Mutations in DIIS5 and the DIIS4-S5 linker ofDrosophila melanogastersodium channel define binding domains for pyrethroids and DDT

Point mutations in the voltage-gated sodium channel gene conferring pyrethroid resistance in China populations of the Dermanyssus gallinae

BACKGROUND

Dermanyssus gallinae, the poultry red mite (PRM), is a worldwide ectoparasite posing significant economic challenges in poultry farming. The extensive use of pyrethroids for PRM control has led to the emergence of pyrethroid resistance. The objective of this study is to detect the pyrethroid resistance and explore its associated point mutations in the voltage-gated sodium channel (VGSC) gene among PRM populations in China.

RESULTS

Several populations of D. gallinae, namely CJF-1, CJP-2, CJP-3, CSD-4 and CLD-5, displayed varying degrees of resistance to beta-cypermethrin compared to a susceptible field population (CBP-5). Mutations of VGSC gene in populations of PRMs associated with pyrethroid resistance were identified through sequencing its fragments IIS4-IIS5 and IIIS6. The mutations I917V, M918T/L, A924G and L925V were present in multiple populations, while no mutations were found at positions T929, I936, F1534 and F1538.

CONCLUSION

The present study confirmed the presence of extremely high levels of pyrethroid resistance in PRM populations in China, and for the first time detected four pyrethroid resistance mutations in the VGSC gene. Identifying pyrethroid resistance in the field population of PRM in China can be achieved through screening for VGSC gene mutations as an early detection method. Our findings underscore the importance of implementing chemical PRM control strategies based on resistance evidence, while also considering the management of acaricide resistance in the control of PRMs. © 2024 Society of Chemical Industry.

Mutations of voltage-gated sodium channel contribute to pyrethroid resistance in Panonychus citri

Insecticide resistance in Panonychus citri is a major obstacle to mite control in citrus orchards. Pyrethroid insecticides are continually used to control mites in China, although resistance to pyrethroids has evolved in some populations. Here, the resistance to the pyrethroid fenpropathrin was investigated and 7 out of 8 field-collected populations of P. citri exhibited a high level of resistance, ranging from 171-fold to 15 391-fold higher than the susceptible (SS) comparison strain. Three voltage-gated sodium channel (VGSC) mutations were identified in the tested populations: L1031V, F1747L, and F1751I. Amplicon sequencing was used to evaluate the frequency of these mutations in the 19 field populations. L1031V and F1751I were present in all populations at frequencies of 11.6%-82.1% and 0.5%-31.8%, respectively, whereas the F1747L mutation was only present in 12 populations from Chongqing, Sichuan, Guangxi, and Yunnan provinces. Introduction of these mutations singly or in combination into transgenic flies significantly increased their resistance to fenpropathrin and these flies also exhibited reduced mortality after exposure to the pyrethroids permethrin and β-cypermethrin. Panonychus citri VGSC homology modeling and ligand docking indicate that F1747 and F1751 form direct binding contacts with pyrethroids, which are lost with mutation, whereas L1031 mutation may diminish pyrethroid effects through an allosteric mechanism. Overall, the results provide molecular markers for monitoring pest resistance to pyrethroids and offer new insights into the basis of pyrethroid actions on sodium channels.

Functional Characterization of Double Mutations T929I/K1774N in the Voltage-Gated Sodium Channel of Megalurothrips usitatus (Bagnall) Related to Pyrethroid Resistance

Megalurothrips usitatus (Bagnall), the main pest on legume vegetables, is controlled by pyrethroids in the field. Field strains of M. usitatus resistant to pyrethroids were collected from three areas in Hainan Province (Haikou, Ledong, and Sanya City), and two mutations, T929I and K1774N, were detected in the voltage-gated sodium channel. In this study, the sodium channel in M. usitatus was first subcloned and successfully expressed in Xenopus oocytes. The single mutation (T929I or K1774N) and double mutation (T929I/K1774N) shifted the voltage dependence of activation in the hyperpolarization direction. The three mutants all reduced the amplitude of tail currents induced by type I (permethrin and bifenthrin) and type II (deltamethrin and λ-cyhalothrin) pyrethroids. Homology modeling analysis of these two mutations shows that they may change the local hydrophobicity and positive charge of the sodium channel. Our data can be used to reveal the causes of the resistance of M. usitatus to pyrethroids and provide guidance for the comprehensive control of M. usitatus in the future.

Geographical distribution of pyrethroid resistance mutations in Varroa destructor across Türkiye and a European overview

Varroa destructor Anderson & Trueman (Acari: Varroidae) is of paramount significance in modern beekeeping, with infestations presenting a primary challenge that directly influences colony health, productivity, and overall apicultural sustainability. In order to control this mite, many beekeepers rely on a limited number of approved synthetic acaricides, including the pyrethroids tau-fluvalinate, flumethrin and organophosphate coumaphos. However, the excessive use of these substances has led to the widespread development of resistance in various beekeeping areas globally. In the present study, the occurrence of resistance mutations in the voltage-gated sodium channel (VGSC) and acetylcholinesterase (AChE), the target-site of pyrethroids and coumaphos, respectively, was examined in Varroa populations collected throughout the southeastern and eastern Anatolia regions of Türkiye. All Varroa samples belonged to the Korean haplotype, and a very low genetic distance was observed based on cytochrome c oxidase subunit I (COI) gene sequences. No amino acid substitutions were determined at the key residues of AChE. On the other hand, three amino acid substitutions, (L925V/I/M), previously associated with pyrethroid resistance, were identified in nearly 80% of the Turkish populations. Importantly, L925M, the dominant mutation in the USA, was detected in Turkish Varroa populations for the first time. To gain a more comprehensive perspective, we conducted a systematic analysis of the distribution of pyrethroid resistance mutations across Europe, based on the previously reported data. Varroa populations from Mediterranean countries such as Türkiye, Spain, and Greece exhibited the highest frequency of resistance mutation. Revealing the occurrence and geographical distribution of pyrethroid resistance mutations in V. destructor populations across the country will enhance the development of more efficient strategies for mite management.

Evolutionary origin and distribution of amino acid mutations associated with resistance to sodium channel modulators in onion thrips, Thrips tabaci

In onion thrips Thrips tabaci, reduced sensitivity of the sodium channel caused by several sodium channel mutations have been correlated with pyrethroid resistance. For this study, using mitochondrial cytochrome c oxidase subunit I gene sequences, we examined the phylogenetic relation among a total of 52 thelytokous and arrhenotokous strains with different genotypes of the sodium channel mutations. Then, we used flow cytometry to estimate their ploidy. Results showed that the strains are divisible into three groups: diploid thelytoky, triploid thelytoky, and diploid arrhenotoky. Using 23 whole genome resequencing data obtained from 20 strains out of 52, we examined their genetic relation further using principal component analysis, admixture analysis, and a fixation index. Results showed that diploid and triploid thelytokous groups are further classifiable into two based on the sodium channel mutations harbored by the respective group members (strains). The greatest genetic divergence was observed between thelytokous and arrhenotokous groups with a pair of T929I and K1774N. Nevertheless, they shared a genomic region with virtually no polymorphism around the sodium channel gene loci, suggesting a hard selective sweep. Based on these findings, we discuss the evolutionary origin and distribution of the sodium channel mutations in T. tabaci.

A review of the molecular mechanisms of acaricide resistance in mites and ticks

The Arachnida subclass of Acari comprises many harmful pests that threaten agriculture as well as animal health, including herbivorous spider mites, the bee parasite Varroa, the poultry mite Dermanyssus and several species of ticks. Especially in agriculture, acaricides are often used intensively to minimize the damage they inflict, promoting the development of resistance. Beneficial predatory mites used in biological control are also subjected to acaricide selection in the field. The development and use of new genetic and genomic tools such as genome and transcriptome sequencing, bulked segregant analysis (QTL mapping), and reverse genetics via RNAi or CRISPR/Cas9, have greatly increased our understanding of the molecular genetic mechanisms of resistance in Acari, especially in the spider mite Tetranychus urticae which emerged as a model species. These new techniques allowed to uncover and validate new resistance mutations in a larger range of species. In addition, they provided an impetus to start elucidating more challenging questions on mechanisms of gene regulation of detoxification associated with resistance.

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.

