Baseline toxicity of metaflumizone and lack of cross resistance between indoxacarb and metaflumizone in diamondback moth (Lepidoptera: Plutellidae)

Identification and detection of the V1848I indoxacarb resistance mutation in the beet armyworm, Spodoptera exigua

Indoxacarb is a pivotal insecticide used worldwide to manage Spodoptera exigua, a devastating agricultural pest. This active compound plays a crucial role in resistance management strategies due to its distinctive mode of action. A field population of S. exigua (SH23) from Shanghai, China, exhibited significantly reduced susceptibility to indoxacarb, with a resistance ratio of 113.84-fold in biological assays. Following two rounds of laboratory screening with indoxacarb, the resistance of the new strain (SH23-S2) escalated steeply to 876.15-fold. Genetic analyses of both the SH23 and SH23-S2 strains demonstrated autosomal inheritance and incompletely dominant resistance patterns. Synergist assays indicated a minor role of detoxification enzymes (glutathione s-transferases and cytochrome P450) of SH23-S2 strain in this resistance, implicating target-site resistance as the primary mechanism. To explore the impact of target-site resistance, segment 1-6 of domain IV (IVS1-6) of the sodium channel in S. exigua was cloned, and the sequences from susceptible and indoxacarb-resistant S. exigua were compared. The V1848I mutation, linked to indoxacarb resistance in Plutella xylostella, Tuta absoluta and Liriomyza trifolii, was identified and strongly associated with the indoxacarb-resistant phenotype in the S. exigua SH23-S2 strain, whereas the F1845Y mutation was not detected. Furthermore, a molecular test for the V1848I mutation in field populations was created using an allele-specific PCR (AS-PCR). The discovery of indoxacarb resistance mutation and the creation of diagnostic tool will enable the early detection of indoxacarb resistance, which will facilitate the implementation of targeted resistance management strategies, ultimately delaying the proliferation of resistance.

Sublethal effect and detoxifying metabolism of metaflumizone and indoxacarb on the fall armyworm, Spodoptera frugiperda

The fall armyworm (FAW), Spodoptera frugiperda (J.E. Smith) (Lepidoptera, Noctuidae), is a highly polyphagous invasive pest that damages various crops. Pesticide control is the most common and effective strategy to control FAW. In this study, we evaluated the toxicity of metaflumizone and indoxacarb against third-instar FAW larvae using the insecticide-incorporated artificial diet method under laboratory conditions. Both metaflumizone and indoxacarb exhibited substantial toxicity against FAW, with LC50 values of 2.43 and 14.66 mg/L at 72 h, respectively. The sublethal effects of metaflumizone and indoxacarb on parental and F1 generation FAW were investigated by exposing third-instar larvae to LC10 and LC30 concentrations of these insecticides. Sublethal exposure to these two insecticides significantly shortened adult longevity, extended pupal developmental times and led to reduced pupal weight, pupation rates, and adult fecundity in the treated parental generation and F1 generation at LC10 or LC30 concentrations, in comparison to the control group. The larval developmental times were shortened in the parental generation but prolonged in the F1 generation, after being treated with sublethal concentrations of metaflumizone. Furthermore, larvae exposed to LC10 or LC30 concentrations of indoxacarb exhibited elevated activity levels of cytochrome P450 monooxygenase and glutathione S-transferase, which coincides with the observed synergistic effect of piperonyl butoxide and diethyl maleate. In conclusion, the high toxicity and negative impact of metaflumizone and indoxacarb on FAW provided significant implications for the rational utilization of insecticides against this pest.

Control efficacy and joint toxicity of metaflumizone mixed with chlorantraniliprole or indoxacarb against the fall armyworm, Spodoptera frugiperda

BACKGROUND

The fall armyworm (FAW), Spodoptera frugiperda is the main destructive pest of grain crops, and has led to substantial economic losses worldwide. Chemical pesticides are the most effective way to manage FAW. Here, a laboratory test using an artificial diet-incorporated assay was conducted to determine the toxicity of five insecticides and the joint effect of the binary combination insecticides to FAW larvae. A field plot test using foliar spray was carried out to assess the control efficacy of metaflumizone mixed with chlorantraniliprole or indoxacarb against FAW.

