Population bulk segregant mapping uncovers resistance mutations and the mode of action of a chitin synthesis inhibitor in arthropods

Exposure to teflubenzuron reduces drought tolerance of collembolans

Chitin synthesis inhibitors (CSIs) are commonly used insecticides compromising cuticle formation and structure in arthropods. Arthropods rely on intact cuticles to maintain water balance and cellular homeostasis to survive in different weather conditions. We hypothesized that physiological impacts of CSIs may make arthropods more vulnerable to harsh environmental conditions, such as extreme heat, cold or drought. The aim of this study was to investigate if pre-exposure to teflubenzuron (a common CSI) would influence Folsomia candida's (Collembola: Isotomidae) sensitivity to natural stressors. Here, we exposed adult collembolans to teflubenzuron through food for two weeks, then survivors were immediately divided into three groups for subsequent acute heat, cold, and drought exposure. After acute exposure to these natural stressors, the collembolans were moved to optimal conditions for a one-week recovery period during which their survival, time to regain reproduction, and egg production were examined. We analyzed the interaction between effects of teflubenzuron and natural stressors using a multiplicative model. No interaction between effects of teflubenzuron and heat was observed in any test endpoints. A synergistic interaction between effects of teflubenzuron and cold was observed in the time to regain reproduction. Both survival and egg production, on the other hand, showed synergistic interaction between effects of teflubenzuron and drought, as well as a tendency for longer reproduction recovery times. Our results suggest that pre-exposure to teflubenzuron reduces drought tolerance in F. candida, while its impact on heat or cold tolerance is minor or absent. This study is among the first to explore the combined effects of CSI and natural stressors on soil arthropods, providing more insight on potential risks posed by such chemicals in the environment.

The G932C mutation of chitin synthase 1 gene (CHS1) mediates buprofezin resistance as confirmed by CRISPR/Cas9-mediated knock-in approach in the brown planthopper, Nilaparvata lugens

The brown planthopper (Nilaparvata lugens) is a major destructive rice pest in Asia. High levels of insecticide resistance have been frequently reported, and the G932C mutation in the chitin synthase 1 (CHS1) gene has been found to mediate buprofezin resistance. However, there has been no direct evidence to confirm the functional significance of the single G932C substitution mutation leading to buprofezin resistance in N. lugens. Here, we successfully constructed a knock-in homozygous strain (Nl-G932C) of N. lugens using CRISPR/Cas9 coupled with homology-directed repair (HDR). Compared with the background strain susceptible to buprofezin (Nl-SS), the knock-in strain (Nl-G932C) showed a 94.9-fold resistance to buprofezin. Furthermore, resistant strains (Nl-932C) isolated from the field exhibited a 2078.8-fold resistance to buprofezin, indicating that there are other mechanisms contributing to buprofezin resistance in the field. Inheritance analysis showed that the resistance trait is incomplete dominance. In addition, the Nl-G932C strain had a relative fitness of 0.33 with a substantially decreased survival rate, emergence rate, and fecundity. This study provided in vivo functional evidence for the causality of G932C substitution mutation of CHS1 with buprofezin resistance and valuable information for facilitating the development of resistance management strategies in N. lugens. This is the first example of using CRISPR/Cas9 gene-editing technology in a hemipteran insect to directly confirm the role of a candidate target site mutation in insecticide resistance.

Integrated de novo transcriptome of Culex pipiens mosquito larvae as a resource for genetic control strategies

We present a de novo transcriptome of the mosquito vector Culex pipiens, assembled by sequences of susceptible and insecticide resistant larvae. The high quality of the assembly was confirmed by TransRate and BUSCO. A mapping percentage until 94.8% was obtained by aligning contigs to Nr, SwissProt, and TrEMBL, with 27,281 sequences that simultaneously mapped on the three databases. A total of 14,966 ORFs were also functionally annotated by using the eggNOG database. Among them, we identified ORF sequences of the main gene families involved in insecticide resistance. Therefore, this resource stands as a valuable reference for further studies of differential gene expression as well as to identify genes of interest for genetic-based control tools.

Novel ability of diflubenzuron as an inhibitor of mitochondrial function

Compounds classified as benzoylphenylurea (BPU), such as diflubenzuron (DFB), are used as insecticides. Although BPU disrupts molting by inhibiting chitin biosynthesis and exhibits insecticidal activity, their exact mode of action remains unknown. Since epidermal cells proliferate and morphologically change from squamous to columnar cells during the early stages of insect molting, we speculate that a transition similar to that from epithelium to mesenchyme occurs and that BPU may inhibit this transition. Here, we addressed this possibility. We found that DFB decreases actin expression in insect cells (the tissue cultures of insect integument). Detailed analysis in Schneider S2 cells reveals that DFB inhibits the expression of actin isoforms (Act5C and Act42A) and the Drosophila ortholog of myocardin-related transcription factor (Mrtf), leading to cell growth suppression. Proteomics identified the Drosophila ortholog of prohibitin (Phb1D and Phb2E) as one of the DFB-binding proteins. DFB inhibits the interaction between Phb1D and Phb2E and induces mitochondrial dysfunction. The knock-down of Phb2E suppresses the expression of Act5C, Act42A, and Mrtf, leading to cell growth inhibition. Thus, the disruption of Phb function is a possible novel target of DFB.

Enantioselective metabolism of fenpropathrin enantiomers by carboxyl/choline esterase 6 in Tetranychus cinnabarinus

BACKGROUND

Tetranychus cinnabarinus is a polyphagous pest mite commonly found in agriculture. As an excellent acaricide, fenpropathrin (FEN) is frequently used to control T. cinnabarinus in agriculture. However, commercial FEN is a racemate with two enantiomers, R-FEN and S-FEN. Considering that investigations on the metabolism of FEN by T. cinnabarinus are based on racemate FEN, it is important to investigate the enantioselective metabolism of FEN in T. cinnabarinus.

RESULTS

S-FEN was more toxic to T. cinnabarinus than R-FEN by more than 68.8-fold. Moreover, the synergist bioassay revealed that carboxylesterase and cytochrome P450 were the primary enzymes engaged in the detoxification of FEN in T. cinnabarinus, with carboxylesterase playing a leading role. Seven genes were substantially different after the induction of S-FEN and R-FEN. TcCCE06 was screened and selected as a key gene that related to FEN metabolism in T. cinnabarinus. The metabolic results showed that the recombinant TcCCE06 effectively metabolized 32.1% of the R-FEN and 13.8% of the S-FEN within 4 h of incubation. Moreover, R-FEN was demonstrated to have a higher affinity for the TcCCE06 protein than S-FEN based on molecular docking.

CONCLUSION

Our results indicated that TcCCE06 mediates the enantioselective metabolism of FEN in T. cinnabarinus. Our findings will contribute to a more comprehensive understanding of the mechanisms underlying the differential toxicity of the FEN enantiomers against T. cinnabarinus. Furthermore, they also provide a new perspective for the development of enantiomer-enriched acaricides with higher activity and lower pesticide dosage and pollution risks. © 2023 Society of Chemical Industry.

Synthesis and insecticidal activity of N-(5-phenylpyrazin-2-yl)-benzamide derivatives: Elucidation of mode of action on chitin biosynthesis through symptomology and genetic studies

Among the six-membered heterocycles, the pyrazine ring is poorly explored in crop protection and does not feature in any product listed in the current IRAC MoA classification. In an effort to identify new leads for internal research, we synthesized a series of N-(5-phenylpyrazin-2-yl)-benzamide derivatives and evaluated them for their insecticidal activity. N-(5-phenylpyrazin-2-yl)-benzamide derivatives 3 were prepared using an automated two-step synthesis protocol. These compounds were tested for their initial biological activity against a wide range of sucking and chewing insect pests and found to be active against lepidopterans only. More detailed experiments, including symptomology studies on the diamondback moth, Plutella xylostella (L.) and the Egyptian cotton leafworm, Spodoptera littoralis (Boisduval) showed that analog 3q causes severe abnormalities in the lepidopteran cuticle leading to larval mortality. Compound 3q shows strong potency against both P. xylostella and S. littoralis, whereas analog 3i shows better potency against S. littoralis causing also impaired cuticular structure and death of the larvae. Additionally, P. xylostella genetic studies showed that compound 3q resistance is linked to Chitin Synthase 1. Our studies show that N-(5-phenylpyrazin-2-yl)-benzamide derivatives 3, and in particular analogs 3i and 3q, act as insect growth modulator insecticides. Conformational similarities with lufenuron are discussed.