The molecular mechanisms of insecticide resistance in aphid crop pests

Aphids are a group of hemipteran insects that include some of the world's most economically important agricultural pests. The control of pest aphids has relied heavily on the use of chemical insecticides, however, the evolution of resistance poses a serious threat to their sustainable control. Over 1000 cases of resistance have now been documented for aphids involving a remarkable diversity of mechanisms that, individually or in combination, allow the toxic effect of insecticides to be avoided or overcome. In addition to its applied importance as a growing threat to human food security, insecticide resistance in aphids also offers an exceptional opportunity to study evolution under strong selection and gain insight into the genetic variation fuelling rapid adaptation. In this review we summarise the biochemical and molecular mechanisms underlying resistance in the most economically important aphid pests worldwide and the insights study of this topic has provided on the genomic architecture of adaptive traits.

Development of insecticide resistance in Hyalella azteca

Hyalella azteca are epibenthic amphipods that have developed resistance to pyrethroid and organophosphate insecticides due to single amino acid substitutions in the voltage-gated sodium channel and the acetylcholinesterase-1 gene, respectively. Aquatic systems are often contaminated with several different types of insecticides, therefore there is a possibility that H. azteca have also developed resistance to other classes of insecticides. The aims of the current study were to verify that pyrethroid- and organophosphate-resistant H. azteca have retained their resistance after being cultured in the absence of selective pressure for 5 years (Escondido Creek population) and 9 years (Mosher Slough population), to determine if these populations have cross-resistance to carbaryl (carbamate) and 4,4'-dichlorodiphenyltrichloroethane (DDT; organochlorine), and determine whether previous field exposure to fipronil (phenylpyrazole) and imidacloprid (neonicotinoid) caused resistance in cultured pyrethroid- and organophosphate-resistant H. azteca populations. Escondido Creek and Mosher Slough H. azteca populations both maintained high tolerances for bifenthrin due to L925I and I936F amino acid substitutions. Resistance was also found for chlorpyrifos in the Escondido Creek and Mosher Slough populations with lower genotype frequencies of the G119S substitution, indicating that additional factors may be responsible for organophosphate resistance in this study. Mosher Slough H. azteca were moderately resistant to DDT, and Escondido Creek and Mosher Slough H. azteca were moderately resistant to carbaryl, suggesting cross-resistance. No differences were observed in acute toxicity values across the three populations of H. azteca for fipronil and imidacloprid, and this is possibly due to the lack of exposure to toxic concentrations of these insecticides in the field and lack of similar modes of action to pyrethroids and organophosphates. Resistance is known to be associated with fitness costs that can place insecticide-resistant populations at risk for decline through decreased survival and reduced fecundity.

Recent progress in the structural study of ion channels as insecticide targets

Ion channels, many expressed in insect neural and muscular systems, have drawn huge attention as primary targets of insecticides. With the recent technical breakthroughs in structural biology, especially in cryo-electron microscopy (cryo-EM), many new high-resolution structures of ion channel targets, apo or in complex with insecticides, have been solved, shedding light on the molecular mechanism of action of the insecticides and resistance mutations. These structures also provide accurate templates for structure-based insecticide screening and rational design. This review summarizes the recent progress in the structural studies of 5 ion channel families: the ryanodine receptor (RyR), the nicotinic acetylcholine receptor (nAChR), the voltage-gated sodium channel (VGSC), the transient receptor potential (TRP) channel, and the ligand-gated chloride channel (LGCC). We address the selectivity of the channel-targeting insecticides by examining the conservation of key coordinating residues revealed by the structures. The possible resistance mechanisms are proposed based on the locations of the identified resistance mutations on the 3D structures of the target channels and their impacts on the binding of insecticides. Finally, we discuss how to develop "green" insecticides with a novel mode of action based on these high-resolution structures to overcome the resistance.

Predicting Resistance: Quantifying the Relationship between Urban Development, Agricultural Pesticide Use, and Pesticide Resistance in a Nontarget Amphipod

Resistance alleles within the voltage-gated sodium channel (vgsc) have been correlated with pyrethroid resistance in wild populations of the nontarget amphipod, Hyalella azteca from California (CA), U.S.A. In the present study, we expand upon the relationship between land use and the evolution of pesticide resistance in H. azteca to develop a quantitative methodology to target and screen novel populations for resistance allele genotypes in a previously uninvestigated region of the U.S. (New England: NE). By incorporating urban land development and toxicity-normalized agricultural pesticide use indices into our site selection, we successfully identified three amino acid substitutions associated with pyrethroid resistance. One of the resistance mutations has been described in H. azteca from CA (L925I). We present the remaining two (vgsc I936F and I936V) as novel pyrethroid-resistance alleles in H. azteca based on previous work in insects and elevated cyfluthrin resistance in one NE population. Our results suggest that urban pesticide use is a strong driver in the evolution of resistance alleles in H. azteca. Furthermore, our method for resistance allele screening provides an applied framework for detecting ecosystem impairment on a nationwide scale that can be incorporated into ecological risk assessment decisions.

Characterization of two kdr mutations at predicted pyrethroid receptor site 2 in the sodium channels of Aedes aegypti and Nilaparvata lugens

Pyrethroid insecticides prolong the opening of insect sodium channels by binding to two predicted pyrethroid receptor sites (PyR), PyR1 and PyR2. Many naturally-occurring sodium channel mutations that confer pyrethroid resistance (known as knockdown resistance, kdr) are located at PyR1. Recent studies identified two new mutations, V253F and T267A, at PyR2, which co-exist with two well-known mutations F1534C or M918T, at PyR1, in pyrethroid-resistant populations of Aedes aegypti and Nilaparvata lugens, respectively. However, the role of the V253F and T267A mutations in pyrethroid resistance has not been functionally examined. Here we report functional characterization of the V253F and T267A mutations in the Ae. aegypti sodium channel AaNa_v2-1 and the N. lugens sodium channel NlNa_v1 expressed in Xenopus oocytes. Both mutations alone reduced channel sensitivity to pyrethroids, including etofenprox. We docked etofenprox in a homology model of the pore module of the NlNa_v1 channel based on the crystal structure of an open prokaryotic sodium channel NavMs. In the low-energy binding pose etofenprox formed contacts with V253, T267 and a previously identified L1014 within PyR2. Combining of V253F or T267A with F1534C or M918T results in a higher level of pyrethroid insensitivity. Furthermore, both V253F and T267A mutations altered channel gating properties. However, V253F- and T267A-induced gating modifications was not observed in the double mutant channels. Our findings highlight the first example in which naturally-found combinational mutations in PyR1 and PyR2 not only confer higher level pyrethroid insensitivity, but also reduce potential fitness tradeoff in pyrethroid-resistant mosquitoes caused by kdr mutation-induced sodium channel gating modifications.

Pyrethroids in an AlphaFold2 Model of the Insect Sodium Channel

Pyrethroid insecticides stabilize the open state of insect sodium channels. Previous mutational, electrophysiological, and computational analyses led to the development of homology models predicting two pyrethroid receptor sites, PyR1 and PyR2. Many of the naturally occurring sodium channel mutations, which confer knockdown resistance (kdr) to pyrethroids, are located within or close to these receptor sites, indicating that these mutations impair pyrethroid binding. However, the mechanism of the state-dependent action of pyrethroids and the mechanisms by which kdr mutations beyond the receptor sites confer resistance remain unclear. Recent advances in protein structure prediction using the AlphaFold2 (AF2) neural network allowed us to generate a new model of the mosquito sodium channel AaNav1-1, with the activated voltage-sensing domains (VSMs) and the presumably inactivated pore domain (PM). We further employed Monte Carlo energy minimizations to open PM and deactivate VSM-I and VSM-II to generate additional models. The docking of a Type II pyrethroid deltamethrin in the models predicted its interactions with many known pyrethroid-sensing residues in the PyR1 and PyR2 sites and revealed ligand-channel interactions that stabilized the open PM and activated VSMs. Our study confirms the predicted two pyrethroid receptor sites, explains the state-dependent action of pyrethroids, and proposes the mechanisms of the allosteric effects of various kdr mutations on pyrethroid action. The AF2-based models may assist in the structure-based design of new insecticides.

