Genetic and biochemical analysis of a laboratory-selected spirodiclofen-resistant strain of Tetranychus urticae Koch (Acari: Tetranychidae)

Identification and Functional Characterization of Carboxylesterase Genes Involved in Spirodiclofen Resistance in Panonychus citri (McGregor)

Overexpression of carboxyl/cholinesterase (CCE) genes has been reported to be associated with many cases of pesticide resistance in arthropods. However, it has been rarely documented that CCE genes participate in spirodiclofen resistance in Panonychus citri. In previous research, we found that spirodiclofen resistance is related to increased P450 and CCE enzyme activities in P. citri. In this study, we identified two CCE genes, PcCCE3 and PcCCE5, which were significantly upregulated in spirodiclofen-resistant strain and after exposure to spirodiclofen. RNA interference of PcCCE3 and PcCCE5 increased the spirodiclofen susceptibility in P. citri. In vitro metabolism indicated that PcCCE3 and PcCCE5 could interact with spirodiclofen, but metabolites were detected only in the PcCCE3 treatment. Our results indicated that PcCCE3 participates in spirodiclofen resistance through direct metabolism, and PcCCE5 may be involved in the spirodiclofen resistance by passive binding and sequestration, which provides new insights into spirodiclofen resistance in P. citri.

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.

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 detoxification pathway initiated by a nuclear receptor TcHR96h in Tetranychus cinnabarinus (Boisduval)

Understanding the mechanism of detoxification initiation in arthropods after pesticide exposure is crucial. Although the identity of transcription factors that induce and regulate the expression of detoxification genes in response to pesticides is beginning to emerge, whether transcription factors directly interact with xenobiotics is unclear. The findings of this study revealed that a nuclear hormone receptor, Tetranychus cinnabarinus hormone receptor (HR) TcHR96h, regulates the overexpression of the detoxification gene TcGSTm02, which is involved in cyflumetofen resistance. The nuclear translocation of TcHR96h increased after cyflumetofen exposure, suggesting direct binding with cyflumetofen. The direct binding of TcHR96h and cyflumetofen was supported by several independent proteomic assays that quantify interactions with small molecules. Together, this study proposes a model for the initiation of xenobiotic detoxification in a polyphagous agricultural pest. These insights not only provide a better understanding of the mechanisms of xenobiotic detoxification and metabolism in arthropods, but also are crucial in understanding adaptation in polyphagous herbivores.

Negative Impact of Unstable Spiromesifen Resistance on Fitness of Tetranychus urticae (Acari: Tetranychidae)

Two-spotted spider mite, Tetranychus urticae Koch (Acari: Tetranychidae), is a phytophagous haplodiploid mite and its control is largely based on the use of pesticides. But, the short life cycle and high reproductive rate allow them to develop resistance to many pesticides. To design a strategy for resistance management, a fitness cost study was conducted on different populations of T. urticae, i.e., spiromesifen selected (SPIRO-SEL), unselected (Unsel), and reciprocal crosses. After twelve rounds of selections, T. urticae developed high spiromesifen resistance (71.7-fold) compared to the Unsel strain. Results showed a fitness cost for SPIRO-SEL, Cross1 (Unsel ♀ × SPIRO-SEL ♂), and Cross2 (SPIRO-SEL ♀ × Unsel ♂) with a relative fitness values of 0.63, 0.86, and 0.70, respectively. There was a significant increase in the incubation period, quiescent larvae, and egg to adult male and female developmental period of the SPIRO-SEL compared with Unsel strain. Moreover, resistance to spiromesifen was unstable with a decline in resistance value of - 0.05. The presence of unstable spiromesifen resistance associated with fitness costs suggests that intermittent withdrawal of its usage could potentially preserve its effectiveness for management of T. urticae.

Increased metabolism in combination with the novel cytochrome b target-site mutation L258F confers cross-resistance between the Qo inhibitors acequinocyl and bifenazate in Tetranychus urticae

Acequinocyl and bifenazate are potent acaricides acting at the Q_o site of complex III of the electron transport chain, but frequent applications of these acaricides have led to the development of resistance in spider mites. Target-site resistance caused by mutations in the conserved cd1- and ef-helices of the Q_o pocket of cytochrome b has been elucidated as the main resistance mechanism. We therefore monitored Q_o pocket mutations in European field populations of Tetranychus urticae and uncovered a new mutation, L258F. The role of this mutation was validated by revealing patterns of maternal inheritance and by the independently replicated introgression in an unrelated susceptible genetic background. However, the parental strain exhibited higher resistance levels than conferred by the mutation alone in isogenic lines, especially for acequinocyl, implying the involvement of strong additional resistance mechanisms. This was confirmed by revealing a polygenic inheritance pattern with classical genetic crosses and via synergism experiments. Therefore, a genome-wide expression analysis was conducted that identified a number of highly overexpressed detoxification genes, including many P450s. Functional expression revealed that the P450 CYP392A11 can metabolize bifenazate by hydroxylation of the ring structure. In conclusion, the novel cytochrome b target-site mutation L258F was uncovered in a recently collected field strain and its role in acequinocyl and bifenazate resistance was validated. However, the high level of resistance in this strain is most likely caused by a combination of target-site resistance and P450-based increased detoxification, potentially acting in synergism.

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.

Trunk Injection with Insecticides Manages Xylotrechus chinensis (Chevrolat) (Coleoptera: Cerambycidae)

Xylotrechus chinensis (Chevrolat) (Coleoptera: Cerambycidae) is a serious wood-boring insect of mulberry trees (Morus spp.). Larvae of this species enter the trunk of the tree and feed on woody tissues. Xylotrechus chinensis is endemic in several Asian countries, while, in the last decade, it invaded Europe. In the present work, we evaluated trunk injection against this pest. The systemic insecticides fipronil, imidacloprid, and spirotetramat were used in the trials. Abamectin was used as a positive control since it has been found to be effective for the management of X. chinensis. Imidacloprid and fipronil proved to be highly effective against this species in both years (9.5 and 12.1 exit holes/tree in 2021, 5.0 and 8.8 exit holes/tree in 2022, respectively), while spirotetramat was the least effective. The lowest mean number of exit holes was recorded when abamectin was applied in both years (4.7 exit holes/tree in 2021 and 3.3 exit holes/tree in 2022). The percentage of exit holes was reduced by 76.1, 71.8, and 85.6% in trees treated with imidacloprid, fipronil, and abamectin, respectively, after two years of application, while spirotetramat caused a 37.4% reduction. Trunk injection with imidacloprid, fipronil, and abamectin could be used against X. chinensis for long-term control of mulberry trees.

Biochemical and Molecular Analysis of Field Resistance to Spirodiclofen in Panonychus citri (McGregor)

Spirodiclofen is one of the most widely used acaricides in China. The citrus red mite, Panonychus citri (McGregor) (Acari: Tetranychidae), is one of the most destructive citrus pests worldwide and has developed a high resistance to spirodiclofen. However, the molecular mechanism of spirodiclofen resistance in P. citri is still unknown. In this study, we identified a field spirodiclofen-resistant strain (DL-SC) that showed 712-fold resistance to spirodiclofen by egg bioassay compared to the susceptible strain. Target-site resistance was not detected as non-synonymous mutations were not found by amplification and sequencing of the ACCase gene of resistant and susceptible strains; in addition, the mRNA expression levels of ACCase were similar in both resistant and susceptible strains. The activity of detoxifying enzymes P450s and CCEs in the resistant strain was significantly higher than in the susceptible strain. The transcriptome expression data showed 19 xenobiotic metabolisms genes that were upregulated. Stage-specific expression profiling revealed that the most prominent upregulated gene, CYP385C10, in transcriptome data was significantly higher in resistant strains in all stages. Furthermore, functional analysis by RNAi indicated that the mortality caused by spirodiclofen was significantly increased by silencing the P450 gene CYP385C10. The current results suggest that overexpression of the P450 gene, CYP385C10, may be involved in spirodiclofen resistance in P. citri.