RESULTS

The bioassay results showed that metaflumizone had a stronger insecticidal effect than indoxacarb toward FAW larvae. Furthermore, the mixture of metaflumizone and chlorantraniliprole in a volume ratio of 3:7 had the strongest synergistic effect against FAW, with a co-toxicity coefficient (CTC) of 317.18. The best synergistic effect for mixtures of metaflumizone and indoxacarb was observed at a 1:9 volume ratio, with a CTC of 185.98. However, there was an antagonistic effect of metaflumizone mixed with emamectin benzoate and with lufenuron, because the co-toxic factor was less than -20 at volume ratios of 8:2 and 9:1, respectively. According to the results of the field trial, metaflumizone mixed with chlorantraniliprole or indoxacarb at a 50% reduction of the application rate can effectively control FAW with efficacy ranging from 77.73% to 94.65% 1-7 days postapplication.

CONCLUSION

Overall, our findings suggest that metaflumizone and its binary combination insecticides can be utilized in FAW integrated pest management programs. © 2022 Society of Chemical Industry.

Baseline susceptibility of Helicoverpa armigera, Plutella xylostella, and Spodoptera frugiperda to the meta-diamide insecticide broflanilide

Broflanilide is a novel meta-diamide insecticide that acts as a γ-aminobutyric acid-gated chloride channel allosteric modulator. With its unique mode of action, broflanilide has no known cross-resistance with existing insecticides and is expected to be an effective tool for the management of insecticide resistance. Establishing the baseline susceptibility to this insecticide is an essential step for developing and implementing effective resistance management strategies. Here we evaluated the baseline susceptibility to broflanilide for 3 cosmopolitan lepidopteran pest species, Helicoverpa armigera, Plutella xylostella, and Spodoptera frugiperda. Broflanilide exhibited high activity against populations sampled in the major distribution range of these pests in China, with median lethal concentrations (LC₅₀ ) ranging between 0.209 and 0.684, 0.076 and 0.336, and 0.075 and 0.219 mg/L for H. armigera, P. xylostella, and S. frugiperda, respectively. Among-population variability in susceptibility to broflanilide was moderate for H. armigera (3.3-fold), P. xylostella (4.4-fold), and S. frugiperda (2.9-fold). The recommended diagnostic concentrations for H. armigera, P. xylostella, and S. frugiperda were 8, 4, and 2 mg/L, respectively. Little or no cross-resistance to broflanilide was detected in 3 diamide-resistant strains of P. xylostella and 1 spinosyns-resistant strain of S. frugiperda. Our results provide critical information for the development of effective resistance management programs to sustain efficacy of broflanilide against these key lepidopteran pests.

Insecticide Resistance and Control Failure Likelihood Analysis in Plutella xylostella (Lepidoptera: Plutellidae) Populations From Taiwan

The status of insecticide resistance levels is important for applying suitable pest management approaches. The present study investigated the insecticide resistance of Plutella xylostella (Linnaeus) (Lepidoptera: Plutellidae) populations from five major cruciferous growing areas in Taiwan. The surveyed locations were distributed from central to southern Taiwan and included Taichung, Changhua, Yunlin, Chiayi, and Kaohsiung. High levels of resistance to spinosad, indoxacarb, metaflumizone, and chlorantraniliprole were recorded among the surveyed populations except in Taichung. The resistance ratios ranged from 2.376 to 1,236-fold for spinosad and 24.63-1,511-fold for indoxacarb. Similarly, those for metaflumizone and chlorantraniliprole, were 2.563-76.21- and 4.457-683.0-fold, respectively. However, Bacillus thuringiensis subsp. aizawai, Bacillus thuringiensis subsp. kurstaki, emamectin benzoate, and diafenthiuron were still relatively effective against most field populations of P. xylostella. After approximately 10 generations of being maintained in the laboratory without exposure to insecticides, the resistance ratios of the Kaohsiung and Changhua populations declined to approximately 1.4-10-fold, and insecticides with control failure likelihood also began to show a negligible risk of control failure. Although spinosad, indoxacarb, metaflumizone, and chlorantraniliprole have lost their effectiveness in most field populations of P. xylostella in Taiwan, their effectiveness may be recovered in the absence of insecticide-selection pressure for approximately 10 generations. Therefore, we suggest that a constant survey of insecticide resistance and well-designed insecticide rotation based on the survey results are necessary for the effective control and insecticide resistance management of P. xylostella.