Key role of mitochondrial mutation Leu107Ser (COX1) in deltamethrin resistance in salmon lice (Lepeophtheirus salmonis)

The pyrethroid deltamethrin (DTM) is used to treat Atlantic salmon (Salmo salar) against salmon louse (Lepeophtheirus salmonis) infestations. However, DTM resistance has evolved in L. salmonis and is currently common in the North Atlantic. This study aimed to re-assess the association between DTM resistance and mitochondrial (mtDNA) mutations demonstrated in previous reports. Among 218 L. salmonis collected in Scotland in 2018–2019, 89.4% showed DTM resistance in bioassays, while 93.6% expressed at least one of four mtDNA single nucleotide polymorphisms (SNPs) previously shown to be resistance associated. Genotyping at further 14 SNP loci allowed to define three resistance-associated mtDNA haplotypes, named 2, 3 and 4, occurring in 72.0%, 14.2% and 7.3% of samples, respectively. L. salmonis strains IoA-02 (haplotype 2) and IoA-10 (haplotype 3) both showed high levels (~ 100-fold) of DTM resistance, which was inherited maternally in crossing experiments. MtDNA haplotypes 2 and 3 differed in genotype for 17 of 18 studied SNPs, but shared one mutation that causes an amino acid change (Leu107Ser) in the cytochrome c oxidase subunit 1 (COX1) and was present in all DTM resistant while lacking in all susceptible parasites. We conclude that Leu107Ser (COX1) is a main genetic determinant of DTM resistance in L. salmonis.

Towards understanding transfluthrin efficacy in a pyrethroid-resistant strain of the malaria vector Anopheles funestus with special reference to cytochrome P450-mediated detoxification

Malaria vector control interventions rely heavily on the application of insecticides against anopheline mosquitoes, in particular the fast-acting pyrethroids that target insect voltage-gated sodium channels (VGSC). Frequent applications of pyrethroids have resulted in resistance development in the major malaria vectors including Anopheles funestus, where resistance is primarily metabolic and driven by the overexpression of microsomal cytochrome P450 monooxygenases (P450s). Here we examined the pattern of cross-resistance of the pyrethroid-resistant An. funestus strain FUMOZ-R towards transfluthrin and multi-halogenated benzyl derivatives, permethrin, cypermethrin and deltamethrin in comparison to the susceptible reference strain FANG. Transfluthrin and two multi-fluorinated derivatives exhibited micromolar potency - comparable to permethrin - to functionally expressed dipteran VGSC in a cell-based cation influx assay. The activity of transfluthrin and its derivatives on VGSC was strongly correlated with their contact efficacy against strain FUMOZ-R, although no such correlation was obtained for the other pyrethroids due to their rapid detoxification by the resistant strain. The low resistance levels for transfluthrin and derivatives in strain FUMOZ-R were only weakly synergized by known P450 inhibitors such as piperonyl butoxide (PBO), triflumizole and 1-aminobenzotriazole (1-ABT). In contrast, deltamethrin toxicity in FUMOZ-R was synergized > 100-fold by all three P450 inhibitors. The biochemical profiling of a range of fluorescent resorufin and coumarin compounds against FANG and FUMOZ-R microsomes identified 7-benzyloxymethoxy-4-trifluoromethylcoumarin (BOMFC) as a highly sensitive probe substrate for P450 activity. BOMFC was used to develop a fluorescence-based high-throughput screening assay to measure the P450 inhibitory action of potential synergists. Azole fungicides prochloraz and triflumizole were identified as extremely potent nanomolar inhibitors of microsomal P450s, strongly synergizing deltamethrin toxicity in An. funestus. Overall, the present study contributed to the understanding of transfluthrin efficacy at the molecular and organismal level and identified azole compounds with potential to synergize pyrethroid efficacy in malaria vectors.

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Transfluthrin and derivatives lack cross-resistance in resistant Anopheles funestus.

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Pyrethroid resistance in An. funestus is strongly synergized by azole fungicides.

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BOMFC is a highly active fluorescent probe substrate for microsomal cytochrome P450 monooxygenases in An. funestus.

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Azole fungicides are nanomolar inhibitors of microsomal cytochrome P450 monooxygenases in An. funestus.

Biochemical and molecular mechanisms of acaricide resistance in Dermanyssus gallinae populations from Turkey

The poultry red mite, Dermanyssus gallinae, is the most important blood sucking ectoparasite of egg laying hens and causes economic losses in poultry farms worldwide. Although various management methods exist, the control of poultry red mites (PRMs) mainly relies on acaricides such as pyrethroids and organophosphates (OPs) in many regions of the world. However, repeated use of these synthetic chemicals has resulted in resistance development causing control failure of PRM. In this study, we investigated acaricide resistance mechanisms of Turkish PRM populations. First, we obtained the COI sequence from 30 PRM populations from different regions in Turkey and identified four different COI haplotypes. Toxicity assays showed that four field-collected PRM populations were highly resistant to the pyrethroid alpha-cypermethrin, with resistance ratios (RRs) varying between 100- and 400-fold, while two of these populations had a RR of more than 24-fold against the OP acaricide phoxim. Biochemical assays showed a relatively higher activity of glutathione-S-transferases and carboxyl-cholinesterases, two well-known classes of detoxification enzymes, in one of these resistant populations. In addition, we also screened for mutations in the gene encoding the voltage-gated sodium channel (vgsc) and acetylcholinesterase 1 (ace-1), the target-site of pyrethroids and OPs, respectively. In all but two PRM populations, at least one vgsc mutation was detected. A total of four target-site mutations, previously associated with pyrethroid resistance, M918T, T929I, F1534L, F1538L were found in domain II and III of the VGSC. The T929I mutation was present in the vgsc of almost all PRM populations, while the other mutations were only found at low frequency. The G119S/A mutation in ace-1, previously associated with OP resistance, was found in PRM for the first time and present in fourteen populations. Last, both alive and dead PRMs were genotyped after pesticide exposure and supported the possible role of target-site mutations, T929I and G119S, in alpha-cypermethrin and phoxim resistance, respectively. To conclude, our study provides a current overview of resistance levels and resistance mutations in Turkish PRM populations and might aid in the design of an effective resistance management program of PRM in Turkey.

Multiple Mechanisms Conferring Broad-Spectrum Insecticide Resistance in the Tropical Bed Bug (Hemiptera: Cimicidae)

The modern resurgence of the common (Cimex lectularius L.) and tropical bed bugs (C. hemipterus [F.]) is thought to be primarily due to insecticide resistance. While there are many reports on insecticide resistance mechanisms in C. lectularius, such information in C. hemipterus is limited. We examined dichloro-diphenyl-trichloroethane (DDT), malathion, deltamethrin, permethrin, lambda-cyhalothrin resistance, and the underlying mechanisms in several C. hemipterus strains (Australia: Queensland [QLD-AU]; Malaysia: Kuala Lumpur [KL-MY], Tanjung Tokong [TT-MY], Christian [CH-MY], and Green Lane [GL-MY]). We used a surface contact method, synergism studies (utilizing piperonyl butoxide [PBO], S,S,S-tributyl phosphorotrithioate [DEF], and diethyl maleate [DEM]), and molecular detection of kdr mutations. Results demonstrated that all C. hemipterus strains possessed high resistance to DDT and the pyrethroids and moderate to high resistance to malathion. Synergism studies showed that deltamethrin resistance in all strains was significantly (P < 0.05) inhibited by PBO. In contrast, deltamethrin resistance was not affected in DEF or DEM. Similar findings were found with lambda-cyhalothrin resistance. Malathion resistance was significantly (P < 0.05) reduced by DEF in all strains. Resistance to DDT was not affected by DEM in all strains. Multiple kdr mutations (M918I, D953G, and L1014F) were detected by molecular analyses. TT-MY strain was found with individuals possessing three kdr mutation combinations; D953G + L1014F (homozygous susceptible: M918), M918I + D953G + L1014F (heterozygous resistant: I918), and M918I + D953G + L1014F (homozygous resistant: I918). Individuals with M918I + D953G + L1014F (homozygous resistant: I918) survived longer on deltamethrin (>12 h) than those (≤1 h) with other combinations. M918I + L1014F mutations most likely conferred super-kdr characteristic toward pyrethroids and DDT in C. hemipterus.

Sodium channel mutations (T929I and F1534S) found in pyrethroid-resistant strains of the cigarette beetle, Lasioderma serricorne (Coleoptera: Anobiidae)

RNA-seq data analysis of cigarette beetle (Lasioderma serricorne) strains having different sensitivities to pyrethroids identified sodium channel mutations in strains showing pyrethroid resistance: the T929I and F1534S mutations. These results suggest that reduced sensitivity of the sodium channel confers the pyrethroid resistance of L. serricorne. Results also showed that the F1534S mutation mostly occurred concurrently with the T929I mutation. The functional relation between both mutations for pyrethroid resistance is discussed.