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.

Cross resistance, inheritance and fitness advantage of cyetpyrafen resistance in two-spotted spider mite, Tetranychus urticae

Cyetpyrafen belonging to mitochondrial electron transport inhibitors of complex II (METI II) has been widely applied to manage pest mites in China. To investigate the adaption of Tetranychus urticae in the evolution of cyetpyrafen resistance, a study of cross resistance, mode of inheritance and fitness comparison of resistance using indoor cyetpyrafen resistant strain (resistance ratio, RR > 2, 000-fold) was executed. Cyet-R showed serious cross resistance to cyenopyrafen (>2500-fold) and cyflumetofen (~190-fold). The number of resistant genes was evaluated via chi-square (χ²) test and the concentration-response curve regarding goodness-of-fit between observed and the expected mortality. The LC₅₀s of F_1RS (Cyet-R^♀ × Tu-YN^♂) and F_1SR (Tu-YN^♀ × Cyet-R^♂) were 3126.30 mg/L and 2743.97 mg/L, respectively, without significance, suggesting autosomal inheritance. The degree of dominance (D) values of F_1RS and F_1SR ranged between 0 and 1, revealing an incompletely dominant inheritance in the tested population of Tetranychus urticae. Plots of concentration-response data for the orthogonal backcross and reverse backcross progenies showed a significant deviation from the expected lines, pointing out a polygenic inheritance. Besides, lifetable analysis showed a fitness advantage of Cyet-R with a significantly decreased adult preadult period and significantly increased total fecundity. This study suggested that cyetpyrafen resistance against T. urticae was inherited as autosomal, incompletely dominant and multigenetic and characterized with serious cross resistance and fitness advantage. Therefore, rational application and preventive strategy should be considered to sustain the efficacy of cyetpyrafen against T. urticae.

Chemosensory Proteins Are Associated with Thiamethoxam and Spirotetramat Tolerance in Aphis gossypii Glover

Chemosensory proteins (CSPs) are a class of transporters in arthropods. Deeper research on CSPs showed that CSPs may be involved in some physiological processes beyond chemoreception, such as insect resistance to pesticides. We identified two upregulated CSPs in two resistant strains of Aphis gossypii Glover. To understand their role in the resistance of aphids to pesticides, we performed the functional verification of CSP1 and CSP4 in vivo and in vitro. Results showed that the sensitivity of the thiamethoxam-resistant strain to thiamethoxam increased significantly with the silencing of CSP1 and CSP4 by RNAi (RNA interference), and the sensitivity of the spirotetramat-resistant strain to spirotetramat increased significantly with the silencing of CSP4. Transgenic Drosophila melanogaster expressing CSPs exhibited stronger resistance to thiamethoxam, spirotetramat, and alpha-cypermethrin than the control did. In the bioassay of transgenic Drosophila, CSPs showed different tolerance mechanisms for different pesticides, and the overexpressed CSPs may play a role in processes other than resistance to pesticides. In brief, the present results prove that CSPs are related to the resistance of cotton aphids to insecticides.

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.

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.

Resistance to spirodiclofen in Brevipalpus yothersi (Acari: Tenuipalpidae) from Brazilian citrus groves: detection, monitoring, and population performance

BACKGROUND

Brevipalpus yothersi mite is the main vector of Citrus leprosis virus (CiLV), the causal agent of citrus leprosis disease. The acaricide spirodiclofen has been widely used to control this mite. However, failures in control using spirodiclofen have been frequently reported by citrus growers. In this study, we estimated the diagnostic concentration to monitor the resistance to spirodiclofen of B. yothersi populations collected in nine citrus groves in Brazil. We then selected the B. yothersi population that showed lowest mortality with the estimated diagnostic concentration of spirodiclofen to characterize the frequency of resistant individuals, as well as demographic and life table parameters.

RESULTS

Variability was higher between populations in terms of susceptibility to spirodiclofen. The frequency of resistant eggs between populations ranged from 0.7% to 85.8%. The resistance ratio of B. yothersi to spirodiclofen was low to moderate. Survival rates of the immature stage, total adult longevity, oviposition days, and female fecundity were lower in the resistant strain. Furthermore, net reproduction rate, intrinsic rate of increase, finite rate of increase, and mean length of a generation were also lower in the R strain.

CONCLUSION

Although variations in resistance to spirodiclofen were observed between populations, the resistance ratio was low to moderate. Such data can be useful in the development of resistance management strategies for B. yothersi in Brazilian citrus groves. © 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.

The role of detoxification enzymes in the susceptibility of Brevipalpus californicus exposed to acaricide and insecticide mixtures

The intense spraying of pesticides to control different arthropod pests has resulted in negative side effects for the management of pests. It was previously discovered that exposure to non-acaricidal insecticides alone or in a mixture, results in lower efficiency of the acaricide spirodiclofen used for Brevipalpus spp. control. We investigate here whether the induced expression of detoxification enzymes by non-lethal insecticides may antagonize spirodiclofen toxicity. Brevipalpus californicus mites exposed to the insecticide phosmet alone or in combination with spirodiclofen showed increased activity of P450 monooxygenases (P450s). No antagonistic effects in mite mortality were observed by the combination of phosmet and spirodiclofen. On the other hand, mites exposed to the insecticide imidacloprid alone or in combination with spirodiclofen showed an increase in the activity of P450s, carboxylcholinesterases (CCE), and glutathione-S-transferases (GST). An antagonistic effect on mite mortality was observed when mites were exposed to the LC₂₅ of spirodiclofen combined with the field rate treatment of imidacloprid. The addition of PBO (a P450 monooxygenase inhibitor) to the mixture of spirodiclofen and imidacloprid resulted in a synergistic effect over mite mortality but the addition of DEM (a GST inhibitor) resulted in an antagonist effect. Taken together, this study showed that the combination of imidacloprid with spirodiclofen is antagonistic for the control of B. californicus, and this results from the induction of detoxification enzymes, such as P450s, CCE, and GST. The use of inhibitors highlights the role of these enzymes in the antagonism of the mixture.