Indoxacarb resistance-associated mutation of Liriomyza trifolii in Hainan, China

Liriomyza trifolii, which has been recently prevalent in China, harms more than 300 plant species, especially cowpea in Hainan. This pest also affects the quality and production of vegetables in winter. Indoxacarb is the first commercial oxadiazine pesticide, which is a new efficient insecticide used to control pests of Diptera, including L. trifolii. The unique mechanism of indoxacarb is that indenyl is transformed into N-demethoxycarbonyl metabolite (DCJW) in insects and acts on inactivated sodium channel; DCJW could then destroy the conduction of nerve impulses, which leads to movement disorders, feeding stoppage, paralysis, and eventually the death of pests. The field population of L. trifolii developed resistance by 769 times higher than the sensitive population in Sanya, Hainan. Results revealed the existence of a mutation (i.e., V1848I) in the sixth transmembrane segment of Domain IV of the sodium channel in the field population. The homozygous resistant genotype frequency for the V1848I mutation was 10-15% among the three field-collected populations. This paper reports for the first time the presence of the kdr mutation V1848I in resistant populations of L. trifolii to indoxacarb. The present study will contribute to the understanding of the evolution of indoxacarb resistance and contribute to the development of resistance management practices for winter vegetables in Hainan.

Historical changes in the lethal effects of insecticides against the diamondback moth, Plutella xylostella (L.)

BACKGROUND

The diamondback moth, Plutella xylostella (L.), is one of the most destructive pests owing to the rapid development of resistance to various groups of insecticides. Although the rotation strategy has become common, resistance management continues to be a critical issue.

RESULTS

In Japan, historical changes in the susceptibility of P. xylostella to insecticides were estimated using 82 references. The meta-analysis using generalized linear mixed models suggested that each mode of action (MoA) had different patterns of changes to the susceptibility, which might be due to the prevalence of the rotation strategy since the 2010s and the fitness cost of the resistance gene. For example, spinosyns and bacterial insecticide Bacillus thuringiensis (Bt), for which resistance has a high fitness cost, maintained high lethal effects on P. xylostella throughout several decades. However, pyrethroids and benzoylureas, for which resistance has no or little fitness cost, remained at low levels of susceptibility regardless of the rotation strategy.

CONCLUSION

Our findings suggest optimal combinations of MoA for a sustainable rotation strategy based on their stability and recovery of effectiveness. Susceptibility monitoring using our method and further study of fitness costs will improve the rotation strategy concerning P. xylostella in the future. © 2021 Society of Chemical Industry.

Baseline Response, Monitoring, and Cross-Resistance of Spodoptera frugiperda (Lepidoptera: Noctuidae) to Sodium Channel Blocker Insecticides in Brazil

Spodoptera frugiperda (J.E. Smith) is one of the key cross-crop pests in Brazilian agroecosystems. Field-evolved resistance of S. frugiperda to some conventional insecticides and Bt proteins has already been reported. Thus, the use of insecticides with new mode of action such as sodium channel blockers (indoxacarb and metaflumizone) could be an important tool in insecticide resistance management (IRM) programs. To implement a proactive IRM, we conducted baseline response and monitoring to indoxacarb and metaflumizone in 87 field populations of S. frugiperda collected from major maize-growing regions of Brazil from 2017 to 2020, estimated the frequency of resistance alleles to indoxacarb, and evaluated cross-resistance of indoxacarb and metaflumizone to some selected insecticides and Bt proteins. Low variation in susceptibility to indoxacarb (4.6-fold) and metaflumizone (2.6-fold) was detected in populations of S. frugiperda in 2017. The frequency of the resistance allele to indoxacarb was 0.0452 (0.0382-0.0527 CI 95%), by using F2 screen method. The mean survival at diagnostic concentration, based on CL99, varied from 0.2 to 12.2% for indoxacarb and from 0.0 to 12.7% for metaflumizone, confirming high susceptibility of S. frugiperda to these insecticides in Brazil. No cross-resistance was detected between sodium channel blocker insecticides and other insecticides (organophosphate, pyrethroid, benzoylurea, spinosyn, and diamide) and Bt proteins. These findings showed that sodium channel blocker insecticides are important candidates to be exploited in IRM strategies of S. frugiperda in Brazil.

Insecticide Rotation and Adaptive Fitness Cost Underlying Insecticide Resistance Management for Spodoptera frugiperda (Lepidoptera: Noctuidae)

