Complete maternal inheritance of bifenazate resistance in Tetranychus urticae Koch (Acari: Tetranychidae) and its implications in mode of action considerations

A novel mutation in mitochondrial cytochrome b conferring resistance to bifenazate in two-spotted spider mite Tetranychus urticae Koch (Acarina: Tetranychidae)

BACKGROUND

The two-spotted spider mite Tetranychus urticae causes significant damage to ornamental, cotton, sugarcane and horticultural crops in Australia. It has a long history of developing resistance to many acaricides including bifenazate. A mutation in the conserved cd1- and ef-helices of the Qo pocket of cytochrome b is recognized as the primary mechanism of bifenazate resistance. To investigate the resistance mechanisms against bifenazate in Australian two-spotted spider mite, we sequenced the complete mitochondrion genome of five mite strains including a susceptible and bifenazate-resistant strain.

RESULTS

We identified a novel mutation D252N in the G126S background at cytochrome b being the cause of bifenazate resistance in a bifenazate-resistant strain, Bram. We validated the role of this mutation combination by reciprocal crosses between a bifenazate resistant and susceptible strain. By doing these crosses we confirmed the pattern of inheritance was maternal. Additionally, mitochondrial heteroplasmy was not observed by single mite genotyping of the mutations in cytb in a known bifenazate-resistant strain Bram. The phylogenetic analysis with the complete mitochondrion genome sequences revealed that Australian two-spotted spider mite strains are closely related to the green form of T. urticae found in China.

CONCLUSIONS

The novel mutation D252N found in the cytochrome b in the G126S background was revealed to be the main cause of bifenazate resistance in the Australian T. urticae strain Bram. © 2024 Society of Chemical Industry.

Monitoring detoxification enzyme levels and resistance of Tetranychus urticae against some METI-group chemicals in Türkiye cotton fields

Acaricides used against Tetranychus urticae Koch, 1836 (Acari: Tetranychidae) in cotton fields cause control failure over time. To determine the resistance status of T. urticae populations to tebufenpyrad and bifenazate, different populations collected from Aydın (AYD), Adana (ADA), Şanlıurfa (SAN), and Diyarbakır (DIY) provinces of Türkiye, between 2019 and 2020, were subjected to diagnostic dose bioassays. Firstly, the spider mites were eliminated with a discriminating dose. Afterwards, LC50 and LC90 of the remaining populations were determined and the ten highest resistant populations were selected. The highest phenotypic resistance to bifenazate was observed in AYD4 and DIY2 (LC50 57.14 mg L- 1 with 85.01-fold and LC50 30.15 mg L- 1with 44.86-fold, respectively), while the lowest phenotypic resistance was found in SAN6 (LC50 1.5 mg L- 1; 2.28-fold). Considering the phenotypic resistance to tebufenpyrad, the highest resistance was found in AYD4 population (LC50 96.81 mg L- 1; 12.92-fold), while the lowest - in DIY28 population (LC50 21.23 mg L- 1; 2.83-fold). In pharmacokinetic studies, the ADA16 population was compared with the sensitive German Susceptible Strain population and it was determined that carboxylesterase activity was statistically higher (1.46 ± 0.04 nmol/min/mg protein enzyme activation 2.70-fold). The highest activation of glutathione S-transferase was detected in ADA16 (1.49 ± 0.01 nmol/min/mg protein; 2.32-fold). No mutations were found in PSST (METI 1), the point mutation site for tebufenpyrad, and Cytb (METI 3), the point mutation site for bifenazate. In terms of phenotypic resistance, bifenazate was found to be moderately resistant in two populations (85.01 and 44.86-fold), while tebufenpyrad was moderately resistant in one population (12.92-fold). This study showed that both acaricides are still effective against T. urticae populations.

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.

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.

Molecular Basis for the Selectivity of the Succinate Dehydrogenase Inhibitor Cyflumetofen between Pest and Predatory Mites

Acaricides that act as inhibitors of the mitochondrial succinate dehydrogenase (SDHIs) provide excellent control of phytophagous mites but display limited toxicity to predatory mites and other beneficial organisms. However, the molecular mechanism of selectivity is not fully understood. Here, we first confirm that SDHI acaricides are over 10,000-fold more toxic to spider mites than predatory mites. Next, we show that differential penetration, pro-acaricide activation, or metabolism are most likely not the main reason for this selectivity. In contrast, the inhibition of AB-1 on the SDH target is approximately 200-fold more potent in spider mites compared to predatory mites, revealing strong target-site selectivity. Strikingly, a key motif associated with differential binding was identified and validated by gene editing in Drosophila. Our findings contribute to understanding the selectivity of SDHIs, which can be used for the rational design of selective acaricides in support of an integrated pest management.