Beauvericin potentiates the activity of pesticides by neutralizing the ATP-binding cassette transporters in arthropods

Multi-drug resistance is posing major challenges in suppressing the population of pests. Many herbivores develop resistance, causing a prolonged survival after exposure to a previously effective pesticide. Consequently, resistant pests reduce the yield of agricultural production, causing significant economic losses and reducing food security. Therefore, overpowering resistance acquisition of crop pests is a must. The ATP binding cassette transporters (ABC transporters) are considered as the main participants to the pesticide efflux and their neutralization will greatly contribute to potentiate failed treatments. Real-Time PCR analysis of 19 ABC transporter genes belonging to the ABCB, ABCC, ABCG, and ABCH revealed that a broad range of efflux pumps is activated in response to the exposure to pesticides. In this study, we used beauvericin (BEA), a known ABC transporters modulator, to resensitize different strains of Tetranychus urticae after artificial selection for resistance to cyflumetofen, bifenazate, and abamectin. Our results showed that the combinatorial treatment of pesticide (manufacturer's recommended doses) + BEA (sublethal doses: 0.15 mg/L) significantly suppressed the resistant populations of T. urticae when compared to single-drug treatments. Moreover, after selective pressure for 40 generations, the LC₅₀ values were significantly reduced from 36.5, 44.7, and 94.5 (pesticide) to 8.3, 12.5, and 23.4 (pesticide + BEA) for cyflumetofen, bifenazate, and abamectin, respectively. While the downstream targets for BEA are still elusive, we demonstrated hereby that it synergizes with sub-lethal doses of different pesticides and increases their effect by inhibiting ABC transporters. This is the first report to document such combinatorial activity of BEA against higher invertebrates paving the way for its usage in treating refractory cases of resistance to pesticides. Moreover, we demonstrated, for the first time, using in silico techniques, the higher affinity of BEA to ABC transformers subfamilies when compared to xenobiotics; thus, elucidating the pathway of the mycotoxin.

Transcriptome analysis revealed that multiple genes were related to the cyflumetofen resistance of Tetranychus cinnabarinus (Boisduval)

Metabolic resistance is one of the main causes of acaricide resistance. Many previous studies focused on the function of specific genes in insecticides/acaricides resistance. However, during the development of resistance, the overall dynamic of expression levels of detoxification enzyme genes in mites is still unclear. Tetranychus cinnabarinus, a major agricultural pest, which is notorious for developing resistance to acaricides rapidly. In this study, a field susceptible strain (YS) was continuously selected for 16, 25 and 32 generations, and developed to low resistance (7.83-fold, L), medium resistance (17.23-fold, M) and high resistance (86.05-fold, H), respectively. Transcriptome sequencing was performed in YS, L, M and H strains. Overall, compared with YS strain, the number of differential expression genes increased slightly with the development of cyflumetofen-resistance. As for detoxification genes, the median of fold change of up-regulated P450、CCE and GST genes was higher than those of all up-regulated genes in three resistance level, but only the number and the median of fold change of up-regulated P450 genes was increased slightly with the development of resistance. In addition, synergism experiments also proved that P450 and GST genes were the major contributors to the metabolic resistance of cyflumetofen of T. cinnabarinus. These results showed that the resistance of T. cinnabarinus to cyflumetofen was related to many resistant genes, among which P450 genes could play crucial roles in cyflumefen resistance.

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.

Long-term studies on the evolution of resistance of Myzus persicae (Hemiptera: Aphididae) to insecticides in Greece

The aphid Myzus persicae s.l. (Hemiptera: Aphididae) is an important pest of many crops worldwide with a complex life cycle, intensely controlled by chemical pesticides, and has developed resistance to almost all used insecticides. In Greece, the aphid exhibits high genetic variation and adaptability and it is a classic example of evolution in the making. We have been studying M. persicae for over 20 years, on different host plants and varying geographical areas, analyzing its bio-ecology and the ability to develop resistance to insecticides. In this review, we present new and historical data on the effectiveness of insecticides from seven chemical groups used to control the aphid in Greece and the incidence of seven resistance mechanisms, including the new fast-spreading R81T point mutation of the postsynaptic nicotinic acetylcholine receptor. Thousands of samples were tested by biological, biochemical and molecular assays. The aphid populations were found to have developed and maintain resistance at medium to high levels to organophosphates, carbamates, pyrethroids and neonicotinoids for decades. In the latter group, a marked increase is recorded during an ~10-year period. The data analyzed and the extensive bibliography, advocate the difficulty to control the aphid making the design and application of IPM/IRM programs a challenge. We discuss principles and recommendations for the management of resistance, including the use of compounds such as flonicamid, spirotetramat, flupyradifurone and sulfoxaflor. We emphasize that resistance is a dynamic phenomenon, changing in time and space, requiring, therefore, continuous monitoring.

Genetic analysis and screening of detoxification-related genes in an amitraz-resistant strain of Panonychus citri

Panonychus citri (McGregor) is the most common pest in citrus-producing regions. Special low-toxicity acaricides, such as spirocyclic tetronic acids and mite growth inhibitors, have been used for a long time in China. However, pesticide resistance in mites is a growing problem due to the lack of new acaricide development. Wide-spectrum insecticides, such as amitraz have gained acceptance among fruit growers. An amitraz-resistant strain of P. citri was obtained by indoor screening to examine field resistance monitoring of mites to acaricides and to explore the resistant mechanism of mites against amitraz. The amitraz-resistant strain of P. citri had an LC50 value of 2361.45 mg l-1. The resistance ratio was 81.35 times higher in the resistant strain of P. citri compared with the sensitive strain. Crossing experiments between the sensitive and resistant strains of P. citri were conducted, resulting in a D value of 0.11 for F1 SS♀×RS♂ and 0.06 for F1 RS♀×SS♂. Reciprocal cross experiments showed that the dose-mortality curves for the F1 generations coincided, indicating that the resistance trait was not affected by cytoplasmic inheritance. The dose-expected response relationship was evaluated in the backcross generation and a significant difference was observed compared with the actual value. The above results indicate that the inheritance of resistance trait was incompletely dominant, governed by polygenes on the chromosome. Synergism studies demonstrated that cytochrome P450s and esterase may play important roles in the detoxification of amitraz. Based on differential gene analysis, 23 metabolism-related genes of P. citri were identified, consistent with the results of synergism studies. Real-time PCR verification implied that P450s, ABC transporters, and acetylcholinesterase might influence the detoxification of amitraz by P. citri. These results provide the genetic and molecular foundation for the management of pest mite resistance.

Physiological responses of plants and mites to salicylic acid improve the efficacy of spirodiclofen for controlling Tetranychus urticae (Acari: Tetranychidae) on greenhouse tomatoes

Salicylic acid (SA) is a signaling molecule that can induce plant resistance to certain herbivores. Although the role of jasmonic acid in mediating mite-tomato plant interactions has been well studied, the role of salicylic acid has not. This study examined how the application of exogenous SA, via its effects on tomato plant physiology, alters the activity of mite digestive enzymes, mite energy reserves, and mite susceptibility to spirodiclofen. Enzymatic activity-including superoxide dismutase, ascorbate peroxidase, guaiacol peroxidase, polyphenol oxidase, and phenylalanine ammonia-lyase-along with contents of total phenolic, hydrogen peroxide, and total chlorophyll significantly increased in plants 24 h after treatment with 2 mM of SA. In contrast, catalase activity significantly decreased in treated plants, and malondialdehyde content was unaffected. Mites fed on tomato plants treated with SA had significantly lower glutathione S-transferase, esterase, α-amylase, and aminopeptidase activities than those fed on control plants. Energy reserve analyses demonstrated a significant decrease in contents of lipid, protein, and glycogen in mites fed on SA-treated plants, whereas carbohydrate content significantly increased. The LC₅₀ of spirodiclofen was decreased 1.8-fold for Tetranychus urticae fed on SA-treated tomato plants compared to controls. Treatment of adult mites with 2 mM SA on leaf discs did not cause any direct mortality after 24 h. Finally, a greenhouse bioassay confirmed that spider mite mortality following exposure to spirodiclofen was significantly higher on SA plants than on control plants. Mortality of mites exposed to half of the recommended rate of spirodiclofen was similar to those exposed to the recommended rate when they were held on treated plants. These results have valuable implications for T. urticae management programs in tomato production.