Rotation of insecticide modes of action is recommended to delay selection for insecticide resistance. In this study, larvae of Spodoptera frugiperda (J.E. Smith) were exposed alternately to spinetoram and metaflumizone with insecticidal and biological response determined. Five generations (G) were evaluated with at least 200 larvae assayed per treatment. The experiment consisted of rearing field-collected and untreated larvae (M-Control), larvae with successive applications of either metaflumizone (M-MET) or spinetoram (M-SPI), and alternation of these insecticides (M-Rotation treatment) consisting of the following treatments: G₂ = exposure to SPI, G₃ = exposure to MET, G₄ = exposure to SPI, G₅ = exposure to MET, and G₆ = exposure to SPI. Four days after application, those surviving larvae were used to compose the following generations. In the G₇, evaluations were made on the selected populations M-MET, M-SPI, M-Rotation, and control larvae to determine biological characteristics used to calculate the fertility life table parameters and further comparisons across treatments. The frequency of resistance was increased at rate of 49.5% and 29.2% after five generations of selection pressure with MET and SPI, respectively. However, rotation of insecticides reduced the frequency of resistance about 50% over the generations. Individuals originated from successive exposures to the insecticides exhibited delayed egg incubation, greater percentage of pupae with deformation, longer larval and pupal periods, and reduction in emergence rate and longevity of adults, suggesting adaptive costs associated with resistance.

Biochemical Mechanisms, Cross-resistance and Stability of Resistance to Metaflumizone in Plutella xylostella

The diamondback moth, Plutella xylostella (L.) is an important pest of cruciferous crops worldwide. It has developed resistance to many conventional and novel insecticide classes. Metaflumizone belongs to the new chemical class of semicarbazone insecticides. To delay the development of metaflumizone resistance in P. xylostella and to guide insecticide use in the field, the biochemical mechanisms, cross-resistance spectrum, and stability of resistance to metaflumizone were studied in a laboratory-selected resistant strain (metaflu-SEL). Synergism tests with the carboxylesterase inhibitor triphenyl phosphate (TPP), the glutathione S-transferase depletor diethyl maleate (DEM), and the P450 inhibitor piperonyl butoxide(PBO) had no obvious effect on metaflumizone in the metaflu-SEL strain and the susceptible strain (SS) of P. xylostella, with synergism ratios that ranged from 1.02 to 1.86. Biochemical studies revealed that the cytochrome P450-dependent monooxygenase increased only 1.13-fold in the metaflu-SEL strain compared with the UNSEL stain; meanwhile, carboxylesterase and glutathione S-transferase activity showed no difference. These results suggest that these detoxification enzymes may be not actively involved in metaflumizone resistance. Furthermore, the metaflu-SEL population showed a moderate level of cross-resistance to indoxacarb (11.63-fold), but only very low cross-resistance to spinosad (1.75-fold), spinetoram (3.52-fold), abamectin (2.81-fold), beta-cypermethrin (0.71-fold), diafenthiuron (0.79-fold), chlorantraniliprole (2.16-fold), BT (WG-001) (3.34-fold), chlorfenapyr (0.49-fold), and chlorfluazuron (0.97-fold). Moreover, metaflumizone resistance decreased from 1087.85- to 1.23-fold in the metaflu-SEL strain after 12 generations without exposure to metaflumizone. These results are useful for formulating insecticide resistance management strategies to control P. xylostella and to delay the development of metaflumizone resistance in the field.

A systemic study of indoxacarb resistance in Spodoptera litura revealed complex expression profiles and regulatory mechanism

The tobacco cutworm, Spodoptera litura, is an important pest of crop and vegetable plants worldwide, and its resistance to insecticides have quickly developed. However, the resistance mechanisms of this pest are still unclear. In this study, the change in mRNA and miRNA profiles in the susceptible, indoxacarb-resistant and field indoxacarb-resistant strains of S. litura were characterized. Nine hundred and ten co-up-regulated and 737 co-down-regulated genes were identified in the resistant strains. Further analysis showed that 126 co-differentially expressed genes (co-DEGs) (cytochrome P450, carboxy/cholinesterase, glutathione S-transferase, ATP-binding cassette transporter, UDP-glucuronosyl transferase, aminopeptidase N, sialin, serine protease and cuticle protein) may play important roles in indoxacarb resistance in S. litura. In addition, a total of 91 known and 52 novel miRNAs were identified, and 10 miRNAs were co-differentially expressed in the resistant strains of S. litura. Furthermore, 10 co-differentially expressed miRNAs (co-DEmiRNAs) had predicted co-DEGs according to the expected miRNA-mRNA negative regulation pattern and 37 indoxacarb resistance-related co-DEGs were predicted to be the target genes. These results not only broadened our understanding of molecular mechanisms of insecticide resistance by revealing complicated profiles, but also provide important clues for further study on the mechanisms of miRNAs involved in indoxacarb resistance in S. litura.

