Monitoring of resistance to spirodiclofen and five other acaricides in Panonychus citri collected from Chinese citrus orchards

Sublethal and transgenerational effects of broflanilide on the citrus red mite, Panonychus citri

BACKGROUND

The citrus red mite, Panonychus citri is a serious pest of the citrus industry and has developed resistance to many acaricides. Broflanilide is a novel meta-diamide insecticide that binds to a new site on the γ -aminobutyric acid receptor with high potency against pests. However, little information has been reported about its effect on the citrus red mite.

RESULTS

Broflanilide exhibited higher toxicity to female adults and eggs of a laboratory strain of P. citri The median lethal concentration (LC50), 9.769 mg/L and 4.576 mg/L, respectively) than other commonly used acaricides and was also toxic to two P. citri field strains. Broflanilide treatment with LC10, LC20, and LC30 significantly decreased the fecundity and longevity of female adults of F0 P. citri compared with the control. The duration of larva, protonymph, deutonymph and adult, and total life span in the F1 generation were significantly reduced after treatment of F0 with broflanilide. Population parameters, including the intrinsic rate of increase (r) and finite rate of increase (λ), were significantly increased, and the mean generation time (T) of F1 progeny was significantly reduced in the LC20 treatment. The predicted population size of F1 increased when parental female adults were treated with sublethal concentrations.

CONCLUSION

Broflanilide had high acaricidal activity toward P. citri, and exposure to a sublethal concentration significantly inhibited the population growth of F0. The transgenerational hormesis effect is likely to cause population expansion of F1. More attention should be paid when broflanilide is applied to control P. citri in citrus orchards. © 2024 Society of Chemical Industry.

Overexpression of a nuclear receptor HR96 contributes to spirodiclofen susceptibility in Panonychus citri (McGregor)

The citrus red mite, Panonychus citri, is one of the most notorious and devastating citrus pests around the world that has developed resistance to multiple chemical acaricides. In previous research, we found that spirodiclofen-resistant is related to overexpression of P450, CCE, and ABC transporter genes in P. citri. However, the regulatory mechanisms of these detoxification genes are still elusive. This study identified all hormone receptor 96 genes of P. citri. 8 PcHR96 genes contained highly conserved domains. The expression profiles showed that PcHR96h was significantly upregulated in spirodiclofen resistant strain and after exposure to spirodiclofen. RNA interference of PcHR96h decreased expression of detoxification genes and increased spirodiclofen susceptibility in P. citri. Furthermore, molecular docking, heterologous expression, and drug affinity responsive target stability demonstrated that PcHR96h can interact with spirodiclofen in vitro. Our research results indicate that PcHR96h plays an important role in regulating spirodiclofen susceptibility and provides theoretical support for the resistance management of P. citri.

Identification and Functional Characterization of an Omega-Class Glutathione Gene PcGSTO1 Associated with Cyetpyrafen Resistance in Panonychus citri (McGregor)

Cyetpyrafen is a recently developed acaricide. The citrus red mite, Panonychus citri (McGregor), has developed significant resistance to cyetpyrafen. However, the molecular mechanism underlying the cyetpyrafen resistance in P. citri remains unclear. Glutathione S-transferases (GSTs) play a critical role in arthropod pesticide resistance. This study showed that GSTs were potentially related to the resistance of P. citri to cyetpyrafen through synergistic experiments and enzyme activity analysis. An omega-family GST gene, PcGSTO1, was significantly up-regulated in the egg, nymph, and adult stages of the cyetpyrafen-resistant strain. Additionally, silencing of PcGSTO1 significantly increased the mortality of P. citri to cyetpyrafen and recombinant PcGSTO1 demonstrated the ability to metabolize cyetpyrafen. Our results indicated that the overexpression of PcGSTO1 is associated with cyetpyrafen resistance in P. citri, and they also provided valuable information for managing resistance in P. citri.

Genome-wide analysis of Panonychus citri microRNAs with a focus on potential insecticidal activity of 4 microRNAs to eggs and nymphs

Panonychus citri McGregor (Acari: Tetranychidae), a destructive citrus pest, causes considerable annual economic losses due to its short lifespan and rapid resistance development. MicroRNA (miRNA)-induced RNA interference is a promising approach for pest control because of endogenous regulation of pest growth and development. To search for miRNAs with potential insecticidal activity in P. citri, genome-wide analysis of miRNAs at different developmental stages was conducted, resulting in the identification of 136 miRNAs, including 73 known and 63 novel miRNAs. A total of 17 isomiRNAs and 12 duplicated miRNAs were characterized. MiR-1 and miR-252-5p were identified as reference miRNAs for P. citri and Tetranychus urticae. Based on differential expression analysis, treatments with miR-let-7a and miR-315 mimics and the miR-let-7a antagomir significantly reduced the egg hatch rate and resulted in abnormal egg development. Overexpression or downregulation of miR-34-5p and miR-305-5p through feeding significantly decreased the adult eclosion rate and caused molting defects. The 4 miRNAs, miR-let-7a, miR-315, miR-34-5p, and miR-305-5p, had important regulatory functions and insecticidal properties in egg hatching and adult eclosion. In general, these data advance our understanding of miRNAs in mite biology, which can assist future studies on insect-specific miRNA-based green pest control technology.

Cross-resistance, inheritance and biochemical mechanism of abamectin resistance in a field-derived strain of the citrus red mite, Panonychus citri (Acari: Tetranychidae)

BACKGROUND

The citrus red mite, Panonychus citri (McGregor), a global pest of citrus, has developed different levels of resistance to various acaricides in the field. Abamectin is one of the most important insecticides/acaricides worldwide, targetting a wide number of insect and mite pests. The evolution of abamectin resistance in P. citri is threatening the sustainable use of abamectin for mite control.

RESULTS

The abamectin resistant strain (NN-Aba), derived from a field strain NN by consistent selection with abamectin, showed 4279-fold resistance to abamectin compared to a relatively susceptible strain (SS) of P. citri. Cross-resistance of NN-Aba was observed between abamectin and emamectin benzoate, pyridaben, fenpropathrin and cyflumetofen. Inheritance analyses indicated that abamectin resistance in the NN-Aba strain was autosomal, incompletely recessive and polygenic. The synergy experiment showed that abamectin toxicity was synergized by piperonyl butoxide (PBO), diethyl maleate (DEM) and tributyl phosphorotrithiotate (TPP) in the NN-Aba strain, and synergy ratios were 2.72-, 2.48- and 2.13-fold, respectively. The glutathione-S-transferases activity in the NN-Aba strain were significantly increased by 2.08-fold compared with the SS strain.

CONCLUSION

The abamectin resistance was autosomal, incompletely recessive and polygenic in P. citri. The NN-Aba strain showed cross-resistance to various acaricides with different modes of action. Metabolic detoxification mechanism participated in abamectin resistance in NN-Aba strain. These findings provide useful information for resistance management of P. citri in the field. © 2023 Society of Chemical Industry.