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.

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.

Sodium selenite-carbon dots nanocomposites enhance acaricidal activity of fenpropathrin: Mechanism and application

As an essential trace element, selenium can be used to protect crops from pests, while, in nature, most crops cannot accumulate enough selenium from the soil to reach the effective dose for pest control. In this study, carbon dots modified with arginine in nano-scale was prepared and characterized, then, it was combined with sodium selenite to form selenium-carbon dots (Se-CDs). Function evaluation of Se-CDs showed that it could increase the absorption of selenium in plant leaves, promote the control efficiency of fenpropathrin, and protect plant from damage caused by Tetranychus cinnabarinus. In addition, we found that expressions of P450 genes and activity of P450 enzyme both decreased in selenium treated mites. In vivo, the acaricidal activity of fenpropathrin increased significantly when one of the P450 genes, CYP389B1, was silenced, and the recombinant protein of CYP389B1 could metabolize fenpropathrin in vitro. The results suggested that inhibiting the expression of P450 gene and repressing the detoxification of T. cinnabarinus was the molecular mechanism that how selenium promoted the acaricidal activity of fenpropathrin. The application of Se-CDs in the field will decrease the use of chemicals acaricides, reduce chemical residues, and ensure the safety of agricultural products.

Bifenazate induces developmental and immunotoxicity in zebrafish

Bifenazate is a widely used acaricide, but its biological safety remains unknown. In the present study, the immunotoxic effects of exposure to bifenazate on zebrafish larvae were evaluated for the first time. Firstly, after exposure to bifenazate, the body length of the zebrafish larvae became shorter and the yolk sac swelled. Secondly, the number of innate immune cells and adaptive immune cells was greatly reduced. Following exposure to bifenazate, oxidative stress levels in the zebrafish increased significantly, antioxidant activity was inhibited, and the expression of genes related to antioxidants, such as those of the glutathione metabolism pathway, changed, including gclm, prdx1, serpine1, and gss. In addition, inflammatory factors such as CXCL-c1c, IFN-γ, iL-8, iL-6, and MYD88 were abnormally expressed. The use of astaxanthin was effective in rescuing the developmental toxicity caused by bifenazate exposure. In summary, bifenazate exposure is immunotoxic and can cause oxidative stress in zebrafish larvae.

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.

Identification and characterization of new mutations in mitochondrial cytochrome b that confer resistance to bifenazate and acequinocyl in the spider mite Tetranychus urticae

BACKGROUND

In spider mites, mutations in the mitochondrial cytochrome b Q_o pocket have been reported to confer resistance to the Q_o inhibitors bifenazate and acequinocyl. In this study, we surveyed populations of the two-spotted spider mite Tetranychus urticae for mutations in cytochrome b, linked newly discovered mutations with resistance and assessed potential pleiotropic fitness costs.

RESULTS

We identified two novel mutations in the Q_o site: G132A (equivalent to G143A in fungi resistant to strobilurins) and G126S + A133T (previously reported to cause bifenazate and acequinocyl resistance in Panonychus citri). Two T. urticae strains carrying G132A were highly resistant to bifenazate but not acequinocyl, whereas a strain with G126S + A133T displayed high levels of acequinocyl resistance, but only moderate levels of bifenazate resistance. Bifenazate and acequinocyl resistance were inherited maternally, providing strong evidence for the involvement of these mutations in the resistance phenotype. Near isogenic lines carrying G132A revealed several fitness penalties in T. urticae; a lower net reproductive rate (R₀ ), intrinsic rate of increase (rm) and finite rate of increase (LM); a higher doubling time (DT); and a more male-biased sex ratio.

CONCLUSIONS

Several lines of evidence were provided to support the causal role of newly discovered cytochrome b mutations in bifenazate and acequinocyl resistance. Because of the fitness costs associated with the G132A mutation, resistant T. urticae populations might be less competitive in a bifenazate-free environment, offering opportunities for resistance management. © 2019 Society of Chemical Industry.

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.

Field-evolved resistance and cross-resistance of the two-spotted spider mite, Tetranychus urticae, to bifenazate, cyenopyrafen and SYP-9625

The acaricide bifenazate acts as complex III inhibitor whereas cyenopyrafen and SYP-9625 act as complex II inhibitors. All these acaricides are commonly used to control two-spotted spider mite (TSSM), Tetranychus urticae Koch. We examined field-evolved and laboratory-selected resistance of TSSM to these three acaricides and determined cross-resistance among them. Six field populations of TSSM showed low levels of resistance to bifenazate with resistance ratios ranging from 2.20 to 10.65 compared to a susceptible strain. SYP-9625, structurally similar to cyenopyrafen, showed slightly higher activity to TSSMs but significant cross-resistance in both field populations and a laboratory-selected strain by SYP-9625. However, low levels of resistance to these two chemicals were found in field populations even when used for short time periods. Cross-resistance was not found between bifenazate and Complex II inhibitors, cyenopyrafen and SYP-9625, in both field populations and the laboratory-selected strain. Field-evolved resistance of TSSM to the tested acaricides is still low and should be delayed by the implementation of resistance management practices. Cross-resistance between cyenopyrafen and SYP-9625 is obvious, so they should not be used together in resistance management strategies based on mode of action rotation.

