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

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.

Functional analysis of three odorant receptors in Plutella xylostella response to repellent activity of 2,3-dimethyl-6-(1-hydroxy)-pyrazine

Plutella xylostella is an important pest showing resistance to various chemical pesticides, development of botanical pesticides is an effective strategy to resolve above problem and decrease utilization of chemical pesticides. Previous study showed that 2,3-dimethyl-6-(1-hydroxy)-pyrazine has significant repellent activity to P. xylostella adult which mainly effect to the olfactory system, however the molecular targets and mechanism are still unclear. Based on the RNA-Seq and RT-qPCR data, eight ORs (Odorant receptor) in P. xylostella were selected as candidate targets response to repellent activity of 2,3-dimethyl-6-(1-hydroxy)-pyrazine. Here, most of the ORs in P. xylostella were clustered into three branches, which showed similar functions such as recognition, feeding, and oviposition. PxylOR29, PxylOR31, and PxylOR46 were identified as the potential molecular targets based on the results of repellent activity and EAG response tests to the adults which have been injected with dsRNA, respectively. Additionally, the three ORs were higher expressed in antenna of P. xylostella, followed by those in the head segment. Furthermore, it was found that the bindings between these three ORs and 2,3-dimethyl-6-(1-hydroxy)-pyrazine mainly depend on the hydrophobic effect of active cavities, and the binding to PxylOR31 was more stabler and easier with an energy of -16.34 kcal/mol, together with the π-π T-shaped interaction at PHE195 site. These findings pave the way for the complete understanding of pyrazine repellent mechanisms.

Characterization of Spodoptera littoralis (Lepidoptera: Noctuidae) resistance to indoxacarb: inheritance mode, realized heritability, and fitness costs

Spodoptera littoralis (Boisd.) (Lepidoptera: Noctuidae) is a major economic pest attacking a variety of crops in Egypt and other Mediterranean countries. S. littoralis has developed resistance to both traditional and novel insecticides. The current study investigated S. littoralis resistance to indoxacarb regarding inheritance mode, realized heritability (h2), and fitness costs. An indoxacarb-resistant strain (Indoxa-SEL) was obtained by selecting a field strain with indoxacarb. Indoxa-SEL strain outperformed the susceptible one (Indoxa-S) by 29.77-fold after 16 consecutive generations of selection. Based on the LC50 values of the progenies of reciprocal crosses F1 (R♂ × S♀) and F1' (R♀ × S♂), S. littoralis resistance to indoxacarb was found to be autosomal and partially recessive. Chi-square tests for goodness-of-fit between observed and expected mortalities of self-bred F1 and resistant strain reciprocal crosses revealed that the resistance was controlled by multiple genes. The resistant strain had a relative fitness of 0.80, with significantly increased total preovipositional period of females, egg, larvae, pupae, preadult, adult, and total longevity period. The estimated realized heritability value in the Indoxa-SEL strain was 0.21. The current study will contribute to sustaining indoxacarb efficacy and designing effective resistance management programs against S. littoralis.

Comprehensive analysis of the overexpressed cytochrome P450-based insecticide resistance mechanism in Spodoptera litura

Cytochrome P450s play critical roles in the metabolic resistance of insecticides in insects. Previous findings showed that enhanced P450 activity was an important mechanism mediating indoxacarb resistance, and multiple P450 genes were upregulated in indoxacarb resistant strains of Spodoptera litura. However, the functions of these P450 genes in insecticide resistance remain unknown. Here, the P450 inhibitor PBO effectively decreased the resistance of S. litura to indoxacarb. Ten upregulated P450 genes were characterized, all of which were overexpressed in response to indoxacarb induction. Knockdown of nine P450 genes decreased cell viability against indoxacarb, and further silencing of three genes (CYP339A1, CYP340G2, CYP321A19) in larvae enhanced the sensitivity to indoxacarb. Transgenic overexpression of these three genes increased resistance to indoxacarb in Drosophila melanogaster. Moreover, molecular modeling and docking predicted that these three P450 proteins could bind tightly to indoxacarb and N-decarbomethoxylated metabolite (DCJW). Interestingly, these three P450 genes may also mediate cross-resistance to chlorantraniliprole, λ-cyhalothrin and imidacloprid. Additionally, heterologous expression and metabolic assays confirmed that three recombinant P450s could effectively metabolize indoxacarb and DCJW. This study strongly demonstrates that multiple overexpressed mitochondrial and microsomal P450 genes were involved in insecticide resistance in S. litura.

Indoxacarb resistance in Iranian populations of Tuta absoluta (Lepidoptera: Gelechiidae): Cross-resistance, biochemical and molecular mechanisms

Tuta absoluta (Meyrick) is an invasive tomato pest that occurs worldwide, including Iran. This study investigates the occurrence of resistance to indoxacarb, an oxadiazine insecticide, and the underlying mechanisms in Iranian populations of T. absoluta. Bioassays were performed on second-instar larvae using indoxacarb alone and in combination with three synergists: Piperonyl butoxide (PBO), diethyl maleate (DEM), and triphenyl phosphate (TPP). The activities of the main detoxification enzymes, including glutathione S-transferases (GST), general carboxylesterases (CarEs), and P450 monooxygenases (P450s), were evaluated. In addition, the presence of known amino acid substitutions in the IV segment 6 domain of the T. absoluta sodium channel was investigated. The results showed that resistance rates to indoxacarb in Iranian populations ranged from 2.37- to 14.45-fold. However, pretreatment with synergists did not significantly increase the toxicity of indoxacarb. Enzyme assays showed that Ardabil (Ar) and Kerman (Kr) populations had the highest CarEs activity, while Ar population showed the highest P450 activity. However, the observed increases in enzyme activities were <2-fold. Two indoxacarb resistance mutations, F1845Y and V1848I, were detected. Apart from a significant and positive correlation between LC50 values of indoxacarb and thiocyclam hydrogen oxalate, no cross-resistance between indoxacarb and other insecticides was detected. Overall, these results suggest that populations of T. absoluta in Iran have developed resistance to indoxacarb, primarily through changes at the target site.

Integrated mass spectrometry imaging and metabolomics reveals sublethal effects of indoxacarb on the red fire ant Solenopsis invicta

BACKGROUND

Indoxacarb, representing an efficient insecticide, is normally made into a bait to spread the poison among red fire ants so that it can be widely applied in the prevention and control of Solenopsis invicta. However, the potential toxicity mechanism of S. invicta in response to indoxacarb remains to be explored. In this study, we integrated mass spectrometry imaging (MSI) and untargeted metabolomics methods to reveal disturbed metabolic expression levels and spatial distribution within the whole-body tissue of S. invicta treated with indoxacarb.

RESULTS

Metabolomics results showed a significantly altered level of metabolites after indoxacarb treatment, such as carbohydrates, amino acids and pyrimidine and derivatives. Additionally, the spatial distribution and regulation of several crucial metabolites resulting from the metabolic pathway and lipids can be visualized using label-free MSI methods. Specifically, xylitol, aspartate, and uracil were distributed throughout the whole body of S. invicta, while sucrose-6'-phosphate and glycerol were mainly distributed in the abdomen of S. invicta, and thymine was distributed in the head and chest of S. invicta. Taken together, the integrated MSI and metabolomics results indicated that the toxicity mechanism of indoxacarb in S. invicta is closely associated with the disturbance in several key metabolic pathways, such as pyrimidine metabolism, aspartate metabolism, pentose and glucuronate interconversions, and inhibited energy synthesis.

CONCLUSION

Collectively, these findings provide a new perspective for the understanding of toxicity assessment between targeted organisms S. invicta and pesticides. © 2023 Society of Chemical Industry.

The chemosensory protein 1 contributes to indoxacarb resistance in Plutella xylostella (L.)

