Acetylcholinesterase point mutations in European strains of Tetranychus urticae (Acari: Tetranychidae) resistant to organophosphates

Expression reduction and a variant of a P450 gene mediate chlorpyrifos resistance in Tetranychus urticae Koch

INTRODUCTION

Understanding how insects and mites develop resistance to chlorpyrifos is crucial for effective field management. Although extensive research has demonstrated that T. urticae exhibits high resistance to chlorpyrifos, the specific resistance mechanism remains elusive. Investigating this mechanism could provide valuable insights for pest control strategies.

OBJECTIVES

This study aimed to reveal the mechanism of chlorpyrifos resistance in T. urticae.

METHODS

The expression level of the CYP392D8 gene in T. urticae strains were analyzed using real- time quantitative PCR and western blot techniques. Functional validation of CYP392D8 was conducted through RNAi and heterogeneous expression. The production of chlorpyrifos-oxon in both resistant and susceptible strains were quantified using LC-MS/MS. Furthermore, the metabolic capabilities of CYP392D8 variants were verified using HPLC-MS and molecular docking.

RESULTS

The results showed the expression of CYP392D8 was reduced in some Chinese resistant populations and mites with knocked down CYP392D8 showed decreased susceptibility to chlorpyrifos. Chlorpyrifos-oxon, the active metabolite of chlorpyrifos, was generated when chlorpyrifos was incubated with recombinant CYP392D8 protein in vitro. And a higher efficiency of chlorpyrifos-oxon formation was observed with the CYP392D8-S variant from susceptible mites compared to the CYP392D8-R variant from resistant mites. After treatment with low doses of chlorpyrifos, susceptible mite extracts produced substantial amounts of chlorpyrifos-oxon, while resistant mites only showed trace amounts. In addition, molecular docking studies showed that CYP392D8-S had a higher binding capacity to chlorpyrifos than the CYP392D8-R variant.

CONCLUSION

This study reveals a mechanism of insecticide resistance due to the bioactivation reduction in combination with the sequence variation in a pest mite. These findings provide an important theoretical bias for management strategies of mites in the field and comprehensive control.

S431F mutation on AChE1 and overexpression of P450 genes confer high pirimicarb resistance in Sitobion miscanthi

Sitobion miscanthi is a destructive wheat pest responsible for significant wheat yield losses. Pirimicarb, one of the most important representatives of N, N-dimethylcarbamate insecticides, is widely used to control wheat aphids. In present work, heterozygous S431F mutation of acetylcholinesterase 1 (AChE1) was identified and verified in three pirimicarb-resistant S. miscanthi populations (two field populations (HA and HS, >955.8-fold) and one lab-selected population (PirR, 486.1-fold)), which has not been reported in S. miscanthi yet. The molecular docking results revealed that AChE1 containing the S431F mutation of S. miscanthi (SmAChE1S431F) showed higher free binding energy to three insecticides (pirimicarb, omethoate, and methomyl) than wild-type AChE1 of S. miscanthi (SmAChE1). Enzyme kinetic and inhibition experiments showed that the recombinant SmAChE1S431F was more insensitive to pirimicarb and omethoate than the recombinant SmAChE1. Furthermore, two overexpression P450 genes (CYP6K1 and CYP6A14) associated with pirimicarb resistance of S. miscanthi were verified by RNAi. These results suggested both target alteration and enhanced metabolism contributed to high pirimicarb resistance of S. miscanthi in the field and laboratory. These findings lay a foundation for further elucidating the mechanism of pirimicarb resistance in S. miscanthi, and have important implications for the resistance management of S. miscanthi control.

Discovery of insecticide resistance in field-collected populations of the aphid pest, Acyrthosiphon kondoi Shinji

BACKGROUND

The bluegreen aphid (Acyrthosiphon kondoi) is a worldwide pest of alfalfa, pulses, and other legume crops. An overreliance on insecticides to control A. kondoi has potentially placed populations under selection pressure favouring resistant phenotypes, but to date, there have been no documented cases of insecticide resistance. Recently, Australian growers began reporting that conventional insecticides were failing to adequately control A. kondoi populations, prompting this laboratory-based investigation into whether these populations have evolved resistance.

RESULTS

We discovered four A. kondoi populations with moderate resistance (10-40-fold) to three different insecticide groups: organophosphates, carbamates and pyrethroids. However, A. kondoi populations showed no resistance to the butenolide, flupyradifurone. We were unable to identify general metabolic mechanisms using synergist assays (cytochromes P450, glutathione S-transferases, or esterases), indicating that further detailed molecular investigations to characterise the putative resistance mechanism are needed.

CONCLUSION

Insecticide-resistant A. kondoi present an emerging challenge to Australian agriculture. Growers require new tools and updated strategies, including access to newer chemistries, to alleviate their reliance on the few insecticides currently registered against A. kondoi. The implications of insecticide resistant A. kondoi for future management, the potential mechanisms of resistance, and future research priorities are discussed. © 2023 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Ace and ace-like genes of invasive redlegged earth mite: copy number variation, target-site mutations, and their associations with organophosphate insensitivity

BACKGROUND

Invasive Australian populations of redlegged earth mite, Halotydeus destructor (Tucker), are evolving increasing organophosphate resistance. In addition to the canonical ace gene, the target gene of organophosphates, the H. destructor genome contains many radiated ace-like genes that vary in copy number and amino acid sequence. In this work, we characterise copy number and target-site mutation variation at the canonical ace and ace-like genes and test for potential associations with organophosphate insensitivity. This was achieved through comparisons of whole-genome pool-seq data from alive and dead mites following organophosphate exposure.

RESULTS

A combination of increased copy number and target-site mutations at the canonical ace was associated with organophosphate insensitivity in H. destructor. Resistant populations were segregating for G119S, A201S, F331Y at the canonical ace. A subset of populations also had copy numbers of canonical ace > 2, which potentially helps overexpress proteins carrying these target-site mutations. Haplotypes possessing different copy numbers and target-site mutations of the canonical ace gene may be under selection across H. destructor populations. We also detected some evidence that increases in copy number of radiated ace-like genes are associated with organophosphate insensitivity, which might suggest potential roles in sequestration or breakdown of organophosphates.

CONCLUSION

Different combinations of target-site mutations and (or) copy number variation in the canonical ace and ace-like genes may provide non-convergent ways for H. destructor to respond to organophosphate selection. However, these changes may only play a partial role in organophosphate insensitivity, which appears to have a polygenic architecture. © 2023 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

The host plant strongly modulates acaricide resistance levels to mitochondrial complex II inhibitors in a multi-resistant field population of Tetranychus urticae

The two-spotted spider mite Tetranychus urticae is a polyphagous pest with an extraordinary ability to develop acaricide resistance. Here, we characterize the resistance mechanisms in a T. urticae population (VR-BE) collected from a Belgian tomato greenhouse, where the grower was unsuccessful in chemically controlling the mite population resulting in crop loss. Upon arrival in the laboratory, the VR-BE population was established both on bean and tomato plants as hosts. Toxicity bioassays on both populations confirmed that the population was highly multi-resistant, recording resistance to 12 out of 13 compounds tested from various mode of action groups. DNA sequencing revealed the presence of multiple target-site resistance mutations, but these could not explain resistance to all compounds. In addition, striking differences in toxicity for six acaricides were observed between the populations on bean and tomato. The highest difference was recorded for the complex II inhibitors cyenopyrafen and cyflumetofen, which were 4.4 and 3.3-fold less toxic for VR-BE mites on tomato versus bean. PBO synergism bioassays suggested increased P450 based detoxification contribute to the host-dependent toxicity. Given the involvement of increased detoxification, we subsequently determined genome-wide gene expression levels of VR-BE on both hosts, in comparison to a reference susceptible population, revealing overexpression of a large set of detoxification genes in VR-BE on both hosts compared to the reference. In addition, a number of mainly detoxification genes with higher expression in VR-BE on tomato compared to bean was identified, including several cytochrome P450s. Together, our work suggests that multi-resistant field populations can accumulate a striking number of target-site resistance mutations. We also show that the host plant can have a profound effect on the P450-associated resistance levels to cyenopyrafen and cyflumetofen.

A review of the molecular mechanisms of acaricide resistance in mites and ticks

The Arachnida subclass of Acari comprises many harmful pests that threaten agriculture as well as animal health, including herbivorous spider mites, the bee parasite Varroa, the poultry mite Dermanyssus and several species of ticks. Especially in agriculture, acaricides are often used intensively to minimize the damage they inflict, promoting the development of resistance. Beneficial predatory mites used in biological control are also subjected to acaricide selection in the field. The development and use of new genetic and genomic tools such as genome and transcriptome sequencing, bulked segregant analysis (QTL mapping), and reverse genetics via RNAi or CRISPR/Cas9, have greatly increased our understanding of the molecular genetic mechanisms of resistance in Acari, especially in the spider mite Tetranychus urticae which emerged as a model species. These new techniques allowed to uncover and validate new resistance mutations in a larger range of species. In addition, they provided an impetus to start elucidating more challenging questions on mechanisms of gene regulation of detoxification associated with resistance.