Acequinocyl Resistance Associated With I256V and N321S Mutations in the Two-Spotted Spider Mite (Acari: Tetranychidae)

The two-spotted spider mite, Tetranychus urticae Koch (Acari: Tetranychidae), is one of the most important pest species, because it devastates many horticultural and ornamental crops and fruit trees. In the present study, we explored a field strain that was collected in January 2001 and then selected for 16 years for acequinocyl resistance. The resistance ratios calculated for the LC50 value in the laboratory-selected acequinocyl-resistant (LSAR16) strain was 4,237-fold higher than that of the susceptible strain. Pretreatment with the synergists piperonyl butoxide and S,S,S-tributyl-phosphorotrithioate significantly increased the toxicity of acequinocyl to the LSAR16 strain. Crossing experiments revealed that the resistance in the LSAR16 strain was maternally inherited, dominant, and monogenic. Furthermore, among individuals in the LSAR16 strain, 85.5-98.5% had the I256V mutation and 98-99% had the N321S mutation in mitochondrial cytochrome b. These results suggest that these two new point mutations contribute to acequinocyl resistance in T. urticae.

Independently evolved and gene flow-accelerated pesticide resistance in two-spotted spider mites

Pest species are often able to develop resistance to pesticides used to control them, depending on how rapidly resistance can emerge within a population or spread from another resistant population. We examined the evolution of bifenazate resistance in China in the two-spotted spider mite (TSSM) Tetranychus uticae Koch (Acari: Tetranychidae), one of the most resistant arthropods, by using bioassays, detection of mutations in the target cytb gene, and population genetic structure analysis using microsatellite markers. Bioassays showed variable levels of resistance to bifenazate. The cytb mutation G126S, which confers medium resistance in TSSM to bifenazate, had previously been detected prior to the application of bifenazate and was now widespread, suggesting likely resistance evolution from standing genetic variation. G126S was detected in geographically distant populations across different genetic clusters, pointing to the independent origin of this mutation in different TSSM populations. A novel A269V mutation linked to a low-level resistance was detected in two southern populations. Widespread resistance associated with a high frequency of the G126S allele was found in four populations from the Beijing area which were not genetically differentiated. In this case, a high level of gene flows likely accelerated the development of resistance within this local region, as well as into an outlying region distant from Beijing. These findings, therefore, suggest patterns consistent with both local evolution of pesticide resistance as well as an impact of migration, helping to inform resistance management strategies in TSSM.

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

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

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.

Deltamethrin resistance in the salmon louse, Lepeophtheirus salmonis (Krøyer): Maternal inheritance and reduced apoptosis

Resistance towards deltamethrin (DMT) in the crustacean ectoparasite Lepeophtheirus salmonis (Caligidae) is a problem on fish farms lining the North Atlantic Ocean. Two Norwegian strains with different susceptibility towards DMT were crossed in the parental generation (P0), females from a sensitive strain were crossed with males from a resistant strain and vice versa. Individual susceptibility towards DMT was assessed in the second filial generation (F2). DMT resistance was only found in F2 descendants when the P0 females were from the resistant strain, pointing to maternal inheritance. Since maternal inheritance might be linked to the mitochondrial (mt) genome, the nucleotide sequences and the gene expressions of mt-genes were analysed. Twenty non-synonymous single nucleotide polymorphisms (SNPs) were identified in mt-transcripts from resistant F2 parasites, including SNPs in two cytochrome C oxidase subunits (COX1 and COX3) and two subunits of the NADH dehydrogenase complex (ND1 and ND5) previously linked to DMT resistance in the salmon louse. Differential expression analysis between the sensitive and resistant strain revealed strain effect in seven out of twelve mt-genes. The current study also show that DNA fragmentation (indicating apoptosis) was affected by DMT exposure in skeletal muscle tissue and that resistant parasites undergo less apoptosis than sensitive parasites.