BACKGROUND

Insecticide resistance continuously poses a threat to agricultural production. Chemosensory protein-mediated resistance is a new mechanism of insecticide resistance discovered in recent years. In-depth research on resistance mediated by chemosensory proteins (CSPs) provides new insight into aid insecticide resistance management.

RESULTS

Chemosensory protein 1 in Plutella xylostella (PxCSP1) was overexpressed in the two indoxacarb-resistant field populations and PxCSP1 has a high affinity with indoxacarb. PxCSP1 was upregulated when exposed to indoxacarb and the knockdown of this gene elevated sensitivity to indoxacarb, which demonstrate that PxCSP1 is involved in the indoxacarb resistance. Considering that CSPs may confer resistance in insects via binding or sequestering, we explored the binding mechanism of indoxacarb in PxCSP1-mediated resistance. Using molecular dynamics simulations and site-directed mutation, we found that indoxacarb forms a solid complex with PxCSP1 mainly through van der Waals interactions and electrostatic interactions. Between these, the electrostatic interaction provided by the Lys100 side chain in PxCSP1, and especially the hydrogen bonding between the NZ atom and the O of the carbamoyl carbonyl group of indoxacarb, are the key factors for the high affinity of PxCSP1 to indoxacarb.

CONCLUSIONS

The overexpression of PxCPS1 and its high affinity to indoxacarb is partially responsible for indoxacarb resistance in P. xylostella. Modification of indoxacarb's carbamoyl group has the potential to alleviate indoxacarb resistance in P. xylostella. These findings will contribute to solving chemosensory protein-mediated indoxacarb resistance and provide a better understanding of the insecticide resistance mechanism. © 2023 Society of Chemical Industry.

Toward Overcoming Pyrethroid Resistance in Mosquito Control: The Role of Sodium Channel Blocker Insecticides

Diseases spread by mosquitoes lead to the death of 700,000 people each year. The main way to reduce transmission is vector control by biting prevention with chemicals. However, the most commonly used insecticides lose efficacy due to the growing resistance. Voltage-gated sodium channels (VGSCs), membrane proteins responsible for the depolarizing phase of an action potential, are targeted by a broad range of neurotoxins, including pyrethroids and sodium channel blocker insecticides (SCBIs). Reduced sensitivity of the target protein due to the point mutations threatened malaria control with pyrethroids. Although SCBIs-indoxacarb (a pre-insecticide bioactivated to DCJW in insects) and metaflumizone-are used in agriculture only, they emerge as promising candidates in mosquito control. Therefore, a thorough understanding of molecular mechanisms of SCBIs action is urgently needed to break the resistance and stop disease transmission. In this study, by performing an extensive combination of equilibrium and enhanced sampling molecular dynamics simulations (3.2 μs in total), we found the DIII-DIV fenestration to be the most probable entry route of DCJW to the central cavity of mosquito VGSC. Our study revealed that F1852 is crucial in limiting SCBI access to their binding site. Our results explain the role of the F1852T mutation found in resistant insects and the increased toxicity of DCJW compared to its bulkier parent compound, indoxacarb. We also delineated residues that contribute to both SCBIs and non-ester pyrethroid etofenprox binding and thus could be involved in the target site cross-resistance.

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.

Equivalent intensity but differential dominance of sodium channel blocker insecticide resistance conferred by F1845Y and V1848I mutations of the voltage-gated sodium channel in Plutella xylostella

Two point mutations (F1845Y and V1848I) in the voltage-gated sodium channel gene of Plutella xylostella are involved in the target-site resistance to sodium channel blocker insecticides (SCBIs). The contribution of the individual mutations to the SCBI resistance and the associated inheritance modes is as yet unclear. Through 2 rounds of single-pair crossing and marker-assisted selection, 2 P. xylostella strains (1845Y and 1848I) bearing homozygous F1845Y or V1848I mutant alleles were successfully established from a field-collected population, and the contribution of each mutation to SCBI resistance, as well as associated inheritance patterns, was determined. When compared with the susceptible SZPS strain, each of the mutations individually conferred equally high-level resistance to indoxacarb (378 and 313 fold) and metaflumizone (734 and 674 fold), respectively. However, dominance levels of resistance to SCBIs were significantly different between the 2 resistant strains. Resistance of the 1845Y strain to indoxacarb and metaflumizone was inherited as an autosomal and incompletely dominant trait (D values ranged from 0.43 to 0.76). In contrast, that of the 1848I strain followed an autosomal but incompletely recessive to semidominant mode (D values: -0.24 to 0.09). Our findings enriched the current understanding of inheritance and mechanisms of SCBI resistance in P. xylostella, and will help develop resistance management programs for P. xylostella and other economic pests.

A comprehensive assessment of insecticide resistance mutations in source and immigrant populations of the diamondback moth Plutella xylostella (L.)

BACKGROUND

The diamondback moth (DBM) Plutella xylostella has developed resistance to almost all insecticides used to control it. Populations of DBM in temperate regions mainly migrate from annual breeding areas. However, the distribution pattern of insecticide resistance of DBM within the context of long-distance migration remains unclear.

RESULTS

In this study, we examined the frequency of 14 resistance mutations for 52 populations of DBM collected in 2010, 2011, 2017 and 2018 across China using a high-throughput KASP genotyping method. Mutations L1041F and T929I conferring pyrethroid resistance, and mutations G4946E and E1338D conferring chlorantraniliprole resistance were near fixation in most populations, whereas resistant alleles of F1020S, M918I, A309V and F1845Y were uncommon or absent in most populations. Resistance allele frequencies were relatively stable among different years, although the frequency of two mutations decreased. Principal component analysis based on resistant allele frequencies separated a southern population as an outlier, whereas the immigrants clustered with other populations, congruent with the migration pattern of northern immigrants coming from the Sichuan area of southwestern China. Most resistant mutations deviated from Hardy-Weinberg equilibrium due to a lower than expected frequency of heterozygotes. The deviation index of heterozygosity for resistant alleles was significantly higher than the index obtained from single nucleotide polymorphisms across the genome. These findings suggest heterogeneous selection pressures on resistant mutations.

CONCLUSION

Our results provide a picture of resistant mutation patterns in DBM shaped by insecticide usage and migration of this pest, and highlight the widespread distribution of resistance alleles in DBM. © 2022 Society of Chemical Industry.

Characterization of Indoxacarb Resistance in the Fall Armyworm: Selection, Inheritance, Cross-Resistance, Possible Biochemical Mechanisms, and Fitness Costs

The fall armyworm, Spodoptera frugiperda (J.E. Smith), is a voracious insect pest that is difficult to control due to resistance to insecticides and Bt proteins. We assessed cross-resistance, resistance mechanism, and fitness costs based on the life history traits of S. frugiperda. We established an S. frugiperda strain selected for resistance to indoxacarb (Ind-SEL) from a field-collected population and an unselected strain, Ind-UNSEL. Results indicated that after 24 generations of selection, the resistance to indoxacarb was increased by 472.67-fold as compared to the Ind-UNSEL. There was high cross-resistance to deltamethrin (31.23-fold) with very low or negligible cross-resistance to chlorantraniliprole, emamectin benzoate, and/or methoxyfenozide in the Ind-SEL population. Butoxide synergist increased susceptibility to indoxacarb, indicating that P450 enzymes may be involved in indoxacarb resistance. Significantly longer developmental time of larvae extended pupal duration, shorter adult longevity, and lower fecundity were observed in Ind-SEL as compared with the Ind-UNSEL population. The Net reproductive rate (R₀) was the only growth parameter that differs between crosses of Ind-SEL♂ × Ind-UNSEL♀ (176 ± 46) and Ind-SEL♀ × Ind-UNSEL♂ (328 ± 57). On the other hand, all population growth parameters differ between Ind-SEL and Ind-UNSEL strains. Our work contributes to the growing body of research that demonstrates the importance of strain genetics in fitness cost experiments and helps resistance management programs make decisions.