Resistance Risk Evaluated by Metaflumizone Selection and the Effects on Toxicities Over Other Insecticides in Spodoptera exigua (Lepidoptera: Noctuidae)

Metaflumizone is a novel semicarbazone insecticide. It functions as a sodium channel blocker insecticide (SCBI) with excellent insecticidal activity on most economically important lepidopterous pests. This study assessed the resistance risk of Spodoptera exigua (Hübner) (Lepidoptera: Noctuidae) to metaflumizone in the laboratory and the effects of metaflumizone selection on toxicities to other insecticides. Spodoptera exigua collected from a field population at Huizhou in 2012 were successively challenged by metaflumizone to evaluate the risk of resistance evolution. Twelve generations of selection increased resistance to metaflumizone by 3.4-fold and threshold trait analysis revealed that the realized heritability (h2) of this resistance was 0.086. When h2 was equal to 0.086 and 90% of individuals were killed at each generation, LC50 to metaflumizone increased by 10-fold after 15 generations. The selection by metaflumizone did not increase the resistance to indoxacarb, chlorantraniliprole, spinosad, methomyl, or endosulfan, suggesting a lack of cross-resistance. However, metaflumizone challenge upheld the recession of resistance to emamectin benzoate, chlorfluazuron, and tebufenozide. The block of resistance drops by metaflumizone exposure implied a possible cross-resistance between metaflumizone and these three insecticides. These results contribute to integrated resistance management of S. exigua.

Gossypol-induced fitness gain and increased resistance to deltamethrin in beet armyworm, Spodoptera exigua (Hübner)

BACKGROUND

In plant-insect interactions, phytotoxins such as gossypol, exert a defensive role on behalf of the plant by interfering with the essential metabolic, biochemical and physiological pathways of herbivorous insects. The beet armyworm, Spodoptera exigua (Hübner), is a key pest for many important crops including a range of vegetables, ornamentals, and cotton. In this study, we investigated how resistance to deltamethrin relates to enzyme activity in gossypol-pretreated larvae of S. exigua.

RESULTS

Following selection with deltamethrin insecticides on gossypol-pretreated larvae for 10 generations, the Gos-SEL population developed a 113.29-fold resistance. Under the same conditions, the Delta-SEL selected population showed a 69.76-fold increase in resistance along with corresponding levels of xenobiotic defense enzyme activity. Similarly, the fecundity of the Delta-SEL population together with male and female longevity were found to be significantly lower when compared with the Gos-SEL population and the laboratory susceptible-strain group (SS-Strain). In addition, the activities of cytochrome P450s in S. exigua were significantly enhanced when the insects were fed on a deltamethrin and gossypol-pretreated diet compared with being fed on deltamethrin alone.

CONCLUSION

The reproductive capacity of S. exigua is significantly reduced in Delta-SEL and Gos-SEL populations compared with the control group (SS-Strain). Elevation of the major detoxification enzyme cytochrome P450 monooxygenase and esterase might have an important role in inducing tolerance to deltamethrin in gossypol-fed S. exigua populations. This study enhances our understanding of detoxification enzyme pathways for S. exigua gene expression and their role in responses to insecticides and plant secondary metabolites. © 2018 Society of Chemical Industry.

Monitoring, Cross-Resistance, Inheritance, and Synergism of Plutella xylostella (Lepidoptera: Plutellidae) Resistance to Pyridalyl in China

Pyridalyl is an insecticide that shows significant efficacy against Plutella xylostella, a notorious pest insect worldwide. In this study, we monitored resistance of P. xylostella to pyridalyl in China from 2016 to 2017, determined cross-resistance, inheritance, and synergism of pyridalyl resistance in two pyridalyl-resistant populations, one field-evolved resistant population (ZL-PR) and one laboratory-selected resistant population (XY-PR). We found that variation in susceptibility among 15 field populations in China from 2016 to 2017 was high, with mean LC50 values ranging from 1.839 to 1,652 mg/liter. The laboratory-selected XY-PR strain showed 31.3-fold resistance to pyridalyl and moderate cross-resistance to fipronil. The ZL-PR displayed 1,050.2-fold resistance to pyridalyl and high resistance to all tested insecticides. Genetic analysis illustrated that pyridalyl resistance in ZL-PR was autosomally inherited and incompletely recessive. However, pyridalyl resistance in the XY-PR strain was autosomally inherited but incompletely dominant. Moreover, piperonyl butoxide significantly inhibited pyridalyl resistance in the XY-PR strain. In conclusion, P. xylostella field populations from South China have high levels of resistance to pyridalyl and different modes of inheritance of resistance were found in XY-PR and ZL-PR. Moreover, enhanced oxidative metabolism is possibly involved in resistance of the XY-PR strain but not in the ZL-PR strain.