The effect of insecticide synergist treatment on genome-wide gene expression in a polyphagous pest

Synergists can counteract metabolic insecticide resistance by inhibiting detoxification enzymes or transporters. They are used in commercial formulations of insecticides, but are also frequently used in the elucidation of resistance mechanisms. However, the effect of synergists on genome-wide transcription in arthropods is poorly understood. In this study we used Illumina RNA-sequencing to investigate genome-wide transcriptional responses in an acaricide resistant strain of the spider mite Tetranychus urticae upon exposure to synergists such as S,S,S-tributyl phosphorotrithioate (DEF), diethyl maleate (DEM), piperonyl butoxide (PBO) and cyclosporin A (CsA). Exposure to PBO and DEF resulted in a broad transcriptional response and about one third of the differentially expressed genes (DEGs), including cytochrome P450 monooxygenases and UDP-glycosyltransferases, was shared between both treatments, suggesting common transcriptional regulation. Moreover, both DEF and PBO induced genes that are strongly implicated in acaricide resistance in the respective strain. In contrast, CsA treatment mainly resulted in downregulation of Major Facilitator Superfamily (MFS) genes, while DEGs of the DEM treatment were not significantly enriched for any GO-terms.

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.

Maternal inheritance of deltamethrin resistance in the salmon louse Lepeophtheirus salmonis (Krøyer) is associated with unique mtDNA haplotypes

Parasitic infections by the salmon louse, Lepeophtheirus salmonis (Krøyer), cause huge economic damage in salmon farming in the northern hemisphere, with combined treatment costs and production losses in 2014 having been estimated at US$ 350 million for Norway (annual production 1.25 million tonnes). The control of L. salmonis relies significantly on medicinal treatments, supplemented by non-pharmacological approaches. However, efficacy losses have been reported for several delousing agents, including the pyrethroid deltamethrin. The aim of the present study was to analyse the genetic basis of deltamethrin resistance in L. salmonis. Deltamethrin median effective concentrations (EC50) were 0.28 μg L-1 in the drug susceptible L. salmonis strain IoA-00 and 40.1 μg L-1 in the pyrethroid resistant strain IoA-02. IoA-00 and IoA-02 were crossed to produce families spanning one parental and three filial generations (P0, F1-F3). In three families derived from P0 crosses between an IoA-00 sire and an IoA-02 dam, 98.8% of F2 parasites (n = 173) were resistant, i.e. remained unaffected after exposure to 2.0 μg L-1 deltamethrin. F3 parasites from these crosses showed a deltamethrin EC50 of 9.66 μg L-1. In two families of the inverse orientation at P0 (IoA-02 sire x IoA-00 dam), 16.7% of F2 parasites were resistant (n = 84), while the deltamethrin EC50 in F3 animals was 0.26 μg L-1. The results revealed a predominantly maternal inheritance of deltamethrin resistance. The 15,947-nt mitochondrial genome was sequenced and compared among six unrelated L. salmonis strains and parasites sampled from wild salmon in 2010. IoA-02 and three further deltamethrin resistant strains, established from isolates originating from different regions of Scotland, showed almost identical mitochondrial haplotypes. In contrast, the mitochondrial genome was variable among susceptible strains and L. salmonis from wild hosts. Deltamethrin caused toxicity and depletion of whole body ATP levels in IoA-00 but not IoA-02 parasites. The maternal inheritance of deltamethrin resistance and its association with mitochondrial haplotypes suggests that pyrethroid toxicity in L. salmonis may involve molecular targets encoded by mitochondrial genes.

Chance and design in proinsecticide discovery

Many insecticides are inactive on their target sites in the form that is sold and applied, needing first to be bioactivated. This proinsecticide strategy has often been achieved by design, through systematic derivatization of intrinsically active molecules with protecting groups that mask their toxic effects until their selective removal in target insects by metabolic enzymes generates the toxiphore. Proinsecticides can be designed to gain selectivity between target and non-target organisms, or to improve bioavailability by enhancing plant or insect uptake. In most cases, however, chance trumps design in proinsecticide discovery: most first-in-class products that we now know to be proinsecticides were only discovered a posteriori to be such, often after having been on the market for years. Knowing the active form of an insecticide is essential to mode of action identification, and early mode of action studies on novel chemotypes should take into account the possibility that the compounds might be proinsecticides. This paper reviews examples of proinsecticides in the marketplace, strategies for making proinsecticides and techniques for unmasking proinsecticides in mode of action studies. Our analysis of global agrochemical sales data shows that 34% of the dollar value of crop insecticides used in 2015 were proinsecticides. © 2016 Society of Chemical Industry.

