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

Rapid insecticide resistance bioassays for three major urban insects in Taiwan

BACKGROUND

Taiwan's warm and humid climate and dense population provide a suitable environment for the breeding of pests. The three major urban insects in Taiwan are house flies, cockroaches, and mosquitoes. In cases where a disease outbreak or high pest density necessitates chemical control, selecting the most effective insecticide is crucial. The resistance of pests to the selected environmental insecticide must be rapidly assessed to achieve effective chemical control and reduce environmental pollution.

METHODS

In this study, we evaluated the resistance of various pests, namely, house flies (Musca domestica L.), cockroaches (Blattella germanica L. and Periplaneta americana), and mosquitoes (Aedes aegypti and Ae. albopictus) against 10 commonly used insecticides. Rapid insecticide resistance bioassays were performed using discriminating doses or concentrations of the active ingredients of insecticides.

RESULTS

Five field strains of M. domestica (L.) are resistant to all 10 commonly used insecticides and exhibit cross- and multiple resistance to four types of pyrethroids and three types of organophosphates, propoxur, fipronil, and imidacloprid. None of the five field strains of P. americana are resistant to any of the tested insecticides, and only one strain of B. germanica (L.) is resistant to permethrin. One strain of Ae. albopictus is resistant to pirimiphos-methyl, whereas five strains of Ae. aegypti exhibit multiple resistance to pyrethroids, organophosphates, and other insecticides.

CONCLUSIONS

In the event of a disease outbreak or high pest density, rapid insecticide resistance bioassays may be performed using discriminating doses or concentrations to achieve precise and effective chemical control, reduce environmental pollution, and increase control efficacy.

The complex II resistance mutation H258Y in succinate dehydrogenase subunit B causes fitness penalties associated with mitochondrial respiratory deficiency

BACKGROUND

The acaricides cyflumetofen, cyenopyrafen and pyflubumide inhibit the mitochondrial electron transport chain at complex II [succinate dehydrogenase (SDH) complex]. A target site mutation H258Y was recently discovered in a resistant strain of the spider mite pest Tetranychus urticae. H258Y causes strong cross-resistance between cyenopyrafen and pyflubumide, but not cyflumetofen. In fungal pests, fitness costs associated with substitutions at the corresponding H258 position that confer resistance to fungicidal SDH inhibitors have not been uncovered. Here, we used H258 and Y258 near-isogenic lines of T. urticae to quantify potential pleiotropic fitness effects on mite physiology.

RESULTS

The H258Y mutation was not associated with consistent significant changes of single generation life history traits and fertility life table parameters. In contrast, proportional Sanger sequencing and droplet digital polymerase chain reaction showed that the frequency of the resistant Y258 allele decreased when replicated 50:50 Y258:H258 experimentally evolving populations were maintained in an acaricide-free environment for approximately 12 generations. Using in vitro assays with mitochondrial extracts from resistant (Y258) and susceptible (H258) lines, we identified a significantly reduced SDH activity (48% lower activity) and a slightly enhanced combined complex I and III activity (18% higher activity) in the Y258 lines.

CONCLUSION

Our findings suggest that the H258Y mutation is associated with a high fitness cost in the spider mite T. urticae. Importantly, while it is the most common approach, it is clear that only comparing life history traits and life table fecundity does not allow to reliably estimate fitness costs of target site mutations in natural pest populations. © 2023 Society of Chemical Industry.

Evaluation of the Predatory Mite Neoseiulus barkeri against Spider Mites Damaging Rubber Trees