Entomopathogenic Fungi-Mediated AgNPs: Synthesis and Insecticidal Effect against Plutella xylostella (Lepidoptera: Plutellidae)

The insect Plutella xylostella is known worldwide to cause severe damage to brassica plantations because of its resistance against several groups of chemicals and pesticides. Efforts have been conducted to overcome the barrier of P. xylostella genetic resistance. Because of their easy production and effective insecticidal activity against different insect orders, silver nanoparticles are proposed as an alternative for agricultural pest control. The use of entomopathogenic fungi for nanoparticle production may offer additional advantages since fungal biomolecules may synergistically improve the nanoparticle's effectiveness. The present study aimed to synthesize silver nanoparticles using aqueous extracts of Beauveria bassiana, Metarhizium anisopliae, and Isaria fumosorosea isolates and to evaluate their insecticidal activity against P. xylostella, as innovative nano-ecofriendly pest control. The produced silver nanoparticles were evaluated by measuring the UV-vis spectrum and the mean particle size by dynamic light scattering (DLS). I. fumosorosea aqueous extract with 3-mM silver nitrate solution showed the most promising results (86-nm mean diameter and 0.37 of polydispersity). Scanning electron microscopy showed spherical nanoparticles and Fourier-Transform Infrared Spectroscopy revealed the presence of amine and amide groups, possibly responsible for nanoparticles' reduction and stabilization. The CL₅₀ value of 0.691 mg mL^-1 was determined at 72-h for the second-instar larvae of the P. xylostella, promoting a 78% of cumulative mortality rate after the entire larval stage. From our results, the synthesis of silver nanoparticles mediated by entomopathogenic fungi was successful in obtaining an efficient product for insect pest control. The I. fumosorosea was the most suitable isolate for the synthesis of silver nanoparticles contributing to the development of a green nanoproduct and the potential control of P. xylostella.

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.

Contribution of multiple overexpressed carboxylesterase genes to indoxacarb resistance in Spodoptera litura

BACKGROUND

As an important family of detoxification enzymes, carboxylesterases (CarEs) have important roles in the development of insecticide resistance in almost all agricultural pests. Previous studies have suggested that enhancement of CarE activity is an important mechanism mediating indoxacarb resistance in Spodoptera litura, and several CarE genes have been found to be overexpressed in indoxacarb-resistant strains. However, the functions of these CarE genes in indoxacarb resistance needs to be further investigated.

RESULTS

The synergist triphenyl phosphate effectively reduced the resistance of S. litura to indoxacarb, suggesting an involvement of CarEs in indoxacarb resistance. Among seven identified S. litura CarE genes (hereafter SlituCOE), six were overexpressed in two indoxacarb-resistant strains, but there were no significant differences in gene copy number. Knockdown of SlituCOE009 and SlituCOE050 enhanced indoxacarb sensitivity in both susceptible and resistant strains, whereas knockdown of SlituCOE090, SlituCOE093 and SlituCOE074 enhanced indoxacarb sensitivity in only the resistant strain. Knockdown of the sixth gene, SlituCOE073, did not have any effect. Furthermore, simultaneous knockdown of the five SlituCOE genes had a greater effect on increasing indoxacarb sensitivity than silencing them individually. By contrast, overexpression of the five SlituCOE genes individually in Drosophila melanogaster significantly decreased the toxicity of indoxacarb to transgenic fruit flies. Furthermore, modeling and docking analysis indicated that the catalytic pockets of SlituCOE009 and SlituCOE074 were ideally shaped for indoxacarb and N-decarbomethoxylated metabolite (DCJW), but the binding affinity for DCJW was stronger than for indoxacarb.

CONCLUSION

This study reveals that multiple overexpressed CarE genes are involved in indoxacarb resistance in S. litura.

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.

Influence of seasonal migration on evolution of insecticide resistance in Plutella xylostella

The diamondback moth, Plutella xylostella (L.), is one of the most destructive migratory pest species of cruciferous vegetables worldwide and has developed resistance to most of the insecticides used for its control. The migration regularity, migratory behavior, and relationship between flight and reproduction of P. xylostella have been widely reported. However, the effect of migration on insecticide resistance in this pest is still unclear. In this study, the effect of migration on P. xylostella resistance to seven insecticides was investigated using populations across the Bohai Sea that were collected in the early and late seasons during 2017-2019. The bioassay results showed that the early season populations of P. xylostella from south China possessed much higher resistance to insecticides because of intensive insecticide application; alternatively, the late season populations migrated from northeast China, where the insecticides were only used occasionally, showed much lower insecticide resistance. The genome re-sequencing results revealed that, among the eight mutations involved in insecticide resistance, the frequencies of two acetylcholinesterase mutations (A298S and G324A) responsible for organophosphorus insecticide resistance were significantly decreased in the late season populations. The results indicated that P. xylostella migration between tropical and temperate regions significantly delayed the development of insecticide resistance. These findings illustrated the effect of regional migration on the evolution of insecticide resistance in P. xylostella, and provided foundational information for further research on the relationship between migration and insecticide resistance development in other insects.

Comprehensive Detoxification Mechanism Assessment of Red Imported Fire Ant (Solenopsis invicta) against Indoxacarb

The red imported fire ant (Solenopsis invicta) is one of the deadliest invasive ant species that threatens the world by disrupting biodiversity, important functions within a natural ecosystem, and community structure. They are responsible for huge economic losses in the infested countries every year. Synthetic insecticides, especially indoxacarb, have been broadly used to control S. invicta for many years. However, the biochemical response of S. invicta to indoxacarb remains largely undiscovered. Here, we used the sublethal doses of indoxacarb on the S. invicta collected from the eight different cities of Southern China. The alteration in the transcriptome profile of S. invicta following sublethal dosages of indoxacarb was characterized using high-throughput RNA-seq technology. We created 2 libraries, with 50.93 million and 47.44 million clean reads for indoxacarb treatment and control, respectively. A total of 2018 unigenes were regulated after insecticide treatment. Results indicated that a total of 158 differentially expressed genes (DEGs) were identified in the indoxacarb-treated group, of which 100 were significantly upregulated and 58 were downregulated, mostly belonging to the detoxification enzymes, such as AChE, CarE, and GSTs. Furthermore, results showed that most of these DEGs were found in several KEGG pathways, including steroid biosynthesis, other drug metabolizing enzymes, glycerolipid metabolism, chemical carcinogenesis, drug-metabolizing cytochrome P450, glutathione metabolism, glycerophospholipid metabolism, glycolysis/gluconeogenesis, and metabolism of xenobiotics. Together, these findings indicated that indoxacarb causes significant alteration in the transcriptome profile and signaling pathways of S. invicta, providing a foundation for further molecular inquiry.

Determinants of Insecticide Resistance Evolution: Comparative Analysis Among Heliothines

It is increasingly clear that pest species vary widely in their propensities to develop insecticide resistance. This review uses a comparative approach to analyze the key pest management practices and ecological and biochemical or genetic characteristics of the target that contribute to this variation. We focus on six heliothine species, three of which, Helicoverpa armigera, Heliothis virescens, and Helicoverpa zea, have developed resistances to many pesticide classes. The three others, Helicoverpa punctigera, Helicoverpa assulta, and Helicoverpa gelotopoeon, also significant pests, have developed resistance to very few pesticide classes. We find that host range and movement between alternate hosts are key ecological traits that influence effective selection intensities for resistance. Operational issues are also critical; area-wide, cross-pesticide management practices that account for these ecological factors are key to reducing selection intensity. Without such management, treatment using broad-spectrum chemicals serves to multiply the effects of host plant preference, preadaptive detoxification ability, and high genetic diversity to create a pesticide treadmill for the three high-propensity species.Without rigorous ongoing management, such a treadmill could still develop for newer, more selective chemistries and insecticidal transgenic crops.