SYNCAS: Efficient CRISPR/Cas9 gene-editing in difficult to transform arthropods

The genome editing technique CRISPR/Cas9 has led to major advancements in many research fields and this state-of-the-art tool has proven its use in genetic studies for various arthropods. However, most transformation protocols rely on microinjection of CRISPR/Cas9 components into embryos, a method which is challenging for many species. Alternatively, injections can be performed on adult females, but transformation efficiencies can be very low as was shown for the two-spotted spider mite, Tetranychus urticae, a minute but important chelicerate pest on many crops. In this study, we explored different CRISPR/Cas9 formulations to optimize a maternal injection protocol for T. urticae. We observed a strong synergy between branched amphipathic peptide capsules and saponins, resulting in a significant increase of CRISPR/Cas9 knock-out efficiency, exceeding 20%. This CRISPR/Cas9 formulation, termed SYNCAS, was used to knock-out different T. urticae genes - phytoene desaturase, CYP384A1 and Antennapedia - but also allowed to develop a co-CRISPR strategy and facilitated the generation of T. urticae knock-in mutants. In addition, SYNCAS was successfully applied to knock-out white and white-like genes in the western flower thrips, Frankliniella occidentalis. The SYNCAS method allows routine genome editing in these species and can be a game changer for genetic research in other hard to transform arthropods.

Buprofezin delayed the molting of Pardosa pseudoannulata, a predatory enemy for insect pests, by suppressing chitin synthase 1 expression

Spiders, the major predatory enemies of insect pests in fields, are vulnerable to insecticides. In this study, we observed that the recommended dose of buprofezin delayed the molting of the pond wolf spider Pardosa pseudoannulata, although it had no lethal effect on the spiders. Since buprofezin is an insect chitin biosynthesis inhibitor, we identified two chitin synthase genes (PpCHS1 and PpCHS2) in P. pseudoannulata. Tissue-specific expression profiling showed that PpCHS1 was most highly expressed in cuticle. In contrast, PpCHS2 showed highest mRNA levels in the midgut and fat body. RNAi knockdown of PpCHS1 significantly delayed the molting of 12-days old spiderlings, whereas no significant effect on the molting was observed in the PpCHS2-silencing spiderlings. The expression of PpCHS1 was significantly suppressed in the spiderlings treated with buprofezin, but rescued by exogenous ecdysteroid ponasterone A (PA). Consistent with this result, the molting delay caused by buprofezin was also rescued by PA. The results revealed that buprofezin delayed the molting of spiders by suppressing PpCHS1 expression, which will benefit the protection of P. pseudoannulate and related spider species.

Contrasting roles of cytochrome P450s in amitraz and chlorfenapyr resistance in the crop pest Tetranychus urticae

The molecular mechanisms of amitraz and chlorfenapyr resistance remain only poorly understood for major agricultural pests and vectors of human diseases. This study focusses on a multi-resistant field strain of the crop pest Tetranychus urticae, which could be readily selected in the laboratory to high levels of amitraz and chlorfenapyr resistance. Toxicity experiments using tralopyril, the active toxophore of chlorfenapyr, suggested decreased activation as a likely mechanism underlying resistance. Starting from the same parental strain, transcriptome profiling revealed that a cluster of detoxifying genes was upregulated after amitraz selection, but unexpectedly downregulated after chlorfenapyr selection. Further functional validation associated the upregulation of CYP392A16 with amitraz metabolism and the downregulation of CYP392D8 with reduced activation of chlorfenapyr to tralopyril. Genetic mapping (QTL analysis by BSA) was conducted in an attempt to unravel the genetic mechanisms of expression variation and resistance. This revealed that chlorfenapyr resistance was associated with a single QTL, while 3 QTLs were uncovered for amitraz resistance. Together with the observed contrasting gene expression patterns, we argue that transcriptional regulators most likely underly the distinct expression profiles associated with resistance, but these await further functional validation.

Advances in understanding insect chitin biosynthesis

Chitin, a natural polymer of N-acetylglucosamine chains, is a principal component of the apical extracellular matrix in arthropods. Chitin microfibrils serve as structural components of natural biocomposites present in the extracellular matrix of a variety of invertebrates including sponges, molluscs, nematodes, fungi and arthropods. In this review, we summarize the frontier advances of insect chitin synthesis. More specifically, we focus on the chitin synthase (CHS), which catalyzes the key biosynthesis step. CHS is also known as an attractive insecticidal target in that this enzyme is absent in mammals, birds or plants. As no insect chitin synthase structure have been reported so far, we review recent studies on glycosyltransferase domain structures derived from fungi and oomycetes, which are conserved in CHS from all species containing chitin. Auxiliary proteins, which coordinate with CHS in chitin biosynthesis and assembly, are also discussed.

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.

Expedient discovery of novel oxime ester derivatives of piperine/piperine analogs as potent pesticide candidates and their mode of action against Tetranychus cinnabarinus Boisduval

BACKGROUND

Nowadays, pest infestation and resistance have appeared as a consequence of repeated and extensive use of pesticides. Thus, development of new effective pesticide candidates in crop protection is highly desirable. Herein, a series of new piperine derivatives containing oxime ester scaffolds were regioselectively and stereoselectively prepared as pesticidal agents.

RESULTS

Steric configurations of compounds 2, 5z and 13e were definitively determined by single-crystal X-ray diffraction. Against Tetranychus cinnabarinus, notably, compounds 5f [median lethal concentration (LC50 ) = 0.14 mg mL-1 ] and 5v (LC50  = 0.13 mg mL-1 ) showed >107-fold greater acaricidal activity than piperine (LC50  = 15.02 mg mL-1 ), which were comparable to the commercial acaricide spirodiclofen. Against Aphis citricola, compound 5d (LD50  = 19.12 ng aphid-1 ) exhibited 6.1-fold more potent aphicidal activity than piperine (LD50  = 116.06 ng aphid-1 ). Additionally, through scanning electron microscopy, the toxicology study suggested that the acaricidal activity of piperine derivatives may be related to damage of the cuticle layer crest of T. cinnabarinus.

CONCLUSION

The structure-activity relationships suggested that 3,4-dioxymethylene of piperine was crucial for its acaricidal activity; and introduction of a certain length of aliphatic chain at the C-2 position was beneficial to the aphicidal and acaricidal activities. Compounds 5f and 5v are potential leads for further structural modification as acaricidal agents. © 2023 Society of Chemical Industry.

A nuclear receptor HR96-related gene underlies large trans-driven differences in detoxification gene expression in a generalist herbivore

The role, magnitude, and molecular nature of trans-driven expression variation underlying the upregulation of detoxification genes in pesticide resistant arthropod populations has remained enigmatic. In this study, we performed expression quantitative trait locus (eQTL) mapping (n = 458) between a pesticide resistant and a susceptible strain of the generalist herbivore and crop pest Tetranychus urticae. We found that a single trans eQTL hotspot controlled large differences in the expression of a subset of genes in different detoxification gene families, as well as other genes associated with host plant use. As established by additional genetic approaches including RNAi gene knockdown, a duplicated gene with a nuclear hormone receptor HR96-related ligand-binding domain was identified as causal for the expression differences between strains. The presence of a large family of HR96-related genes in T. urticae may enable modular control of detoxification and host plant use genes, facilitating this species' known and rapid evolution to diverse pesticides and host plants.

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.

A high-throughput KASP assay provides insights into the evolution of multiple resistant mutations in populations of the two-spotted spider mite Tetranychus urticae across China

BACKGROUND

The two-spotted spider mite (TSSM), Tetranychus urticae (Acari: Tetranychidae), is a cosmopolitan phytophagous pest in agriculture and horticulture. It has developed resistance to many acaricides by target-site mutations. Understanding the status and evolution of resistant mutations in the field is essential for resistance management. Here, we applied a high-throughput Kompetitive allele-specific polymerase chain reaction (KASP) method for detecting six mutations conferring resistance to four acaricides of the TSSM. We genotyped 3274 female adults of TSSM from 43 populations collected across China in 2017, 2020, and 2021.

RESULTS

The KASP genotyping of 24 testing individuals showed 99% agreement with Sanger sequencing results. KASP assays showed that most populations had a high frequency of mutations conferring avermectin (G314D and G326E) and pyridaben (H92R) resistance. The frequency of mutation conferring bifenazate (A269V and G126S) and etoxazole (I1017F) resistance was relatively low. Multiple mutations were common in the TSSM, with 70.2% and 24.6% of individuals having 2-6 and 7-10 of 10 possible resistant alleles, respectively. No loci were linked in most populations among the six mutations, indicating the development of multiple resistance is mainly by independent selection. However, G314D and I1017F on the nuclear genome deviated from Hardy-Weinberg equilibrium in most populations, indicating significant selective pressure on TSSM populations by acaricides or fitness cost of the mutations in the absence of acaricide selection.