UDP-glycosyltransferases contribute to spirotetramat resistance in Aphis gossypii Glover

Uridine diphosphate (UDP)-glycosyltransferases (UGTs) catalyze the conjugation of small lipophilic endogenous and exogenous compounds with sugars to produce water-soluble glycosides, playing an important role in insect endobiotic regulation and xenobiotic detoxification. In this study, two UGT-inhibitors, sulfinpyrazone and 5-nitrouracil, significantly increased spirotetramat toxicity against third instar nymphs of resistant Aphis gossypii, whereas there were no synergistic effects in apterous adult aphids, suggesting UGT involvement in spirotetramat resistance in cotton aphids. Furthermore, the UHPLC-MS/MS was employed to determine the content of spirotetramat and its four metabolites (S-enol, S-glu, S-mono, S-keto) in the honeydew of resistant cotton aphids under spirotetramat treatment. No residual spirotetramat was detected in the honeydew, while its four metabolites were detected at a S-enol: S-glu: S-mono: S-keto ratio of 69.30: 6.54: 1.44: 1.00. Therefore, glycoxidation plays a major role in spirotetramat inactivation and excretion in resistant aphids. Compared with the susceptible strain, the transcriptional levels of UGT344M2 were significantly upregulated in nymphs and adults of the resistant strain. RNA interference of UGT344M2 dramatically increased spirotetramat toxicity in nymphs, but no such effect were found in the resistant adult aphids. Overall, UGT-mediated glycoxidation were found to be involved in spirotetramat resistance. The suppression of UGT344M2 significantly increased the sensitivity of resistant nymphs to spirotetramat, suggesting that UGT344M2 upregulation might be associated with spirotetramat detoxification. This study provides an overview of the involvement of metabolic factors, UGTs, in the development of spirotetramat resistance.

Overexpression of an alternative allele of carboxyl/choline esterase 4 (CCE04) of Tetranychus urticae is associated with high levels of resistance to the keto-enol acaricide spirodiclofen

BACKGROUND

Spirodiclofen is an acaricide that targets lipid biosynthesis by inhibiting acetyl-coenzyme A carboxylase. Spirodiclofen resistance in spider mites has been previously documented and was associated with overexpression of CYP392E10, a cytochrome P450 mono-oxygenase that metabolizes spirodiclofen. However, additional mechanisms have been suggested in several studies and a carboxyl/choline esterase gene, CCE04, was shown to be overexpressed in two genetically different strains, SR-VP and SR-TK, both exhibiting high spirodiclofen resistance levels.

RESULTS

We identified two different CCE04 alleles in both resistant strains, CCE04^SR-VP and CCE04^London , with CCE04^SR-VP being highly overexpressed. Isoelectric focusing analysis confirmed the overexpression of a single esterase isozyme, while copy number and random fragment length polymorphism analysis revealed that CCE04^SR-VP overexpression was more likely due to selection for the CCE04^SR-VP allele rather than gene amplification. Both CCE04 alleles were functionally expressed using the Pichia expression system. Functional enzyme assays revealed only limited kinetic differences between CCE04 isoforms for model substrates. In addition, inhibition/competition experiments with spirodiclofen suggested a similar interaction with both enzymes, whereas its active metabolite, spirodiclofen enol, did not inhibit enzyme activity.

CONCLUSION

Our study suggests that selection with spirodiclofen results in enrichment of a specific allele of CCE04 (CCE04^SR-VP ) in two genetically independent strains, which is highly overexpressed. Based on kinetic enzyme data, however, quantitative rather than qualitative differences between CCE04^SR-VP and CCE04^London seem more likely to be involved in resistance. Our findings are discussed in the light of a possible spirodiclofen resistance mechanism, with sequestration of spirodiclofen by CCE04^SR-VP being a likely hypothesis. © 2019 Society of Chemical Industry.

Physiological resistance alters behavioral response of Tetranychus urticae to acaricides

Multiple acaricide resistance in Tetranychus urticae continues to threaten crop production globally, justifying the need to adequately study resistance for sustainable pest management. Most studies on acaricide resistance have focused on the acute contact toxicity of acaricides with little or no information on the behavioral responses elicited after acaricide exposure. Furthermore, the impact of physiological resistance on these behavioral responses remains unknown in most pest species, including T. urticae. We tested the effect of acaricide resistance on contact toxicity, irritancy and repellency of mitochondrial electron transport inhibitor of complex I (MET-I) and mite growth inhibitor (MGI) acaricides on multiple T. urticae strains. We also tested whether acaricides with similar physiological target site/mode of action also elicit similar behavioral effects on T. urticae strains. MET-I acaricides (fenazaquin, fenpyroximate, and pyrabiden) and MGIs (clofentezine, hexythiazox and etoxazole) elicited a dose-dependent irritant and repellent effect on T. urticae. Selection of strains for physiological resistance to these acaricides affected the behavioral response of T. urticae, especially in MET-I resistant strains, that showed reduced irritancy and repellency to MET-I acaricides. Behavioral response also affected the oviposition of T. urticae, where strains generally showed preferential oviposition away from the acaricides. The outcome of this study highlights negative consequences of acaricide resistance that can potentially affect T. urticae management.

Downregulation of carboxylesterase contributes to cyflumetofen resistance in Tetranychus cinnabarinus (Boisduval)

BACKGROUND

Increased expression or point mutations of carboxyl/cholinesterases (CCEs) have been involved in many cases of insecticide and acaricide resistance. However, it has been only rarely documented that downregulation of CCE genes is associated with resistance, although many insecticides and acaricides need hydrolytic activation in vivo. Previously, expression analysis of a laboratory-selected cyflumetofen-resistant strain of Tetranychus cinnabarinus indicated that resistance was associated with increased expression of a CCE gene of TcCCE04, but also the downregulation of two CCE genes, TcCCE12 and TcCCE23.

RESULTS

Synergism experiments revealed the importance of ester hydrolysis in cyflumetofen toxicity, because treatment with S,S,S-tributylphosphorotrithioate (DEF) caused strong inhibition of cyflumetofen hydrolysis, in both the susceptible and resistant strains. Moreover, silencing expression of TcCCE12 and TcCCE23 via RNAi further decreased the susceptibility of mites to cyflumetofen significantly, suggesting that downregulated CCE genes could be involved in cyflumetofen resistance. In addition, it was shown that recombinant TcCCE12 protein could hydrolyze cyflumetofen effectively.

CONCLUSION

Decreased esterase activity via downregulation of specific CCE genes most likely contributes to cyflumetofen resistance by decreased activation of cyflumetofen to its active metabolite. Mixtures of cyflumetofen and esterase-inhibition acaricides (e.g. organophosphates or carbamates) should be avoided in field applications. © 2019 Society of Chemical Industry.

