Flubendiamide resistance and Bi-PASA detection of ryanodine receptor G4946E mutation in the diamondback moth (Plutella xylostella L.)

A Sex-Specific Trade-Off Between Pesticide Resistance and Tolerance to Heat-Induced Sterility in Tetranychus urticae

Current pest management relies extensively on pesticide application worldwide, despite the frequent rise of pesticide resistance in crop pests. This is particularly worrisome because resistance is often not costly enough to be lost in populations after pesticide application, resulting in increased dependency on pesticide application. As climate warming increases, effort should be put into understanding how heat tolerance will affect the persistence of pesticide resistance in populations. To address this, we measured heat tolerance in two populations of the spider mite crop pest Tetranychus urticae that differ in the presence or absence of a target-site mutation conferring resistance to etoxazole pesticide. We found that developmental time and fertility, but not survival, were negatively affected by increasing temperatures in the susceptible population. Furthermore, we found no difference between resistant and susceptible populations in all life-history traits when both sexes developed at control temperature, nor when females developed at high temperature. Resistant heat-stressed males, in contrast, showed lower fertility than susceptible ones, indicating a sex-specific trade-off between heat tolerance and pesticide resistance. This suggests that global warming could lead to reduced pesticide resistance in natural populations. However, resistant females, being as affected by high temperature as susceptible individuals, may buffer the toll in resistant male fertility, and the shorter developmental time at high temperatures may accelerate adaptation to temperature, the pesticide or the cost thereof. Ultimately, the complex dynamic between these two factors will determine whether resistant populations can persist under climate warming.

Insecticide resistance and characteristics of mutations related to target site insensitivity of diamondback moths in Taiwan

Diamondback moth (DBM, Plutella xylostella) is the most significant pest of cruciferous vegetables as they rapidly develop high-level resistance to many insecticides. Monitoring DBM susceptibility and target-site mutation frequency is essential for pest control. In this study, 10 insecticides were tested on 11 field populations. Frequencies of target-site mutations (including para, ace1, Rdl1, RyR1, and nAChRα6 genes) were estimated by pyrosequencing. Insecticides registered after 2007 for DBM control in Taiwan, i.e., spinetoram, chlorantraniliprole, chlorfenapyr, metaflumizone, and flubendiamide, showed >80% mortality toward several populations; Bacillus thurigiensis, emamectin benzoate, and chlorfluazuron showed medium to low efficacy in all populations; and tolfenpyrad and mevinphos were highly ineffective. Susceptibility to insecticides varied substantially among populations: eight out of nine populations were highly susceptible to spinetoram, but only one was susceptible to flubendiamide. Target-site mutations related to organophosphates, pyrethroids, fipronil, and diamides were detected in all populations, but there were few spinosad and spinetoram mutations. Our three-year field study demonstrated rapid efficacy loss for all insecticides tested, particularly for more toxic insecticides. Skipped-generation selection of a field DBM strain to emamectin benzoate, metaflumizone, chlorantraniliprole, and flubendiamide revealed that mortality rates dropped from 60 to 80% to <10% after 6 generations. Next-generation sequencing was performed to identify possible target gene mutations. A resistance management program that considers the instability of resistance to some chemicals and pertinent data on resistance mechanisms should be established. Identifying compounds to overcome high-frequency field DBM point mutations could be beneficial for pest control.

Rapid detection of kdr mutation F1534C in Aedes aegypti using recombinase polymerase amplification and lateral flow dipsticks