CONCLUSION

Our study revealed that the high frequency of TSSMs evolved multiple resistant mutations in population and individual levels by independent selection across China, alarming for managing multiple-acaricides resistance. © 2023 Society of Chemical Industry.

Eco-friendly deacetylated chitosan base siRNA biological-nanopesticide loading cyromazine for efficiently controlling Spodoptera frugiperda

Spodoptera frugiperda is a serious threat to various crops, such as corn and rice, and results in severe economic losses. Herein, a chitin synthase sfCHS highly expressed in the epidermis of S. frugiperda was screened, and when interfered by an sfCHS-siRNA nanocomplex, most individuals could not ecdysis (mortality rate 53.3 %) or pupate (abnormal pupation 80.6 %). Based on the results of structure-based virtual screening, cyromazine (CYR, binding free energy -57.285 kcal/mol) could inhibit ecdysis (LC₅₀, 19.599 μg/g). CYR-CS/siRNA nanoparticles encapsulating CYR and SfCHS-siRNA with chitosan (CS) were successfully prepared, as confirmed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and 74.9 mg/g CYR was characterized in the core of CYR-CS/siRNA by high-performance liquid chromatography and Fourier transform infrared spectroscopy. Small amounts of prepared CYR-CS/siRNA containing only 1.5 μg/g CYR could better inhibit chitin synthesis in the cuticle and peritrophic membrane (mortality rate 84.4 %). Therefore, chitosan/siRNA nanoparticle-loaded pesticides were useful for pesticide reduction and comprehensive control of S. frugiperda.

Molecular diagnostics for monitoring insecticide resistance in the western flower thrips Frankliniella occidentalis

BACKGROUND

Insecticide resistance has emerged in various western flower thrips (WFT) populations across the world, threatening the efficiency of chemical control applications. Elucidation of insecticide resistance mechanisms at the molecular level provides markers for the development of diagnostics to monitor the trait and support evidence-based resistance management.

RESULTS

TaqMan and Droplet Digital polymerase chain reaction (ddPCR) diagnostics were developed and validated, against Sanger sequencing, in individual and pooled WFT samples respectively, for the G275E mutation (nicotinic acetylcholine receptor α6 gene, nAChR α6) associated with resistance to nAChR allosteric modulators, site I (spinosyns); L1014F, T929I, T929C and T292V mutations (voltage-gated sodium channel gene, vgsc) linked with pyrethroid resistance; and I1017M (chitin synthase 1 gene, chs1) conferring resistance to growth inhibitors affecting CHS1 (benzoylureas). The detection limits of ddPCR assays for mutant allelic frequencies (MAF) were in the range of 0.1%-0.2%. The assays were applied in nine WFT field populations from Crete, Greece. The G275E (MAF = 29.66%-100.0%), T929I and T929V (combined MAF = 100%), L1014F (MAF = 11.01%-37.29%), and I1017M (MAF = 17.74%-51.07%) mutations were present in all populations.

CONCLUSION

The molecular diagnostics panel that was developed in this study can facilitate the quick and sensitive resistance monitoring of WFT populations at the molecular level, to support evidence-based insecticide resistance management strategies. © 2022 Society of Chemical Industry.

Intraspecific diversity in the mechanisms underlying abamectin resistance in a cosmopolitan pest

Pesticide resistance relies on a myriad of mechanisms, ranging from single mutations to a complex and polygenic architecture, and it involves mechanisms such as target-site insensitivity, metabolic detoxification, or a combination of these, with either additive or synergistic effects. Several resistance mechanisms against abamectin, a macrocyclic lactone widely used in crop protection, have been reported in the cosmopolitan pest Tetranychus urticae. However, it has been shown that a single mechanism cannot account for the high levels of abamectin resistance found across different mite populations. Here, we used experimental evolution combined with bulked segregant analyses to map quantitative trait loci (QTL) associated with abamectin resistance in two genetically unrelated populations of T. urticae. In these two independent QTL mapping experiments, three and four QTLs were identified, of which three were shared between experiments. Shared QTLs contained genes encoding subunits of the glutamate-gated chloride channel (GluCl) and harboured previously reported mutations, including G314D in GluCl1 and G326E in GluCl3, but also novel resistance candidate loci, including DNA helicases and chemosensory receptors. Surprisingly, the fourth QTL, present only in only one of the experiments and thus unique for one resistant parental line, revealed a non-functional variant of GluCl2, suggesting gene knock-out as resistance mechanism. Our study uncovers the complex basis of abamectin resistance, and it highlights the intraspecific diversity of genetic mechanisms underlying resistance in a cosmopolitan pest.

Activity, selection response and molecular mode of action of the isoxazoline afoxolaner in Tetranychus urticae

BACKGROUND

Afoxolaner is a novel representative of the isoxazolines, a class of ectoparasiticides which has been commercialized for the control of tick and flea infestations in dogs. In this study, the biological efficacy of afoxolaner against the two-spotted spider mite Tetranychus urticae was evaluated. Furthermore, as isoxazolines are known inhibitors of γ-aminobutyric acid-gated chloride channels (GABACls), the molecular mode of action of afoxolaner on T. urticae GABACls (TuRdls) was studied using functional expression in Xenopus oocytes followed by two-electrode voltage-clamp (TEVC) electrophysiology, and results were compared with inhibition by fluralaner, fipronil and endosulfan. To examine the influence of known GABACl resistance mutations, H301A, I305T and A350T substitutions in TuRdl1 and a S301A substitution in TuRdl2 were introduced.

RESULTS

Bioasassays revealed excellent efficacy of afoxolaner against all developmental stages and no cross-resistance was found in a panel of strains resistant to most currently used acaricides. Laboratory selection over a period of 3 years did not result in resistance. TEVC revealed clear antagonistic activity of afoxolaner and fluralaner for all homomeric TuRdl1/2/3 channels. The introduction of single, double or triple mutations to TuRdl1 and TuRdl2 did not lower channel sensitivity. By contrast, both endosulfan and fipronil had minimal antagonistic activities against TuRdl1/2/3, and channels carrying single mutations, whereas the sensitivity of double and triple TuRdl1 mutants was significantly increased.

CONCLUSIONS

Our results demonstrate that afoxolaner is a potent antagonist of GABACls of T. urticae and has a powerful mode of action to control spider mites. © 2022 Society of Chemical Industry.

Using targeted sequencing and TaqMan approaches to detect acaricide (bifenthrin, bifenazate, and etoxazole) resistance associated SNPs in Tetranychus urticae collected from peppermint fields and hop yards

Tetranychus urticae (Koch) is an economically important pest of many agricultural commodities world-wide. Multiple acaricides, including bifenazate, bifenthrin, and extoxazole, are currently registered to control T. urticae. However, populations of T. urticae in many different growing regions have developed acaricide resistance through multiple mechanisms. Within T. urticae, single nucleotide polymorphisms (SNPs) have been documented in different genes which are associated with acaricide resistance phenotypes. The detection of these mutations through TaqMan qPCR has been suggested as a practical, quick, and reliable tool to inform agricultural producers of acaricide resistance phenotypes present within their fields and have potential utility for making appropriate acaricide application and integrated pest management decisions. Within this investigation we examined the use of a TaqMan qPCR-based approach to determine genotypes which have been previously associated with acaricide resistance in field-collected populations of T. urticae from peppermint fields and hop yards in the Pacific Northwest of the United States and confirmed the results with a multiplex targeted sequencing. The results suggest that a TaqMan qPCR approach accurately genotypes T. urticae populations for SNPs that have been linked to Bifenazate, Bifenthrin, and Etoxazole resistance. The results also demonstrated that different populations of mites in Washington and Idaho displayed varying frequencies of the examined SNPs. While we were able to detect the SNPs associated with the examined acaricides, the mutation G126S was not an appropriate or accurate indicator for bifenazate resistance.