The genetic architecture of target-site resistance to pyrethroid insecticides in the African malaria vectors Anopheles gambiae and Anopheles coluzzii

Resistance to pyrethroid insecticides is a major concern for malaria vector control. Pyrethroids target the voltage-gated sodium channel (VGSC), an essential component of the mosquito nervous system. Substitutions in the amino acid sequence can induce a resistance phenotype. We use whole-genome sequence data from phase 2 of the Anopheles gambiae 1000 Genomes Project (Ag1000G) to provide a comprehensive account of genetic variation in the Vgsc gene across 13 African countries. In addition to known resistance alleles, we describe 20 other non-synonymous nucleotide substitutions at appreciable population frequency and map these variants onto a protein model to investigate the likelihood of pyrethroid resistance phenotypes. Thirteen of these novel alleles were found to occur almost exclusively on haplotypes carrying the known L995F kdr (knock-down resistance) allele and may enhance or compensate for the L995F resistance genotype. A novel mutation I1527T, adjacent to a predicted pyrethroid-binding site, was found in tight linkage with V402L substitutions, similar to allele combinations associated with resistance in other insect species. We also analysed genetic backgrounds carrying resistance alleles, to determine which alleles have experienced recent positive selection, and describe ten distinct haplotype groups carrying known kdr alleles. Five of these groups are observed in more than one country, in one case separated by over 3000 km, providing new information about the potential for the geographical spread of resistance. Our results demonstrate that the molecular basis of target-site pyrethroid resistance in malaria vectors is more complex than previously appreciated, and provide a foundation for the development of new genetic tools for insecticide resistance management.

Unique post-translational modifications diversify the sodium channels in peach aphid (Myzus persicae [Sulzer])

BACKGROUND

Myzus persicae (Sulzer), a worldwide pest, has caused remarkable damage to agriculture. Among the various control methods, chemical control (especially pyrethroids) is most commonly used. The targets of pyrethroids are voltage-gated sodium channels (Na_v s). Unlike those of other insects, all Na_v s of aphids (including two genes), such as Myzus persicae, are unique.

RESULTS

In this study, three interlock patterns, I(918)-F(1014), L(918)-L(1014), and T(918)-F(1014), were found at sites 918 and 1014 in the sensitive Myzus persicae strain. Similar to that of other aphids, the Na_v of Myzus persicae (MpNa_v ) consisted of two parts, that is MpNa_v -I and MpNa_v -II, which were embedded with an atypical 'DENS' ion selectivity filter and a conventional 'MFM' inactivation gate, respectively. MpNa_v had 11 alternative exons, including two mutually exclusive exons (k and l) and three exons (w, x, and t), which were located in domains I and III, respectively. In addition, various RNA editing events, A503T and V588A, appearing between the connection of domains I and II and the S3 of domain IV, respectively, had been described.

CONCLUSION

Overall, MpNa_v was characterized by unique post-translational regulation mode, 918 and 1014 interlocks, and unusually alternative exons. Our research provides a new perspective on the evolution and variation of insect Na_v s. © 2021 Society of Chemical Industry.

The contribution of detoxification pathways to pyrethroid resistance in Hyalella azteca

Chronic exposure to pyrethroid insecticides can result in strong selective pressures on non-target species in aquatic systems and drive the evolution of resistance and population-level changes. Characterizing the underlying mechanisms of resistance is essential to better understanding the potential consequences of contaminant-driven microevolution. The current study found that multiple mechanisms enhance the overall tolerance of Hyalella azteca to the pyrethroid permethrin. In H. azteca containing mutations in the voltage-gated sodium channel (VGSC), both adaptation and acclimation played a role in mitigating the adverse effects of pyrethroid exposures. Pyrethroid resistance is primarily attributed to the heritable mutation at a single locus of the VGSC, resulting in reduced target-site sensitivity. However, additional pyrethroid tolerance was conferred through enhanced enzyme-mediated detoxification. Cytochrome P450 monooxygenases (CYP450) and general esterases (GE) significantly contributed to the detoxification of permethrin in H. azteca. Over time, VGSC mutated H. azteca retained most of their pyrethroid resistance, though there was some increased sensitivity from parent to offspring when reared in the absence of pyrethroid exposure. Permethrin median lethal concentrations (LC50s) declined from 1809 ng/L in parent (P₀) individuals to 1123 ng/L in the first filial (F₁) generation, and this reduction in tolerance was likely related to alterations in acclimation mechanisms, rather than changes to target-site sensitivity. Enzyme bioassays indicated decreased CYP450 and GE activity from P₀ to F₁, whereas the VGSC mutation was retained. The permethrin LC50s in resistant H. azteca were still two orders-of-magnitude higher than non-resistant populations indicating that the largest proportion of resistance was maintained through the inherited VGSC mutation. Thus, the noted variation in tolerance in H. azteca is likely associated with inducible traits controlling enzyme pathways. A better understanding of the mechanistic and genomic basis of acclimation is necessary to more accurately predict the ecological and evolutionary consequences of contaminant-driven change in H. azteca.

Differential state-dependent effects of deltamethrin and tefluthrin on sodium channels in central neurons of Helicoverpa armigera

The cotton bollworm, Helicoverpa armigera is one of the worldwide pests. Electrophysiological properties of voltage-gated sodium channels in central neurons of sensitive and pyrethroid resistant H. armigera were investigated using whole-cell patch clamp technique. The modification effects of pyrethroid insecticides deltamethrin and tefluthrin on sodium channels were also compared. The V_0.5 of voltage dependence of activation of resistant H. armigera sodium channels (resistant channels) exhibited an obvious depolarizing shift by 13.52 mV compared to that of sensitive H. armigera sodium channels (sensitive channels). In contrast, the V_0.5 of the voltage dependence of steady-state inactivation of the resistant channels showed a significant hyperpolarizing shift by 7.59 mV in comparison with that of the sensitive channels. The time course of recovery from inactivation for the resistant channels was prolonged significantly, by 0.17 ms, compared with that for the sensitive channels. We also assessed the use-dependent effects of deltamethrin and tefluthrin on sensitive sodium channels. Repetitive depolarization remarkably increased the extent of the sensitive channel modification by 10 μM deltamethrin by ~4.61-fold but had no effect on the extent of sensitive channel modifications by 10 μM tefluthrin. These results provide more direct evidence for the presence of nerve insensitivity in resistant H. armigera strains in North of China. The sodium channels of the resistant H. armigera differ from those of the sensitive H. armigera in the fundamental electrophysiological properties, and correspondingly, have a different response to the modification of pyrethroids. Both deltamethrin and tefluthrin have effects on the closed state of the sensitive sodium channels, but deltamethrin has higher affinity to the open state of these channels.

Novel CONCOMITANT mutations L932F and I936V in the Voltage-Gated Sodium Channel and Its Association With Pyrethroid Resistance in Culex quinquefasciatus (Diptera: Culicidae)

Highly residual pyrethroids such as permethrin have been used for controlling mosquitoes that transmit infectious diseases. However, the selective pressure from such insecticides may result in cross-resistance against other pyrethroids used for household insecticides. In this study, we investigated the susceptibility of Culex quinquefasciatus Say collected from Brazil and Myanmar to permethrin in addition to four types of household pyrethroids. Both strains exhibited high resistance against all pyrethroids tested, indicating cross-resistance. Furthermore, we detected the knockdown resistance (kdr) mutations L932F+I936V in the voltage-gated sodium channel gene (VGSC) in the Brazilian strain. Notably, the L932F+I936V haplotype has previously been observed in in silico data, but it should be detected not directly from living insects. In comparison, a common kdr mutation, L1014F, was detected from the Myanmar strain. Although L1014F was also detected from the Brazilian strain, the allele frequency was too low to affect resistance. Both strains harbored the resistance-associated haplotypes of the cytochrome P450 gene, CYP9M10. The Brazilian strain demonstrated comparable resistance against pyrethroids as that of the Myanmar strain even when a cytochrome P450 inhibitor, piperonyl butoxide was added to the bioassay. Our results suggested that the L932F+I936V mutations confer the Brazilian strain of Cx. Quiquefasciatus with resistance at a comparable level to that conferred by the well-recognized kdr mutation L1014F in the Myanmar strain. The identification of unexplored mutations may improve the diagnosis and understanding of resistance of this medically important species.