High-resolution QTL mapping in Tetranychus urticae reveals acaricide-specific responses and common target-site resistance after selection by different METI-I acaricides

Arthropod herbivores cause dramatic crop losses, and frequent pesticide use has led to widespread resistance in numerous species. One such species, the two-spotted spider mite, Tetranychus urticae, is an extreme generalist herbivore and a major worldwide crop pest with a history of rapidly developing resistance to acaricides. Mitochondrial Electron Transport Inhibitors of complex I (METI-Is) have been used extensively in the last 25 years to control T. urticae around the globe, and widespread resistance to each has been documented. METI-I resistance mechanisms in T. urticae are likely complex, as increased metabolism by cytochrome P450 monooxygenases as well as a target-site mutation have been linked with resistance. To identify loci underlying resistance to the METI-I acaricides fenpyroximate, pyridaben and tebufenpyrad without prior hypotheses, we crossed a highly METI-I-resistant strain of T. urticae to a susceptible one, propagated many replicated populations over multiple generations with and without selection by each compound, and performed bulked segregant analysis genetic mapping. Our results showed that while the known H92R target-site mutation was associated with resistance to each compound, a genomic region that included cytochrome P450-reductase (CPR) was associated with resistance to pyridaben and tebufenpyrad. Within CPR, a single nonsynonymous variant distinguished the resistant strain from the sensitive one. Furthermore, a genomic region linked with tebufenpyrad resistance harbored a non-canonical member of the nuclear hormone receptor 96 (NHR96) gene family. This NHR96 gene does not encode a DNA-binding domain (DBD), an uncommon feature in arthropods, and belongs to an expanded family of 47 NHR96 proteins lacking DBDs in T. urticae. Our findings suggest that although cross-resistance to METI-Is involves known detoxification pathways, structural differences in METI-I acaricides have also resulted in resistance mechanisms that are compound-specific.

Long-Term Population Studies Uncover the Genome Structure and Genetic Basis of Xenobiotic and Host Plant Adaptation in the Herbivore Tetranychus urticae

Pesticide resistance arises rapidly in arthropod herbivores, as can host plant adaptation, and both are significant problems in agriculture. These traits have been challenging to study as both are often polygenic and many arthropods are genetically intractable. Here, we examined the genetic architecture of pesticide resistance and host plant adaptation in the two-spotted spider mite, Tetranychus urticae, a global agricultural pest. We show that the short generation time and high fecundity of T. urticae can be readily exploited in experimental evolution designs for high-resolution mapping of quantitative traits. As revealed by selection with spirodiclofen, an acetyl-CoA carboxylase inhibitor, in populations from a cross between a spirodiclofen-resistant and a spirodiclofen-susceptible strain, and which also differed in performance on tomato, we found that a limited number of loci could explain quantitative resistance to this compound. These were resolved to narrow genomic intervals, suggesting specific candidate genes, including acetyl-CoA carboxylase itself, clustered and copy variable cytochrome P450 genes, and NADPH cytochrome P450 reductase, which encodes a redox partner for cytochrome P450s. For performance on tomato, candidate genomic regions for response to selection were distinct from those responding to the synthetic compound and were consistent with a more polygenic architecture. In accomplishing this work, we exploited the continuous nature of allele frequency changes across experimental populations to resolve the existing fragmented T. urticae draft genome to pseudochromosomes. This improved assembly was indispensable for our analyses, as it will be for future research with this model herbivore that is exceptionally amenable to genetic studies.

Spiromesifen and spirotetramat resistance in field populations of Bemisia tabaci Gennadius in Spain

BACKGROUND

Spiromesifen and spirotetramat are novel insecticides belonging to the chemical class of tetronic and tetramic acid derivatives. Both compounds have proven very effective against field populations of Bemisia tabaci around the world. However, several growers have recently reported control failures in Spain. Therefore, we studied the resistance level to these insecticides in field populations reporting control failures. In addition, we further selected a spiromesifen-resistant strain to study the mechanisms involved and the cross-resistance pattern.

RESULTS

All the new field populations collected were significantly more resistant to spiromesifen than the susceptible population, confirming the presence of resistance. Several populations showing high levels of resistance to spiromesifen (>10 000-fold), exhibited cross-resistance to spirotetramat, but resistance ratios were much lower (130-fold). The spiromesifen laboratory-selected strain was very resistant to spiromesifen (LC₅₀ > 30 000 mg L^-1 ) and spirotetramat (LC₅₀ = 368.1 mg L^-1 ), but lacks any cross-resistance to other insecticides, thus providing options for resistance management. None of the synergists tested significantly restored the susceptibility of B. tabaci to either spiromesifen or spirotetramat.

CONCLUSION

This is the first report of resistance to spiromesifen and spirotetramat in B. tabaci, and such high levels of resistance have not been reported before in any field collected pest. Our results suggest that enhanced detoxification does not critically contribute to resistance to ketoenols in B. tabaci. The obvious lack of a metabolic resistance mechanism either suggests a target-site resistance mechanism or a metabolic mechanism insensitive to the synergists tested. © 2018 Society of Chemical Industry.

Arylpyrrole and fipronil analogues that inhibit the motility and/or development of Haemonchus contortus in vitro

Due to widespread drug resistance in parasitic nematodes, there is a need to develop new anthelmintics. Given the cost and time involved in developing a new drug, the repurposing of known chemicals can be a promising, alternative approach. In this context, we tested a library (n = 600) of natural product-inspired pesticide analogues against exsheathed third stage-larvae (xL3s) of Haemonchus contortus (barber's pole worm) using a whole-organism, phenotypic screening technique that measures the inhibition of motility and development in treated larvae. In the primary screen, we identified 32 active analogues derived from chemical scaffolds of arylpyrrole or fipronil. The seven most promising compounds, selected based on their anthelmintic activity and/or limited cytotoxicity, are arylpyrroles that reduced the motility of fourth-stage larvae (L4s) with significant potency (IC₅₀ values ranged from 0.04 ± 0.01 μM to 4.25 ± 0.82 μM, and selectivity indices ranged from 10.6 to 412.5). Since the parent structures of the active compounds are uncouplers of oxidative phosphorylation, we tested the effect of selected analogues on oxygen consumption in xL3s using the Seahorse XF24 flux analyser. Larvae treated with the test compounds showed a significant increase in oxygen consumption compared with the untreated control, demonstrating their uncoupling activity. Overall, the results of the present study have identified natural product-derived molecules that are worth considering for chemical optimisation as anthelmintic drug leads.

•

Pesticide analogues were assessed for anthelmintic activity.

•

Inhibition of motility and development of larval stages of Haemonchus contortus.

•

Uncoupling of oxidative phosphorylation in larvae.

Resistant inheritance and cross-resistance of cyflumetofen in Tetranychus cinnabarinus (Boisduval)

As a new acaricide, cyflumetofen can effectively control Tetranychus, Panonychus, as well as other phytophagous mites. But its risk and the way of genetic and resistant inheritance in mites are not clear. In this study, two cyflumetofen-resistant strains (CyR and YN-CyR) were selected for 104 and 12 generations, and developed 104.7-fold and 25.6-fold resistance, respectively. Three crossing groups (CyR_80 × SS, CyR_104 × SS, YN-CyR × SS) were conducted to explore the resistant inheritance of cyflumetofen in T. cinnabarinus changed along with resistant level or not. The results of reciprocal crosses and backcrosses revealed that the incomplete recessive and multiple genes trait involved in two resistant strains. The different stage of resistance also has a same genetic trait. A cross-resistance study revealed that there was no cross-resistance between cyflumetofen and other four acaricides including avermectin, fenpropathrin, propargite and bifenazate respectively, but the cross-resistance to pyridaben reached a high level with 63.8-fold, which indicates an underlying mechanism that can both mediate cyflumetofen- and pyridaben-resistance in T. cinnabarinus.