A mutation in the PSST homologue of complex I (NADH:ubiquinone oxidoreductase) from Tetranychus urticae is associated with resistance to METI acaricides

The acaricidal compounds pyridaben, tebufenpyrad and fenpyroximate are frequently used in the control of phytophagous mites such as Tetranychus urticae, and are referred to as Mitochondrial Electron Transport Inhibitors, acting at the quinone binding pocket of complex I (METI-I acaricides). Because of their very frequent use, resistance evolved fast more than 20 years ago, and is currently wide-spread. Increased activity of P450 monooxygenases has been often associated with resistance, but target-site based resistance mechanisms were never reported. Here, we report the discovery of a mutation (H92R) in the PSST homologue of complex I in METI-I resistant T. urticae strains. The position of the mutation was studied using the high-resolution crystal structure of Thermus thermophilus, and was located in a stretch of amino acids previously photo-affinity labeled by fenpyroximate. Selection experiments with a strain segregating for the mutant allele, together with marker-assisted back-crossing of the mutation in a susceptible background, confirmed the involvement of the mutation in METI-I resistance. Additionally, an independent genetic mapping approach; QTL analysis identified the genomic region of pyridaben resistance, which included the PSST gene. Last, we used CRISPR-Cas9 genome editing tools to introduce the mutation in the Drosophila PSST homologue.

The economic importance of acaricides in the control of phytophagous mites and an update on recent acaricide mode of action research

Acaricides are one of the cornerstones of an efficient control program for phytophagous mites. An analysis of the global acaricide market reveals that spider mites such as Tetranychus urticae, Panonychus citri and Panonychus ulmi are by far the most economically important species, representing more than 80% of the market. Other relevant mite groups are false spider mites (mainly Brevipalpus), rust and gall mites and tarsonemid mites. Acaricides are most frequently used in vegetables and fruits (74% of the market), including grape vines and citrus. However, their use is increasing in major crops where spider mites are becoming more important, such as soybean, cotton and corn. As revealed by a detailed case study of the Japanese market, major shifts in acaricide use are partially driven by resistance development and the commercial availability of compounds with novel mode of action. The importance of the latter cannot be underestimated, although some compounds are successfully used for more than 30 years. A review of recent developments in mode of action research is presented, as such knowledge is important for devising resistance management programs. This includes spirocyclic keto-enols as inhibitors of acetyl-CoA carboxylase, the carbazate bifenazate as a mitochondrial complex III inhibitor, a novel class of complex II inhibitors, and the mite growth inhibitors hexythiazox, clofentezine and etoxazole that interact with chitin synthase I.

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

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

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

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

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.

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.

A horizontally transferred cyanase gene in the spider mite Tetranychus urticae is involved in cyanate metabolism and is differentially expressed upon host plant change

The genome of the phytophagous two-spotted spider mite Tetranychus urticae was recently sequenced, representing the first complete chelicerate genome, but also the first genome of a highly polyphagous agricultural pest. Genome analysis revealed the presence of an unexpected high number of cases of putative horizontal gene transfers, including a gene that encodes a cyanase or cyanate lyase. In this study we show by recombinant expression that the T. urticae cyanase remained functionally active after horizontal gene transfer and has a high affinity for cyanate. Cyanases were also detected in other plant parasitic spider mites species such as Tetranychus evansi and Panonychus citri, suggesting that an ancient gene transfer occurred before the diversification within the Tetranychidae family. To investigate the potential role of cyanase in the evolution of plant parasitic spider mites, we studied cyanase expression patterns in T. urticae in relation to host plant range and cyanogenesis, a common plant defense mechanism. Spider mites can alter cyanase expression levels after transfer to several new host plants, including the cyanogenic Phaseolus lunatus. However, the role of cyanase is probably not restricted to cyanide response, but likely to the plant nutritional quality as a whole. We finally discuss potential interactions between cyanase activity and pyrimidine and amino acid synthesis.

Resistance to acaricides in Italian strains of Tetranychus urticae: toxicological and enzymatic assays

Problems with Tetranychus urticae are frequently reported in protected crops in Italy, particularly in roses where many introduced acaricides show a progressive loss of effectiveness. We have conducted bioassays to assess the response of some Italian strains of T. urticae to a number of acaricides. These include compounds that were widespread and frequently used in the past, but also some recently registered compounds. We investigated two T. urticae strains collected from rose growers where control failures were reported (SAN and PSE), together with a strain collected from unsprayed vegetables (BOSA). Adult females of the rose strains (SAN and PSE) were resistant to tebufenpyrad (Resistant Ratio-RR, RR(50) = 48.4 and 163.6) and fenpyroximate (RR(50) = 74.1 and 25.9) when compared to the susceptible BOSA strain. Lethal concentrations for these products were higher than the registered field rate. The PSE strain proved to be highly resistant to abamectin (RR(50) = 1,294.1). Variation in bifenazate susceptibility was detected amongst strains, but LC(90) values of SAN and PSE were still in the range of the registered field rate. In egg bioassays, the SAN and PSE strains exhibited high resistance levels to clofentezine (RR(50) = 66,473 and 170,714), hexythiazox (RR(50) = 70,244 and 159,493) and flufenoxuron (RR(50) = 61.9 and 117.9). But the recently introduced ovi/larvicides etoxazole and spirodiclofen exhibited high activity on all strains. The activity of detoxifying enzymes such as esterases, glutathione-S-transferases (GSTs) and cytochrome P450 monooxygenases (MFOs) was determined in these strains as a preliminary attempt to identify potential resistance mechanisms. Enzymatic assays showed that the rose strains exhibited 2.66 and 1.95-fold increased MFOs activity compared to the susceptible strain. Assays for GSTs revealed that only the SAN strain exhibited a significantly higher activity. In contrast, only the PSE strain showed a significant higher hydrolysis of 1-naphthyl acetate.