Cuticle Modifications and Over-Expression of the Chitin-Synthase Gene in Diflubenzuron-Resistant Phenotype

Insecticide resistance is a major threat challenging the control of harmful insect species. The study of resistant phenotypes is, therefore, pivotal to understand molecular mechanisms underpinning insecticide resistance and plan effective control and resistance management strategies. Here, we further analysed the diflubenzuron (DFB)-resistant phenotype due to the point-mutation I1043M in the chitin-synthase 1 gene (chs1) in the mosquito Culex pipiens. By comparing susceptible and resistant strains of Cx. pipiens through DFB bioassays, molecular analyses and scanning electron microscopy, we showed that the I1043M-resistant mosquitoes have: (i) a striking level of DFB resistance (i.e., resistance ratio: 9006); (ii) a constitutive 11-fold over-expression of the chs1 gene; (iii) enhanced cuticle thickness and cuticular chitin content. Culex pipiens is one of the most important vector species in Europe and the rapid spread of DFB resistance can threaten its control. Our results, by adding new data about the DFB-resistant phenotype, provide important information for the control and management of insecticide resistance.

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.

High-Resolution Genetic Mapping Combined with Transcriptome Profiling Reveals That Both Target-Site Resistance and Increased Detoxification Confer Resistance to the Pyrethroid Bifenthrin in the Spider Mite Tetranychus urticae

Pyrethroids are widely applied insecticides in agriculture, but their frequent use has provoked many cases of resistance, in which mutations in the voltage-gated sodium channel (VGSC), the pyrethroid target-site, were shown to play a major role. However, for the spider mite Tetranychus urticae, it has also been shown that increased detoxification contributes to resistance against the pyrethroid bifenthrin. Here, we performed QTL-mapping to identify the genomic loci underlying bifenthrin resistance in T. urticae. Two loci on chromosome 1 were identified, with the VGSC gene being located near the second QTL and harboring the well-known L1024V mutation. In addition, the presence of an L925M mutation in the VGSC of a highly bifenthrin-resistant strain and its loss in its derived, susceptible, inbred line indicated the importance of target-site mutations in bifenthrin resistance. Further, RNAseq experiments revealed that genes encoding detoxification enzymes, including carboxyl/choline esterases (CCEs), cytochrome P450 monooxygenases and UDP-glycosyl transferases (UGTs), were overexpressed in resistant strains. Toxicity bioassays with bifenthrin (ester pyrethroid) and etofenprox (non-ester pyrethroid) also indicated a possible role for CCEs in bifenthrin resistance. A selection of CCEs and UGTs were therefore functionally expressed, and CCEinc18 was shown to metabolize bifenthrin, while teturUGT10 could glycosylate bifenthrin-alcohol. To conclude, our findings suggest that both target-site and metabolic mechanisms underlie bifenthrin resistance in T. urticae, and these might synergize high levels of resistance.

Knockdown of GFAT disrupts chitin synthesis in Hyphantria cunea larvae

Glutamine-fructose-6-phosphate transaminase (GFAT) has been reported to regulate the hexosamine biosynthetic pathway as the first rate-limiting enzyme. As a key enzyme that catalyzes the substrate of glycosylation modification, which has a wide-ranging effect on cellular functions. However, there are few studies on the relationship between GFAT and chitin metabolism in insects. In the present study, the GFAT gene from Hyphantria cunea was identified based on transcriptome and bioinformatic analysis. The role of HcGFAT in regulating development and chitin synthesis was analyzed by RNA interference (RNAi) in H. cunea larvae. The full-length HcGFAT gene (2028 bp) encodes a 676 amino acid (aa) polypeptide had typical structural features of the SIS and Gn_AT_II superfamily. Phylogenetic analyses showed that GFAT of H. cunea shares the highest homology and identity with GFAT of Ostrinia furnacalis. Expression profiles indicated that HcGFAT was expressed throughout larval, pupal and three tissues (midgut, fat body, epidermis), and highly expressed in the last instar of larvae and strongly expressed in epidermis among three tissues. Bioassay results showed that knockdown of HcGFAT repressed larval growth and development, resulting in a significant loss of larval body weight. Meanwhile, HcGFAT knockdown also significantly caused larval developmental deformity. Knockdown of HcGFAT regulated the expression of four other critical genes in the chitin synthesis pathway (HcGNA, HcPAGM, HcUAP, HcCHSA), and ultimately resulted in decreased chitin content in the epidermis. In summary, these findings indicated that GFAT plays a critical role in larval growth and development, as well as chitin synthesis in H. cunea.

Detection of Chitin Synthase Mutations in Lufenuron-Resistant Spodoptera frugiperda in China

Spodoptera frugiperda (J. E. Smith), is commonly known as fall armyworm, native to tropical and subtropical regions of America, is an important migratory agricultural pest. It is important to understand the resistance and internal mechanism of action of S. frugiperda against lufenuron in China. Lufenuron is one of the main insecticides recommended for field use in China and has a broad prospect in the future. We conducted a bioassay using the diet-overlay method and found that the current S. frugiperda in China are still at a low level of resistance to lufenuron. Secondly, we examined whether the mutation I1040M (I1042M in Plutella xylostella), associated with lufenuron resistance, was produced in the field. And then we tested the expression of chitin synthase SfCHSA and SfCHSB in different tissues, and the changes of these two genes after lufenuron induction. The results showed that there is still no mutation generation in China and there is a significant change in the expression of SfCHSA under the effect of lufenuron. In conclusion, our study suggests that field S. frugiperda populations in 2019 and 2020 were less resistant to lufenuron. In fall armyworm, chitin synthases included SfCHSA and SfCHSB genes, and after induction treatment with lufenuron, the expression of the SfCHSA gene was significantly increased. In SfCHSA, no mutation has been detected in the site associated with lufenuron resistance. Secondly, in S. frugiperda larvae, the SfCHSA gene was the highest in the head of the larvae, followed by the integument; while the SfCHSB gene was mainly concentrated in the midgut. Therefore, we believe that the SfCHSA gene plays a greater role in the resistance of S. frugiperda to lufenuron than the SfCHSB gene. It is worth noting that understanding the level of resistance to lufenuron in China, the main mechanism of action of lufenuron on larvae, and the mechanism of resistance to lufenuron in S. frugiperda will help in crop protection as well as in extending the life span of this insecticide.

Utility and challenges of using whole‐genome resequencing to detect emerging insect and mite resistance in agroecosystems

Arthropods that invade agricultural ecosystems systematically evolve resistance to the control measures used against them, and this remains a significant and ongoing challenge for sustainable food production systems. Early detection of resistance evolution could prompt remedial action to slow the spread of resistance alleles in the landscape. Historical approaches used to detect emerging resistance included phenotypic monitoring of agricultural pest populations, as well as monitoring of allele frequency changes at one or a few candidate pesticide resistance genes. In this article, I discuss the successes and limitations of these traditional monitoring approaches and then consider whether whole‐genome scanning could be applied to samples collected from agroecosystems over time for resistance monitoring. I examine the qualities of agroecosystems that could impact application of this approach to pesticide resistance monitoring and describe a recent retrospective analysis where genome scanning successfully detected an oligogenic response to selection by pesticides years prior to pest management failure. I conclude by considering areas of further study that will shed light on the feasibility of applying whole‐genome scanning for resistance risk monitoring in agricultural pest species.

A mutation in chitin synthase I associated with etoxazole resistance in the citrus red mite Panonychus citri (Acari: Tetranychidae) and its uneven geographical distribution in Japan

BACKGROUND

High-levels of etoxazole resistance have not yet been frequently reported in Panonychus citri. Although a highly resistant strain was discovered in 2014, etoxazole resistance has not become a significant problem in areas of citrus production in Japan. A target site mutation in chitin synthase 1 (CHS1), I1017F, is a major etoxazole-resistance factor in Tetranychus urticae. To investigate the mechanisms of etoxazole resistance and the dispersal of resistance genes, we analyzed target-site mutations in a highly resistant strain and their geographical distribution in Japan.

RESULTS

High-level etoxazole resistance was completely recessive. The I1017F mutation was detected in CHS1 of the highly resistant strain, and its frequency was correlated with the hatchability of eggs treated with etoxazole. Sequencing and variant frequency analyses of local populations by quantitative polymerase chain reaction revealed that I1017F is restricted to the Ariake Sea area of Kyushu Island. Although a new nonsynonymous substitution, S1016L, accompanied by I1017F was found in CHS1 of the highly resistant strain, CRISPR/Cas9 engineering of flies showed that S1016L had no effect on the etoxazole resistance conferred by I1017F.

CONCLUSION

I1017F is a major target site mutation that confers high-level etoxazole resistance on P. citri. Dispersion of I1017F possibly was suppressed as a result of the completely recessive inheritance of resistance together with low gene flow between local populations. © 2022 Society of Chemical Industry.