Resistance of Panonychus citri (McGregor) (Acari: Tetranychidae) to pyridaben in China: monitoring and fitness costs

BACKGROUND

Panonychus citri is a major citrus pest worldwide. The short life cycle and high reproductive potential of P. citri, combined with heavy acaricide use, have led to high levels of resistance to acaricides, posing a threat to global resistance management programs. Here, resistance monitoring was established to determine the pyridaben resistance status of ten P. citri populations in China from 2014 to 2021 using a leaf-dipping assay. Four characterized strains-the susceptible strain (Lab_S), the resistant strain (Pyr_R), as well as the segregated resistant strain (Pyr_Rs) and the segregated susceptible strain (Pyr_Control) derived from the crossing of the Lab_S and Pyr_R strains, were used to evaluate the life-history characteristics using age-stage, two-sex life tables.

RESULTS

Most P. citri populations developed high resistance to pyridaben. Resistance levels exceeded 1000-fold in Yuxi, Anyue, Nanning, and Ganzhou populations compared with the Lab_S strain. Compared with Pyr_Control, two key fitness cost criteria, developmental period and fecundity, showed significant differences in Pyr_Rs under consistent conditions. The intrinsic rate of increase, net reproductive rate and gross reproductive rate were lower in the resistant strain compared with the Pyr_Control strain. The Pyr_Rs strain had a lower relative fitness of 0.934 compared with the Pyr_Control. Moreover, the life-history traits and population parameters of the Pyr_R strain also showed significant differences compared with the Lab_S strain.

CONCLUSION

The resistance levels to pyridaben varied greatly among the different P. citri populations and showed regional differences. Substantial fitness costs are associated with pyridaben resistance. This study provides potential implications for developing strategies for resistance management in P. citri. © 2022 Society of Chemical Industry.

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

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

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

BACKGROUND

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

RESULTS

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

CONCLUSION

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

Evaluation of the Impact of Different Management Methods on Tetranychus urticae (Acari: Tetranychidae) and Their Predators in Citrus Orchards

To evaluate the effectiveness of eco-friendly treatments based on detergents classified as non-hazardous and black soap on the pest Tetranychus urticae Koch 1836, and their predators (Euseius stipulatus Athias-Henriot, 1960, Typhlodromus sp., Phytoseiulus persimilis Athias-Henriot, 1957), different treatments were applied to citrus orchards planted with Valencia late (Orange) in the Mechraa Belksiri region of Morocco (T0 = control experiment; T1 = spirodiclofen 0.5 L/Ha; T2 = 125 L/Ha (5%) of black soap; T3 = detergent; 4 L/Ha of Oni product + 2 L/Ha of Tide product). The results obtained during the whole monitoring period indicated that the three treatments used, namely spirodiclofen, black soap, and detergents, ensured a reduction in the rate of population of the pest T. urticae compared to the untreated plot. In the untreated plot, the average was 45.01 A± 4.90 mobile forms, while the plot treated with spirodiclofen it was only 21.10 C ± 2.71, the black soap 31.49 B ± 3.35, and in the plot treated with detergents, the average was similar to that obtained by spirodiclofen (22.90 C ± 2.18). On the predators (E. stipulatus, P. persimilis, and Typhlodropmus sp.), the black soap and the treatment with detergents were less harmful compared to the chemical spirodiclofen.

Lethal and sublethal effects of emamectin benzoate on life-table and physiological parameters of citrus red mite, Panonychus citri

The citrus red mite (Panonychus citri) is a challenge to manage in citrus orchards due to resistance against several pesticides. There is a necessity therefore to find new pesticides for effective control of P. citri. This study was designed to evaluate the lethal and sublethal effects of emamectin benzoate against P. citri. The results showed that the LC50 of emamectin benzoate to adults of P. citri was 0.35 (0.26-0.43) mg a.i. L-1 and the LC90 was 1.44 (1.16-1.96) mg a.i. L-1. The sublethal concentration exposures (LC10 and LC30) had a significant negative impact on the larval, protonymph, and deutonymph developmental periods. Male longevity was much lower in LC30 treatments than in the controls. Although female longevity was unaffected, the fecundity (eggs per female) was decreased in the sublethal concentration treatments. Results revealed that the adult pre-oviposition period (APOP) and total pre-oviposition period (TPOP) were increased. Other growth parameters r, λ, and R0 decreased, whereas mean generation time (T) increased due to pesticide exposure. The survival rate (Sxj), age-specific fecundity and net maternity, life expectancy (Exj), and reproduction (Vxj) was reduced by LC10 and LC30 exposure. An increase in malondialdehyde (MDA) contents with increasing emamectin benzoate concentration demonstrates that emamectin benzoate induces oxidative stress in P. citri. The activity of antioxidant enzymes (superoxide dismutase, SOD and catalase, CAT) was decreased due to LC30 and LC10 treatments compared to the control. Detoxification enzyme activity (cytochrome P450, glutathione-S-transferases, GST and acetylcholinesterase, AChE) was increased in treated mites compared to the control. This study demonstrates that emamectin benzoate has both a lethal effect on citrus red mite and sublethal effects on its biology and physiology. It is, therefore, potentially an effective pesticide for management of P. citri.

Identification and the potential roles of long non-coding RNAs in regulating acetyl-CoA carboxylase ACC transcription in spirotetramat-resistant Aphis gossypii

Long non-coding RNAs (lncRNAs) represent the largest class of non-coding transcripts. They act a pivotal part in various insect developmental processes and stress responses. However, the investigation of lncRNA functions in insecticide resistant remains at an early phase. Herein, we conducted whole-transcriptome RNA sequencing for two cotton aphid (Aphis gossypii Glover) strains, i.e., insecticide-susceptible (SS) and spirotetramat-resistant (SR). We discovered 6059 lncRNAs in the RNA-Seq data, and 874 lncRNAs showed differential expression. In addition, 5 lncRNAs among 874 lncRNAs were predicted as targets of acetyl-CoA carboxylase (ACC). Reverse transcription real-time quantitative PCR (RT-qPCR) combined with RNA interference (RNAi) confirmed that selected ACC lncRNA was related to the expression of ACC. Moreover, we also identified two transcription factors, i.e., C/EBP and C/EBPzeta, that regulate the transcription level of ACC lncRNA. These results provide a good basis for the study of cotton aphid lncRNA functions in insecticide resistance development.

Whole genome sequencing and bulked segregant analysis suggest a new mechanism of amitraz resistance in the citrus red mite, Panonychus citri (Acari: Tetranychidae)

BACKGROUND

Amitraz is a broad-spectrum insecticide/acaricide for the control of aphids, psyllids, ticks and mites. Current evidence suggests that ticks and phytophagous mites have developed strong resistance to amitraz. Previous studies have shown that multiple mechanisms are associated with amitraz resistance in ticks, but very few reports have involved Panonychus citri. We therefore used whole genome sequencing and bulked segregant analysis (BSA) to identify the mechanism underlying P. citri's resistance to amitraz.