Contribution of cytochrome P450 monooxygenase CYP380C6 to spirotetramat resistance in Aphis gossypii Glover

The cytochrome P450 monooxygenases play a key role in detoxification mechanism for spirotetramat resistance in Aphis gossypii Glover. However, only one P450 genes (CYP6DA2), among thirty-five P450 genes identified from Aphis gossypii transcriptome database, has been reported to play important role in spirotetramat resistance in previous resistance level until now. In this study, after the confirmation of the rise of resistance level and important roles of P450s in spirotetramat resistance by the synergism analysis, the gene expression changes were determined for P450 genes in spirotetramat susceptible and resistant strains. Compared with the susceptible strain, CYP6CY4, CYP6CY14, CYP6CY18 and CYP6DC1 in CYP3 Clade were up-regulated in resistant nymphs, with the CYP6CY14, CYP6CY4, CYP6DC1, and CYP6CY18 increased to 2.54-, 1.51-, 1.31- and 1.29-fold, respectively. Eight genes in CYP3 Clade, three genes in CYP4 Clade and one gene in Mito Clade were down-regulated. In resistant adult aphids, CYP380C6 in CYP4 Clade, CYP353B1 in CYP2 Clade, and CYP307A1 in Mito Clade were up-regulated under spirotetramat stress, with the CYP380C6, CYP353B1 and CYP307A1 increased to 2.89-, 1.91-, and 1.38-fold, respectively. In contrast, the other P450 genes were almost down-regulated, especially these P450 genes in CYP3 Clade, CYP4 Clade and Mito Clade. RNA interference of CYP380C6 significantly increased the sensitivity of the resistant adults and nymphs to spirotetramat, while suppression of CYP6CY14 could not increase the toxicity of spirotetramat. These results indicate the possible involvement of the CYP380C6 genes in spirotetramat resistance at present very high resistance levels. Screening the expression changes of P450 genes under different spirotetramat resistance levels in the genome-scale will provide an overall view on the possible metabolic factors in the resistance development. The results may facilitate further work to validate the roles of P450 in spirotetramat resistance with heterologous expression.

Mechanisms of resistance to three mite growth inhibitors of Tetranychus urticae in hops

Mite growth inhibitors (MGIs), such as etoxazole and hexythiazox, are valuable IPM tools for Tetranychus urticae control in hops due to their unique mode of action and selectivity. Hence, it is necessary to standardize bioassay methods to evaluate the efficacy of MGIs, monitor resistance, and identify mechanisms underlying MGI resistance in the field. Here, we developed a three-tiered approach for evaluating ovicidal toxicity of MGIs to T. urticae, which simulated different MGI exposure scenarios in the field. The most effective bioassay method was direct exposure of T. urticae eggs to MGIs. With this method, four field-collected T. urticae populations showed low-to-moderate resistance to MGIs. Cross-resistance among MGIs and from MGIs to bifenazate and bifenthrin was detected. Besides target site insensitivity, enhanced cytochrome P450 and esterase activities also contribute to the MGI resistance in hop yard-collected T. urticae populations. Low-to-moderate MGI resistance in T. urticae populations may be mediated by multiple mechanisms. Positive selection pressure on the I1017F mutation is moderate in field-collected T. urticae populations. Further studies are required to identify metabolic detoxification genes that confer resistance to MGIs for precise resistance monitoring.

Evolution of imidacloprid resistance in Myzus persicae in Greece and susceptibility data for spirotetramat

BACKGROUND

Myzus persicae s.l. is a major crop pest globally and has evolved resistance to a range of insecticide classes making it increasingly difficult to control in some areas. Here we compare bioassay monitoring data for two important compounds, imidacloprid and spirotetramat, on field samples/clones collected in Greece.

RESULTS

A total of 122 aphid samples/clones from central and northern Greece were examined in dose-response bioassays with imidacloprid. There was an overall increase in the level of resistance (resistance factor = 15-40) within tobacco-collected samples from 78.7% in 2007 to 86.7% in 2015. The corresponding frequencies for peach samples were 13.3% and 6.7%. These results were confounded however by the first identification of the R81T target mutation in Greece during 2015 (4.3% as heterozygotes in peach) and 2016 (21.3% as heterozygotes in peach). No resistance to spirotetramat was found at the 60 clones collected in 2015.

CONCLUSION

Resistance to imidacloprid is continuing to increase within Greek M. persicae s.l. populations and the situation is likely to deteriorate further with the recent identification of the R81T resistance mutation. Resistance to spirotetramat has not been found and is therefore a good alternative to neonicotinoids for resistance management. © 2017 Society of Chemical Industry.

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.

Novel mutations and expression changes of acetyl-coenzyme A carboxylase are associated with spirotetramat resistance in Aphis gossypii Glover

Acetyl-coenzyme A carboxylase (ACC) catalyses the carboxylation of acetyl-coenzyme A (acetyl-CoA) to produce malonyl-CoA during the de novo synthesis of fatty acids. Spirotetramat, an inhibitor of ACC, is widely used to control a range of sucking insects, including the Aphis gossypii. In the present study, Reverse transcription quantitative real-time PCR (RT-qPCR) results demonstrated that ACC was significantly overexpressed in a laboratory-selected spirotetramat-resistant strain compared with the susceptible strain. ACC RNA interference significantly suppressed fecundity and led to cuticle formation deficiencies in resistant adults and nymphs compared with the control. The full-length ACC gene was sequenced from both resistant and susceptible cotton aphids, and a strong association was found between spirotetramat resistance and 14 amino acid substitutions in the biotin carboxylase domain and carboxyl transferase domain of the ACC gene. Furthermore, ACC activity was higher in resistant aphids than in the susceptible strain, and ACC in the resistant aphids exhibited significant insensitivity to spirotetramat and spirotetramat-enol. The results indicate that the overexpressed insensitive (mutated) ACC target played an important role in the high levels of spirotetramat resistance observed here. This association of amino acid substitution with resistance is the first report of a potential target site mechanism affecting spirotetramat in the cotton aphid.

miR-276 and miR-3016-modulated expression of acetyl-CoA carboxylase accounts for spirotetramat resistance in Aphis gossypii Glover

Acetyl-coenzyme A carboxylase (acetyl-CoA carboxylase, ACC) catalyses the carboxylation of acetyl-CoA to produce malonyl-CoA during de novo fatty acid synthesis. A laboratory-selected spirotetramat-resistant strain (SR) of cotton aphid was used in this study. RT-qPCR results demonstrated significant increases in the levels of ACC transcript in the resistant strain compared to the susceptible strain. Depletion of overexpressed ACC transcripts by RNAi also significantly enhanced the sensitivity of the resistant aphid to spirotetramat. We hypothesized that ACC gene expression is subject to post-transcriptional regulation. To investigate the underlying mechanism, the 66 known miRNAs of Aphis gossypii were used for target prediction, eight of which were predicted to target ACC. Validation identified two miRNAs, miR-276 and miR-3016, with abundance levels that were highly inversely correlated with ACC transcript levels. This result suggests that the miRNAs miR-276 and miR-3016 may play major roles in the post-transcriptional regulation of the ACC gene. Modulation of the abundance of miR-276 and miR-3016 through addition of inhibitors/mimics of miR-276 or miR-3016 to the artificial diet significantly altered both ACC transcript levels and the tolerance of A. gossypii to spirotetramat, thus confirming the roles of these two miRNAs in the regulation of spirotetramat resistance.