Functional validation of target-site resistance mutations against sodium channel blocker insecticides (SCBIs) via molecular modeling and genome engineering in Drosophila

Sodium channel blocker insecticides (SCBIs) like indoxacarb and metaflumizone offer an alternative insecticide resistance management (IRM) strategy against several pests that are resistant to other compounds. However, resistance to SCBIs has been reported in several pests, in most cases implicating metabolic resistance mechanisms, although in certain indoxacarb resistant populations of Plutella xylostella and Tuta absoluta, two mutations in the domain IV S6 segment of the voltage-gated sodium channel, F1845Y and V1848I have been identified, and have been postulated through in vitro electrophysiological studies to contribute to target-site resistance. In order to functionally validate in vivo each mutation in the absence of confounding resistance mechanisms, we have employed a CRISPR/Cas9 strategy to generate strains of Drosophila melanogaster bearing homozygous F1845Y or V1848I mutations in the para (voltage-gated sodium channel) gene. We performed toxicity bioassays of these strains compared to wild-type controls of the same genetic background. Our results indicate both mutations confer moderate resistance to indoxacarb (RR: 6-10.2), and V1848I to metaflumizone (RR: 8.4). However, F1845Y confers very strong resistance to metaflumizone (RR: >3400). Our molecular modeling studies suggest a steric hindrance mechanism may account for the resistance of both V1848I and F1845Y mutations, whereby introducing larger side chains may inhibit metaflumizone binding.

Molecular Mechanism of Action and Selectivity of Sodium Ch annel Blocker Insecticides

Sodium channel blocker insecticides (SCBIs) are a relatively new class of insecticides that are represented by two commercially registered compounds, indoxacarb and metaflumizone. SCBIs, like pyrethroids and DDT, target voltage-gated sodium channels (VGSCs) to intoxicate insects. In contrast to pyrethroids, however, SCBIs inhibit VGSCs at a distinct receptor site that overlaps those of therapeutic inhibitors of sodium channels, such as local anesthetics, anticonvulsants and antiarrhythmics. This review will recount the development of the SCBI insecticide class from its roots as chitin synthesis inhibitors, discuss the symptoms of poisoning and evidence supporting inhibition of VGSCs as their mechanism of action, describe the current model for SCBI-induced inhibition of VGSCs, present a model for the receptor for SCBIs on VGSCs, and highlight differences between data collected from mammalian and insect experimental models.

Cross-resistance and baseline susceptibility of Spodoptera litura (Fabricius) (Lepidoptera: Noctuidae) to cyantraniliprole in the south of China

BACKGROUND

The oriental leafworm moth, Spodoptera litura Fab. (Lepidoptera: Noctuidae), is a widely distributed polyphagous insect pest in Asia that has been shown to be resistant to various types of insecticide. The newly registered anthranilic diamide cyantraniliprole provided novel insight and great opportunities to control S. litura.

RESULTS

In this study, the susceptibilities of S. litura collected from South China to cyantraniliprole were measured by standard leaf-disc bioassay, and obvious variation in susceptibility was observed among the 17 field populations, with LC50 values varying from 0.206 to 1.336 mg AI L(-1). Significant correlations were detected between the LC50 values of cyantraniliprole and chlorantraniliprole (P < 0.05). However, no significant correlation (P > 0.05) was observed between the two anthranilic diamides and other insecticides with different action mechanisms (delcamethrin, chlorpyrifos, indoxacarb and emamectin benzoate). Piperonyl butoxide showed obvious synergism in Lab-Sus, ZC14 and cyantraniliprole-resistant strains, while diethyl maleate and S,S,S-tributylphorotrithioate had no obvious synergistic effects in any of the strains tested.

CONCLUSION

These results revealed obvious regional variation in cyantraniliprole susceptibilities among populations of S. litura from different areas, and potential cross-resistance to chlorantraniliprole, which suggested that S. litura could develop resistance to cyantraniliprole. Detoxification enzymes might not be involved in the observed tolerance in field-collected populations and the cyantraniliprole-resistant strain.