Insecticide resistance monitoring is essential in assessing the efficacy of vector control measures. However, gold standard PCR-based molecular analyses for insecticide resistance detection are often hindered by time-consuming sample processing, as well as considerable infrastructure and resourcing requirements. In this study, we combined a novel one-step sample preparation reagent with a rapid isothermal molecular test that detects a knock down resistance (kdr) mutation (F1534C) that enables pyrethroid resistance in Aedes aegypti mosquitoes. We trialled the rapid F1534C pyrethroid resistance test using insecticide resistant Ae. aegypti mosquito bodies and compared results to a conventional, allele-specific quantitative PCR (AS-qPCR) coupled with melt curve genotyping in corresponding mosquito heads. From a strain of Ae. aegypti established from an insecticide resistant population in Merida, Mexico (n = 27), all the mosquito bodies (n = 27) tested positive with the rapid F1534C test regardless of whether they were homozygous or heterozygous. To assess diagnostic test specificity, we confirmed that F1534 was not detected in laboratory-reared, fully susceptible Ae. aegypti mosquito bodies (n = 28) using the rapid F1534C test or the conventional AS-qPCR melt curve analysis. All corresponding mosquito heads (n = 28) were homozygous wild-type FF1534. The rapid F1534C test thus demonstrated 100% diagnostic sensitivity (95% CI: 87.23% to 100%) and 100% diagnostic specificity (95% CI: 87.66% to 100.00%) for detection of the F1534C pyrethroid resistant single nucleotide polymorphism (SNP) in both heterozygous and homozygous Ae. aegypti. In the collection of mutant mosquitoes from Mexico, CC1534 homozygous mutants occurred at a frequency of 74.1% (n = 20) and FC heterozygous mutants at a frequency of 25.9% (n = 7). The rapid F1534C test significantly reduced the sample processing and testing time from approximately 6 h for the AS-qPCR melt curve analysis to only 25 min. These results demonstrate significant potential for our approach to resistance testing as a field-based, low-resource, rapid alternative to time-consuming and expensive laboratory-based detection.

Resistance and fitness costs in diamondback moths after selection using broflanilide, a novel meta-diamide insecticide

The diamondback moth (DBM) Plutella xylostella (L.) (Lepidoptera: Plutellidae) is an insect pest found around the world that feeds on cruciferous crops. The DBM has become resistant to most insecticides in current use in the field. Broflanilide is a novel meta-diamide insecticide that binds to a new site on the γ-aminobutyric acid receptor and very efficiently protects against most pests in the order Lepidoptera, including DBM. In this study, the resistance of a laboratory-bred susceptible strain of DBM to broflanilide and the fitness costs posed by broflanilide to the DBM were evaluated. The DBM had no obvious resistance to broflanilide after 10 generations of selection. The realized heritability h² was 0.033, suggesting a low risk of resistance developing in this strain. The F10 generation had no cross-resistance to the insecticides abamectin and endosulfan (which target the γ-aminobutyric acid receptor) and chlorantraniliprole (which targets a non-γ-aminobutyric acid receptor). The specific activities of important detoxification enzymes (cytochrome P450 monooxygenase, esterase, and glutathione S-transferase) were not obviously altered. However, the larval stage was prolonged and the adult stage was shortened significantly in F11 generation than the F0 generation. The total preoviposition period TPOP significantly prolonged 1.90 d in F11 generation. The fitness value R_f (0.93) was lower for the F11 generation than the F0 generation. The results indicated that long-term exposure to broflanilide exerts clear fitness costs in the DBM. This information will be useful in identifying reasonable broflanilide application guidelines for managing broflanilide resistance in the DBM.

Whole genome sequencing and bulked segregant analysis suggest a new mechanism of amitraz resistance in the citrus red mite, Panonychus citri (Acari: Tetranychidae)

BACKGROUND

Amitraz is a broad-spectrum insecticide/acaricide for the control of aphids, psyllids, ticks and mites. Current evidence suggests that ticks and phytophagous mites have developed strong resistance to amitraz. Previous studies have shown that multiple mechanisms are associated with amitraz resistance in ticks, but very few reports have involved Panonychus citri. We therefore used whole genome sequencing and bulked segregant analysis (BSA) to identify the mechanism underlying P. citri's resistance to amitraz.