Heritability and artificial selection on ambulatory dispersal distance in Tetranychus urticae: effects of density and maternal effects

Dispersal distance is understudied although the evolution of dispersal distance affects the distribution of genetic diversity through space. Using the two-spotted spider mite, Tetranychus urticae, we tested the conditions under which dispersal distance could evolve. To this aim, we performed artificial selection based on dispersal distance by choosing 40 individuals (out of 150) that settled furthest from the home patch (high dispersal, HDIS) and 40 individuals that remained close to the home patch (low dispersal, LDIS) with three replicates per treatment. We did not observe a response to selection nor a difference between treatments in life-history traits (fecundity, survival, longevity, and sex-ratio) after ten generations of selection. However, we show that heritability for dispersal distance depends on density. Heritability for dispersal distance was low and non-significant when using the same density as the artificial selection experiments while heritability becomes significant at a lower density. Furthermore, we show that maternal effects may have influenced the dispersal behaviour of the mites. Our results suggest primarily that selection did not work because high density and maternal effects induced phenotypic plasticity for dispersal distance. Density and maternal effects may affect the evolution of dispersal distance and should be incorporated into future theoretical and empirical studies.

Mitochondrial genome sequence of Unionicola parkeri (Acari: Trombidiformes: Unionicolidae): molecular synapomorphies between closely-related Unionicola gill mites

The mitochondrial genome of Unionicola parkeri is a 14,734 bp circular DNA molecule. The sequence and annotation revealed a unique gene order, related to but distinct from the gene order in the closely related species U. foili. Mitochondrial tRNA sequences annotated in this genome predict non-canonical secondary structures for these molecules. The continuing patterns of unique gene orders and unusual tRNA structures in the Trombidiformes in general and Unionicola in particular support the use of phylogenetic approaches that use these types of molecular features as shared, derived character states. Further progress in using these molecular character states to reconstruct phylogeny will depend on careful annotation, especially of tRNA genes.

Parallel evolution of cytochrome b mediated bifenazate resistance in the citrus red mite Panonychus citri

Bifenazate is a recently developed acaricide that is mainly used to control spider mites on a variety of crops. Although first thought to be a neurotoxin, genetic evidence obtained from bifenazate resistant Tetranychus urticae strains suggested an alternative mode of action as a Qo pocket inhibitor of the mitochondrial complex III. In this study, we reveal how bifenazate resistance in strains of Panonychus citri is maternally inherited and can confer cross-resistance to the known Qo inhibitor acequinocyl. The mitochondrial genome of P. citri was sequenced and Qo pocket mutations were shown to be linked with the resistant trait. Parallel evolution of cytochrome b mediated bifenazate resistance corroborates the alternative mode of action and yet again illustrates that care should be taken when employing Qo inhibitors as crop protection compounds.

Acaricide resistance mechanisms in the two-spotted spider mite Tetranychus urticae and other important Acari: a review

The two-spotted spider mite Tetranychus urticae Koch is one of the economically most important pests in a wide range of outdoor and protected crops worldwide. Its control has been and still is largely based on the use of insecticides and acaricides. However, due to its short life cycle, abundant progeny and arrhenotokous reproduction, it is able to develop resistance to these compounds very rapidly. As a consequence, it has the dubious reputation to be the"most resistant species" in terms of the total number of pesticides to which populations have become resistant, and its control has become problematic in many areas worldwide. Insecticide and acaricide resistance has also been reported in the ectoparasite Sarcoptes scabiei, the causative organism of scabies, and other economically important Acari, such as the Southern cattle tick Rhipicephalus microplus, one of the biggest arthropod threats to livestock, and the parasitic mite Varroa destructor, a major economic burden for beekeepers worldwide. Although resistance research in Acari has not kept pace with that in insects, a number of studies on the molecular mechanisms responsible for the resistant phenotype has been conducted recently. In this review, state-of-the-art information on T. urticae resistance, supplemented with data on other important Acari has been brought together. Considerable attention is given to the underlying resistance mechanisms that have been elucidated at the molecular level. The incidence of bifenazate resistance in T. urticae is expanded as an insecticide resistance evolutionary paradigm in arthropods.