RESULTS

High-quality assembly of the whole P. citri genome was completed, resulting in a genome of approximately 83.97 Mb and a contig N50 of approximately 1.81 Mb. Gene structure predictions revealed 11 577 genes, of which 10 940 genes were annotated. Trait-associated regions in the genome were mapped with bulked segregant analysis and 38 candidate SNPs were obtained, of which T752C had the strongest correlation with the resistant trait, located at the 5' untranslated region (UTR) of the β-2R adrenergic-like octopamine receptor gene. The mutation resulted in the formation of a short hairpin loop structure in mRNA and gene expression was down-regulated by more than 50% in the amitraz-resistant strain. Validation of the T752C mutation in field populations of P. citri found that the correlation between the resistance ratio and the base mutation was 94.40%.

CONCLUSION

Our results suggest that this 5' UTR mutation of the β-2R octopamine receptor gene, confers amitraz resistance in P. citri. This discovery provides a new explanation for the mechanism of pest resistance: base mutations in the 5' untranslated region of target gene may regulate the susceptibility of pests to pesticides.

Novel hydrophobic deep eutectic solvent for vortex-assisted liquid phase microextraction of common acaricides in fruit juice followed by HPLC-UV determination

In the present research, several novel and natural hydrophobic deep eutectic solvents (DESs) were prepared using methyltrioctylammonium chloride (MTOAC) as the hydrogen bond acceptor (HBA) and different types of straight chain alcohols as hydrogen bond donors (HBDs). One of the DESs composed of MTOAC and n-butanol was advantageously used to develop a vortex-assisted liquid phase microextraction (VALPME) method combined with high-performance liquid chromatography-ultraviolet detection (HPLC-UV) for the determination of common acaricides in fruit juice samples. Several important parameters influencing extraction efficiency were investigated and optimized, including the type and volume of DES, sample solution pH, effect of salt addition and, extraction and vortex time. Under optimal experimental conditions, the method showed good linearity with the correlation coefficients (R²) of 0.9986–0.9991 in the linear range of 2–300 μg L⁻¹, low limits of detection of 0.5–1 μg L⁻¹ and acceptable extraction recoveries in the range of 85–93%. The proposed method was successfully applied for the extraction and preconcentration of trace acaricides in real fruit juice samples, and the results demonstrated the potential of the synthesized DESs for the extraction and determination of contaminants in aqueous samples.

In this research, several novel hydrophobic deep eutectic solvents (DESs) were prepared using methyltrioctylammonium chloride (MTOAC) as the hydrogen bond acceptor (HBA) and different types of straight chain alcohols as hydrogen bond donors (HBDs).

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

BACKGROUND

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

RESULTS

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

CONCLUSION

Although variations in resistance to spirodiclofen were observed between populations, the resistance ratio was low to moderate. Such data can be useful in the development of resistance management strategies for B. yothersi in Brazilian citrus groves. © 2021 Society of Chemical Industry.

Spirodiclofen ether derivatives: semisynthesis, structural elucidation, and pesticidal activities against Tetranychus cinnabarinus Boisduval, Aphis citricola Van der Goot and Mythimna separata Walker

BACKGROUND

Spirodiclofen is a spirocyclic tetronic acid-type acaricidal agent. Nowadays, serious pests resistance to spirodiclofen and cross-resistance to other acaricides has appeared. To overcome pests resistance and discover new potential agrochemicals, a series of ether derivatives were prepared based on spirodiclofen as a lead compound. Their pesticidal activities were investigated against three typically agricultural pests, Mythimna separata Walker, Aphis citricola Van der Goot and Tetranychus cinnabarinus Boisduval.

RESULTS

Four steric structures of compounds 5e, 5f, 5i and 5j were determined by single-crystal X-ray diffraction. Against T. cinnabarinus, compounds 5b, 5f and 5l exhibited potent acaricidal activity, and their good control effects in the glasshouse were observed when compared with spirodiclofen, especially the control efficiency of compound 5b was comparable to that of spirodiclofen; against M. separata, compound 5j showed > 1.8-fold potent insecticidal activity of spirodiclofen; against A. citricola, compounds 5d and 5j displayed > 2.0-fold potent aphicidal activity of spirodiclofen. The relationships between their structures and agricultural activities were also discussed.

CONCLUSION

Compounds 5b and 5d could be further studied as acaricidal and aphicidal agents, respectively; compound 5j can be considered as a lead compound for the insecticidal and aphicidal activities. This will pave the way for future application of these derivatives as pesticide substitutes for spirodiclofen. © 2021 Society of Chemical Industry.

Behavioral response of Panonychus citri (McGregor) (Acari: Tetranychidae) to synthetic chemicals and oils

Background

Panonychus citri (McGregor) (Acari: Tetranychidae) population outbreaks after the citrus plantation's chemical application is a common observation. Dispersal behavior is an essential tool to understand the secondary outbreak of P. citri population. Therefore, in the current study, the dispersal activity of P. citri was observed on the leaf surfaces of Citrus reticulata (Rutaceae) treated with SYP-9625, abamectin, vegetable oil, and EnSpray 99.

Method

Mites were released on the first (apex) leaf of the plant (adaxial surface) and data were recorded after 24 h. The treated, untreated, and half-treated data were analyzed by combining the leaf surfaces (adaxial right, adaxial left, abaxial right, and abaxial left). All experiments were performed in open-air environmental conditions.

Results

The maximum number of mites was captured on the un-treated or half-treated surfaces due to chemicals repellency. Chemical bioassays of the free-choice test showed that all treatments significantly increased the mortality of P. citri depending on application method and concentration. A significant number of mites repelled away from treated surfaces and within treated surfaces except adaxial left and abaxial right surfaces at LC₃₀. In the no-choice test, SYP-9625 gave maximum mortality and dispersal by oils than others. No significant differences were observed within the adaxial and abaxial except abaxial surface at LC₃₀. Therefore, the presence of tested acaricides interferes with P. citri dispersal within leaf surfaces of plantations depending on the mites released point and a preferred site for feeding.

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

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

RNA Interference-Based Silencing of the Chitin Synthase 1 Gene for Reproductive and Developmental Disruptions in Panonychus citri

Simple Summary

The Chitin Synthase 1 gene, when suppressed with RNAi, imparts differences in the structural development, physiology, and synthesis of epidermal chitin, which ultimately leads to the mortality of the target pest. The results presented here will help to illuminate the molecular mechanism and function of the PcCHS1 gene, which regulates the egg-laying potential in adult citrus red mites. Using the leaf dip method, we found that dsRNA was potent and effective, and significantly reduced the egg-laying potential and hatching of citrus red mite eggs. These results show the potential utility of the PcCHS1 gene in the development of novel RNA interference strategies for controlling the citrus red mite.