The spider mites Eotetranychus sexmaculatus, Eutetranychus orientalis and Oligonychus biharensisin are severe pests of rubber trees in China. The predatory mite Neoseiulus barkeri has been found to be a natural enemy of these three pests, while nothing is known about the biological performance of this phytoseiid predator against these phytophagous mites. In this study, the development, survivorship, reproduction, adult longevity, fecundity, sex ratio and population growth parameters of N. barkeri fed on these pests were evaluated in comparison to the factitious prey Tyrophagus putrescentiae in the laboratory at 25 ± 1 °C, 75 ± 5% relative humidity and a 12:12 (L:D) h photoperiod. The results showed that N. barkeri could develop from egg to adult and reproduced successfully on the three preys. The survival rate of N. barkeri from egg to adult was higher when fed on E. orientalis (100%) and T. putrescentiae (100%) than when fed on O. biharensisin (93.60%) and E. sexmaculatus (71.42%). The shortest and longest generation time for N. barkeri were observed on E. orientalis with 6.67 d and E. sexmaculatus with 12.50 d, respectively. The maximum fecundity (29.35 eggs per female) and highest intrinsic rate of increase (r_m = 0.226) were recorded when N. barkeri fed on E. orientalis, while feeding on E. sexmaculatus gave the minimum fecundity (1.87 eggs per female) and lowest reproduction rate (r_m = 0.041). The values of these parameters for N. barkeri evaluated on O. biharensisin were found to be comparable to those obtained on T. putrescentiae. The sex ratio of N. barkeri progeny on the preys mentioned above, apart from O. biharensisin, was female biased. According to the findings, N. barkeri could serve as a promising biocontrol agent against E. orientalis and O. biharensisin, and possibly E. sexmaculatus on rubber trees.

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

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

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

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

Rapid specialization of counter defenses enables two-spotted spider mite to adapt to novel plant hosts

Genetic adaptation, occurring over a long evolutionary time, enables host-specialized herbivores to develop novel resistance traits and to efficiently counteract the defenses of a narrow range of host plants. In contrast, physiological acclimation, leading to the suppression and/or detoxification of host defenses, is hypothesized to enable broad generalists to shift between plant hosts. However, the host adaptation mechanisms used by generalists composed of host-adapted populations are not known. Two-spotted spider mite (TSSM; Tetranychus urticae) is an extreme generalist herbivore whose individual populations perform well only on a subset of potential hosts. We combined experimental evolution, Arabidopsis thaliana genetics, mite reverse genetics, and pharmacological approaches to examine mite host adaptation upon the shift of a bean (Phaseolus vulgaris)-adapted population to Arabidopsis. We showed that cytochrome P450 monooxygenases are required for mite adaptation to Arabidopsis. We identified activities of two tiers of P450s: general xenobiotic-responsive P450s that have a limited contribution to mite adaptation to Arabidopsis and adaptation-associated P450s that efficiently counteract Arabidopsis defenses. In approximately 25 generations of mite selection on Arabidopsis plants, mites evolved highly efficient detoxification-based adaptation, characteristic of specialist herbivores. This demonstrates that specialization to plant resistance traits can occur within the ecological timescale, enabling the TSSM to shift to novel plant hosts.

Effect of broflanilide on the phytophagous mite Tetranychus urticae and the predatory mite Typhlodromips swirskii

BACKGROUND

Two-spotted spider mite (TSSM), Tetranychus urticae Koch, is one of the most serious pests of agricultural crops. Broflanilide exhibits high lethality against various pests and has been marketed worldwide under the Vedira and Tenebenal brands in 2020. Nevertheless, little information has been reported about its effects on agricultural mites.

RESULTS

Broflanilide displayed higher toxicity to TSSM eggs (24 h LC₅₀ , 1.015 mg L^-1 ) and adult females (24 h LC₅₀ , 2.062 mg L^-1 ) than commercial acaricides, including cyflumetofen, bifenazate, and profenofos. In contrast, the adverse effects of broflanilide on the predatory mite, Typhlodromips swirskii Athias-Henriot, was lower than those of fenpyroximate and abamectin. In the sublethal effect study, while adult females were treated with broflanilide, the number of eggs and longevity were reduced in LC₁₀ and LC₃₀ treatments; when eggs were treated with broflanilide, the egg duration and deutonymph duration were prolonged in LC₃₀ treatment. A significant decrease in the total life span and duration and fecundity of adult females was observed in LC₁₀ and LC₃₀ treatments. Furthermore, the number of eggs per adult female was significantly reduced from 103.48 ± 3.69 in the control group to 69.42 ± 2.22 and 48.33 ± 1.75 in LC₁₀ and LC₃₀ treatments, respectively. In the greenhouse bioassay, broflanilide 5% suspension concentrate (MCI-8007) showed excellent acaricidal activity to TSSM, with 99.22% corrected control, compared with the MCI-8007 untreated group.

CONCLUSION

Our results indicated that broflanilide has a high acaricidal activity to TSSM and significant inhibition to fecundity of adult female, and could be considered as a potential alternative for TSSM management. © 2021 Society of Chemical Industry.