The Transcriptomic Response of the Boll Weevil, Anthonomus grandis grandis Boheman (Coleoptera: Curculionidae), following Exposure to the Organophosphate Insecticide Malathion

Insecticide tolerance and resistance have evolved countless times in insect systems. Molecular drivers of resistance include mutations in the insecticide target site and/or gene duplication, and increased gene expression of detoxification enzymes. The boll weevil, Anthonomus grandis grandis Boheman (Coleoptera: Curculionidae), is a pest of commercial cotton and has developed resistance in the field to several insecticides; however, the current organophosphate insecticide, malathion, used by USA eradication programs remains effective despite its long-term use. Here, we present findings from an RNA-seq experiment documenting gene expression post-exposure to field-relevant concentrations of malathion, which was used to provide insight on the boll weevil's continued susceptibility to this insecticide. Additionally, we incorporated a large collection of boll weevil whole-genome resequencing data from nearly 200 individuals collected from three geographically distinct areas to determine SNP allele frequency of the malathion target site, as a proxy for directional selection in response to malathion exposure. No evidence was found in the gene expression data or SNP data consistent with a mechanism of enhanced tolerance or resistance adaptation to malathion in the boll weevil. Although this suggests continued effectiveness of malathion in the field, we identified important temporal and qualitative differences in gene expression between weevils exposed to two different concentrations of malathion. We also identified several tandem isoforms of the detoxifying esterase B1 and glutathione S-transferases, which are putatively associated with organophosphate resistance.

The redlegged earth mite draft genome provides new insights into pesticide resistance evolution and demography in its invasive Australian range

Genomic data provide valuable insights into pest management issues such as resistance evolution, historical patterns of pest invasions and ongoing population dynamics. We assembled the first reference genome for the redlegged earth mite, Halotydeus destructor (Tucker, 1925), to investigate adaptation to pesticide pressures and demography in its invasive Australian range using whole-genome pool-seq data from regionally distributed populations. Our reference genome comprises 132 autosomal contigs, with a total length of 48.90 Mb. We observed a large complex of ace genes, which has presumably evolved from a long history of organophosphate selection in H. destructor and may contribute towards organophosphate resistance through copy number variation, target-site mutations and structural variants. In the putative ancestral H. destructor ace gene, we identified three target-site mutations (G119S, A201S and F331Y) segregating in organophosphate-resistant populations. Additionally, we identified two new para sodium channel gene mutations (L925I and F1020Y) that may contribute to pyrethroid resistance. Regional structuring observed in population genomic analyses indicates that gene flow in H. destructor does not homogenize populations across large geographic distances. However, our demographic analyses were equivocal on the magnitude of gene flow; the short invasion history of H. destructor makes it difficult to distinguish scenarios of complete isolation vs. ongoing migration. Nonetheless, we identified clear signatures of reduced genetic diversity and smaller inferred effective population sizes in eastern vs. western populations, which is consistent with the stepping-stone invasion pathway of this pest in Australia. These new insights will inform development of diagnostic genetic markers of resistance, further investigation into the multifaceted organophosphate resistance mechanism and predictive modelling of resistance evolution and spread.

The dynamic changes of genes revealed that persistently overexpressed genes drive the evolution of cyflumetofen resistance in Tetranychus cinnabarinus

Changes in gene expression are associated with the evolution of pesticide resistance in arthropods. In this study, transcriptome sequencing was performed in 3 different resistance levels (low, L; medium, M; and high, H) of cyflumetofen-resistant strain (YN-CyR). A total of 1 685 genes, including 97 detoxification enzyme genes, were upregulated in all 3 stages, of which 192 genes, including 11 detoxification enzyme genes, showed a continuous increase in expression level with resistance development (L to H). RNA interference experiments showed that overexpression of 7 genes (CYP392A1, TcGSTd05, CCE06, CYP389A1, TcGSTz01, CCE59, and CYP389C2) is involved in the development of cyflumetofen resistance in Tetranychus cinnabarinus. The recombinant CYP392A1 can effectively metabolize cyflumetofen, while CCE06 can bind and sequester cyflumetofen in vitro. We compared 2 methods for rapid screening of resistance molecular markers, including short-term induction and 1-time high-dose selection. Two detoxification enzyme genes were upregulated in the field susceptible strain (YN-S) by induction with 20% lethal concentration (LC₂₀ ) of cyflumetofen. However, 16 detoxification enzyme genes were upregulated by 1-time selection with LC₈₀ of cyflumetofen. Interestingly, the 16 genes were overexpressed in all 3 resistance stages. These results indicated that 1 685 genes that were upregulated at the L stage constituted the basis of cyflumetofen resistance, of which 192 genes in which upregulation continued to increase were the main driving force for the development of resistance. Moreover, the 1-time high-dose selection is an efficient way to rapidly obtain the resistance-related genes that can aid in the development of resistance markers and resistance management in mites.

Frequencies and mechanisms of pesticide resistance in Tetranychus urticae field populations in China

The two-spotted spider mite Tetranychus urticate is an important agricultural pest worldwide. It is extremely polyphagous and has developed resistance to many pesticides. Here, we assessed the pesticide resistance of seven field populations of T. urticae in China, their target site mutations and the activities of their detoxification enzymes. The results showed that abamectin and the traditional pesticides pyridaben, profenofos and bifenthrin had higher resistance or lower toxicity than more recently developed pesticides including chlorfenapyr, spinetoram, cyflumetofen, cyenopyrafen, bifenazate and B-azolemiteacrylic. The frequency of point mutations related to abamectin resistance, G314D in the glutamate-gated chloride channel 1 (GluCl1) and G326E in GluCl3, ranged 47%-70% and 0%-97%, respectively. The frequency of point mutations in A1215D and F1538I of the voltage-gated sodium channel gene (VGSC), which may increase resistance to pyrethroids, ranged 88%-100% and 10%-100%, respectively. For target sites related to organophosphate resistance, mutation frequencies ranged 25%-92% for G119S and 0%-23% for A201S in the acetycholinesterase gene (Ace). Mutation G126S in the bifenazate resistance-related cytochrome b gene (Cytb) was observed in three of the seven T. urticae populations. Higher activities of detoxification enzymes (P450, GST, CarEs and UGTs) were observed in two T. urticae populations, with significant difference in the XY-SX population. These results provide useful information on the status of pesticide resistance of T. urticae in China and suggest that T. urticae field populations may have multiple resistance mechanisms.

Associations between acetylcholinesterase-1 mutations and chlorpyrifos resistance in beet armyworm, Spodoptera exigua

Control of the beet armyworm, Spodoptera exigua depends heavily on chemical insecticides. Chlorpyrifos, an acetylcholinesterase (AChE) inhibitor, has been used in beet armyworm control for many years in China. Here we describe high level resistance to chlorpyrifos in a S. exigua strain, FX19-R, which was developed from a field-collected Chinese strain (FX) by selection with chlorpyrifos in the laboratory. FX19-R showed 1001-fold resistance to chlorpyrifos compared with the laboratory reference strain WH-S. The esterase inhibitor triphenyl phosphate (TPP) provided significant but small synergism (only 3.5-fold) for chlorpyrifos and neither of the glutathione s-transferase depletor diethyl maleate and the cytochrome P450s inhibitor piperonyl butoxide provided any detectable synergism, indicating that AChE insensitivity may play the major role in the resistance in FX19-R. Consistent with this, an amino acid substitution, F443Y (F331Y in standard Torpedo californica numbering) in AChE1 was identified in the FX19-R strain and shown to be tightly linked to chlorpyrifos resistance. Precisely homologous substitutions have been associated with organophosphate resistance in other pest species. A novel amino acid substitution, G311S (or G198S in standard numbering), was also identified in the reference strain WH-S. Recombinantly expressed AChE1 proteins carrying the G311S and F443Y substitutions were about 4.2-fold and 210-fold less sensitive to inhibition by chlorpyrifos oxon than wild-type AChE1, respectively. These results enhance our understanding of the mechanisms of chlorpyrifos resistance and provide a basis for resistance management based on monitoring the F443Y and G311S substitutions.

Geographical distribution and origin of acetylcholinesterase mutations conferring acaricide resistance in Tetranychus urticae populations from Turkey

Two-spotted spider mite, Tetranychus urticae Koch (Acari: Tetranychidae), is a cosmopolitan pest species that can feed on more than 1000 host plant species. Historically, organophosphate (OP) and carbamate insecticides have been used to control this extremely polyphagous pest. However, its ability to develop acaricide resistance rapidly has led to failure in control. Mutations in acetylcholinesterase gene (ace), the target-site of OP and carbamate insecticides, have been reported to be one of the major mechanisms underlying this developing resistance. In this study, mutations previously associated with resistance (G119S, A201S, T280A, G328A, F331W/Y) in ace have been screened in 37 T. urticae populations collected across Turkey. All mutations were found in various populations, except G119S. Almost all populations had F331W/Y mutation (being fixed in 32 populations), whereas only two populations harboured A201S mutation, but not fixed. On the other hand, more than half of the populations contained T280A and G328A mutations. In addition, the presence of same haplotypes in populations originating from distinct geographic locations and a wide variety of ace haplotypes might indicate multiple origins of F331W and F331Y mutations; however, this needs further investigation. The results of area-wide screening showed that ace mutations are widely distributed among T. urticae populations. Therefore, the use of this group of insecticides should be limited or only rotational use might be regarded as a resistance management tool due to its different mode of action from other main acaricide groups in T. urticae control across Turkey.