How varying parameters impact insecticide resistance bioassay: An example on the worldwide invasive pest Drosophila suzukii

Monitoring pesticide resistance is essential for effective and sustainable agricultural practices. Bioassays are the basis for pesticide-resistance testing, but devising a reliable and reproducible method can be challenging because these tests are carried out on living organisms. Here, we investigated five critical parameters and how they affected the evaluation of resistance to the organophosphate phosmet or the pyrethroid lambda-cyhalothrin using a tarsal-contact protocol on Drosophila suzukii, a worldwide invasive pest. Three of the parameters were related to insect biology: (i) sex, (ii) age of the imago (adult stage) and (iii) genetic diversity of the tested population. The two remaining parameters were linked to the experimental setup: (iv) the number of individuals tested per dose and (v) the duration of exposure to the active ingredient. Results showed that response to insecticide differed depending on sex, males being twice as susceptible to phosmet as females. Age principally affected young females' susceptibility to phosmet, because 0-24 hour-old flies were twice as susceptible as 24-48 hour-old and 72-96 hour-old females. Genetic diversity had no observable effect on resistance levels. The precision and accuracy of the median lethal dose (LD50) were greatly affected by the number of individuals tested per dose with a threshold effect. Finally, optimal duration of exposure to the active ingredient was 24 h, as we found an underestimation of mortality when assessed between 1 and 5 h after exposure to lambda-cyhalothrin. None of the main known point mutations on the para sodium channel gene associated with a knockdown effect were observed. Our study demonstrates the importance of calibrating the various parameters of a bioassay to develop a reliable method. It also provides a valuable and transferable protocol for monitoring D. suzukii resistance worldwide.

Investigation of deltamethrin resistance in salmon lice (Lepeophtheirus salmonis) provides no evidence for roles of mutations in voltage-gated sodium channels

BACKGROUND

The pyrethroid deltamethrin is used to treat infestations of farmed salmon by parasitic salmon lice, Lepeophtheirus salmonis (Krøyer). However, the efficacy of deltamethrin for salmon delousing is threatened by resistance development. In terrestrial arthropods, knockdown resistance (kdr) mutations of the voltage-gated sodium channel (Na_v ), the molecular target for pyrethroids, can cause deltamethrin resistance. A putative kdr mutation of an L. salmonis sodium channel homologue (LsNa_v 1.3 I936V) has been identified previously. At the same time, deltamethrin resistance of L. salmonis has been shown to be inherited maternally and to be associated with mitochondrial DNA (mtDNA) mutations. This study assessed potential roles of the above putative kdr mutation as a determinant of deltamethrin resistance in laboratory strains and field populations of L. salmonis.

RESULTS

The deltamethrin-resistant L. salmonis strain IoA-02 expresses the LsNa_v 1.3 I936V mutation but was susceptible to the non-ester pyrethroid etofenprox, a compound against which pyrethroid-resistant arthropods are usually cross-resistant if resistance is caused by Na_v mutations. In a family derived from a cross between an IoA-02 male and a drug-susceptible female lacking the kdr mutation, deltamethrin resistance was not associated with the genotype at the LsNa_v 1.3 locus (P > 0.05). Similarly, in Scottish field populations of L. salmonis, LsNa_v 1.3 I936V showed no association with deltamethrin resistance. By contrast, genotypes at the mtDNA loci A14013G and A9030G were significantly associated with deltamethrin resistance (P < 0.001).

CONCLUSION

In the studied L. salmonis isolates, deltamethrin resistance was unrelated to the LsNa_v 1.3 I936V mutation, but showed close association with mtDNA mutations.

The double-mutation（M918I + L1014F）kdr allele is fixed in Cimex hemipterus populations in Guangxi, China

Four putative knockdown resistance (kdr) mutations have been documented in the voltage-gated sodium channel (VGSC) gene of Cimex hemipterus from several countries. However, no information regarding kdr mutations in any Chinese tropical bed bug population is available to date. In this study, a double-mutation（M918I + L1014F）kdr allele was identified in six C. hemipterus populations across Guangxi Zhuang Autonomous Region of China. The frequency of this allele was 100% in all the six examined populations. In addition, only two cytochrome c oxidase I (COI) gene haplotypes, with one synonymous nucleotide variation, were identified in a total of 48 individuals from six locations. The fixation and broad geographic distribution of this resistant allele questions the continued use of pyrethroids in the treatment of tropical bed bug infestations. The very low genetic diversity within and among these populations indicates that these bed bugs may have a single origin.

Three sodium channel mutations from Aedes albopictus confer resistance to Type I, but not Type II pyrethroids

Voltage-gated sodium channels are the major targets of several classes of insecticides, including pyrethroids. However, sensitivities of many insect pest species to pyrethroids have gradually decreased due to overuse in pest management programs. One major mechanism of pyrethroid resistance known as knockdown resistance (kdr) involves mutations in the sodium channel gene. Three new mutations in helix IIIS6 of sodium channel (I1532T and F1534S/L) are recently detected in several pyrethroid-resistant populations of Aedes albopictus. The roles of these mutations in pyrethroid resistance have not been functionally examined. We introduced mutations I1532T and F1534S/L alone or in combination into the pyrethroid-sensitive sodium channel AaNa_v1-1 from Aedes aegypti by site-directed mutagenesis and explored effects of these mutations on the channel gating and sensitivity to pyrethroids. No significant modifications in channel properties were detected, except for a slightly changed activation by F1534S and I1532T + F1534S. However, I1532T and F1534S/L substantially reduced the channel sensitivity to Type I pyrethroids, permethrin and bifenthrin, but not to two Type II pyrethroids, deltamethrin and cypermethrin. The double mutations did not increase the channel resistance to permethrin or bifenthrin. We have built a Na_v1.4-based homology model of the AaNa_v1-1 channel and docked pyrethroids in the model to explain different sensitivities of the mutants to Type I and Type II pyrethroids. The results will assist in developing molecular markers for monitoring pest resistance to pyrethroids. They also provide new insight in the molecular basis of different action of Type I and Type II pyrethroids on sodium channels.

The effects of knock-down resistance mutations and alternative splicing on voltage-gated sodium channels in Musca domestica and Drosophila melanogaster

Voltage-gated sodium channels (VGSCs) are a major target site for the action of pyrethroid insecticides and resistance to pyrethroids has been ascribed to mutations in the VGSC gene. VGSCs in insects are encoded by only one gene and their structural and functional diversity results from posttranscriptional modification, particularly, alternative splicing. Using whole cell patch clamping of neurons from pyrethroid susceptible (wild-type) and resistant strains (s-kdr) of housefly, Musca domestica, we have shown that the V₅₀ for activation and steady state inactivation of sodium currents (I_Na+) is significantly depolarised in s-kdr neurons compared with wild-type and that 10 nM deltamethrin significantly hyperpolarised both of these parameters in the neurons from susceptible but not s-kdr houseflies. Similarly, tail currents were more sensitive to deltamethrin in wild-type neurons (EC₁₅ 14.5 nM) than s-kdr (EC₁₅ 133 nM). We also found that in both strains, I_Na+ are of two types: a strongly inactivating (to 6.8% of peak) current, and a more persistent (to 17.1% of peak) current. Analysis of tail currents showed that the persistent current in both strains (wild-type EC₁₅ 5.84 nM) was more sensitive to deltamethrin than was the inactivating type (wild-type EC₁₅ 35.1 nM). It has been shown previously, that the presence of exon l in the Drosophila melanogaster VGSC gives rise to a more persistent I_Na+ than does the alternative splice variant containing exon k and we used PCR with housefly head cDNA to confirm the presence of the housefly orthologues of splice variants k and l. Their effect on deltamethrin sensitivity was determined by examining I_Na+ in Xenopus oocytes expressing either the k or l variants of the Drosophila para VGSC. Analysis of tail currents, in the presence of various concentrations of deltamethrin, showed that the l splice variant was significantly more sensitive (EC₅₀ 42 nM) than the k splice variant (EC₅₀ 866 nM). We conclude that in addition to the presence of point mutations, target site resistance to pyrethroids may involve the differential expression of splice variants.

Mechanism underlying hooked resurgent-like tail currents induced by an insecticide in human cardiac Nav1.5

Voltage-gated sodium channels are responsible not only for the fast upstroke of the action potential, but they also modify cellular excitability via persistent and resurgent currents. Insecticides act via permanently opening sodium channels to immobilize the animals. Cellular recordings performed decades ago revealed distinctly hooked tail currents induced by these compounds. Here, we applied the classical type-II pyrethroid deltamethrin on human cardiac Nav1.5 and observed resurgent-like currents at very negative potentials in the absence of any pore-blocker, which resemble those hooked tail currents. We show that deltamethrin dramatically slows both fast inactivation and deactivation of Nav1.5 and thereby induces large persistent currents. Using the sea anemone toxin ATx-II as a tool to prevent all inactivation-related processes, resurgent-like currents were eliminated while persistent currents were preserved. Our experiments suggest that, in deltamethrin-modified channels, recovery from inactivation occurs faster than delayed deactivation, opening a brief window for sodium influx and leading to hooked, resurgent-like currents, in the absence of an open channel blocker. Thus, we now explain with pharmacological methods the biophysical gating changes underlying the deltamethrin induced hooked tail currents. SUMMARY: The pyrethroid deltamethrin induces hooked resurgent-like tail currents in human cardiac voltage-gated Nav1.5 channels. Using deltamethrin and ATx-II, we identify additional conducting channel states caused by a faster recovery from inactivation compared to the deltamethrin-induced delayed deactivation.