Field-evolved resistance to 11 insecticides and the mechanisms involved in Helicoverpa armigera (Lepidoptera: Noctuidae)

BACKGROUND

To understand the ongoing resistance of cotton bollworm, Helicoverpa armigera, the sensitivity of five field populations to commonly used insecticides, indoxacarb, abamectin, methoxyfenozide, chlorfenapyr, chlorantraniliprole, spinetoram, lambda-cyhalothrin, carbosulfan, metaflumizone, chlorpyrifos, and flufenoxuron, were evaluated. Furthermore, the biochemical and molecular mechanisms of field-evolved resistance in H. armigera were also investigated.

RESULTS

Five field populations of H. armigera showed moderate resistance to indoxacarb, chlorantraniliprole, metaflumizone, methoxyfenozide, carbosulfan and lambda-cyhalothrin. The resistance ratio (RR) of indoxacarb was significantly correlated with glutathione-S-transferases (GSTs) activity (r = 0.913, P = 0.011). Methoxyfenozide RR was largely correlated with cytochrome P450s activity (r = 0.860, P = 0.028). Besides, six cytochrome P450s genes of CYP4L5 in AQP, CYP6B7 and CYP9A14 in HDP and BDP, CYP9A17V2 in HDP and YSP, CYP332A1 in HDP, LFP, AQP and YSP, CYP337B1 in YSP, and two GSTs genes of GSTd1 and GSTs1 in HDP were overexpressed (>5-fold). Moreover, indoxacarb RR was positively correlated with the overexpression of GSTs1, GSTd1 and CYP9A14 genes (r = 0.880, 0.98 and 0.86, P = 0.021, 0.001 and 0.028, respectively). The transcript of CYP9A17V2 and CYP337B1 were found to be correlated with metaflumizone RR (r = 0.950, P = 0.004) and carbosulfan RR (r = 0.850, P = 0.033), respectively.

CONCLUSION

H. armigera can be effectively controlled using abamectin, chlorfenapyr, chlorpyrifos and spinetoram in Hebei and Shandong provinces. The present study demonstrated that the relative expression level of GSTs1, GSTd1, CYP9A14, CYP9A17V2 and CYP337B1 genes were significantly correlated with the resistance ratio to indoxacarb, metaflumizone and carbosulfan in field H. armigera.

Population structure and insecticide resistance status of Tuta absoluta populations from Turkey

BACKGROUND

Tuta absoluta is a devastating pest in tomato production areas worldwide. After its first introduction to Turkey in 2009, it quickly became the major pest of tomato-growing areas. Although some biocontrol agents have been used, especially in greenhouses, the main control of T. absoluta relies heavily on chemical insecticides. However, failure in chemical control has often been reported due to resistance development. In this study, we investigated (i) the population structure of 22 T. absoluta populations across Turkey by analysing haplotypes, based on the cytochrome oxidase subunit I gene; (ii) the efficacy of three registered insecticides from different classes (metaflumizone, chlorantraniliprole and spinosad) in real field-greenhouse conditions; and (iii) the geographic distribution of target-site mutations associated with insecticide resistance.

RESULTS

The efficacy of spinosad was higher than that of chlorantraniliprole and metaflumizone in the greenhouse trials, as documented by the mortality rates obtained, up to 14 days post application. Known resistance mutations in ryanodine receptors (RyR) (i.e. the I4790M/K and G4946E), nicotinic acetylcholine receptors (G275E), acetylcholinesterases (A201S) and voltage-gated sodium channels (F1845Y and V1848I) were found at various frequencies across the populations genotyped. The I4790K diamide resistance mutation in the RyR has been reported for the first time in T. absoluta populations. Although a total of eight haplotypes were found, the overall mean genetic distance was lower than 0.001, indicating the high genetic homogeneity among Turkish T. absoluta populations.

CONCLUSION

The results will contribute to design area-wide resistance management programs in T. absoluta control in Turkey. However, more monitoring studies are needed to implement evidence-based insecticide resistance management strategies in the frame of integrated pest management. © 2021 Society of Chemical Industry.

Determination of the residue behavior of isocycloseram in Brassica oleracea and soil using the QuEChERS method coupled with HPLC

Isocycloseram is a new isoxazoline insecticide that can efficiently control the diamondback moth in cruciferous crops. The aim of this study was to establish a method for the determination of isocycloseram residues in/on cabbage and in the soil using HPLC-UV at 264 nm. A field test was conducted in December 2019 and 2020 to monitor isocycloseram dissipation in Jiangxi, China. Acetonitrile was used to extract isocycloseram from cabbage and soil. C18 and GCB were used to purify cabbage extracts, whereas soil extracts did not require purification. At the addition level of 0.01-1.0 mg/kg, the average recoveries in cabbage and soil were 91.81-109.95% and 89.89-104.08% respectively. After having applied 10% isocycloseram DC, isocycloseram dissipated faster in the cabbage matrix. Isocycloseram residues on cabbage leaves could be removed through simple cleaning methods, especially by soaking in 2% citric acid.

Comparative characterization of two putative duplicated sodium channel genes in the red flour beetle, Tribolium castaneum

The insect voltage-gated sodium channel is the primary target of pyrethroids and novel efficient insecticides such as indoxacarb and metaflumizone. In this study, we cloned and characterized two putative sodium channel genes, TcNav1 and TcNav2, in Tribolium castaneum. The composite TcNav1 and TcNav2 encode a protein of 2045 and 2037 amino acid residues, sharing 76.1% and 75.5% amino acid identity with Drosophila para, respectively. Comparative analysis of genomic organization showed that TcNav1 and TcNav2 contain 26 and 27 exons, respectively. Analysis of the expression patterns showed that the mRNA levels of TcNav1 and TcNav2 were predominantly expressed in the head. RNAi-mediated knockdown of both TcNav1 and TcNav2 adversely affected adult emergence and significantly decreased sensitivity to deltamethrin. Significantly reduced pupation rate and sensitivity to beta-cypermethrin were observed after injection of siRNA targeting TcNav1 but not TcNav2. Taken together, we provide evidence that sodium channel gene has undergone duplication in T. castaneum, resulting in diversified developmental and toxicological functions.

Cross-resistance, fitness costs, and biochemical mechanism of laboratory-selected resistance to tenvermectin A in Plutella xylostella

BACKGROUND

Tenvermectin A is a new avermectin derivative that has good insecticidal and acaricidal effects. In order to study the resistance of Plutella xylostella to tenvermectin A, a sensitive strain (SS) and a laboratory-selected tenvermectin A-resistant strain (RS, 33.57-fold) were used to evaluate cross-resistance and fitness costs as well as to determine the resistance mechanism.

RESULTS

There was no cross-resistance with common pesticides except for moderate cross-resistance with cypermethrin (resistance ratio = 10.26-fold) observed in RS. The activities of metabolic enzymes were measured, and the results showed that mixed function oxidase (MFO) and carboxylate esterase (CarE) in RS increased significantly by 2.92- and 2.86-fold, respectively, compared with SS. In addition, there was no obvious difference in glutathione-S-transferase (GST), which indicated that enhanced MFO and CarE activities may be the main mechanisms of detoxification. In the four typical resistance-related genes, expression of GluCl (4.86-fold), ABCC2 (3.85-fold), and CYP6 (2.94-fold) in RS were significantly promoted, but expression of GST was not. The clone and sequence of the PxGluClα subunit displayed six mutations that could lead to changes in the amino acid residues.

CONCLUSION

High suitability related to tenvermectin A resistance was observed in RS, and it was found that the developmental stages of RS were significantly shortened and the survival rate of females was reduced. In addition, the mechanism of resistance to tenvermectin A may be regulated by the glutamate-gated chloride channel, ATP-binding cassette transporter, and MFO. In general, the study of resistance and biochemical mechanisms can provide beneficial and rational information for the management of resistance in P. xylostella. © 2021 Society of Chemical Industry.