Multiple TaqMan qPCR and droplet digital PCR (ddPCR) diagnostics for pesticide resistance monitoring and management, in the major agricultural pest Tetranychus urticae

BACKGROUND

Decisions on which pesticide to use in agriculture are expected to become more difficult, as the number of available chemicals is decreasing. For Tetranychus urticae (T. urticae), a major pest for which a number of candidate markers for pesticide resistance are in place, molecular diagnostics could support decision-making for the rational use of acaricides.

RESULTS

A suite of 12 TaqMan qPCR assays [G314D (GluCl1), G326E, I321T (GluCl3), G119S, F331W (Ace-1), H92R (PSST), L1024V, F1538I (VGSC), I1017F (CHS1), G126S, S141F, P262T (cytb)], were validated against Sanger-sequencing, and subsequently adapted for use with the ddPCR technology. The concordance correlation coefficient between the actual and ddPCR measured mutant allelic frequencies was 0.995 (95% CI = 0.991-0.998), and no systematic, proportional, or random differences were detected. The achieved Limit of Detection (LoD) was 0.1% (detection of one mutant in a background of 999 wild type mites). The ddPCR assay panel was then assessed in terms of agreement with phenotypic resistance, through a pilot application in field populations from Crete, with strong correlation and thus predictive and diagnostic value of the molecular assays in some cases (e.g., etoxazole and abamectin resistance). Molecular diagnostics were able to capture incipient resistance that was otherwise missed by phenotypic bioassays. The molecular and phenotypic resistance screening of T. urticae field populations from Crete, revealed both multi-resistant and susceptible populations.

CONCLUSION

The highly sensitive T. urticae molecular diagnostic platforms developed in this study could prove a valuable tool for pesticide resistance management. © 2021 Society of Chemical Industry.

Pirimicarb resistance and associated mechanisms in field-collected and selected populations of Neoseiulus californicus

The predatory mite Neoseiulus californicus McGregor (Acari: Phytoseiidae) is an important natural enemy of phytophagous mites, and naturally established populations are often found in apple orchards. However, insecticide applications to control pests cause side effects to non-target organisms such as N. californicus. Pirimicarb, a widely used carbamate insecticide in apple orchards, is generally considered a selective aphidicide, however, toxicity to beneficial insects and predatory mites has been reported. Furthermore, the molecular basis for this selectivity, if present in N. californicus, is still largely unknown. In this study, 8 field-collected N. californicus populations were investigated and showed up to 27-fold resistance compared to a susceptible laboratory population. Selection in the laboratory for 5 consecutive generations resulted in a 69-fold pirimicarb resistance. Although there were no significant difference in terms of the acetlycholinesterase (AChE) activity between susceptible and field-collected populations, the selected population exhibited a significantly higher AChE activity. In addition, gene copy number variation of acetylcholinesterase (ace) gene among populations was detected and ranged from 1.6 to 2.1-fold relative to the susceptible population. All field-collected populations, but not the selected population, had a significantly higher ace copy number compared to the susceptible population (t-test, p < 0.05). Molecular analysis of the target-site (AChE) revealed, for the first time, a phenylalanine to tryptophan substition at position 331 in AChE (Torpedo californica numbering), both in field-collected and the selected population, but not in the susceptible population. Last, the selected F5 population consumed significantly more Tetranychusurticae adults than the parental population. Together, the results of this study shed light on the molecular determinants of acaricide selectivity in predatory mites, and will contribute to a better design of an integrated mite management program, including the use of pesticide resistant N. californicus in apple orchards.

Common substitution mutation F348Y of acetylcholinesterase gene contributes to organophosphate and carbamate resistance in Cimex lectularius and C. hemipterus

Bed bug control highly depends on insecticides with a limited number of modes of action, especially since the global prevalence of pyrethroid resistance. De facto insecticide options against bed bugs in Japan are acetylcholinesterase inhibitors (AChEis) that consist of organophosphates and carbamates. However, the status of AChEi resistance and the mechanisms involved have not been ascertained. An amino acid substitution mutation, F348Y (or F331Y in standard numbering), occurring at an acyl-binding site of the paralogous AChE gene (p-Ace), was identified among AChEi-resistant colonies of both common and tropical bed bugs (Cimex lectularius and C. hemipterus, respectively). This mutation was genetically associated with propoxur and fenitrothion resistance in F348Y-segregating colonies of C. hemipterus. Inhibition of heterologously expressed C. lectularius p-Ace with insecticides revealed that the sensitivities of F348Y-carrying AChE decreased by orders of 10- to more than 100-fold for diazoxon, carbaryl, fenitroxon, paraoxon, chlorpyrifos-methyl, malaoxon, azamethiphos, methyl-paraoxon, and propoxur. In contrast, the mutant AChE showed a slightly decreased degree of sensitivity for dichlorvos and almost unchanged sensitivity for metoxadiazone. Further studies are needed to ascertain whether the practical efficacies of dichlorvos and metoxadiazone are ensured against F348Y-carrying bed bugs and whether other resistance mechanisms are involved.

Population structure and insecticide resistance status of Tuta absoluta populations from Turkey

BACKGROUND

Tuta absoluta is a devastating pest in tomato production areas worldwide. After its first introduction to Turkey in 2009, it quickly became the major pest of tomato-growing areas. Although some biocontrol agents have been used, especially in greenhouses, the main control of T. absoluta relies heavily on chemical insecticides. However, failure in chemical control has often been reported due to resistance development. In this study, we investigated (i) the population structure of 22 T. absoluta populations across Turkey by analysing haplotypes, based on the cytochrome oxidase subunit I gene; (ii) the efficacy of three registered insecticides from different classes (metaflumizone, chlorantraniliprole and spinosad) in real field-greenhouse conditions; and (iii) the geographic distribution of target-site mutations associated with insecticide resistance.

RESULTS

The efficacy of spinosad was higher than that of chlorantraniliprole and metaflumizone in the greenhouse trials, as documented by the mortality rates obtained, up to 14 days post application. Known resistance mutations in ryanodine receptors (RyR) (i.e. the I4790M/K and G4946E), nicotinic acetylcholine receptors (G275E), acetylcholinesterases (A201S) and voltage-gated sodium channels (F1845Y and V1848I) were found at various frequencies across the populations genotyped. The I4790K diamide resistance mutation in the RyR has been reported for the first time in T. absoluta populations. Although a total of eight haplotypes were found, the overall mean genetic distance was lower than 0.001, indicating the high genetic homogeneity among Turkish T. absoluta populations.

CONCLUSION

The results will contribute to design area-wide resistance management programs in T. absoluta control in Turkey. However, more monitoring studies are needed to implement evidence-based insecticide resistance management strategies in the frame of integrated pest management. © 2021 Society of Chemical Industry.

Dimethoate induces genotoxicity as a result of oxidative stress: in vivo and in vitro studies

Dimethoate ([O,O-dimethyl S-(N-methylcarbamoylmethyl) phosphorodithioate]) is an organophosphate insecticide and acaricide widely used for agricultural purposes. Genotoxicity refers to the ability of a chemical agent interact directly to DNA or act indirectly leading to DNA damage by affecting spindle apparatus or enzymes involved in DNA replication, thereby causing mutations. Taking into consideration the importance of genotoxicity induced by dimethoate, the purpose of this manuscript was to provide a mini review regarding genotoxicity induced by dimethoate as a result of oxidative stress. The present study was conducted on studies available in MEDLINE, PUBMED, EMBASE, and Google scholar for all kind of articles (all publications published until May, 2020) using the following key words: dimethoate, omethoate, DNA damage, genetic damage, oxidative stress, genotoxicity, mutation, and mutagenicity. The results showed that many studies were published in the scientific literature; the approach was clearly demonstrated in multiple tissues and organs, but few papers were designed in humans. In summary, new studies within the field are important for better understanding the pathobiological events of genotoxicity on human cells, particularly to explain what cells and/or tissues are more sensitive to genotoxic insult induced by dimethoate.

Acetylcholinesterase target sites for developing environmentally friendly insecticides against Tetranychus urticae (Acari: Tetranychidae)

The non-target toxicity and resistance problems of acetylcholinesterase (AChE) insecticides, such as organophosphates and carbamates, are of growing concern. To explore the potential targets for achieving inhibitor selectivity, the AChE structures at or near the catalytic pocket of Tetranychus urticae (TuAChE), honey bees, and humans were compared. The entrances to the AChE catalytic pocket differ significantly because of their different peripheral sites. The role of these potential mite-specific sites in AChE function was further elucidated by site-directed mutagenesis of these sites and then examining the catalytic activities of TuAChE mutants. The spider mite E₃₁₆, H₃₆₉, and V₁₀₅ active sites are important for AChE function. By further analyzing their physostigmine inhibitory properties and the detailed interaction between physostigmine and TuAChE, the peripheral site H₃₆₉ locating near the gorge entrance, and S₁₅₄ at the oxyanion hole, affects substrate and inhibitor trafficking. The discovery of conserved mite-specific residues in Tetranychus will enable the development of safer, effective pesticides that target residues present only in mite AChEs, potentially offering effective control against this important agricultural pest.

Impaired neuromuscular function by conjoint actions of organophosphorus insecticide metabolites omethoate and cyclohexanol with implications for treatment of respiratory failure

BACKGROUND

Ingestion of agricultural organophosphorus insecticides is a significant cause of death in rural Asia. Patients often show acute respiratory failure and/or delayed, unexplained signs of neuromuscular paralysis, sometimes diagnosed as "Intermediate Syndrome". We tested the hypothesis that omethoate and cyclohexanol, circulating metabolites of one agricultural formulation, cause muscle weakness and paralysis.