RESULTS

High-quality assembly of the whole P. citri genome was completed, resulting in a genome of approximately 83.97 Mb and a contig N50 of approximately 1.81 Mb. Gene structure predictions revealed 11 577 genes, of which 10 940 genes were annotated. Trait-associated regions in the genome were mapped with bulked segregant analysis and 38 candidate SNPs were obtained, of which T752C had the strongest correlation with the resistant trait, located at the 5' untranslated region (UTR) of the β-2R adrenergic-like octopamine receptor gene. The mutation resulted in the formation of a short hairpin loop structure in mRNA and gene expression was down-regulated by more than 50% in the amitraz-resistant strain. Validation of the T752C mutation in field populations of P. citri found that the correlation between the resistance ratio and the base mutation was 94.40%.

CONCLUSION

Our results suggest that this 5' UTR mutation of the β-2R octopamine receptor gene, confers amitraz resistance in P. citri. This discovery provides a new explanation for the mechanism of pest resistance: base mutations in the 5' untranslated region of target gene may regulate the susceptibility of pests to pesticides.

Crystal structure of 3-iodo-N 2-(2-methyl-1-(methylsulfonyl)propan-2-yl)-N 1-(2-methyl-4-(perfluoropropan-2-yl)phenyl)phthalamide, C23H22F7I1N2O4S1

8[C23H22F7I1N2O4S1], monoclinic, Cc (no. 9), a = 19.762(3) Å, b = 24.690(4) Å, c = 12.106(2) Å, β = 113.696(2)°, V = 5408.8(15) Å3, Z = 1, R gt (F) = 0.0488, wR ref (F 2) = 0.1411, T = 296(2) K.

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.

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.

Characterization and functional analysis of a β-adrenergic-like octopamine receptor from the oriental armyworm (Mythimna separata Walker)

The β-adrenergic-like octopamine receptor (OA2B2), which binds the biogenic amine octopamine, belongs to the class of G-protein coupled receptors and significantly regulates many physiological and behavioral processes in insects. In this study, the putative open reading frame sequence of the MsOA2B2 gene in Mythimna separata was cloned, the full-length complementary DNA was 1191 bp and it encoded a 396-amino acid protein (GenBank accession number MN822800). Orthologous sequence alignment, phylogenetic tree analysis, and protein sequence analysis all showed that the cloned receptor belongs to the OA2B2 protein family. Real-time quantitative polymerase chain reaction of spatial and temporal expression analysis revealed that the MsOAB2 gene was expressed in all developmental stages of M. separata and was most abundant in egg stages and second and fourth instars compared with other developmental stages, while the expression level during the pupal stage was much lower than that at the other stages. Further analysis with sixth instar M. separata larvae showed that the MsOA2B2 gene was expressed 1.81 times higher in the head than in integument and gut tissues. Dietary ingestion of dsMsOA2B2 significantly reduced the messenger RNA level of MsOA2B2 and decreased mortality following amitraz treatment. This study provides both a pharmacological characterization and the gene expression patterns of OA2B2 in M. separata, facilitating further research for insecticides using MsOA2B2 as a target.

FastD: Fast detection of insecticide target-site mutations and overexpressed detoxification genes in insect populations from RNA-Seq data

Target-site mutations and detoxification gene overexpression are two major mechanisms conferring insecticide resistance. Molecular assays applied to detect these resistance genetic markers are time-consuming and with high false-positive rates. RNA-Seq data contains information on the variations within expressed genomic regions and expression of detoxification genes. However, there is no corresponding method to detect resistance markers at present. Here, we collected 66 reported resistance mutations of four insecticide targets (AChE, VGSC, RyR, and nAChR) from 82 insect species. Next, we obtained 403 sequences of the four target genes and 12,665 sequences of three kinds of detoxification genes including P450s, GSTs, and CCEs. Then, we developed a Perl program, FastD, to detect target-site mutations and overexpressed detoxification genes from RNA-Seq data and constructed a web server for FastD (http://www.insect-genome.com/fastd). The estimation of FastD on simulated RNA-Seq data showed high sensitivity and specificity. We applied FastD to detect resistant markers in 15 populations of six insects, Plutella xylostella, Aphis gossypii, Anopheles arabiensis, Musca domestica, Leptinotarsa decemlineata and Apis mellifera. Results showed that 11 RyR mutations in P. xylostella, one nAChR mutation in A. gossypii, one VGSC mutation in A. arabiensis and five VGSC mutations in M. domestica were found to be with frequency difference >40% between resistant and susceptible populations including previously confirmed mutations G4946E in RyR, R81T in nAChR and L1014F in VGSC. And 49 detoxification genes were found to be overexpressed in resistant populations compared with susceptible populations including previously confirmed detoxification genes CYP6BG1, CYP6CY22, CYP6CY13, CYP6P3, CYP6M2, CYP6P4 and CYP4G16. The candidate target-site mutations and detoxification genes were worth further validation. Resistance estimates according to confirmed markers were consistent with population phenotypes, confirming the reliability of this program in predicting population resistance at omics-level.