Insights into the Use of Eco-Friendly Synergists in Resistance Management of Leptinotarsa decemlineata (Coleoptera: Chrysomelidae)

The Colorado potato beetle (CPB), Leptinotarsa decemlineata (Say), is the most notorious insect pest of potato globally. Injudicious use of insecticides for management of this pest has resulted in resistance to all major groups of insecticides along with many human, animal health, and environmental concerns. Additionally, the input cost of insecticide development/discovery is markedly increasing because each year thousands of chemicals are produced and tested for their insecticidal properties, requiring billions of dollars. For the management of resistance in insect pests, synergists can play a pivotal role by reducing the application dose of most insecticides. These eco-friendly synergists can be classified into two types: plant-based synergists and RNAi-based synergists. The use of plant-based and RNAi-based synergists in resistance management of insect pests can give promising results with lesser environmental side effects. This review summarizes the resistance status of CPB and discusses the potential advantage of plant-based and RNAi-based synergists for CPB resistance management. It will motivate researchers to further investigate the techniques of using plant- and RNAi-based synergists in combination with insecticides.

Intradiol ring cleavage dioxygenases from herbivorous spider mites as a new detoxification enzyme family in animals

Background

Generalist herbivores such as the two-spotted spider mite Tetranychus urticae thrive on a wide variety of plants and can rapidly adapt to novel hosts. What traits enable polyphagous herbivores to cope with the diversity of secondary metabolites in their variable plant diet is unclear. Genome sequencing of T. urticae revealed the presence of 17 genes that code for secreted proteins with strong homology to “intradiol ring cleavage dioxygenases (DOGs)” from bacteria and fungi, and phylogenetic analyses show that they have been acquired by horizontal gene transfer from fungi. In bacteria and fungi, DOGs have been well characterized and cleave aromatic rings in catecholic compounds between adjacent hydroxyl groups. Such compounds are found in high amounts in solanaceous plants like tomato, where they protect against herbivory. To better understand the role of this gene family in spider mites, we used a multi-disciplinary approach to functionally characterize the various T. urticae DOG genes.

Results

We confirmed that DOG genes were present in the T. urticae genome and performed a phylogenetic reconstruction using transcriptomic and genomic data to advance our understanding of the evolutionary history of spider mite DOG genes. We found that DOG expression differed between mites from different plant hosts and was induced in response to jasmonic acid defense signaling. In consonance with a presumed role in detoxification, expression was localized in the mite’s gut region. Silencing selected DOGs expression by dsRNA injection reduced the mites’ survival rate on tomato, further supporting a role in mitigating the plant defense response. Recombinant purified DOGs displayed a broad substrate promiscuity, cleaving a surprisingly wide array of aromatic plant metabolites, greatly exceeding the metabolic capacity of previously characterized microbial DOGs.

Conclusion

Our findings suggest that the laterally acquired spider mite DOGs function as detoxification enzymes in the gut, disarming plant metabolites before they reach toxic levels. We provide experimental evidence to support the hypothesis that this proliferated gene family in T. urticae is causally linked to its ability to feed on an extremely wide range of host plants.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-022-01323-1.

QTL mapping suggests that both cytochrome P450-mediated detoxification and target-site resistance are involved in fenbutatin oxide resistance in Tetranychus urticae

The organotin acaricide fenbutatin oxide (FBO) - an inhibitor of mitochondrial ATP-synthase - has been one of the most extensively used acaricides for the control of spider mites, and is still in use today. Resistance against FBO has evolved in many regions around the world but only few studies have investigated the molecular and genetic mechanisms of resistance to organotin acaricides. Here, we found that FBO resistance is polygenic in two genetically distant, highly resistant strains of the spider mite Tetranychus urticae, MAR-AB and MR-VL. To identify the loci underlying FBO resistance, two independent bulked segregant analysis (BSA) based QTL mapping experiments, BSA MAR-AB and BSA MR-VL, were performed. Two QTLs on chromosome 1 were associated with FBO resistance in each mapping experiment. At the second QTL of BSA MAR-AB, several cytochrome P450 monooxygenase (CYP) genes were located, including CYP392E4, CYP392E6 and CYP392E11, the latter being overexpressed in MAR-AB. Synergism tests further implied a role for CYPs in FBO resistance. Subunit c of mitochondrial ATP-synthase was located near the first QTL of both mapping experiments and harbored a unique V89A mutation enriched in the resistant parents and selected BSA populations. Marker-assisted introgression into a susceptible strain demonstrated a moderate but significant effect of the V89A mutation on toxicity of organotin acaricides. The impact of the mutation on organotin inhibition of ATP synthase was also functionally confirmed by ATPase assays on mitochondrial preparations. To conclude, our findings suggest that FBO resistance in the spider mite T. urticae is a complex interplay between CYP-mediated detoxification and target-site resistance.

freqpcr: Estimation of population allele frequency using qPCR ΔΔCq measures from bulk samples

PCR techniques, both quantitative (qPCR) and nonquantitative, have been used to estimate the frequency of a specific allele in a population. However, the labour required to sample numerous individuals and subsequently handle each sample renders the quantification of rare mutations (e.g., pesticide resistance gene mutations at the early stages of resistance development) challenging. Meanwhile, pooling DNA from multiple individuals as a "bulk sample" combined with qPCR may reduce handling costs. The qPCR output for a bulk sample, however, contains uncertainty owing to variations in DNA yields from each individual, in addition to measurement errors. In this study, we have developed a statistical model to estimate the frequency of the specific allele and its confidence interval when the sample allele frequencies are obtained in the form of ΔΔCq in the qPCR analyses on multiple bulk samples collected from a population. We assumed a gamma distribution as the individual DNA yield and developed an R package for parameter estimation, which was verified using real DNA samples from acaricide-resistant spider mites, as well as a numerical simulation. Our model resulted in unbiased point estimates of the allele frequency compared with simple averaging of the ΔΔCq values. The confidence intervals suggest that dividing the bulk samples into more parts will improve precision if the total number of individuals is equal; however, if the cost of PCR analysis is higher than that of sampling, increasing the total number and pooling them into a few bulk samples may also yield comparable precision.

A H258Y mutation in subunit B of the succinate dehydrogenase complex of the spider mite Tetranychus urticae confers resistance to cyenopyrafen and pyflubumide, but likely reinforces cyflumetofen binding and toxicity

Succinate dehydrogenase (SDH) inhibitors such as cyflumetofen, cyenopyrafen and pyflubumide, are selective acaricides that control plant-feeding spider mite pests. Resistance development to SDH inhibitors has been investigated in a limited number of populations of the spider mite Tetranychus urticae and is associated with cytochrome P450 based detoxification and target-site mutations such as I260 T/V in subunit B and S56L in subunit C of SDH. Here, we report the discovery of a H258Y substitution in subunit B of SDH in a highly pyflubumide resistant population of T. urticae. As this highly conserved residue corresponds to one of the ubiquinone binding residues in fungi and bacteria, we hypothesized that H258Y could have a strong impact on SDH inhibitors toxicity. Marker assisted introgression and toxicity bioassays revealed that H258Y caused high cross resistance between cyenopyrafen and pyflubumide, but increased cyflumetofen toxicity. Resistance associated with H258Y was determined as dominant for cyenopyrafen, but recessive for pyflubumide. In vitro SDH assays with extracted H258 mitochondria showed that cyenopyrafen and the active metabolites of pyflubumide and cyflumetofen, interacted strongly with complex II. However, a clear shift in IC50s was observed for cyenopyrafen and the metabolite of pyflubumide when Y258 mitochondria were investigated. In contrast, the mutation slightly increased affinity of the cyflumetofen metabolite, likely explaining its increased toxicity for the mite lines carrying the substitution. Homology modeling and ligand docking further revealed that, although the three acaricides share a common binding motif in the Q-site of SDH, H258Y eliminated an important hydrogen bond required for cyenopyrafen and pyflubumide binding. In addition, the hydrogen bond between cyenopyrafen and Y117 in subunit D was also lost upon mutation. In contrast, cyflumetofen affinity was enhanced due to an additional hydrogen bond to W215 and hydrophobic interactions with the introduced Y258 in subunit B. Altogether, our findings not only highlight the importance of the highly conserved histidine residue in the binding of SDH inhibitors, but also reveal that a resistance mutation can provide both positive and negative cross-resistance within the same acaricide mode of action group.