Stability and fitness costs associated with etoxazole resistance in Tetranychus urticae (Acari: Tetranychidae)

The two-spotted spider mite, Tetranychus urticae Koch, is one of the most important pests on a wide range of crops worldwide. Studies on stability of resistance and possible fitness costs associated with etoxazole resistance were carried out in T. urticae to provide basic information necessary to define effective acaricide resistance management strategies for this pest. Selection for resistance to etoxazole was performed in a population of T. urticae collected from a commercial rose field, in Holambra County, in the State of São Paulo, Brazil. After five rounds of selection for resistance, the resistance ratio (R/S) at the LC50 reached 8739 fold value in comparison with a susceptible strain (Brazabrantes S Strain). The stability of etoxazole resistance was studied under laboratory conditions, using a population with initial frequency of 75 % of resistant mites. The frequencies of etoxazole resistance were evaluated monthly for a period of 13 months. The frequency of etoxazole resistance decreased from 75 to 37 % in this period. Comparison of biological traits between resistant and susceptible strains indicated the presence of fitness costs associated with etoxazole resistance. The resistant strain showed lower fecundity and a higher proportion of males in the population. The figures for net reproductive rate (R 0), intrinsic rate of natural increase (r m) and finite rate of increase (λ) were higher in the susceptible strain. The instability of etoxazole resistance is a favorable aspect for the acaricide resistance management in the spider mite.

Effects of spirotetramat treatments on fecundity and carboxylesterase expression of Aphis gossypii Glover

Spirotetramat is a novel tetramic acid-based insecticide, belonging to keto-enol pesticide family, with a novel mode of action; it interferes with lipid biosynthesis. Its insecticide activity against various agricultural pest insects have been demonstrated (e.g. on Myzus persicae, Bemisia tabaci and Tetranychus urticae). However, information available is currently limited on the efficacy of spirotetramat on the cotton aphid, Aphis gossypii, a key cotton pest worldwide. We assessed the spirotetramat toxicity on A. gossypii and evaluated its effects on aphid fecundity when exposed to a sublethal concentration (LC10) and to increasing lethal concentrations (LC25, LC50, and LC75). A key mechanism involved in insecticide resistance in aphids relates to esterase activity. We estimated the CarE activity and a CarE gene expression in aphids in response to spirotetramat exposure, then we tested tolerance of offspring to spirotetramat when the parents were exposed to the highest concentration tested in our study (LC75). Results showed that spirotetramat showed increasing toxicity to A. gossypii with exposure duration to treated leaves; LC50 ranged from 23,675.68 to 12.27 mg/L for 1 to 5-days exposure. In addition, spirotetramat reduced aphid daily fecundity, in all concentration treatments, especially with up to 90 % reduction in case of exposure to LC75. Total CarE activity increased dramatically and CarE mRNA expression was also up regulated in aphids after exposure to LC75 spirotetramat. Finally, the tolerance to spirotetramat in offspring (when parents were exposed to the LC75) showed a 2.5-fold increase when compared to control aphids. Consequently, spiroteramat showed potential for pest management of cotton aphids owing to both lethal and sublethal activities, notably strong impact on aphid fecundity. However, we also demonstrated that increased tolerance of A. gossypii to spirotetramat may happen through increased CarE- activity and subsequent metabolic degradation of the insecticide in aphids' body.

Resistance status of the carmine spider mite, Tetranychus cinnabarinus and the twospotted spider mite, Tetranychus urticae to selected acaricides on strawberries

The carmine spider mite, Tetranychus cinnabarinus (Boisduval) and the twospotted spider mite, Tetranychus urticae Koch, are serious pests of strawberries and many other horticultural crops. Control of these pests has been heavily dependent upon chemical acaricides. Objectives of this study were to determine the resistance status of these two pest species to commonly used acaricides on strawberries in a year-round intensive horticultural production region. LC90 of abamectin for adult carmine spider mites was 4% whereas that for adult twospotted spider mites was 24% of the top label rate. LC90s of spiromesifen, etoxazole, hexythiazox and bifenazate were 0.5%, 0.5%, 1.4% and 83% of their respective highest label rates for carmine spider mite eggs, 0.7%, 2.7%, 12.1% and 347% of their respective highest label rates for the nymphs. LC90s of spiromesifen, etoxazole, hexythiazox and bifenazate were 4.6%, 11.1%, 310% and 62% of their respective highest label rates for twospotted spider mite eggs, 3%, 13%, 432,214% and 15% of their respective highest label rates for the nymphs. Our results suggest that T. cinnabarinus have developed resistance to bifenazate and that the T. urticae have developed resistance to hexythiazox. These results strongly emphasize the need to develop resistance management strategies in the region.

Over-expression of CYP6A2 is associated with spirotetramat resistance and cross-resistance in the resistant strain of Aphis gossypii Glover

A laboratory-selected spirotetramat-resistant strain (SR) of cotton aphid developed 579-fold and 15-fold resistance to spirotetramat in adult aphids and 3rd instar nymphs, respectively, compared with a susceptible strain (SS) [26]. The SR strain developed high-level cross-resistance to alpha-cypermethrin and bifenthrin and very low or no cross-resistance to the other tested insecticides. Synergist piperonyl butoxide (PBO) dramatically increased the toxicity of spirotetramat and alpha-cypermethrin in the resistant strain. RT-qPCR results demonstrated that the transcriptional levels of CYP6A2 increased significantly in the SR strain compared with the SS strain, which was consistent with the transcriptome results [30]. The depletion of CYP6A2 transcripts by RNAi also significantly increased the sensitivity of the resistant aphid to spirotetramat and alpha-cypermethrin. These results indicate the possible involvement of CYP6A2 in spirotetramat resistance and alpha-cypermethrin cross-resistance in the cotton aphid. These together with other cross-resistance results have implications for the successful implementation of resistance management strategies for Aphis gossypii.

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.

Spirotetramat resistance adaption analysis of Aphis gossypii Glover by transcriptomic survey

A resistant strain of the cotton aphid (SR) developed 441.26-fold and 11.97-fold resistance to spirotetramat for adult aphids and nymphs, respectively, compared with the susceptible (SS) strain. Solexa sequencing technology was employed to identify differentially expressed genes (DEGs) in the spirotetramat-resistant cotton aphid. Respective totals of 22,430,522 and 21,317,732 clean reads were obtained from SR and SS cDNA libraries and assembled into 35,222 non-redundant (Nr) consensus sequences. A total of 14,913, 9,220, 7,922, 4,314 and 4,686 sequences were annotated using Nr, Swiss-Prot, Gene Ontology (GO), Kyoto Encyclopedia of Genes and Genomes (KEGG) and Clusters of Orthologous Groups (COG), respectively. Compared with the SS strain, the SR strain had 1287 significantly changed unigenes, of which 130 genes were up-regulated and 1157 genes were down-regulated (P ≤ 0.001). Among these genes, 440 unigenes were annotated, consisting of 114 up-regulated and 326 down-regulated genes. The expression levels of heat shock protein 70 (Hsp70) and UDP-glucuronosyltransferase were significantly up-regulated in the SR strain compared to the SS strain. The genes encoding cuticle proteins, salivary glue protein, fibroin heavy chain, energy ATP synthase, and cytochrome c oxidase were dramatically decreased. Among the DEGs, cytochrome P450 6A2 (c20965.graph_c0) was the only P450 gene up-regulated in the SR strain. The expression levels of 10 DEGs were confirmed by real-time qPCR, and the trends in gene expression observed by qPCR matched those of the Solexa expression profiles. The acetyl-CoA carboxylase (ACC) genes in the SR and SS libraries both contain four single nucleotide polymorphisms (SNPs), with three common SNPs: 1227 (C/T), 1811 (A/T: F/Y) and 3759 (C/T); however, 7540 (A/T) and 108 (G/A) occurred solely in the SS and SR strains, respectively.