Reduced fecundity and regulation of reproductive factors in flubendiamide-resistant strains of Plutella xylostella

Diamondback moth (DBM), Plutella xylostella, is an important pest of crucifers worldwide. The extensive use of flubendiamide has led to the development of resistance in field populations and reports of control failures. In this study, the lab-selected (R_f) and field-collected (R_b) flubendiamide-resistant strains of P. xylostella with LC₅₀ resistance ratios of 1890-fold and 1251-fold, respectively, were used, as well as a lab-reared flubendiamide-susceptible strain (S). The results showed that the fecundity of the R_f and R_b-resistant strains was significantly lower than that of S strain. The contents of vitellin and transcripts of P. xylostella vitellogenin (PxVg) and P. xylostella vitellogenin receptor (PxVgR) genes in the R_f and R_b strains were significantly higher than those of S strains at 0-48 h after adult eclosion. At 96 h after eclosion, the content of vitellin in the R_f and R_b strains did not differ significantly from those of S strains, whereas transcripts of the PxVg and PxVgR genes in the R_f and R_b strains were significantly lower than that of the S strain. The content of the juvenile hormone III (JH III), β-ecdysone (20E), and the gene expression level of P. xylostella methoprene tolerant (PxMet) in the R_f and R_b strains were significantly higher than that of the S strain. The activity of trehalase was significantly higher in the R_f and R_b strains than that of the S strain in the first to the third instar larvae, whereas in the fourth instar larvae, there was no significantly difference in the three strains. At different times after adult eclosion, the differences in trehalase activity were erratic between the strains. The transcripts of P. xylostella trehalase (PxTre) gene in the R_f and R_b strains were significantly higher than that of the S strain in most developmental stages. Here, we report differences in fecundity between flubendiamide-resistant and susceptible strains of P. xylostella and discuss gene expression of several reproductive factors, which provides a possible explanation for the mechanism of fecundity reduction concurrent with flubendiamide-resistance in P. xylostella.

Increased Responses of Phenoloxidase in Chlorantraniliprole Resistance of Plutella xylostella (Lepidoptera: Plutellidae)

The diamondback moth (Plutella xylostella, DBM) is an important pest of cruciferous vegetables. The use of chlorantraniliprole has been essential in the management of the DBM. However, in many countries and areas, DBM has become highly resistant to chlorantraniliprole. Three different DBM strains, susceptible (S), chlorantraniliprole-selected (Rc), and field-collected (Rb) resistant strains/populations were studied for the role of phenoloxidase in resistance development to the insecticide. By assaying the activity of phenoloxidase (PO) in the three different DBM strains, the results showed that the PO activity in the Rc strain was increased significantly compared with the S strain. The synergistic effects of quercetin showed that the resistant ratio (RR) of the QRc larvae to chlorantraniliprole was decreased from 423.95 to 316.42-fold compared with the Rc larvae. Further studies demonstrated that the transcriptional and translational expression levels of PxPPO1 (P. xylostella prophenoloxidase-1 gene) and PxPPO2 (P. xylostella prophenoloxidase-2 gene) were increased to varying degrees compared with the S strain, such as the transcriptional expression levels of PxPPO2 were 24.02-fold that of the S strain. The responses of phenoloxidase were significantly different in chlorantraniliprole-resistant DBM.