Abstract

Chitin synthase 1 (CHS1) is an essential gene regulating chitin during different developmental stages of arthropods. In the current study, we explored for the first time the role of CHS1 gene regulation in the citrus red mite, Panonychus citri (McGregor) (Acari: Tetranychidae), by silencing its expression using (RNA interference) RNAi-based strategies. The results reveal that P. citri tested in different developmental stages, including larvae, protonymphs, deutonymphs, and adults fed on sweet orange leaves dipped in various concentrations (200, 400, 600, and 800 ng/μL) of dsRNA-PcCHS1, resulted in a continuous reduction in their gene expression, and the extent of transcript knockdown was positively correlated with the concentration of dsRNA. Concentration–mortality response assays revealed a mortality of more than 50% among all the studied developmental stages, except for adulthood. Furthermore, the target gene dsRNA-PcCHS1 treatment of larvae, protonymphs, deutonymphs, and females at a treatment rate of 800 ng/mL of dsRNA significantly decreased the egg-laying rates by 48.50%, 43.79%, 54%, and 39%, respectively, and the hatching rates were also considerably reduced by 64.70%, 70%, 64%, and 52.90%, respectively. Moreover, using the leaf dip method, we found that the RNA interference effectively reduced the PcCHS1 transcript levels by 42.50% and 42.06% in the eggs and adults, respectively. The results of this study demonstrate that the RNAi of PcCHS1 can dramatically reduce the survival and fecundity of P. citri, but the dsRNA concentrations and developmental stages can significantly influence the RNAi effects. These findings indicate the potential utility of the PcCHS1 gene in causing developmental irregularities, which could aid in the development of effective and novel RNAi-based strategies for controlling P. citri.

Comparative Transcriptome Analysis Reveals the Mechanism Related to Fluazinam Stress of Panonychus citri (Acarina: Tetranychidae)

Simple Summary

The citrus red mite, Panonychus citri, is an important pest that causes serious citrus production losses in China. The insecticide fluazinam has a good control effect on the pest mites; however, its mechanism of action on mites remains unclear. In this study, we analyzed the transcriptomic sequencing and differential expression genes in P. citri treated with fluazinam, and identified some of the genes potential involved in detoxification metabolism related with the fluazinam exposure. Evaluating the efficacy of fluazinam, and analyzing the transcriptome data of P. citri under fluazinam stress, potentially provide a new agent for prevention and control of P. citri, and also preliminary research results for exploring the mechanism of action of fluazinam on P. citri. Given the up-regulated expression levels of genes for Mn-superoxide dismutase and catalase, we speculate that they play an important role in fluazinam-stress action on P. citri.

Abstract

The use of a large number of chemical acaricides to control these pest mites has led to an increasing problem of pesticide resistance, which has always been the difficulty in integrated pest management (IPM). Fluazinam has a good control effect on Panonychus citri, the serious pest on citrus; however, we only know the mechanism of action of fluazinam as a fungicide and its mechanism of action on mites remains unclear. Through analysis using Illumina high-throughput transcriptomic sequencing and differential expression genes in P. citri treated with fluazinam, 59 cytochrome P450 genes, 23 glutathione s-transferase genes, five carboxylate esterase genes, 11 superoxide dismutase genes and 15 catalase genes were identified. The Gene Ontology enrichment and the enrichment of KEGG results showed that the treatment were enrichment for redox enzyme pathways. Evaluating the efficacy of fluazinam, and analyzing the transcriptome data of P. citri under fluazinam stress, potentially provide a new agent for prevention and control of P. citri, and also preliminary research results for exploring the mechanism of action of fluazinam on P. citri. Given the up-regulated expression levels of genes for Mn-superoxide dismutase and catalase, we speculate that they play an important role in fluazinam-stress action on P. citri.

Monitoring the Resistance of the Citrus Red Mite (Acari: Tetranychidae) to Four Acaricides in Different Citrus Orchards in China

The citrus red mite, Panonychus citri (McGregor), is an important spider mite pest in citrus producing areas. Owing to long-term acaricide exposure, resistance has evolved rapidly in recent years. To evaluate the extent of resistance, seven field mite populations sampled from various geographical locations in China during 2015-2018 were tested using the leaf-dip bioassay method to determine their susceptibilities to four acaricides. In comparison with the susceptible strain maintained in the laboratory, low or moderate levels of fenpropathrin resistance, while no resistance to abamectin or cyflumetofen, were found among populations sampled from Liangping, Wanzhou, Daying, and Anyue in Southwestern China during the test period. High levels (>1,000-fold, with LC50 values that were greater than the recommended concentration) of resistance to fenpropathrin had evolved in field populations from Southern China, including Guilin, Nanning, and Yuxi, when compared with that of the susceptible strain. Populations from Guilin and Nanning also evolved high resistance levels to abamectin (1,088-fold and 1,401-fold) and cyflumetofen (2,112-fold and 9,093-fold). All the populations sampled in 2018 showed a moderate or high resistance to bifenazate. Generally, field populations of citrus red mites from Southwestern China were more sensitive to the tested acaricides than those of Southern China. The data provide a foundation for developing acaricide resistance management strategies in these regions.

Humic acid functionalized hyperbranched polytriazine based dispersive solid-phase extraction for acaricides determination in tea matrix

Hyperbranched polytriazine functionalized with humic acid was prepared and developed as new sorbents for dispersive solid-phase extraction of three acaricides (clofentezine, fenpyroximate, and pyridaben) in tea samples combined with high-performance liquid chromatography detection. The sorbents were characterized by scanning electron microscopy, energy dispersive spectroscopy, Zeta-potential, and Fourier transform infrared spectroscopy. The extraction parameters (extraction time, ionic strength, desorption conditions) were optimized. The adsorption mechanism was evaluated utilizing Fourier transform infrared spectra. Under optimum conditions, satisfactory analytical performances were achieved, which included high precision (1.33-9.62%), low limits of detection (0.19-3.54 µg/L), and wide linear range (2.5-500 µg/L) for the analysis of the acaricides. Moreover, the proposed method proved highly effective for the determination of acaricides in tea samples, with the relative recoveries in the range of 65.20-108.13% and relative standard deviations < 9.87%. The method has great application potential for the detection of acaricides in tea samples.

Effects of spirodiclofen on life history traits and population growth of a spider mite predator Oligota flavicornis (Coleoptera: Staphyllinidae) based on the age-stage two-sex life table theory

BACKGROUND

Knowledge of the compatibility between spirodiclofen and the predator Oligota flavicornis is an important aspect for the management of spider mites.

RESULTS

We used the age-stage, two-sex life table to assess the effects of spirodiclofen on the life history traits and population growth of O. flavicornis. At the maximum recommended concentration (60 mg a.i. L^-1 ) and also at twice the maximum recommended dosage (120 mg a.i. L^-1 ), the preadult stages of O. flavicornis were significantly lengthened, while the adult longevity and fecundity decreased significantly. The finite rate (λ), intrinsic rate of increase (r), and net reproduction rate (R₀ ) decreased, while the mean generation time (T) was longer after both the 60 and 120 mg a.i. L^-1 treatments than it was in the control and 30 mg a.i. L^-1 treatments. Life expectancy and reproductive value were higher in the control and 30 mg a.i. L^-1 treatment than in the 60 and 120 mg a.i. L^-1 treatments; the two higher concentrations were detrimental to the development of O. flavicornis.