Enhanced metabolism and selection of pyrethroid-resistant western corn rootworms (Diabrotica virgifera virgifera LeConte)

Western corn rootworm (WCR) pyrethroid resistance has been previously reported in the United States (US) western Corn Belt, and cross-resistance and synergism studies suggested that both target site insensitivity and enhanced metabolism may be conferring WCR resistance to pyrethroids. The present study aimed to investigate the potential mechanisms of WCR pyrethroid resistance and to estimate the heritability of the resistance trait. Biochemical assays using model substrates and spectrophotometry revealed 2-4-fold higher activity of P450s and esterases in pyrethroid-resistant WCR populations, whereas the biological activity of glutathione S-transferase was similar between populations tested. No mutation in the voltage-gated sodium channel was detected in pyrethroid-resistant WCR individuals by sequencing PCR products containing the para-homologous L1014, T929, and M918 amino acid positions that are commonly associated with target site mutations in other pyrethroid-resistant insects. A pilot estimation of pyrethroid resistance heritability obtained during laboratory selection of a WCR population suggested a major genetic component of the resistance trait and predicted a 10-fold increase in WCR bifenthrin resistance within ~7 generations of insecticide lethal exposure. Results support earlier indirect evidence that enhanced metabolism may be contributing to WCR resistance to pyrethroids and illustrates the potential of WCR pyrethroid resistance evolution.

Conservation of the voltage-sensitive sodium channel protein within the Insecta

The voltage-sensitive sodium channel (VSSC) is essential for the generation and propagation of action potentials. VSSC kinetics can be modified by producing different splice variants. The functionality of VSSC depends on features such as the voltage sensors, the selectivity filter and the inactivation loop. Mutations in Vssc conferring resistance to pyrethroid insecticides are known as knockdown resistance (kdr). We analysed the conservation of VSSC in both a broad scope and a narrow scope by three approaches: (1) we compared conservation of sequences and of differential exon use across orders of the Insecta; (2) we determined which kdr mutations were possible with a single nucleotide mutation in nine populations of Aedes aegypti; and (3) we examined the individual VSSC variation that exists within a population of Drosophila melanogaster. There is an increasing amount of transcript diversity possible from Diplura towards Diptera. The residues of the voltage sensors, selectivity filter and inactivation loop are highly conserved. The majority of exon sequences were >88.6% similar. Strain-specific differences in codon constraints exist for kdr mutations in nine strains of A. aegypti. Three Vssc mutations were found in one population of D. melanogaster. This study shows that, overall, Vssc is highly conserved across Insecta and within a population of an insect, but that important differences do exist.

Identification and geographical distribution of pyrethroid resistance mutations in the poultry red mite Dermanyssus gallinae

BACKGROUND

The poultry red mite (PRM) Dermanyssus gallinae is the most common ectoparasite on poultry and causes high economic losses in poultry farming worldwide. Pyrethroid acaricides have been widely used for its control and, consequently, pyrethroid resistance has arisen. In this study we aim to investigate the occurrence of resistance and study the geographical distribution of pyrethroid resistance mutations across PRM populations in Europe.

RESULTS

Full dose-response contact bioassays revealed very high levels of resistance against several pyrethroids (α-cypermethrin, fluvalinate, and cyfluthrin) in two PRM populations from Greece, compared to a susceptible reference strain. Resistance was associated with mutations in the gene encoding the target site of pyrethroids, the voltage-gated sodium channel (VGSC). Mutations, M918L and L925V in domain IIS4-S5 and F1534L in domain IIIS6, were found at positions known to play a role in pyrethroid resistance in other arthropod species. Subsequent screening by sequencing VGSC gene fragments IIS4-S5 and IIIS6 revealed the presence and distribution of these mutations in many European populations. In some populations, we identified additional or different mutations including M918V/T, L925M, T929I, I936F, and F1538L. The latter mutation is a possible alternative for F1538I that has been previously associated with pyrethroid resistance in other Acari species.

CONCLUSION

We report very high levels of pyrethroid resistance in PRM populations from Greece, as well as the identification and geographical distribution of 10 pyrethroid resistance mutations in PRM populations across Europe. Our results draw attention to the need for an evidence-based implementation of PRM control, taking acaricide resistance management into consideration. © 2019 Society of Chemical Industry.

Low Frequency of Knockdown Resistance Mutations in Musca domestica (Muscidae: Diptera) Collected From Northwestern Iran

Musca domestica L., the common housefly, is a very important mechanical vector of pathogens. Continuous exposure to pyrethroid insecticides has led to insecticide resistance in houseflies. Some mutations in the voltage-gated sodium channel gene (vgsc) reduce the binding affinity of pyrethroids target site insensitivity. We collected houseflies from the Urmia district of Northwestern Iran. Following DNA extraction, 580 bp regions of the vgsc known to contain knockdown resistance (kdr) mutations were amplified and sequenced using specific primers. The amplified region contained two exons (211-bp and 248-bp) and three introns. There were eight polymorphic sites between M. domestica insecticide-susceptible (MDU38813), super-kdr (NW_004774263) and aabys (KT897924) strains from GenBank in comparison with our sequences. Two amino acid substitutions were detected, N967Y (% polymorphism = 9.5%) and L1014H (% polymorphism = 4.7%) that can be associated with resistance. The common and previously reported mutations L1014F and M918T+L1014F were not detected. Diagnosis based on sequence analysis is useful for monitoring the frequency of pyrethroid resistance mutations, which will be helpful in avoiding overuse of this class of insecticides in house fly control.

Mutations in voltage-gated sodium channels from pyrethroid resistant salmon lice (Lepeophtheirus salmonis)

BACKGROUND

Parasitic salmon lice (Lepeophtheirus salmonis) cause high economic losses in Atlantic salmon farming. Pyrethroids, which block arthropod voltage-gated sodium channels (Na_v 1), are used for salmon delousing. However, pyrethroid resistance is common in L. salmonis. The present study characterized Na_v 1 homologues in L. salmonis in order to identify channel mutations associated to resistance, called kdr (knockdown) mutations.

RESULTS

Genome scans identified three L. salmonis Na_v 1 homologues, LsNa_v 1.1, LsNa_v 1.2 and LsNa_v 1.3. Arthropod kdr mutations map to specific Na_v 1 regions within domains DI-III, namely segments S5 and S6 and the linker helix connecting S4 and S5. The above channel regions were amplified by RT-PCR and sequenced in deltamethrin-susceptible and deltamethrin-resistant L. salmonis. While LsNa_v 1.1 and LsNa_v 1.2 lacked nucleotide polymorphisms showing association to resistance, LsNa_v 1.3 showed a non-synonymous mutation in S5 of DII occurring in deltamethrin-resistant parasites. The mutation is homologous to a previously described kdr mutation (I936V, numbering according to Musca domestica Vssc1) and was present in two pyrethroid-resistant L. salmonis strains (allele frequencies of 0.800 and 0.357), but absent in two pyrethroid-susceptible strains.

CONCLUSIONS

The present study indicates that a kdr-mutation in LsNa_V 1.3 may contribute to deltamethrin resistance in L. salmonis. © 2018 Society of Chemical Industry.

Voltage-gated sodium channels from the bees Apis mellifera and Bombus terrestris are differentially modulated by pyrethroid insecticides

Recent experimental and in-field evidence of the deleterious effects of insecticides on the domestic honey bee Apis mellifera have led to a tightening of the risk assessment requirements of these products, and now more attention is being paid to their sublethal effects on other bee species. In addition to traditional tests, in vitro and in silico approaches may become essential tools for a comprehensive understanding of the impact of insecticides on bee species. Here we present a study in which electrophysiology and a Markovian multi-state modelling of the voltage-gated sodium channel were used to measure the susceptibility of the antennal lobe neurons from Apis mellifera and Bombus terrestris, to the pyrethroids tetramethrin and esfenvalerate. Voltage-gated sodium channels from Apis mellifera and Bombus terrestris are differentially sensitive to pyrethroids. In both bee species, the level of neuronal activity played an important role in their relative sensitivity to pyrethroids. This work supports the notion that honey bees cannot unequivocally be considered as a surrogate for other bee species in assessing their neuronal susceptibility to insecticides.