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.

Molecular cloning, characterization and functional analysis of GluCl from the oriental armyworm, Mythimna separata Walker

Glutamate-gated chloride channels (GluCls) mediate inhibitory synaptic transmission in invertebrate nervous systems, and only one GluCl gene has been found in insects. Therefore, insect GluCls are one of the major targets of insecticides including avermectins. In the present study, a 1347 bp full-length cDNA encoding a 449-amino acid protein (named MsGluCl, GenBank ID: MK336885) was cloned from the oriental armyworm, Mythimna separata, and characterized two alternative splicing variants of MsGluCl. The protein shares 76.9-98.6% identity with other insect GluCl isoforms. Spatial and temporal expression analysis revealed that MsGluCl was highly expressed in the 3rd instar and adult head. Dietary ingestion of dsMsGluCl significantly reduced the mRNA level of MsGluCl and decreased abamectin mortality. Thus, our results reveal that MsGluCl could be the molecular target of abamectin and provide the basis for further understanding the resistance mechanism to abamectin in arthropods.

Molecular cloning and characterization of GABA receptor and GluCl subunits in the western flower thrips, Frankliniella occidentalis

To understand the role of target site insensitivity in abamectin resistance in the western flower thrips (WFT), Frankliniella occidentalis (Pergande), cDNAs encoding gamma-aminobutyric acid receptor subunit (FoRdl) and glutamate-gated chloride channel (FoGluCl) were cloned from WFT, and both single nucleotide polymorphisms (SNPs) and mRNA expression levels of FoRdl and FoGluCl were detected in a susceptible strain (ABA-S) and a laboratory selected strain (ABA-R) displaying 45.5-fold resistance to abamectin. Multiple cDNA sequence alignment revealed three alternative splicing variants of FoRdl and two alternative splicing variants of FoGluCl generated by alternative splicing of exon 3. While sequence comparison of FoRdl and FoGluCl in ABA-S and ABA-R strains identified no resistance-associated mutations, the expression level of FoGluCl in ABA-R strain was 2.63-fold higher than that in ABA-S strain. Thus, our preliminary results provide the evidence that the increased mRNA expression of FoGluCl could be an important factor in FoGluCl-mediated target site insensitivity in WFT.

Baseline susceptibility of Mediterranean strains of Trialeurodes vaporariorum (Westwood) to cyantraniliprole

BACKGROUND

Cyantraniliprole is a novel anthranilic diamide insecticide that acts on a broad spectrum of insect pests, exclusively by activating their ryanodine receptors. Cyantraniliprole is very effective against whitefly and it presents a favorable ecotoxicological profile. In this study, the baseline susceptibility to cyantraniliprole of Trialeurodes vaporariorum populations from the Mediterranean area was established in nymphal systemic uptake bioassays.

RESULTS

The bioassay data showed that the susceptibility to cyantraniliprole varied among the strains collected across the Mediterranean basin. The 50% lethal concentration (LC₅₀ ) range of cyantraniliprole for 16 field populations was from 0.017 to 0.194 mg L^-1 , a 11.4-fold natural variability between the least and most sensitive populations. These LC₅₀ values are similar to those reported in a previous study of the use of cyantraniliprol against another species of whitefly, Bemisia tabaci [LC₅₀  = 0.048 (0.034-0.063) mg L^-1 ].

CONCLUSION

The current study confirmed the effectiveness of cyantraniliprole against T. vaporarioum strains, adding to the evidence that cyantraniliprole is a promising tool for use in integrated pest management programs. Future shifts in the susceptibility of whitefly field populations to cyantraniliprole may be documented according to the baseline susceptibility range of the populations tested in this research. © 2018 Society of Chemical Industry.

Fitness costs and life table parameters of highly insecticide-resistant strains of Plutella xylostella (L.) (Lepidoptera: Plutellidae) at different temperatures

BACKGROUND

In many cases, resistance alleles have been associated with fitness costs and are often dependent on environmental factors such as temperature. Here, we studied the effects of temperature on the overall fitness, including development, survival and reproduction, of three insecticide-resistant and one susceptible strain of diamondback moth (DBM), Plutella xylostella (L.).