CONCLUSION

A proper combination of the O. flavicornis and spirodiclofen to control the spider mite, while avoiding the side effect of spirodiclofen, could be achieved based on the knowledge of life tables. © 2018 Society of Chemical Industry.

Identification of Dialeurodes citri as a Vector of Citrus yellow vein clearing virus in China

In 2009, a new citrus viral disease caused by Citrus yellow vein clearing virus (CYVCV) was first discovered in China and now CYVCV is widely distributed in the field. CYVCV is transmissible by grafting and is spread by aphids from lemon to bean, and from bean to bean. However, until now, no vector has been shown to transmit CYVCV from citrus to citrus. In this study, after a 24-h acquisition access period (AAP), CYVCV was tested for in Dialeurodes citri (Ashmead), Panonychus citri McGregor, and Aphis citricidus (Kirkaldy) by quantitative RT-PCR. After an AAP of 48 h, groups of adults of D. citri, P. citri, and A. citricidus were given a 48 h inoculation access period on cultivar Daidai sour orange seedlings. Three, 6, and 12 months post-transmission by D. citri, CYVCV was detected in the receptor plants, and the mean incidence of infected trees was 31.9, 39.1, and 39.1%, respectively. CYVCV was not transmitted to citrus by P. citri or A. citricidus. This is the first report of the ability of D. citri to transmit CYVCV from infected to healthy citrus under laboratory conditions.

Lethal and sublethal effects of pesticides used in Brazilian citrus groves on Panonychus citri (Acari: Tetranychidae)

ABSTRACT: Pesticides have been blamed as the principal factor responsible for biological disequilibrium favoring the population increase of Panonychus citri (McGregor) in Brazilian citrus groves. In order to generate subsidies for the integrated management of this pest, we evaluated the lethal and sublethal effects of pesticides used in citrus on P. citri. We tested 18 pesticides by evaluating mortality of adult female, egg viability and adult avoidance. For imidacloprid, pyriproxyfen, dimethoate, deltamethrin, lambda-cyhalothrin and lime sulfur, the principal insecticides used in Brazilian orchards, the oviposition behavior and the incidence of adult females on discontinuous pesticide residues were evaluated. The pesticides which caused adult P. citri mortality were: abamectin (94%), dimethoate (86%), lime sulfur (69%), fenpropathrin (44%), diafenthiuron (25%) and lambda-cyhalothrin (23%). The pesticides affecting egg viability were: fenpropathrin (53%), dimethoate (30%) and lime sulfur (22%). Fenpropathrin, lambda-cyhalothrin and lime sulfur caused repellent effects of 55, 45 and 22%, respectively. P. citri preferred areas untreated with deltamethrin, lambda-cyhalothrin and lime sulfur for oviposition and permanence, but the species could not distinguish areas treated with imidacloprid, pyriproxyfen and dimethoate. The insecticides abamectin, dimethoate and diafenthiuron affected survival and did not cause changes in P. citri behavior. These pesticides should be used mainly in periods P. citri occurs. On the other hand, fenpropathrin, lambda-cyhalothrin, lime sulfur and deltamethrin affect dispersal behavior, oviposition and P. citri incidence and their use should be avoided.

Synthesis and Acaricidal Activities of Scopoletin Phenolic Ether Derivatives: QSAR, Molecular Docking Study and in Silico ADME Predictions

Thirty phenolic ether derivatives of scopoletin modified at the 7-hydroxy position were synthesized, and their structures were confirmed by IR, ¹H-NMR, 13C-NMR, MS and elemental analysis. Preliminary acaricidal activities of these compounds against female adults of Tetranychus cinnabarinus (Boisduval) were evaluated using the slide-dip method. The results indicated that some of these compounds exhibit more pronounced acaricidal activity than scopoletin, especially compounds 32, 20, 28, 27 and 8 which exhibited about 8.41-, 7.32-, 7.23-, 6.76-, and 6.65-fold higher acaricidal potency. Compound 32 possessed the the most promising acaricidal activity and exhibited about 1.45-fold higher acaricidal potency against T. cinnabarinus than propargite. Statistically significant 2D-QSAR model supports the observed acaricidal activities and reveals that polarizability (HATS5p) was the most important parameter controlling bioactivity. 3D-QSAR (CoMFA: q² = 0.802, r² = 0.993; CoMSIA: q² = 0.735, r² = 0.965) results show that bulky substituents at R₄, R₁, R₂ and R₅ (C₆, C₃, C₄, and C₇) positions, electron positive groups at R₅ (C₇) position, hydrophobic groups at R₁ (C₃) and R₂ (C₄), H-bond donors groups at R₁ (C₃) and R₄ (C₆) will increase their acaricidal activity, which provide a good insight into the molecular features relevant to the acaricidal activity for further designing novel acaricidal agents. Molecular docking demonstrates that these selected derivatives display different bide modes with TcPMCA1 from lead compound and they interact with more key amino acid residues than scopoletin. In silico ADME properties of scopoletin and its phenolic ether derivatives were also analyzed and showed potential to develop as good acaricidal candidates.

Antioxidant Role of PcGSTd1 in Fenpropathrin Resistant Population of the Citrus Red Mite, Panonychus citri (McGregor)

The citrus red mite, Panonychus citri, a major citrus pest distributed worldwide, has evolved severe resistance to various classes of chemical acaricides/insecticides including pyrethroids. It is well known that the resistance to pyrethroids is mainly caused by point mutations of voltage-gated sodium channel gene in a wide range of pests. However, increasing number of evidences support that pyrethroids resistance might also be resulted from the integrated mechanisms including metabolic mechanisms. In this study, firstly, comparative analysis of RNA-seq data showed that multiple detoxification genes, including a GSTs gene PcGSTd1, were up-regulated in a fenpropathrin-resistant population compared with the susceptible strain (SS). Quantitative real time-PCR results showed that the exposure of fenpropathrin had an induction effect on the transcription of PcGSTd1 in a time-dependent manner. In vitro inhibition and metabolic assay of recombinant PcGSTd1 found that fenpropathrin might not be metabolized directly by this protein. However, its antioxidant role in alleviating the oxidative stress caused by fenpropathrin was demonstrated via the reversely genetic experiment. Our results provide a list of candidate genes which may contribute to a multiple metabolic mechanisms implicated in the evolution of fenpropathrin resistance in the field population of P. citri. Furthermore, during the detoxification process, PcGSTd1 plays an antioxidant role by detoxifying lipid peroxidation products induced by fenpropathrin.