Detoxification enzymes of Bemisia tabaci B and Q: biochemical characteristics and gene expression profiles

BACKGROUND

The sweetpotato whitefly, Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae), is one of the most invasive and destructive pests of field crops worldwide. The sibling species B and Q are the two most damaging members of the B. tabaci species complex. That Q is more resistant than B to many insecticides has been well documented. Over the last decade, Q has gradually displaced B and has become the dominant form of B. tabaci in field agricultural systems in most parts of China. To help understand the differences in insecticide resistance, the activities and gene expression profiles of detoxification enzymes in B. tabaci B and Q were investigated.

RESULTS

The activity of P450 towards 7-ethoxycoumarin was significantly higher (1.46-fold higher) in Q than in B. The expression of 43 of 65 P450 genes was higher (>1-fold) in Q than in B, and expression for eight P450 genes was more than 50-fold greater in Q than in B. The increased expression of selected P450 genes in Q relative to B was confirmed with two other B strains and two other Q strains. On the other hand, carboxylesterase (CarE) activity was significantly lower (0.71-fold lower) in Q than in B; the Km value of CarE was significantly lower in B than in Q, but the opposite was true for the Vmax value of CarE. Glutathione S-transferase activity and values of Km and Vmax did not differ between B and Q.

CONCLUSION

Enhanced metabolic detoxification of insecticides by P450s may be an important reason why B. tabaci Q is more resistant than B. tabaci B to insecticides.

High resolution genetic mapping uncovers chitin synthase-1 as the target-site of the structurally diverse mite growth inhibitors clofentezine, hexythiazox and etoxazole in Tetranychus urticae

The acaricides clofentezine, hexythiazox and etoxazole are commonly referred to as 'mite growth inhibitors', and clofentezine and hexythiazox have been used successfully for the integrated control of plant mite pests for decades. Although they are still important today, their mode of action has remained elusive. Recently, a mutation in chitin synthase 1 (CHS1) was linked to etoxazole resistance. In this study, we identified and investigated a Tetranychus urticae strain (HexR) harboring recessive, monogenic resistance to each of hexythiazox, clofentezine, and etoxazole. To elucidate if there is a common genetic basis for the observed cross-resistance, we adapted a previously developed bulk segregant analysis method to map with high resolution a single, shared resistance locus for all three compounds. This finding indicates that the underlying molecular basis for resistance to all three compounds is identical. This locus is centered on the CHS1 gene, and as supported by additional genetic and biochemical studies, a non-synonymous variant (I1017F) in CHS1 associates with resistance to each of the tested acaricides in HexR. Our findings thus demonstrate a shared molecular mode of action for the chemically diverse mite growth inhibitors clofentezine, hexythiazox and etoxazole as inhibitors of an essential, non-catalytic activity of CHS1. Given the previously documented cross-resistance between clofentezine, hexythiazox and the benzyolphenylurea (BPU) compounds flufenoxuron and cycloxuron, CHS1 should be also considered as a potential target-site of insecticidal BPUs.

Cross-resistance between cyenopyrafen and pyridaben in the twospotted spider mite Tetranychus urticae (Acari: Tetranychidae)

BACKGROUND

Cyenopyrafen is an inhibitor of complex II of the mitochondrial electron transport chain. It has a molecular structure that shares some common features with frequently used complex I inhibitors such as pyridaben. To evaluate whether this similarity in structure poses a cross-resistance risk that might complicate resistance management, we selected for pyridaben and cyenopyrafen resistance in the laboratory and characterized resistance.

RESULTS

The selection for cyenopyrafen conferred cross-resistance to pyridaben and vice versa. Resistance towards these both acaricides was incompletely dominant in adult females. However, in eggs maternal effects were observed in pyridaben resistance, but not in the cyenopyrafen-resistance (completely dominant). In the cyenopyrafen resistant strain, the LC50 of eggs remained lower than the commercially recommended concentration. The common detoxification mechanisms by cytochrome P450 was involved in resistance to these acaricides. Carboxyl esterases were also involved in cyenopyrafen resistance as a major factor.

CONCLUSIONS

Although cross-resistance suggests that pyridaben resistance would confer cyenopyrafen cross-resistance, susceptibility in eggs functions to delay the development of cyenopyrafen resistance.

Determination of the inheritance, cross-resistance and detoxifying enzyme levels of a laboratory-selected, spiromesifen-resistant population of the predatory mite Neoseiulus californicus (Acari: Phytoseiidae)

BACKGROUND

Neoseiulus californicus of the family Phytoseiidae is an effective predatory mite species that is used to control pest mites.

RESULTS

The LC50 and LC60 values of spiromesifen were determined on N. californicus using a leaf-disc method and spraying tower. A laboratory selection population designated SPR13 was found to have a 52.08-fold resistance to spiromesifen following 13 selection cycles. This population developed low resistance to hexythiazox and moderate cross-resistance to propargite, clofentezine, spirodiclofen, etoxazole and milbemectin. PBO, IBP and DEM synergised resistance 3.75-, 2.54- and 1.93-fold respectively. Crossing experiments revealed that spiromesifen resistance in the SPR13 population was intermediately dominant and monogenic. In addition, detoxifying enzyme activities were increased 2.74-fold for esterase, 3.09-fold for glutathione S-transferase and 2.17-fold for cytochrome P450 monooxygenase in the SPR13 population.

CONCLUSIONS

Selection for spiromesifen under laboratory conditions resulted in the development of spiromesifen resistance in the predatory mite N. californicus. Predatory mites that are resistant to pesticides are considered valuable for use in resistance management programmes within integrated pest control strategies.

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.

Molecular analysis of resistance to acaricidal spirocyclic tetronic acids in Tetranychus urticae: CYP392E10 metabolizes spirodiclofen, but not its corresponding enol

Spirodiclofen is one of the most recently developed acaricides and belongs to the new family of spirocyclic tetronic acids (ketoenols). This new acaricidal family is an important chemical tool in resistance management strategies providing sustainable control of spider mites such as Tetranychus urticae. Spirodiclofen targets lipid biosynthesis mediated by direct inhibition of acetyl coenzyme A carboxylase (ACCase). In this study, we investigated two genetically distant spider mite strains with high resistance to spirodiclofen. Despite the strong resistance levels to spirodiclofen (up to 680-fold), only limited cross-resistance with other members of this group such as spiromesifen and spirotetramat could be detected. Amplification and sequencing of the ACCase gene from resistant and susceptible strains did not reveal common non-synonymous mutations, and expression levels of ACCase were similar in both resistant and susceptible strains, indicating the absence of target-site resistance. Furthermore, we collected genome-wide expression data of susceptible and resistant T. urticae strains using microarray technology. Analysis of differentially expressed genes revealed a broad response, but within the overlap of two resistant strains, several cytochrome P450s were prominent. Quantitative PCR confirmed the constitutive over-expression of CYP392E7 and CYP392E10 in resistant strains, and CYP392E10 expression was highly induced by spirodiclofen. Furthermore, stage specific expression profiling revealed that expression levels were not significantly different between developing stages, but very low in eggs, matching the age-dependent resistance pattern previously observed. Functional expression of CYP392E7 and CYP392E10 confirmed that CYP392E10 (but not CYP392E7) metabolizes spirodiclofen by hydroxylation as identified by LC-MS/MS, and revealed cooperative substrate binding and a Km of 43 μM spirodiclofen. CYP392E10 also metabolizes spiromesifen, but not spirotetramat. Surprisingly, no metabolism of the hydrolyzed spirodiclofen-enol metabolite could be detected. These findings are discussed in the light of a likely resistance mechanism.