RESULTS

The broader cross-resistance profile of the resistant strains previously selected by diamide and benzoylurea insecticides was tested. Cohort studies were conducted in the laboratory at three different temperatures (20 ± 1 °C, 25 ± 1 °C and 30 ± 1 °C), and involved fitness costs were estimated. We observed significant differences in the development time, with the susceptible strain showing a shorter developmental period from egg stage to adult stage compared with the resistant strains. Moreover, the resistant strains differed significantly between one another. Additionally, the population growth parameters varied among the strains, with the benzoylurea-resistant strain showing the highest costs affecting the overall fitness of this strain. A temperature of 30 °C was unfavourable for DBM development, resulting in a reduced fitness in all strains.

CONCLUSION

Benzoylurea selection pressure on a diamide-resistant P. xylostella strain resulted in lowest reproduction parameters and the longest generation time, as well as doubling the time among all strains tested. This suggests significant effects on the overall fitness and population growth parameters for diamide-resistant populations pressured by benzoylureas under applied conditions. © 2017 Society of Chemical Industry.

ACE: an efficient and sensitive tool to detect insecticide resistance-associated mutations in insect acetylcholinesterase from RNA-Seq data

Background

Insecticide resistance is a substantial problem in controlling agricultural and medical pests. Detecting target site mutations is crucial to manage insecticide resistance. Though PCR-based methods have been widely used in this field, they are time-consuming and inefficient, and typically have a high false positive rate. Acetylcholinesterases (Ace) is the neural target of the widely used organophosphate (OP) and carbamate insecticides. However, there is not any software available to detect insecticide resistance associated mutations in RNA-Seq data at present.

Results

A computational pipeline ACE was developed to detect resistance mutations of ace in insect RNA-Seq data. Known ace resistance mutations were collected and used as a reference. We constructed a Web server for ACE, and the standalone software in both Linux and Windows versions is available for download. ACE was used to analyse 971 RNA-Seq data from 136 studies in 7 insect pests. The mutation frequency of each RNA-Seq dataset was calculated. The results indicated that the resistance frequency was 30%–44% in an eastern Ugandan Anopheles population, thus suggesting this resistance-conferring mutation has reached high frequency in these mosquitoes in Uganda. Analyses of RNA-Seq data from the diamondback moth Plutella xylostella indicated that the G227A mutation was positively related with resistance levels to organophosphate or carbamate insecticides. The wasp Nasonia vitripennis had a low frequency of resistant reads (<5%), but the agricultural pests Chilo suppressalis and Bemisia tabaci had a high resistance frequency. All ace reads in the 30 B. tabaci RNA-Seq data were resistant reads, suggesting that insecticide resistance has spread to very high frequency in B. tabaci.

Conclusions

To the best of our knowledge, the ACE pipeline is the first tool to detect resistance mutations from RNA-Seq data, and it facilitates the full utilization of large-scale genetic data obtained by using next-generation sequencing.

Electronic supplementary material

The online version of this article (doi:10.1186/s12859-017-1741-6) contains supplementary material, which is available to authorized users.

Environment polluting conventional chemical control compared to an environmentally friendly IPM approach for control of diamondback moth, Plutella xylostella (L.), in China: a review

The diamondback moth, Plutella xylostella, is recognized as a widely distributed destructive insect pest of Brassica worldwide. The management of this pest is a serious issue, and an estimated annual cost of its management has reached approximately US$4 billion. Despite the fact that chemicals are a serious threat to the environment, lots of chemicals are applied for controlling various insect pests especially P. xylostella. An overreliance on chemical control has not only led to the evolution of resistance to insecticides and to a reduction of natural enemies but also has polluted various components of water, air, and soil ecosystem. In the present scenario, there is a need to implement an environmentally friendly integrated pest management (IPM) approach with new management tactics (microbial control, biological control, cultural control, mating disruption, insecticide rotation strategies, and plant resistance) for an alternative to chemical control. The IPM approach is not only economically beneficial but also reduces the environmental and health risks. The present review synthesizes published information on the insecticide resistance against P. xylostella and emphasizes on adopting an alternative environmentally friendly IPM approach for controlling P. xylostella in China.