Biofunctional analysis of Vitellogenin and Vitellogenin receptor in citrus red mites, Panonychus citri by RNA interference

Panonychus citri is one of the most damaging pests of horticultural crops. Conventional control of this pest population through pesticides has led to the enhanced pest resistance. Management of P. citri population through RNAi, is still largely unknown. In oviparous organisms, fabrication and development of yolk protein play a vital role in the reproduction. Vitellin (Vn) is the source of eggs storage that helps in proper functioning of Vitellogenin (Vg) and Vitellogenin receptor (VgR). VgR is very compulsory protein for the development of Vg into oocytes. In the current study, Vg (PcVg) and VgR (PcVgR) genes were studied and their expressions at different developmental stages were quantified by RT-qPCR. Females treated with dsRNA of PcVg and PcVgR genes exhibited reduction in gene expression. Down regulation of target genes significantly effected oviposition and reduced the egg laying capacity up to 48% as compared to control (ds-egfp). Synergistic effect of target gene’s dsRNA was also accessed that reduced the egg laying up to 60.42%. Furthermore, combination of target dsRNA on deutonymph and protonymph also resulted in 67% and 70% reduction in eggs, respectively. Synergistic effect of dsRNA at 1000 ng/ul resulted in longer life span as compared to control treatments. This study suggests to develop a new strategy of P. citri population control by reducing its reproduction.

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

BACKGROUND

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

RESULTS

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

CONCLUSION

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

Abamectin treatment affects glutamate decarboxylase expression and induces higher GABA levels in the citrus red mite, Panonychus citri

The citrus red mite, Panonychus citri, is one of the most economically and globally destructive mite pests of citrus. Acaricide resistance has been a growing problem in controlling this pest. As the main inhibitory neurotransmitter in organisms, γ-aminobutyric acid (GABA) is synthesized from the amino acid glutamate by the action of glutamate decarboxylases (GADs). In the present study, one novel GAD gene, PcGAD, was identified and characterized from P. citri. The opening reading frame of PcGAD contained 1548 nucleotides that encode 515 amino acids. The subsequent spatiotemporal expression pattern by RT-qPCR revealed that the expression levels of PcGAD were significantly higher in larvae than in adults. Challenging with various concentrations of abamectin resulted in the upregulation of PcGAD transcript levels. Furthermore, biochemical characterization indicated that changes in GAD activity coincided with its mRNA levels. High-performance liquid chromatography confirmed that the GABA contents of P. citri increased upon abamectin treatment. The application of abamectin induces PcGAD expression and activates GAD activity, thereby resulting in an increase in GABA content in P. citri, which contributes to the adaptability of the mite to abamectin challenge.

3D-QSAR and Molecular Docking Studies on the TcPMCA1-Mediated Detoxification of Scopoletin and Coumarin Derivatives

The carmine spider mite, Tetranychus cinnabarinus (Boisduval), is an economically important agricultural pest that is difficult to prevent and control. Scopoletin is a botanical coumarin derivative that targets Ca2+-ATPase to exert a strong acaricidal effect on carmine spider mites. In this study, the full-length cDNA sequence of a plasma membrane Ca2+-ATPase 1 gene (TcPMCA1) was cloned. The sequence contains an open reading frame of 3750 bp and encodes a putative protein of 1249 amino acids. The effects of scopoletin on TcPMCA1 expression were investigated. TcPMCA1 was significantly upregulated after it was exposed to 10%, 30%, and 50% of the lethal concentration of scopoletin. Homology modeling, molecular docking, and three-dimensional quantitative structure-activity relationships were then studied to explore the relationship between scopoletin structure and TcPMCA1-inhibiting activity of scopoletin and other 30 coumarin derivatives. Results showed that scopoletin inserts into the binding cavity and interacts with amino acid residues at the binding site of the TcPMCA1 protein through the driving forces of hydrogen bonds. Furthermore, CoMFA (comparative molecular field analysis)- and CoMSIA (comparative molecular similarity index analysis)-derived models showed that the steric and H-bond fields of these compounds exert important influences on the activities of the coumarin compounds.Notably, the C3, C6, and C7 positions in the skeletal structure of the coumarins are the most suitable active sites. This work provides insights into the mechanism underlying the interaction of scopoletin with TcPMCA1. The present results can improve the understanding on plasma membrane Ca2+-ATPase-mediated (PMCA-mediated) detoxification of scopoletin and coumarin derivatives in T. cinnabarinus, as well as provide valuable information for the design of novel PMCA-inhibiting acaricides.

Spirotetramat Resistance Selected in the Phenacoccus solenopsis (Homoptera: Pseudococcidae): Cross-Resistance Patterns, Stability, and Fitness Costs Analysis

The Phenacoccus solenopsis Tinsley (Homoptera: Pseudococcidae) is a major agricultural and horticultural pest of crops throughout the world. To develop a better resistance management strategy for P. solenopsis, we conducted a study on life history parameters of different populations of this pest, one selected with spirotetramat (Spiro-SEL), an unselected (UNSEL) population, and their reciprocal crosses. We also studied the cross-resistance and the stability of spirotetramat resistance. The Spiro-SEL of P. solenopsis exhibited a 328.69-fold resistance compared to the susceptible population (Lab-PK). The Spiro-SEL population also displayed a moderate level of cross-resistance to profenofos and bifenthrin and a high level of cross-resistance to abamectin. Resistance to spirotetramat in Spiro-SEL was unstable in the absence of selection. The study of life history parameters showed that there was a significant reduction in fitness parameters of Spiro-SEL population with a relative fitness value of 0.14. There was a significant decrease in survival rate, pupal weight, fecundity, egg hatching percentage, male and female generation time, intrinsic rate of population increase of males and females, biotic potential, and mean relative growth rate. It is concluded that selection with spirotetramat had marked effect on resistance development in P. solenopsis and upon removal of selection pressure spirotetramat resistance declined significantly, indicating unstable resistance. Development of resistance led to high fitness costs for the spirotetramat-selected population. Our study may provide the basic information on spirotetramat resistance and its mechanism to help develop the resistance management strategies.

Identification of responsive proteins in Panonychus citri exposed to abamectin by a proteomic approach

Abamectin is a microbial-derived pesticide widely used for control of agricultural pests. However, sustained use of abamectin has led to the development of resistance in some target species. Previous studies on arthropod resistance to abamectin have mainly used traditional biochemical and molecular approaches. To understand the responses of citrus red mite, Panonychus citri, exposed to abamectin, comparative proteomic analysis was conducted using two-dimensional electrophoresis (2-DE). A total of 26 distinct protein spots were present in response to abamectin exposure. Tandem mass spectrometry (MS/MS) identified 16 proteins that were mainly involved in energy metabolism and detoxification. Some remaining proteins were not identifiable, suggesting that they may be novel. The expression levels of transcripts associated with proteins were analyzed by quantitative reverse transcription PCR (qRT-PCR). Furthermore, to validate the proteomic data obtained in the present study, Western-blot experiment was performed and the expression of sHsp and PcE1 proteins were confirmed, respectively.

BIOLOGICAL SIGNIFICANCE

The citrus red mite has developed resistance to many acaricides, including abamectin. In the current study, we used the proteomic approaches involving 2-DE, matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF), and MS/MS to document changes in adult P. citri during 24h of abamectin exposure. Abamectin stress induced a total of 16 differentially regulated proteins. The proteomic results were validated in mRNA expression patterns using qRT-PCR. This is the first analysis of differentially expressed proteins in P. citri exposed to abamectin. The results help clarify the physiological mechanisms of P. citri responses to abamectin exposure.