Oxazosulfyl, a Novel Sulfyl Insecticide, Binds to and Stabilizes the Voltage-Gated Sodium Channels in the Slow-Inactivated State

Oxazosulfyl is the first representative of a novel sulfyl class of insecticides with a potent and cross-spectrum insecticidal activity, albeit with an unclear mechanism of action. As a potential agent of pest control in rice fields, we investigated the action of oxazosulfyl on the nervous system and voltage-gated sodium channels in insects. After the injection of 10 μg of oxazosulfyl, American cockroaches (Periplaneta americana) were quickly paralyzed, which persisted for more than 7 days. Extracellular recordings revealed a depressed spontaneous nerve activity in the cockroaches injected with oxazosulfyl, which specifically affected the voltage-gated sodium channels (in German cockroaches (Blattella germanica) expressed in Xenopus oocytes) in the slow-inactivated state resulting in the inhibition of sodium currents. The potency of oxazosulfyl and other sodium channel blockers to block sodium channels was consistent with their insecticidal activity. Thus, we conclude that the action mode of oxazosulfyl involves the state-dependent blockage of voltage-gated sodium channels.

Acute and Chronic Toxicity of Indoxacarb in Two Populations of Plutella xylostella (Lepidoptera: Plutellidae)

The diamondback moth, Plutella xylostella (Linnaeus, 1758), is one of the main pests of brassicas, and various insecticides, such as indoxacarb, are used to control it. However, frequent insecticide applications favor the selection of resistant individuals. Thus, the residual and sublethal effects of indoxacarb in two populations of P. xylostella, one collected in the field (FP) and one from the laboratory (LP), were evaluated and compared. The objective of this research was to investigate the toxicity of indoxacarb at residual and sublethal levels in a field population of P. xylostella from Brazil and a population from the laboratory. Leaf-dip bioassays showed high toxicity, with LC50 values after 48 h of 3.7 and 6.9 mg/liter for the LP and FP, respectively. Sublethal effects were indicated by significant reduction in the survival of larvae, pupae, and offspring. There was an increase in foliar consumption and a decrease in adult survival in the LP, and a decrease in fecundity in the FP. For the LP and FP population, the mean values for R0, rm, and λ for the control treatment were significantly higher than for the treatment groups (CL15 and CL25). Exposure of larvae to sublethal indoxacarb concentrations significantly reduced larval and pupal survival. Larval and pupal survival decreased as the indoxacarb concentration increased. Fecundity was significantly lower for the FP at LC15 (96.2 eggs per female) and LC25 (69.2 eggs per female) concentrations compared with the other treatments.

Biogenetic cantharidin is a promising leading compound to manage insecticide resistance of Mythimna separata (Lepidoptera: Noctuidae)

Cantharidin (CTD) is a natural toxin with effective toxicity to lepidopteran pests. Nevertheless, little information is available on whether pests develop resistance to CTD. After being exposed to CTD (50 mg/L to 90 mg/L) or 10 generations, the resistance ratio of laboratory selected cantharidin-resistant Mythimna separata (Cantharidin-SEL) strain was only elevated 1.95-fold. Meanwhile, the developmental time for M. separata was prolonged (delayed1.65 in males and 1.84 days in females). The reported CTD target, the serine/threonine phosphatases (PSPs), have not been shown significant activity variation during the whole process of CTD-treatment. The activity of detoxification enzymes (cytochrome monooxygenase P450, glutathione-S-transferase (GST) and carboxylesterase) were affected by CTD selection, but this change was not mathematically significant. More importantly, no obvious cross-resistance with other commonly used insecticides was observed in the M. separata population treated with CTD for 10 generations (resistance ratios were all lower 2.5). Overall, M. separata is unlikely to produce target-site insensitivity resistance, metabolic resistance to CTD. Meanwhile, cantharidin-SEL is not prone to develop cross-resistance with other insecticides. These results indicate that CTD is a promising biogenetic lead compound which can be applied in the resistance management on M. separata.

Indoxacarb-loaded fluorescent mesoporous silica nanoparticles for effective control of Plutella xylostella L. with decreased detoxification enzymes activities

BACKGROUND

Plutella xylostella L. is a cosmopolitan lepidopteron insect pest for numerous vegetables and crops. The extensive use of insecticides has resulted in the emergence of resistance in P. xylostella. Thus, development of innovative strategies to overcome the insecticide resistance and control P. xylostella effectively is highly desirable. Inspired by the concept and breakthrough of nanomedical strategies to treat multidrug resistance, nanotechnology may find potential application in overcoming or delaying insecticide resistance.

RESULTS

Carbon dots-embedded fluorescent mesoporous silica nanoparticles (FL-SiO₂ NPs) were successfully developed. Indoxacarb-loaded nanoparticles (IN@FL-SiO₂ NPs) were facilely prepared with loading content of 24%. The release of indoxacarb from IN@FL-SiO₂ NPs was pH sensitive. IN@FL-SiO₂ NPs exhibited better insecticidal activity against P. xylostella than indoxacarb technical under the same doses of active ingredient applied. Moreover, the activities of detoxification enzymes including GST, CarE, and P450 of P. xylostella were suppressed by treatment with IN@FL-SiO₂ NPs. Furthermore, the entry of FL-SiO₂ NPs into the midgut of P. xylostella was confirmed by CLSM observation.

CONCLUSIONS

Although there is no absolute correlation between the enzyme activity and resistance, the change in corresponding enzyme activity can afford valuable information on the resistance situation. IN@FL-SiO₂ NPs treated P. xylostella displayed higher mortality, along with decreased enzymes activities, which indicates that nano-based delivery system of insecticide could be potentially applied in insecticide resistance management. © 2020 Society of Chemical Industry.

Mechanisms of Increased Indoxacarb Toxicity in Methoxyfenozide-Resistant Cotton Bollworm Helicoverpa armigera (Lepidoptera: Noctuidae)

Indoxacarb is an important insecticide for the selective control of Helicoverpa armigera. It can be bioactivated to the more effective N-decarbomethoxylated indoxacarb (DCJW) by esterases in pests. It was observed that both field and laboratory selected populations of H. armigera showed negative cross-resistance between indoxacarb and methoxyfenozide. The Handan population exhibited moderate resistance to indoxacarb, but was susceptible to methoxyfenozide; the Baoding and Yishui populations exhibited moderate resistance to methoxyfenozide, but they were susceptible to indoxacarb. Moreover, the toxicity of indoxacarb was enhanced 1.83-fold in the laboratory methoxyfenozide-resistant H. armigera, and susceptibility to methoxyfenozide was increased 2.81-fold in the laboratory indoxacarb-resistant H. armigera. In vivo, DCJW concentrations in the susceptible and methoxyfenozide-selected (laboratory methoxyfenozide-resistant) populations were 4.59- and 4.31-fold greater than in the indoxacarb-resistant Handan population 1 h after dosing. After 2 h, the highest concentrations of DCJW and indoxacarb appeared in the methoxyfenozide-selected population. Meanwhile, increased carboxyl esterase (CarE) and decreased glutathione S-transferase (GST) activities were observed in the methoxyfenozide-selected population. However, the indoxacarb-selected (laboratory indoxacarb-resistant) and Handan populations showed a higher disappearance of indoxacarb and DCJW, and the activity of cytochrome P450 mono-oxygenase in these populations were significantly increased. This study showed that the improved toxicity of indoxacarb, as observed in the methoxyfenozide-selected H. armigera, was correlated with increased CarE activity, decreased GST activity, and the in vivo accumulation of indoxacarb and DCJW. The significantly increased cytochrome P450 activity and higher disappearance of indoxacarb and DCJW in indoxacarb-resistant H. armigera resulted in the decreased toxicity of indoxacarb.