Two novel sodium channel mutations associated with resistance to indoxacarb and metaflumizone in the diamondback moth, Plutella xylostella

Indoxacarb and metaflumizone belong to a relatively new class of sodium channel blocker insecticides (SCBIs). Due to intensive use of indoxacarb, field-evolved indoxacarb resistance has been reported in several lepidopteran pests, including the diamondback moth Plutella xylostella, a serious pest of cruciferous crops. In particular, the BY12 population of P. xylostella, collected from Baiyun, Guangdong Province of China in 2012, was 750-fold more resistant to indoxacarb and 70-fold more resistant to metaflumizone compared with the susceptible Roth strain. Comparison of complementary DNA sequences encoding the sodium channel genes of Roth and BY12 revealed two point mutations (F1845Y and V1848I) in the sixth segment of domain IV of the PxNav protein in the BY population. Both mutations are located within a highly conserved sequence region that is predicted to be involved in the binding sites of local anesthetics and SCBIs based on mammalian sodium channels. A significant correlation was observed among 10 field-collected populations between the mutant allele (Y1845 or I1848) frequencies (1.7% to 52.5%) and resistance levels to both indoxacarb (34- to 870-fold) and metaflumizone (1- to 70-fold). The two mutations were never found to co-exist in the same allele of PxNav , suggesting that they arose independently. This is the first time that sodium channel mutations have been associated with high levels of resistance to SCBIs. F1845Y and V1848I are molecular markers for resistance monitoring in the diamondback moth and possibly other insect pest species.

Mutations in the transmembrane helix S6 of domain IV confer cockroach sodium channel resistance to sodium channel blocker insecticides and local anesthetics

Indoxacarb and metaflumizone are two sodium channel blocker insecticides (SCBIs). They preferably bind to and trap sodium channels in the slow-inactivated non-conducting state, a mode of action similar to that of local anesthetics (LAs). Recently, two sodium channel mutations, F1845Y (F(4i15)Y) and V1848I (V(4i18)I), in the transmembrane segment 6 of domain IV (IVS6), were identified to be associated with indoxacarb resistance in Plutella xylostella. F(4i15) is known to be critical for the action of LAs on mammalian sodium channels. Previously, mutation F(4i15)A in a cockroach sodium channel, BgNav1-1a, has been shown to reduce the action of lidocaine, a LA, but not the action of SCBIs. In this study, we introduced mutations F(4i15)Y and V(4i18)A/I individually into the cockroach sodium channel, BgNav1-1a, and conducted functional analysis of the three mutants in Xenopus oocytes. We found that both the F(4i15)Y and V(4i18)I mutations reduced the inhibition of sodium current by indoxacarb, DCJW (an active metabolite of indoxacarb) and metaflumizone. F(4i15)Y and V(4i18)I mutations also reduced the use-dependent block of sodium current by lidocaine. In contrast, substitution V(4i18)A enhanced the action metaflumizone and lidocaine. These results show that both F(4i15)Y and V(4i18)I mutations may contribute to target-site resistance to SCBIs, and provide the first molecular evidence for common amino acid determinants on insect sodium channels involved in action of SCBIs and LA.

In vivo effects of metaldehyde on Pacific oyster, Crassostrea gigas: comparing hemocyte parameters in two oyster families

Pollutants via run-off into the ocean represent a potential threat to marine organisms, especially bivalves such as oysters living in coastal environments. These organisms filter large volumes of seawater and may accumulate contaminants within their tissues. Pesticide contamination in water could have a direct or indirect toxic action on tissues or cells and could induce alteration of immune system. Bivalve immunity is mainly supported by hemocytes and participates directly by phagocytosis to eliminate pathogens. Some studies have shown that pesticides can reduce immune defences and/or modify genomes in vertebrates and invertebrates. Metaldehyde is used to kill slugs, snails and other terrestrial gastropods. Although metaldehyde has been detected in surface waters, its effects on marine bivalves including the Pacific oyster, Crassostrea gigas, have never been studied. Given the mode of action of this molecule and its targets (molluscs), it could be potentially more toxic to oysters than other pesticides (herbicides, fungicides, insecticides, etc.). Effects of metaldehyde on oyster hemocyte parameters were thus monitored through in vivo experiments based on a short-term exposure. In this work, metaldehyde at 0.1 μg/L, which corresponds to an average concentration detected in the environment, modulated hemocyte activities of Pacific oysters after an in vivo short-term contact. Individuals belonging to two families showed different behaviours for some hemocyte activities after contamination by metaldehyde. These results suggested that effects of pollutants on oysters may differ from an individual to another in relation to genetic diversity. Finally, it appears essential to take an interest in the effects of metaldehyde on a wide variety of aquatic invertebrates including those that have a significant economic impact.

