Correlation of a resistance-associated Rdl mutation in the German cockroach, Blattella germanica (L), with persistent dieldrin resistance in two Danish field populations

First evidence of the A302S Rdl insecticide resistance mutation in populations of the bed bug, Cimex lectularius (Hemiptera: Cimicidae) in North America

The common bed bug, Cimex lectularius (L.) (Hemiptera: Cimicidae), is a pervasive indoor pest with prominent medical, veterinary, and economic impacts. Bed bug infestations are controlled by a wide range of insecticides, including pyrethroids, neonicotinoids, pyrroles, and phenylpyrazoles; however, bed bugs have evolved resistance mechanisms to most of these insecticides. Mutations in the Rdl (resistance to dieldrin) gene, located in a subunit of the γ-amino butyric acid (GABA)-gated chloride channel, have been identified in several pest insects, including the German cockroach. These have been found to confer resistance to fipronil, a phenylpyrazole insecticide commonly used in urban environments, in addition to cyclodienes (eg dieldrin), a class of insecticides banned in most countries since the 1990s. While resistance to dieldrin and fipronil has been reported in bed bugs, both C. lectularius and the tropical bed bug, C. hemipterus, the occurrence of mutations in the Rdl gene has yet to be thoroughly investigated. In this study, we sequence a fragment of the Rdl gene commonly found to harbor cyclodiene and phenylpyrazole conferring mutations from 134 unique populations collected across the United States and Canada spanning a 14-yr period. Homozygous genotypes for the A302S mutation were found in 2 geographically distinct populations. This finding represents the first record of a non-synonymous Rdl mutation in bed bugs and identifies another mechanism by which insecticide resistance may be conferred in this species.

Characterization of insecticide resistance and their mechanisms in field populations of the German cockroach (Blattodea: Ectobiidae) in Taiwan under different treatment regimes

This study investigated how management strategies influence resistance profiles in German cockroach (Blattella germanica (L.)) populations and their impact on the performance of commercial gel baits containing fipronil, imidacloprid, and indoxacarb. Field populations from premises managed under 3 different strategies: Baiting, random insecticide (RI) used, and insecticide rotation (IR) were tested. Almost all populations under RI and IR were resistant to deltamethrin, but low to moderate resistance was observed under the Baiting approach. Cytochrome P450 monooxygenases (P450) were involved in deltamethrin resistance in these resistant populations. All individuals under Baiting and RI were homozygous for the L993F mutation, but the populations under IR lacked homozygous-resistant individuals. Eighty-three percent of field populations with complete homozygosity for the Rdl mutation displayed low mortality upon exposure to 3× LD95 fipronil. The effect of P450 and the Rdl mutation conferred high fipronil resistance in populations under the Baiting approach, recording moderate performance indices (PI) of 44-67 in fipronil bait. By contrast, those populations under RI and IR, in which involve glutathione S-transferases in fipronil resistance, had high PIs of 78-93. Almost 80% of populations exhibited over 90% mortality at 3× LD95 indoxacarb treatment, accompanied by high PIs of 90-100 in indoxacarb bait. Partial mortality from 1× LD95 imidacloprid occurred across all field populations due to the involvement of P450. PIs of imidacloprid bait ranged 5-57 and 20-94 in populations under RI and IR, respectively. Field populations demonstrate different resistance profiles depending on the treatment regimes, and the resistance mechanisms involved influenced gel bait's effectiveness.

Population genetics as a tool to understand invasion dynamics and insecticide resistance in indoor urban pest insects

Many indoor urban pest insects now show a near-global distribution. The reasons for this may be linked to their cryptic behaviors, which make unintentional transport likely, tied to their reliance on human-mediated dispersal that can result in spread over potentially long-distances. Additionally, numerous species exhibit an array of mechanisms that confer insecticide resistance. Using population genetics, it is possible to elucidate the genetic characteristics that define globally successful indoor urban pest insect species. Furthermore, this approach may be used to determine the frequency and distribution of insecticide resistance. Here, I review the recent literature that utilizes population genetic analyses in an effort to identify the characteristics that help explain the success of indoor urban pests.