Functional analysis of a chitinase gene during the larval-nymph transition in Panonychus citri by RNA interference

Chitinases are hydrolytic enzymes that are required for chitin degradation and reconstruction in arthropods. In this study, we report a cDNA sequence encoding a putative chitinase (PcCht1) from the citrus red mite, Panonychus citri. The PcCht1 (564 aa) possessed a signal peptide, a conserver domain, and a chitin-binding domain. Structural and phylogenetic analyses found that PcCht1 had high sequence similarity to chitinases in Tetranychus urticae. Real-time quantitative PCR analyses showed that the transcript levels of PcCht1 peaked periodically in larval and nymph stages. Moreover, significant increase of PcCht1 transcript level in the larvae was observed upon the exposure of diflubenzuron. In contrast, exposures of the larvae to diflubenzuron resulted in the decreased chitin content. Furthermore, through a feeding-based RNA interference approach, we were able to reduce the PcCht1 transcript level by 59.7 % in the larvae, and consequently the treated larvae showed a very low molting rate compared with the control. Our results expanded the understanding of the important role of PcCht1 in the growth and development of P. citri.

Characterization and functional analysis of a novel glutathione S-transferase gene potentially associated with the abamectin resistance in Panonychus citri (McGregor)

The citrus red mite, Panonychus citri (McGregor), a major citrus pest distributed worldwide, has been found to be resistant to various insecticides and acaricides used in China. However, the molecular mechanisms associated with the abamectin resistance in this species have not yet been reported. In this study, results showed over-expression of a novel glutathione S-transferases (GSTs) gene (PcGSTm5) in abamectin-resistant P. citri. Quantitative real-time PCR analysis showed that the transcripts of PcGSTm5 were also significantly up-regulated after exposure to abamectin and the maximum mRNA expression level at nymphal stage. The recombinant protein of PcGSTm5-pET-28a produced by Escherichia coli showed a pronounced activity toward the conjugates of 1-chloro-2,4 dinitrobenzene (CDNB) and glutathione (GSH). The kinetics of CDNB and GSH and its optimal pH and thermal stability were also determined. Reverse genetic study through a new method of leaf-mediated dsRNA feeding further support a link between the expression of PcGSTm5 and abamectin resistance. However, no direct evidence was found in metabolism or inhibition assays to confirm the hypothesis that PcGSTm5 can metabolize abamectin. Finally, it is here speculated that PcGSTm5 may play a role in abamectin detoxification through other pathway such as the antioxidant protection.

Involvement of Three Esterase Genes from Panonychus citri (McGregor) in Fenpropathrin Resistance

The citrus red mite, Panonychus citri (McGregor), is a major citrus pest with a worldwide distribution and an extensive record of pesticide resistance. However, the underlying molecular mechanism associated with fenpropathrin resistance in this species have not yet been reported. In this study, synergist triphenyl phosphate (TPP) dramatically increased the toxicity of fenpropathrin, suggesting involvement of carboxylesterases (CarEs) in the metabolic detoxification of this insecticide. The subsequent spatiotemporal expression pattern analysis of PcE1, PcE7 and PcE9 showed that three CarEs genes were all over-expressed after insecticide exposure and higher transcripts levels were observed in different field resistant strains of P. citri. Heterologous expression combined with 3-(4,5-dimethyl-thiazol-2-yl)-2,5-diphenyltetra-zolium bromide (MTT) cytotoxicity assay in Spodoptera frugiperda (Sf9) cells revealed that PcE1-, PcE7- or PcE9-expressing cells showed significantly higher cytoprotective capability than parental Sf9 cells against fenpropathrin, demonstrating that PcEs probably detoxify fenpropathrin. Moreover, gene silencing through the method of leaf-mediated dsRNA feeding followed by insecticide bioassay increased the mortalities of fenpropathrin-treated mites by 31% (PcE1), 27% (PcE7) and 22% (PcE9), respectively, after individual PcE gene dsRNA treatment. In conclusion, this study provides evidence that PcE1, PcE7 and PcE9 are functional genes mediated in fenpropathrin resistance in P. citri and enrich molecular understanding of CarEs during the resistance development of the mite.

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

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

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

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

Spirotetramat resistance adaption analysis of Aphis gossypii Glover by transcriptomic survey

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

Regulation of three isoforms of SOD gene by environmental stresses in citrus red mite, Panonychus citri

Superoxide dismutase (SOD) is a family of enzymes with multiple isoforms that possess antioxidative abilities in response to environmental stresses. Panonychus citri is one of the most important pest mites and has a global distribution. In this study, three distinct isoforms of SOD were cloned from P. citri and identified as cytoplasmic Cu-ZnSOD (PcSOD1), extracellular Cu-ZnSOD (PcSOD2), and mitochondrial MnSOD (PcSOD3). mRNA expression level analysis showed that all three isoforms were up-regulated significantly after exposure to the acaricide abamectin and to UV-B ultraviolet irradiation. In particular, PcSOD3 was up-regulated under almost all environmental stresses tested. The fold change of PcSOD3 expression was significantly higher than those of the two Cu-ZnSOD isoforms. Taken together, the results indicate that abamectin and UV-B can induce transcripts of all three SOD isoforms in P. citri. Furthermore, PcSOD3 seems to play a more important role in P. citri tolerance to oxidative stress.

Characterization of cell-free extracts from fenpropathrin-degrading strain Bacillus cereus ZH-3 and its potential for bioremediation of pyrethroid-contaminated soils

Synthetic pyrethroid fenpropathrin has received increasing attention because of its environmental contamination and toxic effects on non-target organisms including human beings. Here we report the degradation characteristics of cell-free extracts from fenpropathrin-degrading strain Bacillus cereus ZH-3 and its potential for pyrethroid bioremediation in soils. 50mg·L(-1) of fenpropathrin was decreased to 20.6mg·L(-1) by the enzymatic extracts (869.4mg·L(-1)) within 30min. Kinetic constants Km and Vm were determined to be 1006.7nmol·L(-1) and 56.8nmol·min(-1), respectively. Degradation products were identified as 3-phenoxybenzaldehyde, α-hydroxy-3-phenoxy-benzeneacetonitrile and phenol by gas chromatography-mass spectrometry (GC-MS). In addition to degradation of fenpropathrin, the cell-free extracts could degrade other pyrethroids including beta-cypermethrin, cyfluthrin, deltamethrin and cypermethrin. Additionally, the reaction conditions were optimized. In the sterile and non-sterile soils, 50mg·kg(-1) of fenpropathrin was reduced to 15.3 and 13.9mg·L(-1) in 1d, respectively. Sprayed 100 and 300mg·kg(-1) of fenpropathrin emulsifiable concentrate (EC), up to 84.6% and 92.1% of soil fenpropathrin were removed from soils within 7d, respectively. Taken together, our results depict the biodegradation characteristics of cell-free extracts from B. cereus ZH-3, highlight its promising potential in bioremediation of pyrethroid-contaminated soils and also provide new insights into the utilization of degrading microbes.