Miticidal activity of fenazaquin and fenpyroximate against Varroa destructor, an ectoparasite of Apis mellifera

BACKGROUND

The Varroa mite (Varroa destructor) is an ectoparasite that can affect the health of honey bees (Apis mellifera) and contributes to the loss of colony productivity. The limited availability of Varroacides with different modes of action in Canada has resulted in the development of chemical resistance in mite populations. Therefore, an urgent need to evaluate new potential miticides that are safe for bees and exhibit high efficacy against Varroa exists. In this study, the acute contact toxicity of 26 active ingredients (19 chemical classes), already available on the market, was evaluated on V. destructor and A. mellifera under laboratory conditions using an apiarium bioassay. In this assay, groups of Varroa-infested worker bees were exposed to different dilutions of candidate compounds. In semi-field trials, Varroa-infested honey bees were randomly treated with four vetted candidate compounds from the apiarium assay in mini-colonies.

RESULTS

Among tested compounds, fenazaquin (quinazoline class) and fenpyroximate (pyrazole class) had higher mite mortality and lower bee mortality over a 24 h exposure period in apiariums. These two compounds, plus spirotetramat and spirodiclofen, were selected for semi-field evaluation based on the findings of the apiarium bioassay trials and previous laboratory studies. Consistent with the apiarium bioassay, semi-field results showed fenazaquin and fenpyroximate had high efficacy (>80%), reducing Varroa abundance by 80% and 68%, respectively.

CONCLUSION

These findings suggest that fenazaquin would be an effective Varroacide, along with fenpyroximate, which was previously registered for in-hive use as Hivastan. Both compounds have the potential to provide beekeepers with an alternative option for managing Varroa mites in honey bee colonies. © 2022 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Insights into gene manipulation techniques for Acari functional genomics

Functional genomics is an essential tool for elucidating the structure and function of genes in any living organism. Here, we review the use of different gene manipulation techniques in functional genomics of Acari (mites and ticks). Some of these Acari species inflict severe economic losses to managed crops and health problems to humans, wild and domestic animals, but many also provide important ecosystem services worldwide. Currently, RNA interference (RNAi) is the leading gene expression manipulation tool followed by gene editing via the bacterial type II Clustered Regularly Interspaced Short Palindromic Repeats and associated protein 9 system (CRISPR-Cas9). Whilst RNAi, via siRNA, does not always lead to expected outcomes, the exploitations of the CRISPR systems in Acari are still in their infancy and are limited only to CRISP/Cas9 to date. In this review, we discuss the advantages and disadvantages of RNAi and CRISPR-Cas9 and the technical challenges associated with their exploitations. We also compare the biochemical machinery of RNAi and CRISPR-Cas9 technologies. We highlight some potential solutions for experimental optimization of each mechanism in gene function studies. The potential benefits of adopting various CRISPR-Cas9 systems for expanding on functional genomics experiments in Acari are also discussed.

Selectivity and molecular stress responses to classical and botanical acaricides in the predatory mite Phytoseiulus persimilis Athias-Henriot (Acari: Phytoseiidae)

BACKGROUND

Acaricide application remains an integral component of integrated pest management (IPM) for the two-spotted spider mite Tetranychus urticae. Species and strains of phytoseiid predatory mites vary significantly in their response to acaricides. For the success of IPM, it is imperative to identify the determinants of selectivity and molecular stress responses of acaricides in predatory mites.

RESULTS

The three classical acaricides bifenazate, cyflumetofen, and fenbutatin oxide did not affect the survival and fecundity of Phytoseiulus persimilis regardless of the route of exposure. Selectivity of the orange oil and terpenoid blend-based botanical acaricides was low via a combination of direct exposure, acaricide-laced diet, and residual exposure but improved when limiting exposure only to diet. To gain insights into the molecular stress responses, the transcriptome of P. persimilis was assembled. Subsequent gene expression analysis of predatory mites orally exposed to fenbutatin oxide and orange oil yielded only a limited xenobiotic stress response. In contrast, P. persimilis exhibited target-site resistance mutations, including I260M in SdhB, I1017M in CHS1, and kdr and super-kdr in VGSC. Extending the screen using available Phytoseiidae sequences uncovered I136T, S141F in cytb, G119S in AChE, and A2083V in ACC, well-known target-sites of acaricides.

CONCLUSION

Selectivity of the tested botanical acaricides to P. persimilis was low but could be enhanced by restricting exposure to a single route. Differential gene expression analysis did not show a robust induced stress response after sublethal exposure. In contrast, this study uncovered target-site mutations that may help to explain the physiological selectivity of several classical acaricides to phytoseiid predators.

Combination of target site mutation and associated CYPs confers high-level resistance to pyridaben in Tetranychus urticae

Pyridaben is a mitochondrial electron transport complex I inhibitor. The H110R mutation in the PSST subunit has been reported as a major factor in pyridaben resistance in the two-spotted spider mite, Tetranychus urticae. However, backcross experiments revealed that the mutant PSST alone conferred only moderate resistance. In contrast, inhibition of cytochrome P450 (CYP) markedly reduces resistance levels in a number of highly resistant strains. It was reported previously that maternal factors contributed to the inheritance of pyridaben resistance in the egg stage, but the underlying mechanisms have yet to be elucidated. Here, we studied the combined effects of the PSST H110R mutation and candidate CYPs, as metabolic resistance factors, on pyridaben resistance in T. urticae. We found that the maternal effects of inheritance of resistance in the egg stage were associated with CYP activity. Analysis of differential gene expression by RNA-seq identified CYP392A3 as a candidate causal factor for the high resistance level. Congenic strains, where the alleles of both PSST and CYP392A3 were derived from a resistant strain (RR_i; i = 1 or 2) and a susceptible strain (SS_i) in a common susceptible genetic background, were constructed by marker-assisted backcrossing. RR_i showed upregulation of CYP392A3 and high resistance levels (LC₅₀ > 10,000 mg L^-1), while SS_i had LC₅₀ < 10 mg L^-1. To disentangle the individual effects of PSST and CYP392A3 alleles, we also attempted to uncouple these genes in RR_i. We conclude that given the variation in LC₅₀ values and expression levels of CYP392A3 in the congenic and uncoupled strains, it is likely that the high pyridaben resistance levels are due to a synergistic or cumulative effect of the combination of mutant PSST and associated CYPs, including CYP392A3, but other yet to be discovered factors cannot be excluded.

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.

The G126S substitution in mitochondrially encoded cytochrome b does not confer bifenazate resistance in the spider mite Tetranychus urticae

Several genetic variants of the cd1- and ef-helices of the Q_o site of mitochondrial cytochrome b have been associated with bifenazate resistance in the spider mite Tetranychus urticae, an important crop pest around the world. Maternal inheritance of bifenazate resistance has provided strong evidence for the involvement of many of these mutations alone or in combination. A number of populations highly resistant to bifenazate were uncovered that carried the G126S substitution in combination with other target-site mutations. This G126S mutation has therefore been investigated in several studies in the context of resistance evolution and the development of diagnostic markers. However, experimental data that link bifenazate resistance with the presence of the G126S mutation without additional cd1- and ef-helices mutations, remain very limited. Here, we genotyped 38 T. urticae field populations for cytochrome b and uncovered nine field populations with a fixed or segregating G126S substitution without other target-site mutations in the conserved cd1- and ef-helices of the cytochrome b Q_o pocket. Toxicity bioassays showed that all nine field populations were very susceptible to bifenazate, providing strong evidence that G126S alone does not confer bifenazate resistance. These findings also implicate that previous T. urticae populations with G126S found to be low to moderately resistant to bifenazate, evolved alternative mechanisms of resistance, and more importantly, that this mutation cannot be used as a molecular diagnostic for bifenazate resistance.

Co-occurrence of subunit B and C mutations in respiratory complex II confers high resistance levels to pyflubumide and cyenopyrafen in the two-spotted spider mite Tetranychus urticae (Acari: Tetranychidae)

BACKGROUND

Pyflubumide and cyenopyrafen are respiratory complex II (complex II) inhibitors. Previous quantitative trait locus analyses suggested associations of I260V and S56L in complex II subunit B (B-I260V) and subunit C (C-S56L) with pyflubumide and cyenopyrafen resistance, respectively, in Tetranychus urticae. However, although resistant strains had been selected separately by these acaricides, all strains were homozygous for both B-I260V and C-S56L. Hence, the effects of each mutation on resistance development remain unclear.