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.

Insecticide Resistance Associated with kdr Mutations in Aedes albopictus: An Update on Worldwide Evidences

Insecticide resistance is an increasing problem worldwide that limits the efficacy of control methods against several pests of health interest. Among them, Aedes albopictus mosquitoes are efficient vectors of relevant pathogens causing animal and human diseases worldwide, including yellow fever, chikungunya, dengue, and Zika. Different mechanisms are associated in conferring resistance to chemical insecticides. One of the most widespread and analysed mechanisms is the knockdown resistance (kdr) causing resistance to DDT and pyrethroids. The mechanism is associated with mutations in the voltage sensitive sodium channel, which is involved in beginning and propagation of action potentials in nervous cells. The mechanism was originally discovered in the housefly and then it was found in a large number of arthropods. In 2011, a kdr associated mutation was evidenced for the first time in A. albopictus and afterward several evidences were reported in the different areas of the world, including China, USA, Brazil, India, and Mediterranean Countries. This review aims to update and summarize current evidences on kdr in A. albopictus, in order to stimulate further researches to analyse in depth A. albopictus resistance status across the world, especially in countries where the presence of this vector is still an emerging issue. Such information is currently needed given the well-known vector role of A. albopictus in the transmission of severe infectious diseases. Furthermore, the widespread use of chemical insecticides for control strategies against A. albopictus progressively lead to pressure selection inducing the rise of insecticide resistance-related mutations in the species. Such event is especially evident in some countries as China, often related to a history of uncontrolled use of chemical insecticides. Thus, a careful picture on the diffusion of kdr mutations worldwide represents a milestone for the implementation of control plans and the triggering of novel research on alternative strategies for mosquito-borne infections.

Vector Control and Insecticidal Resistance in the African Malaria Mosquito Anopheles gambiae

Mosquito-borne diseases (including malaria) belong among the leading causes of death in humans. Vector control is a crucial part of the global strategy for management of mosquito-associated diseases, when insecticide use is the most important component in this effort. However, drug and insecticide resistance threaten the successes made with existing methods. Reduction or elimination of malaria is not possible without effective mosquito control. This article reviews current strategies of intervention in vector control to decrease transmission of disease and covers current relevant knowledge in molecular biology, biochemistry, and medicinal chemistry.

Two novel house fly Vssc mutations, D600N and T929I, give rise to new insecticide resistance alleles

The house fly, Musca domestica, is a serious pest because it transmits a large diversity of human and veterinary diseases. Insecticides, particularly pyrethroids, are commonly used to control house flies. However, the evolution of pyrethroid resistance has reduced the effectiveness of these insecticides. A major mechanism of resistance to pyrethroids is target site insensitivity caused by the mutations in the voltage-sensitive sodium channel (Vssc) gene (e.g. kdr [L1014F] and super-kdr [M918T+L1014F]). Recently, two novel Vssc alleles, super-kdr+D600N and kdr+T929I were detected in a field collected resistant house fly population in Kansas, USA in 2013. To determine the levels of resistance that these new alleles confer to pyrethroids, we isolated strains having the unique Vssc alleles, but being otherwise congenic to the susceptible strain, aabys. We compared levels of resistance conferred to 14 pyrethroids and determined the inheritance of resistance to 8 pyrethroids. Our results revealed that super-kdr+D600N conferred higher levels of resistance to seven pyrethroids relative to super-kdr, and kdr+T929I showed super-kdr-like levels of resistance in house flies. Our results are compared with previous studies and reveal that addition of T929I to the kdr mutation (L1014F) increased resistance to all pyrethroids (except etofenprox), and enhanced resistance by ~1000-fold to acrinathrin and flumethrin. The implications of these results on the evolution of resistance are discussed.

Insecticide resistance and resistance mechanisms in bed bugs, Cimex spp. (Hemiptera: Cimicidae)

The worldwide resurgence of bed bugs [both Cimex lectularius L. and Cimex hemipterus (F.)] over the past two decades is believed in large part to be due to the development of insecticide resistance. The transcriptomic and genomic studies since 2010, as well as morphological, biochemical and behavioral studies, have helped insecticide resistance research on bed bugs. Multiple resistance mechanisms, including penetration resistance through thickening or remodelling of the cuticle, metabolic resistance by increased activities of detoxification enzymes (e.g. cytochrome P450 monooxygenases and esterases), and knockdown resistance by kdr mutations, have been experimentally identified as conferring insecticide resistance in bed bugs. Other candidate resistance mechanisms, including behavioral resistance, some types of physiological resistance (e.g. increasing activities of esterases by point mutations, glutathione S-transferase, target site insensitivity including altered AChEs, GABA receptor insensitivity and altered nAChRs), symbiont-mediated resistance and other potential, yet undiscovered mechanisms may exist. This article reviews recent studies of resistance mechanisms and the genes governing insecticide resistance, potential candidate resistance mechanisms, and methods of monitoring insecticide resistance in bed bugs. This article provides an insight into the knowledge essential for the development of both insecticide resistance management (IRM) and integrated pest management (IPM) strategies for successful bed bug management.

Alanine to valine substitutions in the pore helix IIIP1 and linker-helix IIIL45 confer cockroach sodium channel resistance to DDT and pyrethroids

Pyrethroid insecticides exert toxic effects by prolonging the opening of voltage-gated sodium channels. More than 20 sodium channel mutations from arthropod pests and disease vectors have been confirmed to confer pyrethroid resistance. These mutations have been valuable in elucidating the molecular interaction between pyrethroids and sodium channels, including identification of two pyrethroid receptor sites. Previously, two alanine to valine substitutions, one in the pore helix IIIP1 and the other in the linker-helix connecting S4 and S5 in domain III (IIIL45), were found in Drosophila melanogaster mutants that are resistant to DDT and deltamethrin (a type II pyrethroid with an α-cyano group at the phenylbenzyl alcohol position, which is lacking in type I pyrethroids), but their role in target-site-mediated insecticide resistance has not been functionally confirmed. In this study, we functionally examined the two mutations in cockroach sodium channels expressed in Xenopus laevis oocytes. Both mutations caused depolarizing shifts in the voltage dependence of activation, conferred DDT resistance and also resistance to two Type I pyrethroids by almost abolishing the tail currents induced by Type I pyrethroids. In contrast, neither mutation reduced the amplitude of tail currents induced by the Type II pyrethroids, deltamethrin or cypermethrin. However, both mutations accelerated the decay of Type II pyrethroid-induced tail currents, which normally decay extremely slowly. These results provided new insight into the molecular basis of different actions of Type I and Type II pyrethroids on sodium channels. Computer modeling predicts that both mutations may allosterically affect pyrethroid binding.

Elucidation of pyrethroid and DDT receptor sites in the voltage-gated sodium channel

DDT and pyrethroid insecticides were among the earliest neurotoxins identified to act on voltage-gated sodium channels. In the 1960s, equipped with, at the time, new voltage-clamp techniques, Professor Narahashi and associates provided the initial evidence that DDT and allethrin (the first commercial pyrethroid insecticide) caused prolonged flow of sodium currents in lobster and squid giant axons. Over the next several decades, continued efforts by Prof. Narahashi's group as well as other laboratories led to a comprehensive understanding of the mechanism of action of DDT and pyrethroids on sodium channels. Fast forward to the 1990s, genetic, pharmacological and toxicological data all further confirmed voltage-gated sodium channels as the primary targets of DDT and pyrethroid insecticides. Modifications of the gating kinetics of sodium channels by these insecticides result in repetitive firing and/or membrane depolarization in the nervous system. This mini-review focuses on studies from Prof. Narahashi's pioneer work and more recent mutational and computational modeling analyses which collectively elucidated the elusive pyrethroid receptor sites as well as the molecular basis of differential sensitivities of insect and mammalian sodium channels to pyrethroids.