METHODS

Acetylcholinesterase activity of insecticide components and metabolites was measured using purified enzyme from eel electroplaque or muscle homogenates. Mechanomyographic recording of pelvic limb responses to nerve stimulation was made in anaesthetized pigs and isometric force was recorded from isolated nerve-muscle preparations from mice. Omethoate and cyclohexanol were administered intravenously or added to physiological saline bathing isolated muscle. We also assessed the effect of MgSO₄ and cooling on neuromuscular function.

RESULTS

Omethoate caused tetanic fade in pig muscles and long-lasting contractions of the motor innervation zone in mouse muscle. Both effects were mitigated, either by i.v. administration of MgSO₄ in vivo or by adding 5 mM Mg²⁺ to the medium bathing isolated preparations. Combination of omethoate and cyclohexanol initially potentiated muscle contractions but then rapidly blocked them. Cyclohexanol alone caused fade and block of muscle contractions in pigs and in isolated preparations. Similar effects were observed ex vivo with cyclohexanone and xylene. Cyclohexanol-induced neuromuscular block was temperature-sensitive and rapidly reversible.

CONCLUSIONS

The data indicate a crucial role for organophosphorus and solvent metabolites in muscle weakness following ingestion of agricultural OP insecticide formulations. The metabolites omethoate and cyclohexanol acted conjointly to impair neuromuscular function but their effects were mitigated by elevating extracellular Mg²⁺ and decreasing core temperature, respectively. Clinical studies of MgSO₄ therapy and targeted temperature management in insecticide-poisoned patients are required to determine whether they may be effective adjuncts to treatment.

Identification and characterization of striking multiple-insecticide resistance in a Tetranychus urticae field population from Greece

BACKGROUND

Tetranychus urticae is a notorious crop pest with a worldwide distribution that has developed resistance to a wide range of acaricides. Here, we investigated the resistance levels of a T. urticae population collected from an ornamental greenhouse in Peloponnese, Greece, and analyzed its resistance mechanisms at the molecular level.

RESULTS

Toxicological assays showed resistance against compounds with different modes of action, with resistance ratios of: 89-fold for abamectin; > 1000-fold for clofentezine; > 5000-fold for etoxazole; 27-fold for fenpyroximate and pyridaben; 20- and 36-fold for spirodiclofen and spirotetramat, respectively; and 116- and > 500-fold for cyenopyrafen and cyflumetofen, respectively. Bioassays with synergists indicated the involvement of detoxification enzymes in resistance to abamectin, but not to cyflumetofen and spirodiclofen. RNA sequencing (RNA-seq) analysis showed significant over-expression of several genes encoding detoxification enzymes such as cytochrome P450 monooxygenases and UDP-glycosyltransferases, which have been previously associated with acaricide resistance. Known target-site resistance mutations were identified in acetyl-choline esterase, chitin synthase 1 and NDUFS7/psst, but putative novel resistance mutations were also discovered in targets such as glutamate-gated chloride channel subunit 3. Interestingly, target-site resistance mutations against pyrethroids or bifenazate were not identified, possibly indicating a recent reduced selection pressure in Greece, as well as a possible opportunity to rotate these chemistries.

CONCLUSION

We identified and characterized a striking case of multiple acaricide resistance in a field population of T. urticae. Exceptionally strong resistance phenotypes, with accumulation of multiple resistance mutations and over-expression of P450s and other detoxification genes in the same field population are reported.

Dichlorvos Resistance in the House Fly Populations, Musca domestica, of Iranian Cattle Farms

Background:

Insecticide resistance is one of the most important problems associated with the control of Musca domestica, due to the potential of the rapid development of resistance to different chemical insecticides. The present study was carried out to evaluate dichlorvos resistance in the house fly populations collected from central regions of Iran, Isfahan Province and Chaharmahal and Bakhtiari Province, during 2017 to 2019.

Methods:

Bioassays were carried out using a standard topical application method as well as a fumigation method. The Koohrang population (susceptible) with the lowest LD₅₀ values to dichlorvos was chosen to calculate the resistance ratios (RR). Altered sensitivity of acetylcholinesterase (AChE), a target enzyme for dichlorvos, was investigated.

Results:

According to the results, very high levels of dichlorvos resistance were observed in the Mobarake population (RR= 80.25-fold by topical application and 33-fold by fumigation bioassay), and Isfahan population (RR= 107.30-fold by topical application and 43-fold by fumigation bioassay) compared to the Koohrang population. Acetylcholinesterase of the Koohrang population was the most sensitive to inhibition by dichlorvos based on the determination of median inhibitory concentration (IC₅₀), but AChE of Mobarake and Isfahan populations were 741.93- and 343.94- fold less sensitive to inhibition.

Conclusion:

The insensitivity of AChE was possibly involved in dichlorvos resistance in the house fly populations.

Acaricide resistance status and identification of resistance mutations in populations of the two-spotted spider mite Tetranychus urticae from Ethiopia

The intensive use of pesticides is a common practice for the management of the two-spotted spider mite, Tetranychus urticae, in greenhouses and field farms of Ethiopia. However, incidence of resistance and possible resistance mechanisms in T. urticae populations from Ethiopia have not yet been studied. Here, we assessed the toxicity of various acaricides-bifenazate, abamectin, emamectin benzoate, profenofos, fenbutatin oxide, fenpyroximate, amitraz and chlorfenapyr-on T. urticae populations sampled from six flower greenhouse farms, three strawberry greenhouse farms, one field-grown vegetable farm and two wild populations. In parallel, all populations were screened for known target-site mutations. All tested populations were fully susceptible to bifenazate, abamectin, emamectin benzoate and profenofos, but resistant against fenbutatin oxide and fenpyroximate. Four populations showed considerable levels of resistance against amitraz and one population was resistant to chlorfenapyr. Several target-site mutations were identified in the tested populations, including G119S, A201S, T280A, G328A and F331W/C/Y in acetylcholinesterase and the F1538I and L1024V mutation in the voltage-gated sodium channel. The F1538I mutation was found in eight out of 12 populations, whereas the L1024V mutation was only found in two populations. The H92R mutation in the PSST subunit of complex I and the I1017F mutation in chitin synthase 1 was detected in half of the tested populations. The G326E and I321T mutations in the glutamate-gated chloride channel 3 were also detected, but more rarely, whereas mitochondrial cytochrome b mutations were not detected. The current study revealed multiple resistance patterns in Ethiopian T. urticae populations and together with the wide presence of target-site mutations, calls for the wise use of acaricides in the management of T. urticae in Ethiopia.

Co-Expression of a Homologous Cytochrome P450 Reductase Is Required for In Vivo Validation of the Tetranychus urticae CYP392A16-Based Abamectin Resistance in Drosophila

Simple Summary

The two-spotted spider mite, Tetranychus urticae, is one of the most damaging agricultural pests worldwide, feeding on over 1100 plant species and causing extensive damage to several crops. Chemical acaricides remain the most widely used strategy to control this pest. However, T. urticae has developed significant resistance to numerous acaricide compounds, due to certain features of mite biology and extensive acaricide applications that lead to the selection of resistant pests and subsequently the emergence of resistant populations. Several molecular/genetic mechanisms may contribute to these highly resistant phenotypes. Such mechanisms frequently involve expression of P450 detoxification enzymes, which act together with a partner protein named cytochrome P450 reductase (CPR). In this study, we investigated the potential of a mite P450 enzyme, CYP392A16, to confer resistance to the acaricide abamectin in vivo, when expressed in tissues of the model fruit fly Drosophila melanogaster. We confirmed that expression of this enzyme contributes to abamectin resistance in the fruit fly model, but only when a homologous mite CPR is co-expressed. Our findings indicate that the Drosophila model system can be engineered to facilitate validation of the candidate mite P450s, in order to elucidate resistance mechanisms and their underlying interactions.

Abstract

Overexpression of the cytochrome P450 monooxygenase CYP392A16 has been previously associated with abamectin resistance using transcriptional analysis in the two-spotted spider mite Tetranychus urticae, an important pest species worldwide; however, this association has not been functionally validated in vivo despite the demonstrated ability of CYP392A16 to metabolize abamectin in vitro. We expressed CYP392A16 in vivo via a Gal4 transcription activator protein/Upstream Activating Sequence (GAL4/UAS) system in Drosophila melanogaster flies, driving expression with detoxification tissue-specific drivers. We demonstrated that CYP392A16 expression confers statistically significant abamectin resistance in toxicity bioassays in Drosophila only when its homologous redox partner, cytochrome P450 reductase (TuCPR), is co-expressed in transgenic flies. Our study shows that the Drosophila model can be further improved, to facilitate the functional analysis of insecticide resistance mechanisms acting alone or in combination.