RESULTS

We established strains homozygous for B-I260V with C-S56 (B-I260V_I260V/C-S56_S56) and for C-S56L with B-I260 (B-I260_I260/C-S56L_S56L). High resistance levels (LC₅₀  > 1000 mg L^-1 ) to pyflubumide and cyenopyrafen was not conferred by B-I260V or C-S56L alone. Next, we prepared intermixed strains by crossing B-I260V_I260V/C-S56_S56 and B-I260_I260/C-S56L_S56L. Selection of the intermixed strains by either acaricide caused very high resistance levels (LC₅₀  ≥ 10 000 mg L^-1 ) to both acaricides and fixed both mutations. Allele-selected recoupling of the mutations without acaricide selection also conferred very high resistance levels to both acaricides in the intermixed strains. Unlike these, B-I260V or C-S56L alone conferred very high and high resistance levels to cyflumetofen, respectively.

CONCLUSION

We conclude that the effect of individual mutations characteristically varies among complex II inhibitors. Moreover, very high resistance levels to pyflubumide and cyenopyrafen is conferred by the co-occurrence of B-I260V and C-S56L mutations, which alone have limited effects on resistance level.

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.

Inhibition of hyaluronan secretion by novel coumarin compounds and chitin synthesis inhibitors

Elevated plasma levels of hyaluronic acid (HA) is a disease marker in liver pathology and other inflammatory disorders. Inhibition of HA synthesis with coumarin 4-methylumbelliferone (4MU) has a beneficial effect in animal models of fibrosis, inflammation, cancer and metabolic syndrome. 4MU is an active compound of approved choleretic drug hymecromone with low bioavailability and a broad spectrum of action. New, more specific and efficient inhibitors of hyaluronan synthases (HAS) are required. We have tested several newly synthesized coumarin compounds and commercial chitin synthesis inhibitors to inhibit HA production in cell culture assay. Coumarin derivative compound VII (10'-methyl-6'-phenyl-3'H-spiro[piperidine-4,2'-pyrano[3,2-g]chromene]-4',8'-dione) demonstrated inhibition of HA secretion by NIH3T3 cells with the half-maximal inhibitory concentration (IC50) = 1.69 ± 0.75 μΜ superior to 4MU (IC50 = 8.68 ± 1.6 μΜ). Inhibitors of chitin synthesis, etoxazole, buprofezin, triflumuron, reduced HA deposition with IC50 of 4.21 ± 3.82 μΜ, 1.24 ± 0.87 μΜ and 1.48 ± 1.44 μΜ, respectively. Etoxazole reduced HA production and prevented collagen fibre formation in the CCl4 liver fibrosis model in mice similar to 4MU. Bioinformatics analysis revealed homology between chitin synthases and HAS enzymes, particularly in the pore-forming domain, containing the proposed site for etoxazole binding.

Susceptibility to Acaricides and the Frequencies of Point Mutations in Etoxazole- and Pyridaben-Resistant Strains and Field Populations of the Two-Spotted Spider Mite, Tetranychus urticae (Acari: Tetranychidae)

The two-spotted spider mite Tetranychus urticae Koch is a major agricultural pest worldwide and is known to rapidly develop resistance to pesticides. In the present study, we explored a field strain that was collected in 2000 and 2003 and has been exhibiting resistance to etoxazole and pyridaben over the last 16 years. The resistance ratios of the etoxazole- and pyridaben-resistant strains (ER and PR) to etoxazole or pyridaben were more than 5,000,000- and 4109.6-fold higher than that of the susceptible strain, respectively. All field-collected populations showed resistance to etoxazole and pyridaben. The ER and PR strains showed cross-resistance to several acaricides. Both I1017F and H92R point mutations were detected in 7 out of 8 field groups. Spirodiclofen and spiromesifen resulted in more than 77.5% mortality in the 8 field groups. In addition, the genotype frequency of the I1017F point mutation was 100.0% in the ER strain, and that of the H92R point mutation was 97.0% in the PR strain. All of the field populations were found to have a high frequency of I1017F. These results suggest that the observation of resistance patterns will help in designing a sustainable IPM program for T. urticae.

An octopamine receptor confers selective toxicity of amitraz on honeybees and Varroa mites

The Varroa destructor mite is a devastating parasite of Apis mellifera honeybees. They can cause colonies to collapse by spreading viruses and feeding on the fat reserves of adults and larvae. Amitraz is used to control mites due to its low toxicity to bees; however, the mechanism of bee resistance to amitraz remains unknown. In this study, we found that amitraz and its major metabolite potently activated all four mite octopamine receptors. Behavioral assays using Drosophila null mutants of octopamine receptors identified one receptor subtype Octβ2R as the sole target of amitraz in vivo. We found that thermogenetic activation of octβ2R-expressing neurons mimics amitraz poisoning symptoms in target pests. We next confirmed that the mite Octβ2R was more sensitive to amitraz and its metabolite than the bee Octβ2R in pharmacological assays and transgenic flies. Furthermore, replacement of three bee-specific residues with the counterparts in the mite receptor increased amitraz sensitivity of the bee Octβ2R, indicating that the relative insensitivity of their receptor is the major mechanism for honeybees to resist amitraz. The present findings have important implications for resistance management and the design of safer insecticides that selectively target pests while maintaining low toxicity to non-target pollinators.

The fitness advantages of bistrifluron resistance related to chitin synthase A in Spodoptera litura (Fab.) (Noctuidae: Lepidoptera)

BACKGROUND

Spodoptera litura is one of the major agricultural pests in China, and it has developed serious resistance to many traditional chemical insecticides. In the present study, the bistrifluron-resistant (Bis-SEL) strain accompanied by a higher oviposition, 113.8-fold RR compared to the bistrifluron-susceptible (Bis-UNSEL) strain, was obtained by bidirectional screening. A comparison of their gonad coefficiency and genes related to oviposition or resistance was used to elucidate the resurgence mechanism.

RESULTS

The ovarian index, oviposition, and potential egg production in the Bis-SEL strain of female adults were significantly higher than those in the Bis-UNSEL strain, and the length of ovariole in the Bis-SEL strain was also significantly elongated. The protein contents of vitellogenin (Vg) and vitellogenin receptor (VgR) in the Bis-UNSEL strain were lower than those in the Bis-SEL strain, consistent with their gene expressions levels, and there was a significantly positive linear correlation between Vg and VgR protein contents, further confirming that resistant strains have high reproductive fitness. Moreover, the chitin synthase A in the Bis-SEL strain was clearly up-regulated, and a mutation (H866Y) near the QRRRW in the catalytic domain caused a rise in the hydrogen bond between UDP-GlcNAc and chitin synthase, and its chitin content was higher than that in the Bis-UNSEL strain. Nevertheless, the sensitivity of the Bis-SEL strain to bistrifluron was significantly recovered when it was knocked down though RNA interference.

CONCLUSION

The fitness advantages of bistrifluron resistance may be related to the up-regulation and mution of chitin synthase A. © 2021 Society of Chemical Industry.

High-resolution genetic mapping reveals cis-regulatory and copy number variation in loci associated with cytochrome P450-mediated detoxification in a generalist arthropod pest

Chemical control strategies are driving the evolution of pesticide resistance in pest populations. Understanding the genetic mechanisms of these evolutionary processes is of crucial importance to develop sustainable resistance management strategies. The acaricide pyflubumide is one of the most recently developed mitochondrial complex II inhibitors with a new mode of action that specifically targets spider mite pests. In this study, we characterize the molecular basis of pyflubumide resistance in a highly resistant population of the spider mite Tetranychus urticae. Classical genetic crosses indicated that pyflubumide resistance was incompletely recessive and controlled by more than one gene. To identify resistance loci, we crossed the resistant population to a highly susceptible T. urticae inbred strain and propagated resulting populations with and without pyflubumide exposure for multiple generations in an experimental evolution set-up. High-resolution genetic mapping by a bulked segregant analysis approach led to the identification of three quantitative trait loci (QTL) linked to pyflubumide resistance. Two QTLs were found on the first chromosome and centered on the cytochrome P450 CYP392A16 and a cluster of CYP392E6-8 genes. Comparative transcriptomics revealed a consistent overexpression of CYP392A16 and CYP392E8 in the experimental populations that were selected for pyflubumide resistance. We further corroborated the involvement of CYP392A16 in resistance by in vitro functional expression and metabolism studies. Collectively, these experiments uncovered that CYP392A16 N-demethylates the toxic carboxamide form of pyflubumide to a non-toxic compound. A third QTL coincided with cytochrome P450 reductase (CPR), a vital component of cytochrome P450 metabolism. We show here that the resistant population harbors three gene copies of CPR and that this copy number variation is associated with higher mRNA abundance. Together, we provide evidence for detoxification of pyflubumide by cytochrome P450s that is likely synergized by gene amplification of CPR.