Mutations of two acidic residues at the cytoplasmic end of segment IIIS6 of an insect sodium channel have distinct effects on pyrethroid resistance

Mutations in sodium channels are known to confer knockdown resistance to pyrethroid insecticides, such as permethrin and cypermethrin, in various agricultural pests and disease vectors. Double mutations, D3i28V and E3i32G, were detected in cypermethrin-resistant Helicoverpa armigera and Heliothis virescens populations. However, the role of the two mutations in pyrethroid resistance remains unclear. In this study, we introduced the mutations into the cockroach sodium channel, BgNav1-1a, and examined their effects on channel gating and pyrethroid sensitivity in Xenopus oocytes. D3i28V alone and the double mutation, D3i28V/E3i32G, shifted the voltage dependence of activation in the depolarizing direction by 15 mV and 20 mV, respectively, whereas E3i32G had no significant effect. D3i28V reduced the amplitude of tail currents induced by permethrin and NRDC 157 (Type I pyrethroids) and deltamethrin and cypermethrin (Type II pyrethroids), whereas E3i32G alone had no effect. Intriguingly, the amplitude of Type II pyrethroid-induced tail current from D3i28V/E3i32G channels was similar to that of BgNav1-1a channels, but the decay of the tail currents was accelerated. Such effects were not observed for Type I pyrethroid-induced tail currents. Computational analysis based on the model of dual pyrethroid receptors on insect sodium channels predicted D3i28V and E3i32G exert their effects on channel gating and pyrethroid action via allosteric mechanisms. Our results not only illustrate the distinct effect of the D3i28V/E3i32G double mutations on Type I vs. Type II pyrethroids, but also reinforce the concept that accelerated decay of tail currents can be an effective mechanism of pyrethroid resistance to Type II pyrethroids.

Characterization of the voltage-gated sodium channel of the Asian citrus psyllid, Diaphorina citri

The Asian citrus psyllid, Diaphorina citri Kuwayama (Hemiptera: Liviidae), is an important insect pest of citrus. It is the vector of 'Candidatus' Liberibacter asiaticus, a phloem-limited bacterium that infects citrus, resulting in the disease Huanglongbing (HLB). Disease management relies heavily on suppression of D. citri populations with insecticides, including pyrethroids. In recent annual surveys to monitor insecticide resistance, reduced susceptibility to fenpropathrin was identified in several field populations of D. citri. The primary target of pyrethroids is the voltage-gated sodium channel (VGSC). The VGSC is prone to target-site insensitivity because of mutations that either reduce pyrethroid binding and/or alter gating kinetics. These mutations, known as knockdown resistance or kdr, have been reported in a wide diversity of arthropod species. Alternative splicing, in combination with kdr mutations, has been also associated with reduced pyrethroid efficacy. Here we report the molecular characterization of the VGSC in D. citri along with a survey of alternative splicing across developmental stages of this species. Previous studies demonstrated that D. citri has an exquisite enzymatic arsenal to detoxify insecticides resulting in reduced efficacy. The results from the current investigation demonstrate that target-site insensitivity is also a potential basis for insecticide resistance to pyrethroids in D. citri. The VGSC sequence and its molecular characterization should facilitate early elucidation of the underlying cause of an established case of resistance to pyrethroids. This is the first characterization of a VGSC from a hemipteran to this level of detail, with the majority of the previous studies on dipterans and lepidopterans.

Voltage-gated sodium channels as targets for pyrethroid insecticides

The pyrethroid insecticides are a very successful group of compounds that have been used extensively for the control of arthropod pests of agricultural crops and vectors of animal and human disease. Unfortunately, this has led to the development of resistance to the compounds in many species. The mode of action of pyrethroids is known to be via interactions with the voltage-gated sodium channel. Understanding how binding to the channel is affected by amino acid substitutions that give rise to resistance has helped to elucidate the mode of action of the compounds and the molecular basis of their selectivity for insects vs mammals and between insects and other arthropods. Modelling of the channel/pyrethroid interactions, coupled with the ability to express mutant channels in oocytes and study function, has led to knowledge of both how the channels function and potentially how to design novel insecticides with greater species selectivity.

Ion channels as insecticide targets

Ion channels remain the primary target of most of the small molecule insecticides. This review examines how the subunit composition of heterologously expressed receptors determines their insecticide-specific pharmacology and how the pharmacology of expressed receptors differs from those found in the insect nervous system. We find that the insecticide-specific pharmacology of some receptors, like that containing subunits of the Rdl encoded GABA receptor, can be reconstituted with very few of the naturally occurring subunits expressed. In contrast, workers have struggled even to express functional insect nicotinic acetylcholine receptors (nAChRs), and work has therefore often relied upon the expression of vertebrate receptor subunits in their place. We also examine the extent to which insecticide-resistance-associated mutations, such as those in the para encoded voltage-gated sodium channel, can reveal details of insecticide-binding sites and mode of action. In particular, we examine whether mutations are present in the insecticide-binding site and/or at sites that allosterically affect the drug preferred conformation of the receptor. We also discuss the ryanodine receptor as a target for the recently developed diamides. Finally, we examine the lethality of the genes encoding these receptor subunits and discuss how this might determine the degree of conservation of the resistance-associated mutations found.

Sodium Channel Mutations and Pyrethroid Resistance in Aedes aegypti

Pyrethroid insecticides are widely used to control insect pests and human disease vectors. Voltage-gated sodium channels are the primary targets of pyrethroid insecticides. Mutations in the sodium channel have been shown to be responsible for pyrethroid resistance, known as knockdown resistance (kdr), in various insects including mosquitoes. In Aedes aegypti mosquitoes, the principal urban vectors of dengue, zika, and yellow fever viruses, multiple single nucleotide polymorphisms in the sodium channel gene have been found in pyrethroid-resistant populations and some of them have been functionally confirmed to be responsible for kdr in an in vitro expression system, Xenopus oocytes. This mini-review aims to provide an update on the identification and functional characterization of pyrethroid resistance-associated sodium channel mutations from Aedes aegypti. The collection of kdr mutations not only helped us develop molecular markers for resistance monitoring, but also provided valuable information for computational molecular modeling of pyrethroid receptor sites on the sodium channel.

Novel Mutations in the Voltage-Gated Sodium Channel of Pyrethroid-Resistant Varroa destructor Populations from the Southeastern USA

The parasitic mite Varroa destructor has a significant worldwide impact on bee colony health. In the absence of control measures, parasitized colonies invariably collapse within 3 years. The synthetic pyrethroids tau-fluvalinate and flumethrin have proven very effective at managing this mite within apiaries, but intensive control programs based mainly on one active ingredient have led to many reports of pyrethroid resistance. In Europe, a modification of leucine to valine at position 925 (L925V) of the V. destructor voltage-gated sodium channel was correlated with resistance, the mutation being found at high frequency exclusively in hives with a recent history of pyrethroid treatment. Here, we identify two novel mutations, L925M and L925I, in tau-fluvalinate resistant V. destructor collected at seven sites across Florida and Georgia in the Southeastern region of the USA. Using a multiplexed TaqMan® allelic discrimination assay, these mutations were found to be present in 98% of the mites surviving tau-fluvalinate treatment. The mutations were also found in 45% of the non-treated mites, suggesting a high potential for resistance evolution if selection pressure is applied. The results from a more extensive monitoring programme, using the Taqman® assay described here, would clearly help beekeepers with their decision making as to when to include or exclude pyrethroid control products and thereby facilitate more effective mite management programmes.

Overcoming super-knock down resistance (super-kdr) mediated resistance: multi-halogenated benzyl pyrethroids are more toxic to super-kdr than kdr house flies

Target site insensitivity because of mutations in the voltage-sensitive sodium channel gene (Vssc) is a major mechanism of resistance to pyrethroid insecticides in the house fly, Musca domestica. There are three known Vssc alleles that confer resistance to pyrethroids in the house fly: knock down resistance (kdr; L1014F), super-kdr (M918T + L1014F) and kdr-his (L1014H), but there has been no side-by-side comparison of the resistance levels that they confer. We compared the levels of resistance conferred by the three Vssc alleles in congenic strains to 19 structurally diverse pyrethroids, and compared the full-length Vssc cDNA sequences from each strain. Generally, the levels of resistance conferred were kdr-his < kdr < super-kdr. However, there was significant variation in this pattern, especially for super-kdr, for which both high and low resistance ratios were observed for several pyrethroids. We also examined the levels of resistance in heterozygotes. Resistance in each of the hybrids was generally inherited as an incompletely recessive trait, except for the kdr-his/kdr hybrids, which showed incompletely to completely dominant resistance (ie had resistance levels comparable to kdr homozygotes). The importance of these results to understanding the frequencies of these resistance alleles in natural populations, the evolution of insecticide resistance and resistance management strategies are discussed.