Using CRISPR/Cas9 genome modification to understand the genetic basis of insecticide resistance: Drosophila and beyond

Chemical insecticides are a major tool for the control of many of the world's most damaging arthropod pests. However, their intensive application is often associated with the emergence of resistance, sometimes with serious implications for sustainable pest control. To mitigate failure of insecticide-based control tools, the mechanisms by which insects have evolved resistance must be elucidated. This includes both identification and functional characterization of putative resistance genes and/or mutations. Research on this topic has been greatly facilitated by using powerful genetic model insects like Drosophila melanogaster, and more recently by advances in genome modification technology, notably CRISPR/Cas9. Here, we present the advances that have been made through the application of genome modification technology in insecticide resistance research. The majority of the work conducted in the field to date has made use of genetic tools and resources available in D. melanogaster. This has greatly enhanced our understanding of resistance mechanisms, especially those mediated by insensitivity of the pesticide target-site. We discuss this progress for a series of different insecticide targets, but also report a number of unsuccessful or inconclusive attempts that highlight some inherent limitations of using Drosophila to characterize resistance mechanisms identified in arthropod pests. We also discuss an experimental framework that may circumvent current limitations while retaining the genetic versatility and robustness that Drosophila has to offer. Finally, we describe examples of direct CRISPR/Cas9 use in non-model pest species, an approach that will likely find much wider application in the near future.

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.

Disruption of nicotinic acetylcholine receptor α6 mediated by CRISPR/Cas9 confers resistance to spinosyns in Plutella xylostella

BACKGROUND

The clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9 system provides some advantages over other reverse genetic techniques to investigate the causal relationship between insecticide resistance phenotype and candidate gene. Several studies published to date point to the nicotinic acetylcholine receptor (nAChR) α6 subunit strongly associated with spinosyns resistance in insects, including Plutella xylostella. However, reverse genetic verification of the P. xylostella nAChRα6 has not yet been achieved via an in vivo approach.

RESULTS

Here, we successfully constructed a homozygous strain (Pxα6-KO) with a 2-nt deletion mutation of nAChRα6 by CRISPR/Cas9 coupled with non-homologous end joining approach in P. xylostella. The manipulated mutation results in a frame shift in the open reading frame of transcripts, which produces a predicted protein truncated in the TM3-TM4 loop region. When compared to the background strain IPP-S, the knockout strain Pxα6-KO exhibited 229- and 1462-fold resistance to spinosad and spinetoram, respectively, but no or limited (resistance ratios <3-fold) effects on the toxicities of imidacloprid, abamectin, β-cypermethrin, indoxacarb, metaflumizone and chlorantraniliprole. Furthermore, the mode of inheritance of the acquired spinetoram resistance was autosomal recessive and significantly linked with the 2-nt deletion mutation of nAChRα6 in the Pxα6-KO strain.

CONCLUSION

In vivo functional investigation demonstrates the causality of the Pxα6 truncating mutation with high levels of resistance to spinosyns in P. xylostella. Our results suggest the Pxα6-KO strain underlies an autosomal, recessive mode of inheritance for spinetoram resistance, and reinforces the association of this gene to the mode of action of spinosyns. © 2019 Society of Chemical Industry.

Cross-resistance, biochemical mechanism and fitness costs of laboratory-selected resistance to pyridalyl in diamondback moth, Plutella xylostella

Pyridalyl belongs to one novel type of insecticides with uncertain mode of action, and it showed significant efficacy against Plutella xylostella, which has been considered as one notorious insect pest in the world. To characterize pyridalyl resistance in P. xylostella, one susceptible strain XY-PS and one laboratory-selected pyridalyl-resistant strain XY-PR (34.4-fold) were used to establish cross-resistance patterns, and no cross-resistance to a series of popular insecticides in the XY-PR was observed. Activities of metabolic enzymes were measured and results showed that there was an approximate 5.2-fold significant increase in cytochrome P450 monooxygenase (P450) and no significant differences in glutathione S-transferase (GST) and esterase between XY-PR and XY-PS, indicating that the enhanced activity of P450 could be dominant mechanism of detoxification. Furthermore, expression profiles of three previously published resistance-associated P450 genes were established but no one was significantly different expression. Besides, fitness costs associated with pyridalyl resistance was observed in XY-PR, and it had been found that survival rate of larvae and hatchability were reduced in XY-PR. Then, by calculating the net replacement rate (R₀) of XY-PS, the fitness of XY-PR was established as 0.64. In conclusion, above results provided helpful data and information for studying further on mechanism of pyridalyl resistance, and will be conductive to design rational strategies of resistance management in P. xylostella.

Linkage mapping an indoxacarb resistance locus in Helicoverpa armigera (Lepidoptera: Noctuidae) by genotype-by-sequencing

BACKGROUND

A major challenge to sustainable agricultural pest control is the rapid evolution of insecticide resistance. This is caused by mechanisms that reduce insecticide efficacy. Understanding the genetic mechanisms of resistance is essential for DNA-based monitoring of resistance in field populations. One such insecticide is indoxacarb, an important selective control option for Helicoverpa armigera in a range of crops including grain, horticulture and cotton. Recently, a strain of H. armigera (GY7-39) resistant to indoxacarb (198-fold) was isolated from field-collected moth.

RESULTS

To identify the indoxacarb resistance locus, GY7-39 was backcrossed for six generations to susceptible strain New GR. In each generation, only resistant males were used to cross back to New GR. Genotype-by-sequencing was carried out on 95 H. armigera samples. In total, 13 203 tags with 8697 unique locations on the H. armigera genome were obtained. The indoxacarb resistance locus in strain GY7-39 was mapped to a 2.6 Mbp region on chromosome 16. In this region, two closely linked loci (IndoR1 and IndoR2) were found to be associated with indoxacarb resistant GY7-39.

CONCLUSIONS

We mapped indoxacarb resistance in GY7-39 to two closely linked loci IndoR1 and IndoR2 in a narrowed 2.6 Mbp region of H. armigera chromosome 16. The results provide essential background data for future genetic investigations including fine mapping of the indoxacarb resistance gene and the eventual development of an effective DNA-based diagnostic to support resistance management. © 2019 Society of Chemical Industry.

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 and Synergism of Novel Insecticides in Field Populations of Cotton Bollworm Helicoverpa armigera (Lepidoptera: Noctuidae) in Pakistan

The cotton bollworm Helicoverpa armigera (Hübner) is still a serious pest of non-Bt crops in Asia and Africa. It has been a notorious pest in developing resistance to all the insecticide classes applied for its control. Response of field populations of H. armigera to new chemistries having novel modes of action was monitored during 2003-2016 using a leaf-dip bioassay. No or very low level of resistance was found to newer insecticides, such as spinetoram, chlorantraniliprole, and flubendiamide during 2008-2016. There was also no or very low resistance to chlorfenapyr from 2003 to 2016. Resistance to spinosad, abamectin, and emamectin benzoate was none or very low as well during 2003-2013 and then it rose to a low level for spinosad and to a high level for avermectins by the year 2016. Methoxyfenozide resistance was very low during 2003-2010, low during 2011-2013, and moderate during 2014-2016. Resistance to thiocyclam was very low to low during 2009-2011, but it increased to moderate to high levels during 2012-2016. Resistance to indoxacarb was moderate during 2003-2006, which then decreased to low level during 2007-2010, very low level during 2011-2014, and no resistance during 2015 and 2016 corresponding to the reduced use of indoxacarb in the Pakistani agriculture. Piperonyl butoxide and tribufos exhibited a good synergism with indoxacarb and chlorfenapyr but not with spinosad in H. armigera populations. Rotation of new insecticides, having no, very low, and low resistance along with other IPM practices, is recommended to manage resistance to insecticides in H. armigera.