Molecular characterisation of a sodium channel gene and identification of a Phe1538 to Ile mutation in citrus red mite, Panonychus citri

BACKGROUND

The citrus red mite, Panonychus citri (McGregor), is regarded as one of the most serious citrus pests in many countries and has developed high resistance to pyrethroids as a result of the intensive use of these acaricides.

RESULTS

The para sodium channel gene of P. citri (named PcNav ), containing an entire coding region of 6729 bp, was cloned in this study. Three alternative splicing sites and 12 potential RNA editing sites were identified in PcNav . Thus, exons alt 1 and alt 3-v3 were found to be unique to PcNav . Comparison of field fenpropathrin-resistant (WZ) and susceptible (LS) strains identified the point mutation F1538I in IIIS6 of the sodium channel, which is known to confer strong resistance to pyrethroids in mites. Moreover, it was also found that the PcNav mRNA was present during all life stages, and the transcript seems to be more abundant in larvae than in other developmental stages.

CONCLUSION

These results suggest that the F1538I mutation plays an important role in fenpropathrin resistance in citrus red mites. This is the first study of the sodium channel in P. citri and provides abundant information for further research on the mechanism of pyrethroid resistance.

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

BACKGROUND

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

RESULTS

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

CONCLUSIONS

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

Comparative toxicity of an acetogenin-based extract and commercial pesticides against citrus red mite

Acetogenins, a class of natural compounds produced by some Annonaceae species, are potent inhibitors of mitochondrial electron transport systems. Although the cellular respiration processes are an important biochemical site for the acaricidal action of compounds, few studies have been performed to assess the bioactivity of acetogenin-based biopesticides on spider mites, mainly against species that occur in orchards. Using residual contact bioassays, this study aimed to evaluate the bioactivity of an ethanolic extract from Annona mucosa seeds (ESAM) (Annonaceae) against the citrus red mite Panonychus citri (McGregor) (Acari: Tetranychidae), an important pest of the Brazilian citriculture. ESAM is a homemade biopesticide which was previously characterized by its high concentration of acetogenins. It caused both high mortality of P. citri females (LC50 = 7,295, 4,662, 3,463, and 2,608 mg l(-1), after 48, 72, 96, and 120 h of exposure, respectively) and significant oviposition deterrence (EC50 = 3.194,80 mg l(-1)). However, there was no effect on P. citri female fertility (hatching rate). In addition, the ESAM efficacy (in terms of its LC90) was compared with commercial acaricides/insecticides (at its recommended rate) of both natural [Anosom(®) 1 EC (annonin), Derisom(®) 2 EC (karanjin), and Azamax(®) 1.2 EC (azadirachtin + 3-tigloylazadirachtol)] and synthetic origin [Envidor(®) 24 SC (spirodiclofen)]. Based on all of the analyzed variables, the ESAM exhibited levels of activity superior to other botanical commercial acaricides and similar to spirodiclofen. Thus, our results indicate that ESAM may constitute a biorational acaricide for citrus red mite integrated pest management in Brazilian citrus orchards, particularly for local use.

Exposure to diflubenzuron results in an up-regulation of a chitin synthase 1 gene in citrus red mite, Panonychus citri (Acari: Tetranychidae)

Chitin synthase synthesizes chitin, which is critical for the arthropod exoskeleton. In this study, we cloned the cDNA sequences of a chitin synthase 1 gene, PcCHS1, in the citrus red mite, Panonychus citri (McGregor), which is one of the most economically important pests of citrus worldwide. The full-length cDNA of PcCHS1 contains an open reading frame of 4605 bp of nucleotides, which encodes a protein of 1535 amino acid residues with a predicted molecular mass of 175.0 kDa. A phylogenetic analysis showed that PcCHS1 was most closely related to CHS1 from Tetranychus urticae. During P. citri development, PcCHS1 was constantly expressed in all stages but highly expressed in the egg stage (114.8-fold higher than in the adult). When larvae were exposed to diflubenzuron (DFB) for 6 h, the mite had a significantly high mortality rate, and the mRNA expression levels of PcCHS1 were significantly enhanced. These results indicate a promising use of DFB to control P. citri, by possibly acting as an inhibitor in chitin synthesis as indicated by the up-regulation of PcCHS1 after exposure to DFB.

Identification and characterization of seven glutathione S-transferase genes from citrus red mite, Panonychus citri (McGregor)

The citrus red mite, Panonychus citri (McGregor), is a global citrus pest, and has developed severe resistance to several types of acaricides. However, the molecular mechanisms of resistance in this mite remain unknown. In this study, seven full-length cDNAs encoding glutathione S-transferases (GSTs) genes were identified and characterized in P. citri. The effects of pyridaben and fenpropathrin exposure on the expression of these genes were also investigated. Phylogenetic analysis revealed that the seven GSTs genes in P. citri cloned in this study belong to three different cytosolic classes, including four in mu, two in delta and one in zeta. Among these seven GSTs genes, the relative expression level of PcGSTm1 was significantly higher in adult than in the other life stages (egg, larvae and nymph). Compared with the control, the mRNA levels of the seven GST genes did not change significantly following exposure to pyridaben at LC10. However, RT-qPCR results showed that, when exposed to LC10 of fenpropathrin, six GSTs gene (PcGSTm1, PcGSTm3, PcGSTm4, PcGSTd1, PcGSTd2 and PcGSTz1) transcripts increased in a time-dependent manner. This is the first insight into the molecular characteristics of GSTs gene cDNAs in P. citri. The elevated GSTs gene transcripts following exposure to fenpropathrin might be one of the mechanisms involved in detoxification of this acaricide.

Molecular characterization of two carboxylesterase genes of the citrus red mite, Panonychus citri (Acari: Tetranychidae)

The citrus red mite, Panonychus citri, is known for its ability to rapidly evolve resistance to insecticides/acaricides and to adapt to hosts that produce toxins. To get better insight into the detoxification mechanism of P. citri, two carboxylesterase (CarE) genes, PCE1 and PCE2, were isolated and characterized. PCE1 and PCE2 contained open reading frames of 1,653 and 1,392 nucleotides, encoding proteins of 550 and 463 amino acid residues, respectively. Phylogenetic analyses showed that PCE1 and PCE2 were most closely related to the CarE genes from other phytophagous mites. The transcriptional profiles of two CarE genes among developmental stages (egg, larva, nymph, adult female, and adult male), after exposing to four acaricides (avermectin, azocyclotin, pyridaben, and spirodiclofen) and acid rain were investigated using real-time quantitative PCR (qPCR). The results showed that during development, PCE1 was highly expressed at the egg stage, whereas PCE2 was abundantly expressed at the adult stage of males. The expression levels of PCE1 were highly induced upon exposure to acaricides and acid rain. On the other hand, the expression levels of PCE2 were increased after treatment with avermectin and pyridaben. These results suggest that PCE1 and PCE2 may have distinct roles in different developmental stages and participate in the detoxification of acaricides.