Resistance to pyridaben in Canadian greenhouse populations of two-spotted spider mites, Tetranychus urticae (Koch)

Two-spotted spider mite (TSSM) Tetranychus urticae (Koch) is an important agricultural pest that causes considerable yield losses to over 150 field and greenhouse crops. Mitochondrial electron transport inhibitors (METI) acaricides are commonly used to control mite species in commercial Canadian greenhouses. Development of resistance to METIs in TSSM populations have been reported worldwide, but not until recently in Canada. The objectives of this study were to: 1) monitor the acaricide-susceptibility in greenhouse TSSM populations, and 2) investigate the resistance to pyridaben, a METI acaricide, in greenhouse resistant and pyridaben-selected (SRS) mite strains. The increased mortality to the pyridaben sub-lethal concentration (LC₃₀) when SRS mites were exposed to piperonyl butoxide (PBO), a general cytochrome P450 monooxygenase inhibitor, and higher P450 activity compared to the greenhouse strain (RS) mites, indicated that P450s may be at least partially responsible for the resistance. The molecular mechanisms of target site insensitivity-mediated resistance in the pyridaben resistant strain of TSSM were investigated by comparing the DNA sequence of NADH dehydrogenase subunits TYKY and PSST, NADH-ubiquinone oxidoreductase chain 1 and 5 (ND1, ND5) and the NADH-ubiquinone oxidoreductase subunit 49 kDa from SRS to the reference strain (SS) and RS. Despite a number of nucleotide substitutions, none correlated with the pyridaben resistance. Understanding the underlying mechanisms of TSSM adaptation to acaricides is an essential part of resistance management strategy in any IPM program. The findings of this study will encourage growers to apply acaricides with different modes of action to reduce the rate at which acaricide resistance will occur in greenhouse TSSM populations.

Target-Site Mutations and Glutathione S-Transferases Are Associated with Acequinocyl and Pyridaben Resistance in the Two-Spotted Spider Mite Tetranychus urticae (Acari: Tetranychidae)

Simple Summary

The two-spotted spider mite Tetranychus urticae is a difficult-to-control pest due to its short life cycle and rapid resistance development. In this study, we characterized field strains collected in 2001 and 2003 that have been selected for acequinocyl resistance and pyridaben resistance, respectively. These strains displayed resistance ratios of 1798.6 and 5555.6, respectively, and were screened for cross-resistance against several currently used acaricides. The acequinocyl resistant strain exhibited pyridaben cross-resistance, but the pyridaben resistant strain showed no cross-resistance. The acequinocyl resistant strain exhibited point mutations in cytb (I256V and N321S) and PSST (H92R). In contrast, the pyridaben resistant strain exhibited the H92R but not the I256V and N321S point mutations. In addition, the increased GST metabolism and GST delta expression might be related to acequinocyl resistance in Tetranychus urticae. We hope that the data and patterns described here can now be exploited in the continued quest for rational resistance management strategies.

Abstract

The two-spotted spider mite Tetranychus urticae is a difficult-to-control pest due to its short life cycle and rapid resistance development. In this study, we characterized field strains collected in 2001 and 2003 that were selected for acequinocyl resistance (AR) and pyridaben resistance (PR), respectively. These strains displayed resistance ratios of 1798.6 (susceptible vs. AR) and 5555.6 (susceptible vs. PR), respectively, and were screened for cross-resistance against several currently used acaricides. The AR strain exhibited pyridaben cross-resistance, but the PR strain showed no cross-resistance. The AR strain exhibited point mutations in cytb (I256V, N321S) and PSST (H92R). In contrast, the PR strain exhibited the H92R but not the I256V and N321S point mutations. In some cases increased glutathione S-transferase (GST) activity has previously been linked to enhanced detoxification. The AR strain exhibited approximately 2.3-, 1.8-, and 2.2-fold increased GST activity against 1-chloro-2,4-dinitrobenzene (CDNB), 1,2-dichloro-4-nitrobenzene (DCNB), and 4-nitrobenzyl chloride (NBC), respectively. Among the five GST subclass genes (delta, omega, mu, zeta, and kappa), the relative expression of delta class GSTs in the AR strain were significantly higher than the PR and susceptible strain. These results suggest that the I256V and N321S mutations and the increased GST metabolism and GST delta overexpression might be related to acequinocyl resistance in T. urticae.

Identification and Functional Study of Chitin Metabolism and Detoxification-Related Genes in Glyphodes pyloalis Walker (Lepidoptera: Pyralidae) Based on Transcriptome Analysis

Glyphodes pyloalis Walker (Lepidoptera: Pyralididae) is a serious pest in the sericulture industry, which has caused damage and losses in recent years. With the widespread use of insecticides, the insecticide resistance of G. pyloalis has becomes increasingly apparent. In order to find other effective methods to control G. pyloalis, this study performed a transcriptome analysis of the midgut, integument, and whole larvae. Transcriptome data were annotated with KEGG and GO, and they have been shown to be of high quality by RT-qPCR. The different significant categories of differentially expressed genes between the midgut and the integument suggested that the transcriptome data could be used for next analysis. With the exception of Dda9 (GpCDA5), 19 genes were involved in chitin metabolism, most of which had close protein–protein interactions. Among them, the expression levels of 11 genes, including GpCHSA, GpCDA1, GpCDA2, GpCDA4, GPCHT1, GPCHT2a, GPCHT3a, GPCHT7, GpTre1, GpTre2, and GpRtv were higher in the integument than in the midgut, while the expression levels of the last eight genes, including GpCHSB, GpCDA5, GpCHT2b, GpCHT3b, GpCHT-h, GpPAGM, GpNAGK, and GpUAP, were higher in the midgut than in the integument. Moreover, 282 detoxification-related genes were identified and can be divided into 10 categories, including cytochrome P450, glutathione S-transferase, carboxylesterase, nicotinic acetylcholine receptor, aquaporin, chloride channel, methoprene-tolerant, serine protease inhibitor, sodium channel, and calcium channel. In order to further study the function of chitin metabolism-related genes, dsRNA injection knocked down the expression of GpCDA1 and GpCHT3a, resulting in the significant downregulation of its downstream genes. These results provide an overview of chitin metabolism and detoxification of G. pyloalis and lay the foundation for the effective control of this pest in the sericulture industry.

Target-site mutations (AChE-G119S and kdr) in Guangxi Anopheles sinensis populations along the China-Vietnam border

Background

In South Asia, the epidemiology of malaria is complex, and transmission mainly occurs in remote areas near international borders. Vector control has been implemented as a key strategy in malaria prevention for decades. A rising threat to the efficacy of vector control efforts is the development of insecticide resistance, thus it is important to monitor the type and frequency of insecticide resistant alleles in the disease vectors such as An. sinensis along the China-Vietnam border. Such information is needed to synthesize effective malaria vector control strategies.

Methods

A total of 208 adults of An. sinensis, collected from seven sites in southwest Guangxi along the China-Vietnam border, were inspected for the resistance-conferring G119S mutation in acetylcholinesterase (AChE) by PCR-RFLP (polymerase chain reaction restriction fragment length polymorphism) and kdr mutations in the voltage-gated sodium channel (VGSC) by sequencing. In addition, the evolutionary origin of An. sinensis vgsc gene haplotypes was analyzed using Network 5.0.

Results

The frequencies of mutant 119S of AChE were between 0.61–0.85 in the seven An. sinensis populations. No susceptible homozygote (119GG) was detected in three of the seven sites (DXEC, LZSK and FCGDX). Very low frequencies of kdr (0.00–0.01) were detected in the seven populations, with most individuals being susceptible homozygote (1014LL). The 1014F mutation was detected only in the southeast part (FCGDX) at a low frequency of 0.03. The 1014S mutation was distributed in six of the seven populations with frequencies ranging from 0.04 to 0.08, but absent in JXXW. Diverse haplotypes of 1014L and 1014S were found in An. sinensis along the China-Vietnam border, while only one 1014F haplotype was detected in this study. Consistent with a previous report, resistant 1014S haplotypes did not have a single origin.

Conclusions

The G119S mutation of AChE was present at high frequencies (0.61–0.85) in the An. sinensis populations along the China-Vietnam border, suggesting that the vector control authorities should be cautious when considering carbamates and organophosphates as chemicals for vector control. The low frequencies (0.00–0.11) of kdr in these populations suggest that pyrethroids remain suitable for use against An. sinensis in these regions.

Status of pesticide resistance and associated mutations in the two-spotted spider mite, Tetranychus urticae, in China

The two-spotted spider mite, Tetranychus urticae Koch, is a serious agricultural pest that has developed resistance to many pesticides. A leaf dip assay was used to assess the resistance of seven field populations of T. urticae to 11 pesticides in China. The mutation frequencies of target genes related to pesticide resistance were also determined. The results showed that all seven field populations had high or extremely high resistance to abamectin and had low or moderate resistance to newly developed pesticides including bifenazate, cyenopyrafen, chlorfenapyr, B-azolemiteacrylic, and spinetoram. The RF values for the traditional acaricides bifenthrin, pyridaben, and profenofos were low for all seven populations. For target sites related to abamectin resistance, the frequency of the mutations ranged from 28.33 to 63.64% for G314D in the glutamate-gated chloride channel gene 1 (GluCl1), and from 0 to 95% for G326E in the glutamate-gated chloride channel gene 3 (GluCl3). For target sites related to organophosphate resistance, the frequency of mutations ranged from 33.33 to 56.67% for G119S and from 5.00 to 43.33% for A201S in the acetycholinesterase gene (Ace). For target sites related to pyrethroid resistance, the frequency of the mutations ranged from 76.67 to 98.33% for A1215D and from 3.33 to 100% for F1538I in the voltage-gated sodium channel gene (VGSC). No mutations were detected in the bifenazate resistance-related cytochrome b gene (Cytb). These results will be useful for managing T. urticae resistance to pesticides in China.