QTL mapping using microsatellite linkage reveals target-site mutations associated with high levels of resistance against three mitochondrial complex II inhibitors in Tetranychus urticae

The acaricides cyflumetofen, cyenopyrafen, and pyflubumide act as inhibitors of the mitochondrial electron transport system at complex II (succinate dehydrogenase; SDH), a new mode of action in arthropods. The development and mechanisms of low-level resistance against cyenopyrafen and cyflumetofen have been previously reported in Tetranychus urticae. In the present study, we investigated high levels of resistance against three SDH inhibitors in T. urticae field populations and clarify the genetic basis of resistance using quantitative trait locus (QTL) analysis. First, we constructed a microsatellite linkage map comprising 64 markers assembled into three linkage groups (LGs) with total length of 683.8 cM and average marker spacing of 11.03 cM. We then used the linkage map to perform QTL mapping, and identified significant QTLs contributing to resistance to cyflumetofen (one QTL on LG1), cyenopyrafen (one QTL on LG3), and pyflubumide (two QTLs on LG1 and LG3). The QTL peaks on LG1 for cyflumetofen and pyflubumide overlapped and included the SdhB locus. For cyenopyrafen resistance, the QTLs on LG3 included the SdhC locus. For cyflumetofen resistance, we found an I260T mutation in SdhB. For pyflubumide and cyenopyrafen resistance, we detected I260V and S56L substitutions in SdhB and SdhC, respectively, by direct sequencing. Both I260 in SdhB and S56 in SdhC were present in highly conserved regions of the ubiquinone binding site formed at the interface among SdhB, SdhC, and SdhD. Mutations at these positions have been implicated in resistance against fungicides that act as Sdh inhibitors in various pathogens. Therefore, we consider these mutations to be target-site resistance mutations for these acaricidal SDH inhibitors.

Geographical distribution and molecular insights into abamectin and milbemectin cross-resistance in European field populations of Tetranychus urticae

BACKGROUND

Milbemectin and abamectin are frequently used to control the spider mite Tetranychus urticae. The development of abamectin resistance in this major pest has become an increasing problem worldwide, potentially compromising the use of milbemectin. In this study, a large collection of European field populations was screened for milbemectin and abamectin resistance, and both target-site and metabolic (cross-)resistance mechanisms were investigated.

RESULTS

High to very high levels of abamectin resistance were found in one third of all populations, while milbemectin resistance levels were low for most populations. The occurrence of well-known target-site resistance mutations in glutamate-gated chloride channels (G314D in GluCl1 and G326E in GluCl3) was documented in the most resistant populations. However, a new mutation, I321T in GluCl3, was also uncovered in three resistant populations, while a V327G and L329F mutation was found in GluCl3 of one resistant population. A differential gene-expression analysis revealed the overexpression of detoxification genes, more specifically cytochrome P450 monooxygenase (P450) and UDP-glycosyltransferase (UGT) genes. Multiple UGTs were functionally expressed, and their capability to glycosylate abamectin and milbemectin, was tested and confirmed.

CONCLUSIONS

We found a clear correlation between abamectin and milbemectin resistance in European T. urticae populations, but as milbemectin resistance levels were low, the observed cross-resistance is probably not of operational importance. The presence of target-site resistance mutations in GluCl genes was confirmed in most but not all resistant populations. Gene-expression analysis and functional characterization of P450s and UGTs suggests that also metabolic abamectin resistance mechanisms are common in European T. urticae populations. © 2020 Society of Chemical Industry.

Resistance incidence and presence of resistance mutations in populations of Tetranychus urticae from vegetable crops in Turkey

Tetranychus urticae Koch is one of the most common and harmful pests in vegetable production areas. Similar to other countries, control of T. urticae is mainly based on acaricides in Turkey. However, T. urticae rapidly develops resistance and failures in chemical control have occurred frequently. The toxicity of various acaricides was investigated in ten T. urticae populations collected from vegetable crops in Turkey. In addition, populations were screened for the presence of currently known target-site resistance mutations. It was shown that resistance to bifenthrin was the most widespread, but also half of the populations were resistant to abamectin and hexythiazox. Resistance mutations in the voltage-gated sodium channel (VGSC) and chitin synthase 1 were found in various populations. Moreover, for the first time, F1538I and L1024V VGSC mutations were reported for Turkish populations. Mutations that confer resistance to abamectin, bifenazate and METI-I acaricides such as pyridaben were not detected. These results will contribute to the design of an effective resistance management program in Turkey.