Milbemectin resistance in Tetranychus urticae (Acari: Tetranychidae): selection, stability and cross-resistance to abamectin

Studies on artificial laboratory selections with milbemectin, stability of milbemectin resistance and possible cross-resistance with abamectin were carried out with Tetranychus urticae Koch to provide basic information for a milbemectin resistance management program. Selections for resistance and susceptibility to milbemectin were performed in a population of T. urticae, collected from a commercial chrysanthemum field in the State of São Paulo, Brazil. After six selections for resistance and five selections for susceptibility, susceptible (S) and resistant (R) strains of T. urticae to milbemectin were obtained. The resistance ratio (R/S) at the LC(50) reached 409-fold value. The stability of milbemectin resistance was also studied under laboratory conditions, using a population with initial frequency of 75% of resistant mites. The frequencies of milbemectin resistance were evaluated monthly for a period of 7 months. In order to observe possible correlation between milbemectin and abamectin resistance, the frequencies of abamectin resistance were also evaluated for that population, during the same period. The frequency of milbemectin resistance decreased from 75 to 14.5%, while the percentage of abamectin resistant mites decreased from 57 to 9.1%, in 7 months. The frequencies of milbemectin and abamectin resistance were also evaluated in 25 field populations of T. urticae, collected from several crops in the State of São Paulo. The frequencies of milbemectin resistance varied from 4.1 to 89.5%, and of abamectin, from 7.0 to 90.5%. A positive and significant correlation was observed between the frequencies of milbemectin and abamectin resistance, indicating positive cross-resistance between these acaricides. The results indicate that abamectin should be avoided for managing milbemectin resistance in T. urticae. This is the first report on milbemectin resistance in T. urticae in Brazil.

Development of acaricide resistance in Pacific spider mite (Tetranychus pacificus) from California vineyards

In recent years, grape growers in California reported failures of acaricides against Tetranychus pacificus McGregor. We collected T. pacificus populations from four vineyards and tested them for resistance to bifenazate, propargite and pyridaben. In addition, we sequenced part of the cytochrome b gene of bifenazate-resistant and -susceptible T. pacificus to test for the presence of mutations reported to confer resistance to the congeneric T. urticae. None of the mutations conferring resistance to bifenazate in T. urticae were present in resistant T. pacificus. Resistance levels ranged from full susceptibility to statistically significant 11-fold resistance to pyridaben, sevenfold resistance to bifenazate and fourfold resistance to propargite compared to a susceptible population. Despite the relatively low levels of resistance detected, we estimated that under the conditions of our study the highest field rates of bifenazate and pyridaben application would cause less than 58 and 66% mortality of adult females in the most resistant populations, respectively. In contrast, field rates of propargite application would cause close to 100% mortality in the least susceptible population. These results highlight a potential link between resistance development and reduced field effectiveness for bifenazate and pyridaben. Finally, T. pacificus may be more tolerant to bifenazate and propargite than T. urticae, since the LC(50) values for the susceptible population of T. pacificus were several times higher than LC(50)'s reported for susceptible T. urticae.

Mitochondrial genome sequence of Unionicola foili (Acari: Unionicolidae): a unique gene order with implications for phylogenetic inference

The mitochondrial genome of Unionicola foili is circular, 14,738 bp in length, and contains several notable features. The sequence and annotation revealed a unique gene order, continuing a pattern of highly rearranged mitochondrial genomes in the Trombidiformes. U. foili mitochondrial tRNA sequences predict non-canonical secondary structures for these molecules, and our annotation suggests an in-frame fusion between the nad4L and nad5 genes in this genome. The unique gene order and unusual tRNA structures could serve as idiosyncratic characters and have the potential to be phylogenetically informative.

Susceptibility of an organophosphate resistant strain of the two-spotted spider mite (Tetranychus urticae) to mixtures of bifenazate with organophosphate and carbamate insecticides

Bifenazate, a new and frequently used carbazate, is a pro-acaricide which needs to be activated by carboxylesterases. We evaluated the possible antagonism of organophosphate and carbamate insecticides on bifenazate toxicity in Tetranychus urticae applied in mixtures. Two organophosphate resistant strains were used (WI and MR-VL) and several organophosphate (chlorpyrifos, azinphosmethyl and phosmet) and carbamate (carbaryl and methomyl) insecticides were evaluated. Mixing chlorpyrifos with bifenazate decreased bifenazate toxicity in both tested strains. However, in the strain with a higher esterase activity, antagonism decreased after 2 days. Of all other tested chemicals, only methomyl displayed an antagonistic effect 1 day after treatment. These findings indicate that mixing organophosphate and carbamate insecticides with bifenazate may inhibit bifenazate efficacy under field conditions, especially when resistant strains are present.