Squamocin induce histological and ultrastructural changes in the midgut cells of Anticarsia gemmatalis (Lepidoptera: Noctuidae)

Annonaceous acetogenins (Annona squamosa Linnaeus) comprises of a series of natural products which are extracted from Annonaceae species, squamocin proved to be highly efficient among those agents. Squamocin is mostly referred as a lethal agent for midgut cells of different insects, with toxic effects when tested against larva of some insects. In present study, LC₅₀ and LC₉₀ of squamocin for A. gemmatalis Hübner (Lepidoptera: Noctuidae) were calculated using probit analysis. Morphological changes in midgut cells were analyzed under light, fluorescence and transmission electron microscopes when larvae were treated with LC₅₀ and LC₉₀ of squamocin for 24, 48 and 72 h. Results revealed that the maximum damage to midgut cells was found under LC₉₀ where it showed digestive cells with enlarged basal labyrinth, highly vacuolated cytoplasm, damaged apical surface, cell protrusions to the gut lumen, autophagy and cell death. The midgut goblet cells showed a strong disorganization of their microvilli. Likewise, in insects treated with squamocin, mitochondria were not marked with Mitotracker fluorescent probe, suggesting some molecular damage in these organelles, which was reinforced by decrease in the respiration rate in these insects. These results demonstrate that squamocin has potential to induce enough morphological changes in midgut through epithelial cell damage in A. gemmatalis.

The relative contribution of target-site mutations in complex acaricide resistant phenotypes as assessed by marker assisted backcrossing in Tetranychus urticae

The mechanisms underlying insecticide and acaricide resistance in insects and mites are often complex, including additive effects of target-site insensitivity, increased metabolism and transport. The extent to which target-site resistance mutations contribute to the resistance phenotype is, however, not well studied. Here, we used marker-assisted backcrossing to create 30 congenic lines carrying nine mutations (alone, or in combination in a few cases) associated with resistance to avermectins, pyrethroids, mite growth inhibitors and mitochondrial complex III inhibitors (QoI) in a polyphagous arthropod pest, the spider mite Tetranychus urticae. Toxicity tests revealed that mutations in the voltage-gated sodium channel, chitin synthase 1 and cytochrome b confer high levels of resistance and, when fixed in a population, these mutations alone can result in field failure of acaricide treatment. In contrast, although we confirmed the implication of mutations in glutamate-gated chloride channels in abamectin and milbemectin insensitivity, these mutations do not lead to the high resistance levels that are often reported in abamectin resistant strains of T. urticae. Overall, this study functionally validates reported target-site resistance mutations in T. urticae, by uncoupling them from additional mechanisms, allowing to finally investigate the strength of the conferred phenotype in vivo.

Molecular diagnostics for detecting pyrethroid and abamectin resistance mutations in Tetranychus urticae

Avermectin and pyrethroid resistance mutations (the G314D and the G326E in the glutamate gated chloride channels, and the F1538I in the voltage gated sodium channel) have been reported in the spider mite Tetranychus urticae, one of the most devastating pests of protected and open field crops worldwide. We developed three TaqMan molecular diagnostic assays for monitoring the presence and frequency of these mutations in T. urticae field populations. The TaqMan assays were validated against known genotypes and subsequently used to monitor the frequency of the resistance mutations in eleven T. urticae populations from Greece and Cyprus, with variable history of avermectin and pyrethroids applications. The frequency of the F1538I pyrethroid resistance mutation largely varied among samples, with highest frequencies (75%-97%) detected in four populations derived from protected and open field crops from Crete and Peloponnesus, low frequencies in three populations (2.5%-11%) from Attiki, Cyprus and Crete and not detected in four populations from Crete, Peloponnesus and Cyprus. The frequency of the abamectin resistance mutations G314D and G326E also varied across populations (from 0 to 100%), showing fixation in two populations (>97.5% for the G314D and 100% for the G326E), originating from rose greenhouses from Greece, low frequencies in three populations (5%-12.5%) also originating from rose greenhouses (Crete, Peloponnesus and Cyprus) and not detected in six populations from protected and open field vegetable crops. The TaqMan diagnostics showed higher resolution in detecting specific alleles in low frequency, compared to massive quantitative sequencing approaches previously employed. They can be used, together with classical bioassays, to support evidence - based insecticide resistance management strategies.

RNA-Seq Analysis Reveals Candidate Targets for Curcumin against Tetranychus cinnabarinus

Tetranychus cinnabarinus is an important agricultural pest with a broad host range. We previously identified curcumin as a promising acaricidal compound against T. cinnabarinus. However, the acaricidal mechanism of curcumin remains unknown. In this study, RNA-seq was employed to analyze the transcriptome changes in T. cinnabarinus treated with curcumin or the solvent. A total of 105,706,297 clean sequence reads were generated by sequencing, with more than 90% of the reads successfully mapped to the reference sequence. The RNA-seq identified 111 and 96 differentially expressed genes between curcumin- and solvent-treated mites at 24 and 48 h after treatment, respectively. GO enrichment analysis of differentially expressed genes showed that the cellular process was the dominant group at both time points. Finally, we screened 23 differentially expressed genes that were functionally identical or similar to the targets of common insecticide/acaricides or genes that were associated with mite detoxification and metabolism. Calmodulin, phospholipase A₂, and phospholipase C were activated upon curcumin treatment suggesting that the calcium channel related genes might play important roles in mite's response to curcumin. Overall our results revealed the global transcriptional changes in T. cinnabarinus after curcumin treatment to enable further identification of the targets of curcumin in mites.

Survey of organophosphate resistance and an Ala216Ser substitution of acetylcholinesterase-1 gene associated with chlorpyrifos resistance in Apolygus lucorum (Meyer-Dür) collected from the transgenic Bt cotton fields in China

The mirid bug is frequently controlled by the application of organophosphorus insecticides in the transgenic Bt cotton field of China. A topical bioassay method was performed to evaluate the toxicities of chlorpyrifos and malathion towards field-collected Chinese populations of Apolygus lucorum from transgenic Bt cotton fields. For chlorpyrifos, the resistance ratios ranged from 0.8 to 9.4-fold compared to a susceptible strain. For malathion, the resistance levels relative to the susceptible strain ranged from 1.2 to 14.4-fold. Compared to a susceptible strain, the Cangzhou population from Hebei province showed the highest resistance ratios towards these insecticides. A comparison of the detoxifying and target enzyme activities between the Cangzhou population and a susceptible strain revealed that altered acetylcholinesterase possibly account for the chlorpyrifos and malathion resistance in the Cangzhou population. Two acetylcholinesterase (AChE-encoding) genes (designated Alace1 and Alace2) from the green mirid bug (A. lucorum) were identified. The Alace1 and Alace2 genes encoded 597 and 645 amino acids, respectively. Both AChE proteins had conserved motifs including a catalytic triad, a choline-binding site, and an acyl pocket. Quantitative real-time PCR analysis showed that Alace1 had a much higher transcriptional level than Alace2, for the expression profiles of both spatial and time distributions. One amino acid substitution, Ala216Ser in Alace1, was found in the Cangzhou population. These results suggest that the mutation Ala216Ser should be most likely involved in organophosphorus resistance in A. lucorum.

The Glutathione-S-Transferase, Cytochrome P450 and Carboxyl/Cholinesterase Gene Superfamilies in Predatory Mite Metaseiulus occidentalis

Pesticide-resistant populations of the predatory mite Metaseiulus (= Typhlodromus or Galendromus) occidentalis (Arthropoda: Chelicerata: Acari: Phytoseiidae) have been used in the biological control of pest mites such as phytophagous Tetranychus urticae. However, the pesticide resistance mechanisms in M. occidentalis remain largely unknown. In other arthropods, members of the glutathione-S-transferase (GST), cytochrome P450 (CYP) and carboxyl/cholinesterase (CCE) gene superfamilies are involved in the diverse biological pathways such as the metabolism of xenobiotics (e.g. pesticides) in addition to hormonal and chemosensory processes. In the current study, we report the identification and initial characterization of 123 genes in the GST, CYP and CCE superfamilies in the recently sequenced M. occidentalis genome. The gene count represents a reduction of 35% compared to T. urticae. The distribution of genes in the GST and CCE superfamilies in M. occidentalis differs significantly from those of insects and resembles that of T. urticae. Specifically, we report the presence of the Mu class GSTs, and the J’ and J” clade CCEs that, within the Arthropoda, appear unique to Acari. Interestingly, the majority of CCEs in the J’ and J” clades contain a catalytic triad, suggesting that they are catalytically active. They likely represent two Acari-specific CCE clades that may participate in detoxification of xenobiotics. The current study of genes in these superfamilies provides preliminary insights into the potential molecular components that may be involved in pesticide metabolism as well as hormonal/chemosensory processes in the agriculturally important M. occidentalis.