Trends and Challenges in Pesticide Resistance Detection

Pesticide resistance is a crucial factor to be considered when developing strategies for the minimal use of pesticides while maintaining pesticide efficacy. This goal requires monitoring the emergence and development of resistance to pesticides in crop pests. To this end, various methods for resistance diagnosis have been developed for different groups of pests. This review provides an overview of biological, biochemical, and molecular methods that are currently used to detect and quantify pesticide resistance. The agronomic, technical, and economic advantages and drawbacks of each method are considered. Emerging technologies are also described, with their associated challenges and their potential for the detection of resistance mechanisms likely to be selected by current and future plant protection methods.

Rapid selection for resistance to diamide insecticides in Plutella xylostella via specific amino acid polymorphisms in the ryanodine receptor

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Baseline susceptibility of Plutella xylostella to diamide insecticides collated.

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Instances of diamide resistance in P. xylostella summarized.

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Diamide insecticide specificity for the insect ryanodine receptor highlighted.

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Efforts to isolate and characterize the P. xylostella ryanodine receptor described.

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Molecular mechanisms of diamide resistance in P. xylostella discussed.

Diamide insecticides, such as flubendiamide and chlorantraniliprole, are a new class of insecticide with a novel mode of action, selectively activating the insect ryanodine receptor (RyR). They are particularly active against lepidopteran pests of cruciferous vegetable crops, including the diamondback moth, Plutella xylostella. However, within a relatively short period following their commercialisation, a comparatively large number of control failures have been reported in the field. In this review we summarise the current body of knowledge regarding the molecular mechanisms of diamide resistance in P. xylostella. Resistant phenotypes collected from different countries can often be linked to specific target-site mutation(s) in the ryanodine receptors’ transmembrane domain. Metabolic mechanisms of resistance have also been proposed. Rapid resistance development is probably a consequence of over-reliance on this one class of chemistry for diamondback moth control.

Characterization of the Ryanodine Receptor Gene With a Unique 3'-UTR and Alternative Splice Site From the Oriental Fruit Moth

The ryanodine receptor (RyR), the largest calcium channel protein, has been studied because of its key roles in calcium signaling in cells. Insect RyRs are molecular targets for novel diamide insecticides. The target has been focused widely because of the diamides with high activity against lepidopterous pests and safety for nontarget organisms. To study our understanding of effects of diamides on RyR, we cloned the RyR gene from the oriental fruit moth, Grapholita molesta, which is the most serious pest of stone and pome tree fruits throughout the world, to investigate the modulation of diamide insecticides on RyR mRNA expression in G. molesta (GmRyR). The full-length cDNAs of GmRyR contain a unique 3'-UTR with 625 bp and an open reading frame of 15,402 bp with a predicted protein consisting of 5,133 amino acids. GmRyR possessed a high level of overall amino acid homology with insect and vertebrate isoforms, with 77-92% and 45-47% identity, respectively. Furthermore, five alternative splice sites were identified in GmRyR. Diagnostic PCR showed that the inclusion frequency of one optional exon (f) differed between developmental stages, a finding only found in GmRyR. The lowest expression level of GmRyR mRNA was in larvae, the highest was in male pupae, and the relative expression level in male pupae was 25.67 times higher than that of in larvae. The expression level of GmRyR in the male pupae was 8.70 times higher than in female pupae, and that in male adults was 5.70 times higher than female adults.

miRNAs regulated overexpression of ryanodine receptor is involved in chlorantraniliprole resistance in Plutella xylostella (L.)