Long-term monitoring and characterization of resistance to chlorfenapyr in Plutella xylostella (Lepidoptera: Plutellidae) from China

BACKGROUND

Plutella xylostella (L.) is a worldwide pest of crucifer crops. Chlorfenapyr has been used extensively to manage several insect pests in vegetables, including P. xylostella. In this study, 48 field populations were collected from 11 geographic locations of China from 2008 to 2017 for resistance monitoring to chlorfenapyr. Inheritance and synergism of chlorfenapyr resistance were characterized in a field-derived population.

RESULTS

Compared with the susceptible SZ strain, 33 of 39 populations collected from Eastern China were susceptible or developed low levels of resistance to chlorfenapyr (< 10-fold), five populations developed mid-level resistance (11-33-fold), and only one population had high level resistance (208-fold). However, seven of nine populations sampled from Southern China evolved 136-334-fold resistance. Interestingly, the dynamic of the median lethal concentration (LC₅₀ ) values from four continuous sampling sites (NJ, HF, JN and KS) showed a sharp increase in 2009, which implies that adults may have immigrated from chlorfenapyr-resistant regions. A highly resistant field population (GZ15) was further studied. By crossing the GZ15 and susceptible SZ strains we showed that resistance to chlorfenapyr was autosomally inherited and incompletely dominant. Synergism bioassays revealed that metabolic detoxification might not be responsible for chlorfenapyr resistance in the GZ15 population.

CONCLUSIONS

Some field populations of P. xylostella from China have evolved high levels of resistance to chlorfenapyr. Field-evolved resistance to chlorfenapyr is autosomal and dominant in the GZ15 population. These findings provide useful information for the design of effective resistance management strategies against P. xylostella in the field. © 2018 Society of Chemical Industry.

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.

Diversity and convergence of mechanisms involved in pyrethroid resistance in the stored grain weevils, Sitophilus spp

Target-site mutations and changes in insect metabolism or behavior are common mechanisms in insecticide-resistant insects. The co-occurrence of such mechanisms in a pest strain is a prominent threat to their management, particularly when alternative compounds are scarce. Pyrethroid resistance among stored grain weevils (i.e., Sitophilus spp.) is an example of a long-standing concern, for which reports of resistance generally focus on a single mechanism in a single species. Here, we investigated pyrethroid resistance in maize and rice weevils (i.e., Sitophilus zeamais and S. oryzae), exploring potential knockdown resistance (kdr) mutations in their sodium channels (primary site for pyrethroid actions) and potential changes in their detoxification and walking processes. Resistance in pyrethroid-resistant rice weevils was associated with the combination of a kdr mutation (L1014F) and increases in walking and detoxification activities, while another kdr mutation (T929I) combined with increases in walking activity were the primary pyrethroid resistance mechanisms in maize weevils. Our results suggest that the selection of pyrethroid-resistant individuals in these weevil species may result from multiple and differential mechanisms because the L1014F mutation was only detected in Latin American rice weevils (e.g., Brazil, Argentina and Uruguay), not in Australian and Turkish rice weevils or Brazilian maize weevils.

Resistance selection of indoxacarb in Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae): cross-resistance, biochemical mechanisms and associated fitness costs

BACKGROUND

The cotton bollworm Helicoverpa armigera is a worldwide insect pest with the ability to develop resistance to many insecticides. Indoxacarb, a sodium channel blocker, is an important insecticide that is used to control H. armigera. Cross-resistance, metabolic mechanisms and life history traits were established for an indoxacarb-selected (IND-SEL) population of H. armigera.

RESULTS

After 11 generations of selection, the susceptibility to indoxacarb was decreased by 4.43-fold and the estimated realized heritability (h² ) was only 0.072. Interestingly, the IND-SEL population was more susceptible to methoxyfenozide and abamectin than the susceptible population. The activities of cytochrome P450 monooxygenase (P450) and carboxyl esterase (CarE) were significantly increased in IND-SEL H. armigera. Thus, susceptibility to indoxacarb was increased by piperonyl butoxide and S,S,S-tributyl phosphorothioate, showing synergistic ratios of 2.54- and 1.82-fold, respectively. Moreover, the IND-SEL population had a reduced relative fitness (0.67), with a lower growth rate and fecundity than the susceptible population.

CONCLUSIONS

Evidence from this study suggests that decreased susceptibility to indoxacarb may be associated with fitness costs in H. armigera and enhanced activities of P450 and CarE may be important detoxification mechanisms in the development of indoxacarb resistance. Methoxyfenozide and abamectin can be rotationally used to manage indoxacarb resistance. © 2018 Society of Chemical Industry.

Characterization of UDP-glucuronosyltransferase genes and their possible roles in multi-insecticide resistance in Plutella xylostella (L.)

BACKGROUND

Uridine diphosphate-glucuronosyltransferases (UGTs), as multifunctional detoxification enzymes, play important roles in the biotransformation of various compounds. However, their roles in insecticide resistance are still unclear. This study presents a genome-wide identification of the UGTs in diamondback moth, Plutella xylostella (L.), a notorious insect pest of cruciferous crops worldwide. The possible roles of these UGTs in insecticide resistance were evaluated.

RESULTS

A total of 21 putative UGTs in P. xylostella were identified. Quantitative real-time polymerase chain reaction (PCR)-based analyses showed that all the UGT genes were expressed in all tested developmental stages and tissues. Bioassay results indicated that a field-collected population (BL) was resistant to 9 of 10 commonly used insecticides, and 10 of 21 UGT mRNAs were upregulated in the BL population. Exposure to the LC₅₀ of each insecticide affected the expression of most UGT genes. Among these, the expression levels of UGT40V1, UGT45B1 and UGT33AA4 were induced by more than five insecticides, whereas indoxacarb and metaflumizone significantly repressed the expression of most UGT genes.

CONCLUSION

UGTs may play important roles in the metabolism of commonly used insecticides in P. xylostella. These findings provide valuable information for further research on the physiological and toxicological functions of specific UGT genes in P. xylostella. © 2017 Society of Chemical Industry.

Cross-resistance and biochemical mechanisms of resistance to indoxacarb in the diamondback moth, Plutella xylostella

Indoxacarb belongs to a class of insecticides known as oxadiazines and is the first commercialized pyrazoline-type voltage-dependent sodium channel blocker. A moderate level of resistance to indoxacarb has evolved in field populations of Plutella xylostella from Central China. In the present study, cross-resistance, resistance stability and metabolic mechanisms of indoxacarb resistance were investigated in this moth species. A P. xylostella strain with a high level of resistance to indoxacarb was obtained through continuous selection in the laboratory. The strain showed cross-resistance to metaflumizone, beta-cypermethrin and chlorfenapyr, but no resistance to cyantraniliprole, chlorantraniliprole, abamectin, chlorfluazuron, spinosad and diafenthiuron compared with the susceptible strain. Synergism tests revealed that piperonyl butoxide (PBO) (synergistic ratio, SR=7.8) and diethyl maleate (DEF) (SR=3.5) had considerable synergistic effects on indoxacarb toxicity in the resistant strain (F₅₈). Enzyme activity data showed there was an approximate 5.8-fold different in glutathione S-transferase (GST) and a 6.8-fold different in cytochrome P450 monooxygenase between the resistant strain (F₅₈) and susceptible strain, suggesting that the increased activity of these two enzymes is likely the main detoxification mechanism responsible for the species' resistance to indoxacarb. These results will be helpful for insecticide resistance management strategies to delay the development of indoxacarb resistance in fields.

Identification and detection of indoxacarb resistance mutations in the para sodium channel of the tomato leafminer, Tuta absoluta

BACKGROUND

Indoxacarb is an important active ingredient extensively used for the control of Tuta absoluta, a major tomato pest, playing a particular role in insecticide resistance management schemes.