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

BACKGROUND

Spirodiclofen is a selective, non-systemic acaricide from the new chemical class of tetronic acid derivatives. In order to develop strategies to minimise resistance in the field, a laboratory-selected spirodiclofen-resistant strain of the two-spotted spider mite, Tetranychus urticae Koch, was used to determine genetic, toxicological, biochemical and cross-resistance data.

RESULTS

Selecting for spirodiclofen resistance in the laboratory yielded a strain (SR-VP) with a resistance ratio of 274, determined on the larval stage. The egg stage remained far more susceptible. No cross-resistance was found against other established acaricides, except for spiromesifen. Based on synergist experiments and enzyme assays, it appeared that especially P450 monooxygenases, but also esterases and glutathione-S-transferases, could be involved in the metabolic detoxification of spirodiclofen. Genetic analysis showed that the resistance is inherited as an intermediate trait under control of more than one gene.

CONCLUSIONS

Resistance to spirodiclofen exceeded by far the recommended field rate. A good acaricide resistance management programme is necessary to prevent fast resistance build-up in the field. Spirodiclofen can be used in alternation with most established acaricides, except for other tetronic acid derivatives. Without selection pressure, resistance tends to be unstable and can decrease in the presence of susceptible individuals owing to the intermediate, polygenic inheritance mode.

Mutations in the mitochondrial cytochrome b of Tetranychus urticae Koch (Acari: Tetranychidae) confer cross-resistance between bifenazate and acequinocyl

BACKGROUND

Resistance of Tetranychus urticae Koch to bifenazate was recently linked with mutations in the mitochondrial cytochrome b Q(o) pocket, suggesting that bifenazate acts as a Q(o) inhibitor (Q(o)I). Since these mutations might cause cross-resistance to the known acaricidal Q(o)I acequinocyl and fluacrypyrim, resistance levels and inheritance patterns were investigated in several bifenazate-susceptible and bifenazate-resistant strains with different mutations in the cd1 and ef helices aligning the Q(o) pocket.

RESULTS

Cross-resistance to acequinocyl in two bifenazate-resistant strains was shown to be maternally inherited and caused by the combination of two specific mutations in the cytochrome b Q(o) pocket. Although most investigated strains were resistant to fluacrypyrim, resistance was not inherited maternally, but as a monogenic autosomal highly dominant trait. As a consequence, there was no correlation between cytochrome b genotype and fluacrypyrim resistance.

CONCLUSIONS

Although there is no absolute cross-resistance between bifenazate, acequinocyl and fluacrypyrim, some bifenazate resistance mutations confer cross-resistance to acequinocyl. In the light of resistance development and management, high prudence is called for when alternating bifenazate and acequinocyl in the same crop. Maternally inherited cross-resistance between bifenazate and acequinocyl reinforces the likelihood of bifenazate acting as a mitochondrial complex III inhibitor at the Q(o) site.

Mitochondrial heteroplasmy and the evolution of insecticide resistance: non-Mendelian inheritance in action

Genes encoded by mitochondrial DNA (mtDNA) exist in large numbers per cell but can be selected very rapidly as a result of unequal partitioning of mtDNA between germ cells during embryogenesis. However, empirical studies of this "bottlenecking" effect are rare because of the apparent scarcity of heteroplasmic individuals possessing more than one mtDNA haplotype. Here, we report an example of insecticide resistance in an arthropod pest (Tetranychus urticae) being controlled by mtDNA and on its inheritance in a heteroplasmic mite strain. Resistance to the insecticide bifenazate is highly correlated with remarkable mutations in cytochrome b, a mitochondrially encoded protein in the respiratory pathway. Four sites in the Q(o) site that are absolutely conserved across fungi, protozoa, plants, and animals are mutated in resistant mite strains. Despite the unusual nature of these mutations, resistant mites showed no fitness costs in the absence of insecticide. Partially resistant strains, consisting of heteroplasmic individuals, transmit their resistant and susceptible haplotypes to progeny in highly variable ratios consistent with a sampling bottleneck of approximately 180 copies. Insecticide selection on heteroplasmic individuals favors those carrying resistant haplotypes at a frequency of 60% or more. This combination of factors enables very rapid evolution and accounts for mutations being fixed in most field-collected resistant strains. The results provide a rare insight into non-Mendelian mechanisms of mitochondrial inheritance and evolution, relevant to anticipating and understanding the development of other mitochondrially encoded adaptations in arthropods. They also provide strong evidence of cytochrome b being the target site for bifenazate in spider mites.