Metabolic and Behavioral Mechanisms of Indoxacarb Resistance in Sitophilus zeamais (Coleoptera: Curculionidae)

The control of the most important pest of stored maize, the weevil Sitophilus zeamais Motschulsky (Coleoptera: Curculionidae), is mainly achieved with the use of pyrethroid insecticides. However, the intensive use of these compounds has led to the selection of resistant populations and has compromised the control efficacy of this insect pest. Here, the toxicity of indoxacarb for a potential use in the control of S. zeamais was assessed on 13 Brazilian populations. Concentration-mortality bioassays, in the presence of synergists (piperonyl butoxide, triphenyl phosphate, and diethyl maleate), were used to assess potential metabolic-based indoxacarb resistance mechanisms. We also assessed the behavioral (locomotory) responses of these populations to indoxacarb exposure. The results showed significant differences between the populations (LD50 values ranged from 0.06 to 13.99 mg a.i/kg of grains), resulting in resistance ratios of >200-fold between the least (Canarana-MT) and the most (Espirito Santo do Pinhal-SP) susceptible populations. The results obtained with synergized indoxacarb suggest the involvement of esterases and glutathione-S-transferases on indoxacarb action, and also suggest the involvement of cytochrome P450-dependent monooxygenases as a potential indoxacarb resistance mechanism in Brazilian populations of S. zeamais. Although indoxacarb-induced behavioral avoidance varied among populations, some resistant populations (e.g., Canarana-MT) were able to reduce exposure to indoxacarb by spending more time in the nontreated areas. Collectively, our findings indicate that the behavioral (locomotory) and physiological responses of these insects may compromise the control efficacy of oxadiazine insecticides (e.g., indoxacarb) in Brazilian populations of S. zeamais.

Biochemical mechanisms for metaflumizone resistance in beet armyworm, Spodoptera exigua

The metaflumizone, which belongs to the class of voltage-dependent sodium channel blockers, was registered to control Spodoptera exigua on vegetables in China in 2009. The present study revealed S. exigua has developed high resistance to this novel chemistry insecticide shortly after 2-3 years application in Guangdong Province of China. The metabolic mechanisms for metaflumizone resistance in this insect were analysed. The inhibitor of esterases greatly potentiates the toxicity of this chemical against the field resistant populations. The synergism ratio is 5.7 and 3.4-fold for S. exigua collected from Huizhou, Guangdong Province in 2011 and 2012, respectively. The activity of esterases in field populations (HZ12) is also significantly greater than that in the susceptible strain, and further significantly increased by challenge with metaflumizone for 3 generations. However, the inhibitor of P450s or GSTs only has slight synergism on metaflumizone toxicity against resistant populations, and there are no obvious differences in activities of P450s or GSTs between resistant populations and the susceptible strain. These results suggest that esterases might take pivotal role in conferring metabolic resistance to metaflumizone in the field populations of S. exigua, and P450s or GSTs are not involved in this resistance. Moreover, flavin-dependent monooxygenases (FMOs) are discovered to involve in metaflumizone resistance in the field populations of S. exigua. The FMO inhibitor, methimazole, potentiates metaflumizone toxicity in resistant larva of this species substantially. The synergism ratios for methimazole in resistant populations HZ11 and HZ12 were 3.1 and 1.9, respectively. Enzymatic assays also revealed higher FMO activities in resistant populations than in the susceptible strain, and successive selection with metaflumizone further increased the FMO activity in the field resistant population, but not significantly. The higher FMO activities in the older larval stages and in the larval midgut signify the importance of FMO in the detoxification of xenobiotic from food sources. The synergism assay and FMO activity analysis suggest that FMO contributes to metaflumizone detoxification in resistant populations of S. exigua and conferred metaflumizone resistance in S. exigua. A novel mechanism for insecticide resistance by insect was proposed.

Voltage-Gated Sodium Channels as Insecticide Targets

Voltage-gated sodium channels are critical for the generation and propagation of action potentials. They are the primary target of several classes of insecticides, including DDT, pyrethroids and sodium channel blocker insecticides (SCBIs). DDT and pyrethroids preferably bind to open sodium channels and stabilize the open state, causing prolonged currents. In contrast, SCBIs block sodium channels by binding to the inactivated state. Many sodium channel mutations are associated with knockdown resistance (kdr) to DDT and pyrethroids in diverse arthropod pests. Functional characterization of kdr mutations together with computational modelling predicts dual pyrethroid receptor sites on sodium channels. In contrast, the molecular determinants of the SCBI receptor site remain largely unknown. In this review, we summarize current knowledge about the molecular mechanisms of action of pyrethroids and SCBIs, and highlight the differences in the molecular interaction of these insecticides with insect versus mammalian sodium channels.