Acaricidal Activity of Cyclodipeptides from Bacillus amyloliquefaciens W1 against Tetranychus urticae

Bioassay-guided fractionation of the supernatant of the biocontrol strain Bacillus amyloliquefaciens W1 led to the isolation of eight acaricidal cyclodipeptides from the active fractions by column chromatography separation and HPLC purification. The chemical structures of these compounds were identified as cyclo-(Gly-l-Phe), 2, cyclo-(l-Phe- trans-4-OH-l-Pro), 3, cyclo-(Gly-l-Tyr), 4, cyclo-(l-Ala-l-Pro), 5, cyclo-(l-Pro- trans-4-OH-l-Pro), 6, cyclo-(Gly-l-Pro), 7, cyclo-(l-Pro-l-Pro), 8, and cyclo-(l-Tyr- trans-4-OH-l-Pro), 9. Those cyclodipeptides displayed significant acaricidal activities with LC₅₀ values of 13.85-98.24 μM. Cyclo-(l-Tyr- trans-4-OH-l-Pro) (LC₅₀ 13.85 μM) was five times more effective than the positive control abamectin (LC₅₀ 72.06 μM). The results indicated that the hydroxyl group is an important component. This is the first report on the acaricidal capabilities of cyclodipeptides against Tetranychus urticae. The results revealed that the acaricidal activity of the biocontrol strain B. amyloliquefaciens W1 was dependent on its constituent cyclodipeptides, which have the potential to be safe and environmentally friendly acaricides.

Preference and performance of the two-spotted spider mite Tetranychus urticae (Acari: Tetranychidae) on strawberry cultivars

The two-spotted spider mite (TSSM), Tetranychus urticae Koch (Acari: Tetranychidae), is one of the most serious pests of strawberry worldwide. Understanding the preference of TSSM for particular cultivars of strawberry and performance on them helps identify host-plant resistance to this pest mite. In this study, we tested preference, developmental duration, fecundity and population levels of TSSM on 14 strawberry cultivars. TSSM showed strong preference for the Chinese cultivars of Yanxiang, Baixuegongzhu, and Jingtaoxiang. Development of TSSM on the cultivars varied from 32.32 to 36.82 days; it was longest on the cultivars Hongxiutianxiang and Baixuegongzhu, and shortest on Yanxiang, Jingzangxiang, and Darselect as well as on a wild variety (Wuye). TSSM had high fecundity on the cultivars Yanxiang, Taoxun, Hongxiutianxiang, Jingzangxiang, Albion and Baixuegongzhu as well as on Wuye, whereas egg production was lowest on Sweet Charlie, Portola, Akihime, and Benihoppe. After 28 days of plant infestation with 10 pairs of adults, the cultivars Yanxiang, Taoxun, Jingzangxiang, Jingtaoxiang, and Baixuegongzhu had the highest number of mites (> 1000 per plant), whereas mite numbers on Albion and Camarosa were low. The population size of TSSM was correlated with fecundity, but no correlation was found between other preference/performance measures. Our study suggests that a rapid increase of population size of TSSM on cultivars of strawberry is related to high fecundity, and also that there are substantial differences in preference and performance across cultivars.

Effect of acaricidal components isolated from lettuce (Lactuca sativa) on carmine spider mite (Tetranychus cinnabarinus Boisd.)