Multiple Mutations on the Second Acetylcholinesterase Gene Associated With Dimethoate Resistance in the Melon Aphid, Aphis gossypii (Hemiptera: Aphididae)

The melon aphid, Aphis gossypii Glover (Hemiptera: Aphididae), is an important cosmopolitan and extremely polyphagous species capable of causing direct and indirect damage to various crops. Insecticide resistance in melon aphids is of particular concern. To determine the basis of resistance, organophosphate (OP)-resistant strains of A. gossypii were obtained by continuous selection with dimethoate in the laboratory, and resistance mechanisms were investigated along with susceptible strains. Three resistant strains LKR-1, LKR-2, and LKR-3 exhibiting 270-, 243-, and 210-fold resistance obtained after 30 generations of selection with dimethoate, respectively, were utilized in this study. The role of acetylcholinesterase (AChE), a target enzyme for OPs and carbamates (CMs), was investigated. AChE enzyme assay revealed that there was no significant change in the activities of AChE in resistant and susceptible strains. However, AChE inhibitory assay showed that 50% of the enzyme activity in resistant strains was inhibited at significantly higher concentration of dimethoate (131.87, 158.65, and 99.29 µmolL(−1)) as compared with susceptible strains (1.75 and 2.01 µmolL(−1)), indicating AChE insensitivity owing to altered AChE. Molecular diagnostic tool polymerase chain reaction-restriction fragment length polymorphism revealed the existence of two consistent non-synonymous point mutations, single-nucleotide polymorphism, viz., A302S (equivalent to A201 in Torpedo californica Ayres) and S431F (equivalent to F331 in T. californica), in the AChE gene Ace2 of resistant strains. Further, cloning and sequencing of a partial fragment of Ace2 (897 bp) gene from susceptible and resistant strains revealed an additional novel mutation G221A in resistant strains, LKR-1 and LKR-2. Susceptible Ace2 genes shared 99.6 and 98.9% identity at the nucleic acid and amino acid levels with resistant ones, respectively. Functional analysis of these point mutations was assessed by in silico docking studies using the modeled wild-type and naturally mutated AChE2. Computational analysis showed that the conformational changes in AChE2 active site due to structural gene substitutions (A302S, S431F, and G221A) significantly reduced the level of ligand (OP-dimethoate, omethoate, and CM-pirimicarb) binding, suggesting that they are potentially associated with resistance development. These results unambiguously suggested that multiple mutations located in the enzyme active site are responsible for AChE insensitivity to dimethoate and are likely the molecular basis for dimethoate resistance in these selected field populations of A. gossypii.

Molecular mechanisms of Tetranychus urticae chemical adaptation in hop fields

The two-spotted spider mite, Tetranychus urticae Koch is a major pest that feeds on >1,100 plant species. Many perennial crops including hop (Humulus lupulus) are routinely plagued by T. urticae infestations. Hop is a specialty crop in Pacific Northwest states, where 99% of all U.S. hops are produced. To suppress T. urticae, growers often apply various acaricides. Unfortunately T. urticae has been documented to quickly develop resistance to these acaricides which directly cause control failures. Here, we investigated resistance ratios and distribution of multiple resistance-associated mutations in field collected T. urticae samples compared with a susceptible population. Our research revealed that a mutation in the cytochrome b gene (G126S) in 35% tested T. urticae populations and a mutation in the voltage-gated sodium channel gene (F1538I) in 66.7% populations may contribute resistance to bifenazate and bifenthrin, respectively. No mutations were detected in Glutamate-gated chloride channel subunits tested, suggesting target site insensitivity may not be important in our hop T. urticae resistance to abamectin. However, P450-mediated detoxification was observed and is a putative mechanism for abamectin resistance. Molecular mechanisms of T. urticae chemical adaptation in hopyards is imperative new information that will help growers develop effective and sustainable management strategies.

Transcriptome profiling of a spirodiclofen susceptible and resistant strain of the European red mite Panonychus ulmi using strand-specific RNA-seq

BACKGROUND

The European red mite, Panonychus ulmi, is among the most important mite pests in fruit orchards, where it is controlled primarily by acaricide application. However, the species rapidly develops pesticide resistance, and the elucidation of resistance mechanisms for P. ulmi has not kept pace with insects or with the closely related spider mite Tetranychus urticae. The main reason for this lack of knowledge has been the absence of genomic resources needed to investigate the molecular biology of resistance mechanisms.

RESULTS

Here, we provide a comprehensive strand-specific RNA-seq based transcriptome resource for P. ulmi derived from strains susceptible and resistant to the widely used acaricide spirodiclofen. From a de novo assembly of the P. ulmi transcriptome, we manually annotated detoxification enzyme families, target-sites of commonly used acaricides, and horizontally transferred genes implicated in plant-mite interactions and pesticide resistance. In a comparative analysis that incorporated sequences available for Panonychus citri, T. urticae, and insects, we identified radiations for detoxification gene families following the divergence of Panonychus and Tetranychus genera. Finally, we used the replicated RNA-seq data from the spirodiclofen susceptible and resistant strains to describe gene expression changes associated with resistance. A cytochrome P450 monooxygenase, as well as multiple carboxylcholinesterases, were differentially expressed between the susceptible and resistant strains, and provide a molecular entry point for understanding resistance to spirodiclofen, widely used to control P. ulmi populations.

CONCLUSIONS

The new genomic resources and data that we present in this study for P. ulmi will substantially facilitate molecular studies of underlying mechanisms involved in acaricide resistance.

Phenotypic- and Genotypic-Resistance Detection for Adaptive Resistance Management in Tetranychus urticae Koch

Rapid resistance detection is necessary for the adaptive management of acaricide-resistant populations of Tetranychus urticae. Detection of phenotypic and genotypic resistance was conducted by employing residual contact vial bioassay (RCV) and quantitative sequencing (QS) methods, respectively. RCV was useful for detecting the acaricide resistance levels of T. urticae, particularly for on-site resistance detection; however, it was only applicable for rapid-acting acaricides (12 out of 19 tested acaricides). QS was effective for determining the frequencies of resistance alleles on a population basis, which corresponded to 12 nonsynonymous point mutations associated with target-site resistance to five types of acaricides [organophosphates (monocrotophos, pirimiphos-methyl, dimethoate and chlorpyrifos), pyrethroids (fenpropathrin and bifenthrin), abamectin, bifenazate and etoxazole]. Most field-collected mites exhibited high levels of multiple resistance, as determined by RCV and QS data, suggesting the seriousness of their current acaricide resistance status in rose cultivation areas in Korea. The correlation analyses revealed moderate to high levels of positive relationships between the resistance allele frequencies and the actual resistance levels in only five of the acaricides evaluated, which limits the general application of allele frequency as a direct indicator for estimating actual resistance levels. Nevertheless, the resistance allele frequency data alone allowed for the evaluation of the genetic resistance potential and background of test mite populations. The combined use of RCV and QS provides basic information on resistance levels, which is essential for choosing appropriate acaricides for the management of resistant T. urticae.

Identification of Genes Putatively Involved in Chitin Metabolism and Insecticide Detoxification in the Rice Leaf Folder (Cnaphalocrocis medinalis) Larvae through Transcriptomic Analysis

The rice leaf roller (Cnaphalocrocis medinalis) is one of the most destructive agricultural pests. Due to its migratory behavior, it is difficult to control worldwide. To date, little is known about major genes of C. medinalis involved in chitin metabolism and insecticide detoxification. In order to obtain a comprehensive genome dataset of C. medinalis, we conducted de novo transcriptome sequencing which focused on the major feeding stage of fourth-instar larvae, and our work revealed useful information on chitin metabolism and insecticide detoxification and target genes of C. medinalis. We acquired 29,367,797 Illumina reads and assembled these reads into 63,174 unigenes with an average length of 753 bp. Among these unigenes, 31,810 were annotated against the National Center for Biotechnology Information non-redundant (NCBI nr) protein database, resulting in 24,246, 8669 and 18,176 assigned to Swiss-Prot, clusters of orthologous group (COG), and gene ontology (GO), respectively. We were able to map 10,043 unigenes into 285 pathways using the Kyoto Encyclopedia of Genes and Genomes Pathway database (KEGG). Specifically, 16 genes, including five chitin deacetylases, two chitin synthases, five chitinases and four other related enzymes, were identified to be putatively involved in chitin biosynthesis and degradation, whereas 360 genes, including cytochrome P450s, glutathione S-transferases, esterases, and acetylcholinesterases, were found to be potentially involved in insecticide detoxification or as insecticide targets. The reliability of the transcriptome data was determined by reverse transcription quantitative PCR (RT-qPCR) for the selected genes. Our data serves as a new and valuable sequence resource for genomic studies on C. medinalis. The findings should improve our understanding of C. medinalis genetics and contribute to management of this important agricultural pest.

Transcriptome Analysis of the Carmine Spider Mite, Tetranychus cinnabarinus (Boisduval, 1867) (Acari: Tetranychidae), and Its Response to β-Sitosterol

Tetranychus cinnabarinus (Acari: Tetranychidae) is a worldwide polyphagous agricultural pest that has the title of resistance champion among arthropods. We reported previously the identification of the acaricidal compound β-sitosterol from Mentha piperita and Inula japonica. However, the acaricidal mechanism of β-sitosterol is unclear. Due to the limited genetic research carried out, we de novo assembled the transcriptome of T. cinnabarinus using Illumina sequencing and conducted a differential expression analysis of control and β-sitosterol-treated mites. In total, we obtained >5.4 G high-quality bases for each sample with unprecedented sequencing depth and assembled them into 22,941 unigenes. We identified 617 xenobiotic metabolism-related genes involved in detoxification, binding, and transporting of xenobiotics. A highly expanded xenobiotic metabolic system was found in mites. T. cinnabarinus detoxification genes-including carboxyl/cholinesterase and ABC transporter class C-were upregulated after β-sitosterol treatment. Defense-related proteins, such as Toll-like receptor, legumain, and serine proteases, were also activated. Furthermore, other important genes-such as the chloride channel protein, cytochrome b, carboxypeptidase, peritrophic membrane chitin binding protein, and calphostin-may also play important roles in mites' response to β-sitosterol. Our results demonstrate that high-throughput-omics tool facilitates identification of xenobiotic metabolism-related genes and illustration of the acaricidal mechanisms of β-sitosterol.