Our understanding of the causal genetic variants that drive the evolution of quantitative traits, such as polygenic pesticide resistance, remains very limited. Here, we followed a high-resolution genetic mapping approach to localize the genetic variants that cause pyflubumide resistance in the two-spotted spider mite Tetranychus urticae. Three well-supported QTL were uncovered and pointed towards a major role for cytochrome P450-mediated detoxification. Cis-regulatory variation for cytochrome P450s was observed, and in vitro cytochrome P450 experiments showed that pyflubumide was metabolized into a non-toxic derivate. A third QTL centered on cytochrome P450 reductase (CPR), which is required for cytochrome P450 activity, and is amplified in pyflubumide resistant populations. Our results indicate that pyflubumide resistance is mediated by cytochrome P450 detoxification that is enhanced by gene amplification at the CPR locus.

Transposon-mediated insertional mutagenesis unmasks recessive insecticide resistance in the aphid Myzus persicae

The evolution of resistance to insecticides threatens the sustainable control of many of the world's most damaging insect crop pests and disease vectors. To effectively combat resistance, it is important to understand its underlying genetic architecture, including the type and number of genetic variants affecting resistance and their interactions with each other and the environment. While significant progress has been made in characterizing the individual genes or mutations leading to resistance, our understanding of how genetic variants interact to influence its phenotypic expression remains poor. Here, we uncover a mechanism of insecticide resistance resulting from transposon-mediated insertional mutagenesis of a genetically dominant but insecticide-susceptible allele that enables the adaptive potential of a previously unavailable recessive resistance allele to be unlocked. Specifically, we identify clones of the aphid pest Myzus persicae that carry a resistant allele of the essential voltage-gated sodium channel (VGSC) gene with the recessive M918T and L1014F resistance mutations, in combination with an allele lacking these mutations but carrying a Mutator-like element transposon insertion that disrupts the coding sequence of the VGSC. This results in the down-regulation of the dominant susceptible allele and monoallelic expression of the recessive resistant allele, rendering the clones resistant to the insecticide bifenthrin. These findings are a powerful example of how transposable elements can provide a source of evolutionary potential that can be revealed by environmental and genetic perturbation, with applied implications for the control of highly damaging insect pests.

Molecular Characterization of Chitin Synthase Gene in Tetranychus cinnabarinus (Boisduval) and Its Response to Sublethal Concentrations of an Insecticide

Simple Summary

In this study, we identified chitin synthase 1 gene (TcCHS1) from Tetranychus cinnabarinus (Boisduval) and then explored the gene expression levels of TcCHS1 at different developmental stages of T. cinnabarinus. We also investigated the effects of sublethal concentrations of diflubenzuron on the toxicities and survivals of T. cinnabarinus eggs and larvae as well as TcCHS1 expression levels. Our results demonstrated that TcCHS1 was essential for growth and development, and diflubenzuron exposure affected chitin metabolism. This work was undertaken to establish a foundation for further research on the functions of chitin synthase. It will provide a new target for controlling of T. cinnabarinus in the agricultural ecosystem.

Abstract

The carmine spider mite, Tetranychus cinnabarinus (Boisduval), is one of the most important acarine pest species. At present, its control remains primarily dependent on using various chemical insecticides/acaricides in agricultural crops worldwide. To clarify the mechanism whereby T. cinnabarinus responds to insecticide exposure, we identified the chitin synthase 1 gene (TcCHS1) and then explored the gene expression levels of TcCHS1 at different developmental stages of T. cinnabarinus. We also investigated the effects of sublethal concentrations of diflubenzuron on the toxicities and survivals of T. cinnabarinus eggs and larvae as well as TcCHS1 expression levels. The full-length cDNA sequence contains an open reading frame (ORF) of 4881 nucleotides that encoded for a 1474 amino acid residues protein. The predicted TcCHS1 protein had a molecular mass of 168.35 kDa and an isoelectric point of 6.26, and its amino acid sequence contained all the signature motifs (EDR, QRRRW and TWGTR) of chitin synthases. The results of phylogenetic analyses demonstrated that the putative CHS1 amino acid sequence of T. cinnabarinus revealed high similarities with chitin synthases in other insects and mites. Additionally, at the molecular level, transcriptional analysis by real-time quantitative PCR in different developmental stages of T. cinnabarinus revealed that TcCHS1 mRNA was expressed in all stages, and highest in eggs and female adults, but lowest in deutonymphs. Furthermore, the results of toxicity bioassays indicated that diflubenzuron treatment resulted in high mortality rates in eggs and larvae of T. cinnabarinus. The mRNA expression levels of TcCHS1 from the eggs and larvae of T. cinnabarinus were up-regulated in response to sublethal concentrations of diflubenzuron exposures. Together, all these results demonstrate that diflubenzuron has ovicidal and larvicidal effects and TcCHS1 may play an important role in the growth and development of T. cinnabarinus and may disrupt the chitin biosynthesis, thereby controlling T. cinnabarinus populations.

Whitefly hijacks a plant detoxification gene that neutralizes plant toxins

Plants protect themselves with a vast array of toxic secondary metabolites, yet most plants serve as food for insects. The evolutionary processes that allow herbivorous insects to resist plant defenses remain largely unknown. The whitefly Bemisia tabaci is a cosmopolitan, highly polyphagous agricultural pest that vectors several serious plant pathogenic viruses and is an excellent model to probe the molecular mechanisms involved in overcoming plant defenses. Here, we show that, through an exceptional horizontal gene transfer event, the whitefly has acquired the plant-derived phenolic glucoside malonyltransferase gene BtPMaT1. This gene enables whiteflies to neutralize phenolic glucosides. This was confirmed by genetically transforming tomato plants to produce small interfering RNAs that silence BtPMaT1, thus impairing the whiteflies' detoxification ability. These findings reveal an evolutionary scenario whereby herbivores harness the genetic toolkit of their host plants to develop resistance to plant defenses and how this can be exploited for crop protection.

Development of abamectin resistance in Tetranychus urticae in Australian cotton and the establishment of discriminating doses for T. lambi

Since the 1980s Tetranychus urticae Koch has dominated Australian cotton due to its ability to develop resistance. Here we give screening data for a range of chemicals tested against T. urticae including abamectin, bifenthrin, diafenthiuron, etoxazole and propargite and speculate why abamectin resistance emerged without warning. Abamectin resistance was not detected in T. urticae in Australian cotton before season 2007-2008 when a few resistant individuals were detected in a single strain. Resistance was detected again in season 2010-2011 and continued to be detected in every subsequent season comprising 80% of strains tested in 2018-2019. We speculate the reason may relate to prophylactic abamectin use to prevent mite flare with Creontiades dilutes Stål mirid sprays. With the introduction of transgenic Bt-cotton, spraying significantly reduced and anecdotally Tetranychus lambi became more abundant. Although T. lambi may now be more common than T. urticae its response to chemical controls is completely unknown. Tetranychus lambi conspecific dose responses were established to support resistance monitoring against abamectin, diafenthiuron and propargite that generated discriminating dose (DD) estimates of 0.0007 g/L abamectin, 0.03 g/L diafenthiuron and 0.7 g/L propargite. These DD were used in season 2018-2019 but resistance was not detected against any product including abamectin. The reason why T. lambi may now dominate despite T. urticae being still resistant is speculated and thought related to the progressive reduction in insecticide use in Australian cotton and/or the changing weed complex in the transgenic cotton era.

Isoxaben analogs inhibit chitin synthesis in the cultured integument of the rice stem borer Chilo suppressalis

Benzoylphenylureas (BPUs) were discovered as novel type insecticides about a half century ago; many analogs have been launched as insecticides and acaricides. BPUs are known to inhibit chitin synthesis in insects and other arthropods, but they have no effect against microorganisms such as fungi. We designed new chitin synthesis inhibitors based on the hypothesis that biomolecules that play important roles in cellulose and chitin biosynthesis are similar. In the full automatic modeling system (FAMS), the cellulose synthase was selected as a template three-dimensional structure. Thus, we focused on the structure of cellulose synthase inhibitor, isoxaben, to develop new chemistry. The 1,1-diethylethyl [-C(CH₃)(CH₂CH₃)₂] group of isoxaben was changed to a 4-substituted phenyl group bearing Cl, Et, or Ph. These compounds significantly inhibited chitin synthesis in the cultured integument of the rice stem borer Chilo suppressalis. The activity of the 4-ethylphenyl analog was enhanced 30-fold by adding piperonyl butoxide to the culture medium.