2.2-fold. One of the tested populations showed moderate resistance to indoxacarb and emamectin benzoate. We detected the known ryanodine receptor target site resistance mutation, I4743M, in the field populations of S. exigua with different levels of diamide resistance.

Pyrrole-2 carboxamides - A novel class of insect ryanodine receptor activators

The ryanodine receptor (RyR) is an intracellular calcium channel critical to the regulation of insect muscle contraction and the target site of diamide insecticides such as chlorantraniliprole, cyantraniliprole and flubendiamide. To-date, diamides are the only known class of synthetic molecules with high potency against insect RyRs. Target-based screening of an informer library led to discovery of a novel class of RyR activators, pyrrole-2-carboxamides. Efforts to optimize receptor activity resulted in analogs with potency comparable to that of commercial diamides when tested against RyR of the fruit fly, Drosophila melanogaster. Surprisingly, testing of pyrrole-2-carboxamides in whole-insect screens showed poor insecticidal activity, which is partially attributed to differential selectivity among insect receptors and rapid detoxification. Among various lepidopteran species field resistance to diamide insecticides has been well documented and in many cases has been attributed to a single point mutation, G4946E, of the RyR gene. As with diamide insecticides, the G4946E mutation confers greatly reduced sensitivity to pyrrole-2-carboxamides. This, coupled with findings from radioligand binding studies, indicates a shared binding domain between anthranilic diamides and pyrrole-2-carboxamides.

Broflanilide effectively controls Helicoverpa armigera and Spodoptera exigua exhibiting diverse susceptibilities to chlorantraniliprole and emamectin benzoate

BACKGROUND

The cotton bollworm, Helicoverpa armigera (Hübner), and the beet armyworm, Spodoptera exigua (Hübner), are two major polyphagous lepidopteran pests of cultivated crops. They develop various levels of resistance to many frequently applied broad-spectrum insecticides. Here, the larval susceptibilities of a laboratory population and six field-collected populations per pest from Hunan Province, China to three insecticides were determined using a standard leaf-dipping method in the laboratory. Field-plot trials were conducted to verify the control efficacies of broflanilide 100 g L^-1 suspension concentrate (SC), chlorantraniliprole 5% SC and emamectin benzoate 5.7% water-dispersible granule, against H. armigera and S. exigua larvae using foliar sprays in 2013 and 2019.

RESULTS

Variations among H. armigera and S. exigua field populations in their susceptibility levels to the three insecticides were observed in Linli, Yueyang and Changsha counties from 2013 to 2019. They were still high susceptibility to broflanilide only, but developed low and/or moderate levels of resistance to chlorantraniliprole and emamectin benzoate. In addition, broflanilide at 25 g ha^-1 provided an excellent control efficacy of 81.92% to 96.46% against these pests during the 3-14-days period after treatment in both years, whereas chlorantraniliprole at 50 g ha^-1 and emamectin benzoate at 4.5 g ha^-1 exhibited significantly decreased efficacies, which were consistent with their corresponding toxicity levels.

CONCLUSIONS

These results suggest that broflanilide should be an important new tool for the effective control of diamide- and avermectin-resistant H. armigera and S. exigua. © 2020 Society of Chemical Industry.

CRISPR/Cas9 mediated ryanodine receptor I4790M knockin confers unequal resistance to diamides in Plutella xylostella

The diamondback moth Plutella xylostella is a major destructive pest of Brassica worldwide. P. xylostella has evolved resistance to nearly all commercial insecticides used for its control, including the most recent chemical class, diamide insecticides. Several studies show that the G4946E and I4790M mutations of ryanodine receptor (RyR) are strongly associated with diamide resistance in insects. While the pivotal functional role of G4946E in conferring diamide resistance phenotype has confirmed by several studies in different species, no direct evidence has unambiguously confirmed the functional significance of the single I4790M mutation in diamide resistance. Here, we successfully constructed a knockin homozygous strain (I4790M-KI) of P. xylostella using CRISPR/Cas9 coupled with homology directed repair approach to introduce I4790M into RyR. When compared with the background susceptible IPP-S strain, the manipulated I4790M-KI strain exhibited moderate resistance to the phthalic acid diamide flubendiamide (40.5-fold) and low resistance to anthranilic diamides chlorantraniliprole (6.0-fold) and cyantraniliprole (7.7-fold), with no changes to the toxicities of indoxacarb and β-cypermethrin. Furthermore, the acquired flubendiamide resistance was inherited in an autosomally recessive mode and significantly linked with the I4790M mutation of RyR in this I4790M-KI strain. Our findings provide in vivo functional evidence for the causality of I4790M mutation of PxRyR with moderate levels of resistance to flubendiamide in P. xylostella, and support the hypothesis that the diamide classes have different interactions with RyRs.

Ryanodine receptor mutations (G4946E and I4790K) differentially responsible for diamide insecticide resistance in diamondback moth, Plutella xylostella L

This study examined diamondback moth (Plutella xylostella) strains showing high-level resistance to cyantraniliprole (KA17 strain) and to flubendiamide and chlorantraniliprole (KU13 strain). The LC₅₀ value of the KA17 strain against cyantraniliprole was ca. 100-fold higher than that of the KU13 strain. The KA17 strain also exhibited high-level resistance to chlorantraniliprole and flubendiamide equivalent to those in the KU13 strain. The KU13 strain showed a higher LC₅₀ value against cyantraniliprole than the susceptible strains. However, the LC₅₀ value of the KU13 strain against cyantraniliprole was below the agriculturally recommended concentration. Subsequent QTL analysis using ddRAD-seq identified the resistance responsible regions of the KA17 and KU13 strains with different diamide resistance profiles. Ryanodine receptor (RyR) gene was included in the identified regions. Single nucleotide polymorphism calling in the RyR gene using RNA-seq found previously reported G4946E (amino acid mutation from glycine to glutamic acid at amino acid position 4946) and novel I4790K (amino acid mutation from isoleucine to lysine at amino acid position 4790) mutations, respectively, in the RyR of the KU13 and KA17 strains. Functional significance of I4790K in the resistance was confirmed in calcium imaging of the human embryonic kidney 293T cell line expressing Bombyx mori RyR with the mutation. This reporting is the first describing I4790K as a fundamental mechanism responsible for the resistance to the diamides including cyantraniliprole. From this study, we also report up-regulated expression of some degradation enzymes and that of the RyR gene in the KA17 and KU13 strains based on results of RNA-seq data analysis.