This study aimed to evaluate the acaricidal activity of lettuce (Lactuca sativa) extracts against carmine spider mites (Tetranychus cinnabarinus Boisd.) and isolate the acaricidal components. Acaricidal activities of lettuce extracts isolated from different parts (the leaf, root and seed) using various solvents (petroleum ether, acetone and methanol) were evaluated with slide-dip bioassay and relatively high median lethal concentration (LC50) values were detected. Acetone extracts of lettuce leaves harvested in July and September were fractionated and isolated with silica gel and thin-layer chromatography. Consequently, acetone extracts of lettuce leaves harvested in July exhibited higher acaricidal activity than those harvested in September, with an LC50 value of 0.268 mg ml-1 at 72 h post-treatment. A total of 27 fractions were obtained from the acetone extract of lettuce leaves harvested in July, and mite mortalities with the 11th and 12th fractions were higher than those with the other 25 fractions (LC50: 0.751 and 1.258 mg ml-1 at 48 h post-treatment, respectively). Subsequently, active acaricidal components of the 11th fraction were identified by infrared, nuclear magnetic resonance and liquid chromatography/mass spectrometry. Five components were isolated from the 11th fraction, with components 11-a and 11-b showing relatively high acaricidal activities (LC50: 0.288 and 0.114 mg ml-1 at 48 h post-treatment, respectively). Component 11-a was identified as β-sitosterol. In conclusion, acetone extracts of lettuce leaves harvested in July might be used as a novel phytogenic acaricide to control mites.

Efficacy of carbon dioxide treatments for the control of the two-spotted spider mite, Tetranychus urticae, and treatment impact on plant seedlings

To develop a new control method for the two-spotted spider mite (TSSM), Tetranychus urticae, we investigated the effect of controlled atmospheres of carbon dioxide (CO₂) on TSSM mortality under different concentrations and treatment periods, and evaluated the impact of treatments on seedlings of five host plants of TSSM. Egg hatching rate of TSSM was reduced to 37.7, 5.4 or 0% after 24 h treatment involving concentrations of 16.7, 33.3 or 50%, respectively. Mobile stages (nymphs and adult) of TSSM were completely controlled after 24 h treatment at concentrations higher than 33.3%. After 4 h at concentrations of 33.3 or 50%, 1st-day survival rate for all mobile stages was 45.3 or 36.0%, respectively, whereas after 8 or 16 h treatments, all values were decreased to zero. Seedlings of four major host plants of TSSM (cucumber, eggplant, rape, green peppers) were damaged to varying degrees after 24 h at the three concentrations, but strawberry, another host plant, was not damaged. Cucumber suffered the most serious damage, resulting in wilting and death. In conclusion, controlled atmospheres of CO₂ can kill TSSM, particularly at high concentrations and with long treatment times. It can be used to control TSSM on strawberry, but should be used cautiously on other host plants.

A genomic approach to identify and monitor a novel pyrethroid resistance mutation in the redlegged earth mite, Halotydeus destructor

Resistance mechanisms are typically uncovered by identifying sequence variation in known candidate genes, however this strategy can be problematic for species with no reference data in known relatives. Here we take a genomic approach to identify resistance to pyrethroids in the redlegged earth mite, Halotydeus destructor, a member of the Penthalidae family of mites that are virtually uncharacterized genetically. Based on shallow genome sequencing followed by a genome assembly, we first identified contigs of the H. destructor parasodium channel gene. By linking variation in this gene to known resistant phenotypes, we located a single nucleotide polymorphism in resistant mites. This polymorphism results in a leucine (L) to phenylalanine (F) amino acid substitution in the II6 region (predicted) of the gene (L1024F). This novel mutation has not previously been linked to pyrethroid resistance, although other polymorphisms have been identified in the two-spotted spider mite, Tetranychus urticae at the same locus (L1024V). The sequencing approach was successful in generating a candidate polymorphism that was then validated using laboratory bioassays and field surveys. A high throughput Illumina-based sequencing diagnostic was developed to rapidly assess resistance allele frequencies in pools of mites sourced from hundreds of populations across Australia. Resistance was confirmed to be widespread in the southern wheatbelt region of Western Australia. Two different resistance mutations were identified in field populations, both resulting in the same amino acid substitution. The frequency and distribution of resistance amplicon haplotypes suggests at least two, and probably more independent origins of resistance.

Identification of Onosma visianii Roots Extract and Purified Shikonin Derivatives as Potential Acaricidal Agents against Tetranychus urticae

There is an increasing need for the discovery of reliable and eco-friendly pesticides and natural plant-derived products may play a crucial role as source of new active compounds. In this research, a lipophilic extract of Onosma visianii roots extract containing 12% of shikonin derivatives demonstrated significant toxicity and inhibition of oviposition against Tetranychus urticae mites. Extensive chromatographic separation allowed the isolation of 11 naphthoquinone derivatives that were identified by spectral techniques and were tested against Tetranychus urticae. All the isolated compounds presented effects against the considered mite and isobutylshikonin (1) and isovalerylshikonin (2) were the most active, being valuable model compounds for the study of new anti-mite agents.