The amino acid mutations in ryanodine receptor (RyR) and elevated activity of detoxification enzymes have been associated with the diamide insecticide resistance in the diamondback moth, Plutella xylostella (L.). The up-regulation of P. xylostella RyR mRNA (PxRyR) expression has also been reported in field populations of different graphical origin. However, whether the up-regulation of PxRyR is involved in diamide resistance remains unknown. In this paper, 2.28- to 4.14-fold higher expression of PxRyR was detected in five field collected resistant populations, compared to that in a susceptible population. The expression of PxRyR was up-regulated 5.0- and 7.2-fold, respectively, after P. xylostella was treated with LC50 and LC75 of chlorantraniliprole for 12 h. Suppression of PxRyR using RNA interference restored the toxicity of chlorantraniliprole against the fourth instar larvae from the resistant population. More importantly, the expression of PxRyR is regulated by two miRNAs, miR-7a and miR-8519. These findings provide an empirical evidence of the involvement of miRNAs in the regulation of insecticide resistance, and shed light on the novel targets for the sustainable management of this devastating insect pest.

Geographic spread, genetics and functional characteristics of ryanodine receptor based target-site resistance to diamide insecticides in diamondback moth, Plutella xylostella

Anthranilic diamides and flubendiamide belong to a new chemical class of insecticides acting as conformation sensitive activators of the insect ryanodine receptor (RyR). These compounds control a diverse range of different herbivorous insects including diamondback moth, Plutella xylostella (Lepidoptera: Plutellidae), a notorious global pest on cruciferous crops, which recently developed resistance due to target-site mutations located in the trans-membrane domain of the Plutella RyR. In the present study we further investigated the genetics and functional implications of a RyR G4946E target-site mutation we recently identified in a Philippine diamondback moth strain (Sudlon). Strain Sudlon is homozygous for the G4946E mutation and has been maintained under laboratory conditions without selection pressure for almost four years, and still exhibit stable resistance ratios of >2000-fold to all commercial diamides. Its F1 progeny resulting from reciprocal crosses with a susceptible strain (BCS-S) revealed no maternal effects and a diamide susceptible phenotype, suggesting an autosomally almost recessive mode of inheritance. Subsequent back-crosses indicate a near monogenic nature of the diamide resistance in strain Sudlon. Radioligand binding studies with Plutella thoracic microsomal membrane preparations provided direct evidence for the dramatic functional implications of the RyR G4946E mutation on both diamide specific binding and its concentration dependent modulation of [(3)H]ryanodine binding. Computational modelling based on a cryo-EM structure of rabbit RyR1 suggests that Plutella G4946E is located in trans-membrane helix S4 close to S4-S5 linker domain supposed to be involved in the modulation of the voltage sensor, and another recently described mutation, I4790M in helix S2 approx. 13 Å opposite of G4946E. Genotyping by pyrosequencing revealed the presence of the RyR G4946E mutation in larvae collected in 2013/14 in regions of ten different countries where diamide insecticides largely failed to control diamondback moth populations. Thus, our study highlights the global importance of the G4946E RyR target-site mutation, which as a mechanism on its own, confers high-level resistance to diamide insecticides in diamondback moth.

Genotype to phenotype, the molecular and physiological dimensions of resistance in arthropods

The recent accumulation of molecular studies on mutations in insects, ticks and mites conferring resistance to insecticides, acaricides and biopesticides is reviewed. Resistance is traditionally classified by physiological and biochemical criteria, such as target-site insensitivity and metabolic resistance. However, mutations are discrete molecular changes that differ in their intrinsic frequency, effects on gene dosage and fitness consequences. These attributes in turn impact the population genetics of resistance and resistance management strategies, thus calling for a molecular genetic classification. Mutations in structural genes remain the most abundantly described, mostly in genes coding for target proteins. These provide the most compelling examples of parallel mutations in response to selection. Mutations causing upregulation and downregulation of genes, both in cis (in the gene itself) and in trans (in regulatory processes) remain difficult to characterize precisely. Gene duplications and gene disruption are increasingly reported. Gene disruption appears prevalent in the case of multiple, hetero-oligomeric or redundant targets.