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.

Toxicodynamic mechanisms and monitoring of acaricide resistance in the two-spotted spider mite

The two-spotted spider (Tetranychus urticae) is one of the most serious pests world-wide and has developed resistance to many types of acaricides. Various mutations on acaricide target site genes have been determined to be responsible for toxicodynamic resistance, and the genotyping and frequency prediction of these mutations can be employed as an alternative resistance monitoring strategy. A quantitative sequencing (QS) protocol was reported as a population-based genotyping technique, and applied for the determination of resistance allele frequencies in T. urticae field populations. In addition, a modified glass vial bioassay method (residual contact vial bioassay, RCV) was implemented as a rapid on-site resistance monitoring tool. The QS protocol, together with the RCV, would greatly facilitate monitoring of T. urticae resistance. Recent completion of T. urticae genome analysis should facilitate the identification of additional resistance genetic markers that can be employed for molecular resistance monitoring.

Inducible Expression of Mu-Class Glutathione S-Transferases Is Associated with Fenpropathrin Resistance in Tetranychus cinnabarinus

The carmine spider mite, Tetranychus cinnabarinus (Boisduval), is a serious pest on a variety of economically important crops widely distributed in China, and its resistance to acaricides has quickly developed. In this study, we fully sequenced 13 GST genes of T. cinnabarinus (TcGSTs). The phylogenetic tree showed that five of them belonged to the delta class and the other eight belonged to the mu class. The alignment of gene sequences and comparison of gene expressions between a fenpropathrin-resistant strain (FR) and a susceptible strain (SS) showed that neither point mutation nor overexpression was detected in TcGSTs. However, when challenged by a sublethal dose of fenpropathrin, the mRNA levels of three GSTs from the mu class (TCGSTM2, TCGSTM3, and TCGSTM8) highly increased in FR, while in SS, the expression of these genes was still at the same level under the treatment. In conclusion, specific TcGSTs were identified that were inducible to stimulation by fenpropathrin, and proved that TcGSTs in FR were not constantly expressed at a high level, but could react much more quickly under the stress of fenpropathrin than SS.

Global distribution and origin of target site insecticide resistance mutations in Tetranychus urticae

The control of Tetranychus urticae, a worldwide agricultural pest, is largely dependent on pesticides. However, their efficacy is often compromised by the development of resistance. Recent molecular studies identified a number of target site resistance mutations, such as G119S, A201S, T280A, G328A, F331W in the acetylcholinesterase gene, L1024V, A1215D, F1538I in the voltage-gated sodium channel gene, G314D and G326E in glutamate-gated chloride channel genes, G126S, I136T, S141F, D161G, P262T in the cytochrome b and the I1017F in the chitin synthase 1 gene. We examined their distribution, by sequencing the relevant gene fragments in a large number of T. urticae collections from a wide geographic range. Our study revealed that most of the resistance mutations are spread worldwide, with remarkably variable frequencies. Furthermore, we analyzed the variability of the ace locus, which has been subjected to longer periods of selection pressure historically, to investigate the evolutionary origin of ace resistant alleles and determine whether they resulted from single or multiple mutation events. By sequencing a 1540 bp ace fragment, encompassing the resistance mutations and downstream introns in 139 T. urticae individuals from 27 countries, we identified 6 susceptible and 31 resistant alleles which have arisen from at least three independent mutation events. The frequency and distribution of these ace haplotypes varied geographically, suggesting an interplay between different mutational events, gene flow and local selection.

Point mutations associated with organophosphate and carbamate resistance in Chinese strains of Culex pipiens quinquefasciatus (Diptera: Culicidae)

Acetylcholinesterase resistance has been well documented in many insects, including several mosquito species. We tested the resistance of five wild, Chinese strains of the mosquito Culex pipiens quinquefasciatus to two kinds of pesticides, dichlorvos and propoxur. An acetylcholinesterase gene (ace1) was cloned and sequenced from a pooled sample of mosquitoes from these five strains and the amino acids of five positions were found to vary (V185M, G247S, A328S, A391T, and T682A). Analysis of the correlation between mutation frequencies and resistance levels (LC₅₀) suggests that two point mutations, G247S (r² = 0.732, P = 0.065) and A328S (r² = 0.891, P = 0.016), are associated with resistance to propoxur but not to dichlorvos. Although the V185M mutation was not associated with either dichlorvos or propoxur resistance, its RS genotype frequency was correlated with propoxur resistance (r² = 0.815, P = 0.036). And the HWE test showed the A328S mutation is linked with V185M, also with G247S mutation. This suggested that these three mutations may contribute synergistically to propoxur resistance. The T682A mutation was negatively correlated with propoxur (r² = 0.788, P = 0.045) resistance. Knowledge of these mutations may help design strategies for managing pesticide resistance in wild mosquito populations.

Cross-resistance risk of the novel complex II inhibitors cyenopyrafen and cyflumetofen in resistant strains of the two-spotted spider mite Tetranychus urticae

BACKGROUND

Cyflumetofen and cyenopyrafen are novel acaricides acting as complex II inhibitors. This new mode of action is extremely useful for devising efficient resistance management strategies for mite control. The authors determined the cross-resistance risk of both compounds, using a collection of well-characterised resistant strains of Tetranychus urticae, and also selected for cyflumetofen resistance in the laboratory.

RESULTS

Cross-resistance to cyflumetofen and cyenopyrafen was detected in field strains, with LC50 values exceeding the registered field dose. Synergism experiments suggested that P450 monooxygenases are involved in resistance, and that the activation mechanism of the two compounds most likely differs. Laboratory selection with cyflumetofen resulted in a highly resistant T. urticae strain that displayed negative cross-resistance to cyenopyrafen.

CONCLUSIONS

The cross-resistance risk of cyflumetofen and cyenopyrafen documented in this study needs to be integrated in resistance management strategies, especially in regions or crops with a history of frequent acaricide applications, in order to safeguard the efficacy of these compounds with a valuable new mode of action.

De novo characterization of the Dialeurodes citri transcriptome: mining genes involved in stress resistance and simple sequence repeats (SSRs) discovery

The citrus whitefly, Dialeurodes citri (Ashmead), is one of the three economically important whitefly species that infest citrus plants around the world; however, limited genetic research has been focused on D. citri, partly because of lack of genomic resources. In this study, we performed de novo assembly of a transcriptome using Illumina paired-end sequencing technology (Illumina Inc., San Diego, CA, USA). In total, 36,766 unigenes with a mean length of 497 bp were identified. Of these unigenes, we identified 17,788 matched known proteins in the National Center for Biotechnology Information database, as determined by Blast search, with 5731, 4850 and 14,441 unigenes assigned to clusters of orthologous groups (COG), gene ontology (GO), and SwissProt, respectively. In total, 7507 unigenes were assigned to 308 known pathways. In-depth analysis of the data showed that 117 unigenes were identified as potentially involved in the detoxification of xenobiotics and 67 heat shock protein (Hsp) genes were associated with environmental stress. In addition, these enzymes were searched against the GO and COG database, and the results showed that the three major detoxification enzymes and Hsps were classified into 18 and 3, 6, and 8 annotations, respectively. In addition, 149 simple sequence repeats were detected. The results facilitate the investigation of molecular resistance mechanisms to insecticides and environmental stress, and contribute to molecular marker development. The findings greatly improve our genetic understanding of D. citri, and lay the foundation for future functional genomics studies on this species.

Transcriptome sequencing and annotation of the predatory mite Metaseiulus occidentalis (Acari: Phytoseiidae): a cautionary tale about possible contamination by prey sequences

Next-generation sequencing was applied to the transcriptome of the phytoseiid Metaseiulus occidentalis to characterize gene expression in all life stages reared under different conditions to optimize the recovery of as many genes as possible. One production and one titration run produced a total of 862,069 reads (average size: 314.87 bp), which generated 255.6 Mbp of sequences on the GS-FLX Titanium sequencing platform. After removal of putative prey sequences 850,543 reads were used in NewBler and PTA assemblies to produce 74,172 non-redundant sequences, including 30,691 contigs and 43,481 singlets with 11,994 contigs consisting of more than 500 bp and 37,278 sequences >300 bp, constituting 48.7 % of all sequences. There were 25,888 hits with the NCBI non-redundant database and 15,376 unique transcripts. There were 26,225 hits with the Ixodes scapularis genome and 6,634 unique transcripts. There were 22,225 hits with the RefSeq of Homo sapiens with 6,465 unique transcripts, and 23,656 hits with the RefSeq of Drosophila melanogaster with 9,216 unique transcripts. Selected ESTs corresponding to genes of interest were analyzed including those related to transposable elements, GPCRs, Sox transcription factors, diapause and foraging behavior, and pesticide resistances. Novel and important genes appear to have been discovered that provide insight into the evolution, biology, and physiology of this important predator of pest mites in agriculture and will be useful in analyzing complete genome sequences of this natural enemy.