RESULTS

Reduced susceptibility to indoxacarb was identified (1794-fold resistance) through toxicological bioassays in a field population from Greece and evolved rapidly to resistance after short laboratory selection. Combined bioassays with synergists and biochemical analysis suggested only a partial involvement of detoxification enzymes in the resistant phenotype. To investigate the role of target-site resistance, segment 6 of domain IV of the sodium channel in T. absoluta was cloned and the sequences compared between susceptible and indoxacarb-resistant T. absoluta insects. The presence of the F1845Y and the V1848I indoxacarb resistance mutations was detected and was strongly associated with the phenotype. These amino acid substitutions correspond to recently characterised indoxacarb resistance mutations in diamondback moth (Plutella xylostella). Robust and accurate PCR-RFLP assays were subsequently developed and successfully validated for detecting both indoxacarb resistance mutations in field T. absoluta populations.

CONCLUSION

The identification of indoxacarb resistance mutations and the development of diagnostic tools will allow early detection of indoxacarb resistance, facilitating implementation of appropriate resistance management strategies, thus delaying the spread of resistance. © 2016 Society of Chemical Industry.

Mutations on M3 helix of Plutella xylostella glutamate-gated chloride channel confer unequal resistance to abamectin by two different mechanisms

Abamectin is one of the most widely used avermectins for agricultural pests control, but the emergence of resistance around the world is proving a major threat to its sustained application. Abamectin acts by directly activating glutamate-gated chloride channels (GluCls) and modulating other Cys-loop ion channels. To date, three mutations occurring in the transmembrane domain of arthropod GluCls are associated with target-site resistance to abamectin: A309V in Plutella xylostella GluCl (PxGluCl), G323D in Tetranychus urticae GluCl1 (TuGluCl1) and G326E in TuGluCl3. To compare the effects of these mutations in a single system, A309V/I/G and G315E (corresponding to G323 in TuGluCl1 and G326 in TuGluCl3) substitutions were introduced individually into the PxGluCl channel. Functional analysis using Xenopus oocytes showed that the A309V and G315E mutations reduced the sensitivity to abamectin by 4.8- and 493-fold, respectively. In contrast, the substitutions A309I/G show no significant effects on the response to abamectin. Interestingly, the A309I substitution increased the channel sensitivity to glutamate by one order of magnitude (∼12-fold). Analysis of PxGluCl homology models indicates that the G315E mutation interferes with abamectin binding through a steric hindrance mechanism. In contrast, the structural consequences of the A309 mutations are not so clear and an allosteric modification of the binding site is the most likely mechanism. Overall the results show that both A309V and G315E mutations may contribute to target-site resistance to abamectin and may be important for the future prediction and monitoring of abamectin resistance in P. xylostella and other arthropod pests.

Fitness and inheritance of metaflumizone resistance in Plutella xylostella

The diamondback moth, Plutella xylostella (L.) has developed resistance to many types of insecticides in the field. To study inheritance and fitness cost of metaflumizone resistance, a susceptible strain of diamondback moth was continuously selected with metaflumizone during 37 generations under laboratory conditions. The resistance to metaflumizone was at a high level (resistance ratios from 250.37 to 1450.47-fold). We investigated a metaflumizone resistance strain (G₂₇) and a susceptible strain of P. xylostella, using the age-stage, two-sex life table approach. Compared to the susceptible strain, egg duration, the developmental time of the first and second instar larvae, pupae duration, adult preoviposition period (APOP), total preoviposition period (TPOP), egg hatchability, the survival rate of second instar larva and the mean generation time (T) were significantly differences in the resistant strain. The resistant strain had a relative fitness of 0.78. The inheritance of metaflumizone resistance was also studied by crossing the metaflumizone resistant and susceptible populations. Results revealed an autosomal and incompletely recessive mode of inheritance for metaflumizone resistance in the resistant population of P. xylostella. The present study provided useful information for planning potential management strategies to delay development of metaflumizone resistance in P. xylostella.

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.

The Receptor Site and Mechanism of Action of Sodium Channel Blocker Insecticides

Sodium channels are excellent targets of both natural and synthetic insecticides with high insect selectivity. Indoxacarb, its active metabolite DCJW, and metaflumizone (MFZ) belong to a relatively new class of sodium channel blocker insecticides (SCBIs) with a mode of action distinct from all other sodium channel-targeting insecticides, including pyrethroids. Electroneutral SCBIs preferably bind to and trap sodium channels in the inactivated state, a mechanism similar to that of cationic local anesthetics. Previous studies identified several SCBI-sensing residues that face the inner pore of sodium channels. However, the receptor site of SCBIs, their atomic mechanisms, and the cause of selective toxicity of MFZ remain elusive. Here, we have built a homology model of the open-state cockroach sodium channel BgNav1-1a. Our computations predicted that SCBIs bind in the inner pore, interact with a sodium ion at the focus of P1 helices, and extend their aromatic moiety into the III/IV domain interface (fenestration). Using model-driven mutagenesis and electrophysiology, we identified five new SCBI-sensing residues, including insect-specific residues. Our study proposes the first three-dimensional models of channel-bound SCBIs, sheds light on the molecular basis of MFZ selective toxicity, and suggests that a sodium ion located in the inner pore contributes to the receptor site for electroneutral SCBIs.

Insecticide resistance and nutrition interactively shape life-history parameters in German cockroaches

Fitness-related costs of evolving insecticide resistance have been reported in a number of insect species, but the interplay between evolutionary adaptation to insecticide pressure and variable environmental conditions has received little attention. We provisioned nymphs from three German cockroach (Blattella germanica L.) populations, which differed in insecticide resistance, with either nutritionally rich or poor (diluted) diet throughout their development. One population was an insecticide-susceptible laboratory strain; the other two populations originated from a field-collected indoxacarb-resistant population, which upon collection was maintained either with or without further selection with indoxacarb. We then measured development time, survival to the adult stage, adult body size, and results of a challenge with indoxacarb. Our results show that indoxacarb resistance and poor nutritional condition increased development time and lowered adult body size, with reinforcing interactions. We also found lower survival to the adult stage in the indoxacarb-selected population, which was exacerbated by poor nutrition. In addition, nutrition imparted a highly significant effect on indoxacarb susceptibility. This study exemplifies how poor nutritional condition can aggravate the life-history costs of resistance and elevate the detrimental effects of insecticide exposure, demonstrating how environmental conditions and resistance may interactively impact individual fitness and insecticide efficacy.

A three amino acid deletion in the transmembrane domain of the nicotinic acetylcholine receptor α6 subunit confers high-level resistance to spinosad in Plutella xylostella

Spinosad is a macrocyclic lactone insecticide that acts primarily at the nicotinic acetylcholine receptors (nAChRs) of target insects. Here we describe evidence that high levels of resistance to spinosad in the diamondback moth (Plutella xylostella) are associated with a three amino acid (3-aa) deletion in the fourth transmembrane domain (TM4) of the nAChR α6 subunit (Pxα6). Following laboratory selection with spinosad, the SZ-SpinR strain of P. xylostella exhibited 940-fold resistance to spinosad. In addition, the selected insect population had 1060-fold cross-resistance to spinetoram but, in contrast, no cross-resistance to abamectin was observed. Genetic analysis indicates that spinosad resistance in SZ-SpinR is inherited as a recessive and autosomal trait, and that the 3-aa deletion (IIA) in TM4 of Pxα6 is tightly linked to spinosad resistance. Because of well-established difficulties in functional expression of cloned insect nAChRs, the analogous resistance-associated deletion mutation was introduced into a prototype nAChR (the cloned human α7 subunit). Two-electrode voltage-clamp recording with wild-type and mutated nAChRs expressed in Xenopus laevis oocytes indicated that the mutation causes a complete loss of agonist activation. In addition, radioligand binding studies indicated that the 3-aa deletion resulted in significantly lower-affinity binding of the extracellular neurotransmitter-binding site. These findings are consistent with the 3-amino acid (IIA) deletion within the transmembrane domain of Pxα6 being responsible for target-site resistance to spinosad in the SZ-SpinR strain of P. xylostella.