Adaptation of acaricide stress facilitates Tetranychus urticae expanding against Tetranychus cinnabarinus in China

The two‐spotted spider mite, Tetranychus urticae, and the carmine spider mite, Tetranychus cinnabarinus, are invasive and native species in China, respectively. Compared with T. cinnabarinus, T. urticae has expanded into most parts of China and has become the dominant species of spider mite since 1983, when it was first reported in China. However, the mechanism of the demographic conversion has not been illuminated. In this study, one T. urticae field population and one T. cinnabarinus field population were isolated from the same plant in the same field, and the toxicological characteristics were compared between these two species. Laboratory bioassays demonstrated that T. urticae was more tolerant to commonly used acaricides than T. cinnabarinus. The activities of detoxification enzymes were significantly greater in T. urticae, and the fold changes of enzymes activities in T. urticae were also greater following exposure to acaricides. Furthermore, more metabolism‐related genes were upregulated at a basal level, and more genes were induced in T. urticae following exposure to acaricides. The comparison of proteins and genes between both species led credence to the hypothesis that T. urticae was more resistant to acaricides, which was the reason explaining the expansion of invasive T. urticae against native T. cinnabarinus. Laboratory simulation experiments demonstrated that following the application of acaricides, the composition of a mixed T. urticae/T. cinnabarinus population would change from a T. cinnabarinus‐dominant to a T. urticae‐dominant population. This study not only reveals that T. urticae possesses stronger detoxification capacity than its sibling species T. cinnabarinus, which facilitated its persistent expansion in China, but also points to the need to accurately identify Tetranychus species and to develop species‐specific management strategies for these pests.

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

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

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

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

Screening of target genes for RNAi in Tetranychus urticae and RNAi toxicity enhancement by chimeric genes

Due to its rapid development of resistance to nearly all arrays of acaricide, Tetranychus urticae is extremely hard to control using conventional acaricides. As an alternative control measure of acaricide-resistant mites, RNA interference (RNAi)-based method has recently been suggested. A double-stranded RNA (dsRNA) delivery method using multi-unit chambers was established and employed to screen the RNAi toxicity of 42 T. urticae genes. Among them, the dsRNA treatment of coatomer I (COPI) genes, such as coatomer subunit epsilon (COPE) and beta 2 (COPB2), resulted in high mortality [median lethal time (LT50)=89.7 and 120.3h, respectively]. The transcript level of the COPE gene was significantly (F3,9=16.2, P=0.001) reduced by up to 24% following dsRNA treatment, suggesting that the toxicity was likely mediated by the RNAi of the target gene. As a toxicity enhancement strategy, the recombinant dsRNA was generated by reciprocally recombining half-divided fragments of COPE and COPB2. The two recombinant dsRNAs exhibited higher toxicity than the respective single dsRNA treatments as determined by LT50 values (79.2 and 81.5h, respectively). This finding indicates that the recombination of different genes can enhance RNAi toxicity and be utilized to generate synthetic dsRNA with improved RNAi efficacy.

The Molecular Evolution of Xenobiotic Metabolism and Resistance in Chelicerate Mites

Chelicerate mites diverged from other arthropod lineages more than 400 million years ago and subsequently developed specific and remarkable xenobiotic adaptations. The study of the two-spotted spider mite, Tetranychus urticae, for which a high-quality Sanger-sequenced genome was first available, revealed expansions and radiations in all major detoxification gene families, including P450 monooxygenases, carboxyl/cholinesterases, glutathione-S-transferases, and ATP-binding cassette transporters. Novel gene families that are not well studied in other arthropods, such as major facilitator family transporters and lipocalins, also reflect the evolution of xenobiotic adaptation. The acquisition of genes by horizontal gene transfer provided new routes to handle toxins, for example, the β-cyanoalanine synthase enzyme that metabolizes cyanide. The availability of genomic resources for other mite species has allowed researchers to study the lineage specificity of these gene family expansions and the distinct evolution of genes involved in xenobiotic metabolism in mites. Genome-based tools have been crucial in supporting the idiosyncrasies of mite detoxification and will further support the expanding field of mite-plant interactions.