Activity, selection response and molecular mode of action of the isoxazoline afoxolaner in Tetranychus urticae

BACKGROUND

Afoxolaner is a novel representative of the isoxazolines, a class of ectoparasiticides which has been commercialized for the control of tick and flea infestations in dogs. In this study, the biological efficacy of afoxolaner against the two-spotted spider mite Tetranychus urticae was evaluated. Furthermore, as isoxazolines are known inhibitors of γ-aminobutyric acid-gated chloride channels (GABACls), the molecular mode of action of afoxolaner on T. urticae GABACls (TuRdls) was studied using functional expression in Xenopus oocytes followed by two-electrode voltage-clamp (TEVC) electrophysiology, and results were compared with inhibition by fluralaner, fipronil and endosulfan. To examine the influence of known GABACl resistance mutations, H301A, I305T and A350T substitutions in TuRdl1 and a S301A substitution in TuRdl2 were introduced.

RESULTS

Bioasassays revealed excellent efficacy of afoxolaner against all developmental stages and no cross-resistance was found in a panel of strains resistant to most currently used acaricides. Laboratory selection over a period of 3 years did not result in resistance. TEVC revealed clear antagonistic activity of afoxolaner and fluralaner for all homomeric TuRdl1/2/3 channels. The introduction of single, double or triple mutations to TuRdl1 and TuRdl2 did not lower channel sensitivity. By contrast, both endosulfan and fipronil had minimal antagonistic activities against TuRdl1/2/3, and channels carrying single mutations, whereas the sensitivity of double and triple TuRdl1 mutants was significantly increased.

CONCLUSIONS

Our results demonstrate that afoxolaner is a potent antagonist of GABACls of T. urticae and has a powerful mode of action to control spider mites. © 2022 Society of Chemical Industry.

Multiple Mechanisms Confer Fipronil Resistance in the German Cockroach: Enhanced Detoxification and Rdl Mutation

Populations of Blattella germanica (L.) (German cockroach) have been documented worldwide to be resistant to a wide variety of insecticides with multiple modes of action. The phenylpyrazole insecticide fipronil has been used extensively to control German cockroach populations, exclusively in baits, yet the highest reported fipronil resistance is 38-fold in a single population. We evaluated five populations of German cockroaches, collected in 2018-2019 in apartments in North Carolina and assayed in 2019, to determine the status of fipronil resistance in the state. Resistance ratios in field-collected strains ranged from 22.4 to 37.2, indicating little change in fipronil resistance over the past 20 yr. In contrast, resistance to pyrethroids continues to escalate. We also assessed the roles of detoxification enzymes in fipronil resistance with four synergists previously shown to diminish metabolic resistance to various insecticides in German cockroaches-piperonyl butoxide, S,S,S-tributyl phosphorotrithioate, diethyl maleate, and triphenyl phosphate. These enzymes appear to play a variable role in fipronil resistance. We also sequenced a fragment of the Rdl (resistant to dieldrin) gene that encodes a subunit of the GABA receptor. Our findings showed that all field-collected strains are homozygous for a mutation that substitutes serine for an alanine (A302S) in Rdl, and confers low resistance to fipronil. Understanding why cockroaches rapidly evolve high levels of resistance to some insecticides and not others, despite intensive selection pressure, will contribute to more efficacious pest management.

Combined metabolic and target-site resistance mechanisms confer fipronil and deltamethrin resistance in field-collected German cockroaches (Blattodea: Ectobiidae)

Despite insecticide resistance issues, pyrethroids and fipronil have continued to be used extensively to control the German cockroach, Blattella germanica (L.) (Blattodea: Ectobiidae) for more than two decades. We evaluated the physiological insecticide resistance in five German cockroach populations collected from 2018 to 2020 and measured the extent of metabolic detoxification and target-site insensitivity resistance mechanisms. Topically applied doses of the 3 x LD₉₅ of deltamethrin, fipronil, DDT, or dieldrin of a susceptible strain (UCR, Diagnostic Dose) failed to cause >23% mortality, and the 10 x LD₉₅ of deltamethrin or fipronil failed to cause >53% mortality. All field-collected strains possessed a combination of metabolic and target-site insensitivity mechanisms that cause reduced susceptibility. Elevated activities of esterase and glutathione S-transferase were measured, and the synergists piperonyl butoxide or S,S,S-tributyl phosphorotrithioate increased topical mortality up to 100% for deltamethrin and 93% for fipronil 10 x LD₉₅. The target-site mutations L993F of the para-homologous sodium channel and A302S of the GABA-gated chloride channel associated with pyrethroid and fipronil resistance, respectively, were found at ~80-100% frequency in field populations. Pyrethroid and fipronil spray formulations also were ineffective in a choice box assay against field-collected strains suggesting that these treatments would fail to control cockroaches under field conditions.

Distinct roles of two RDL GABA receptors in fipronil action in the diamondback moth (Plutella xylostella)

The phenylpyrazole insecticide fipronil blocks resistance to dieldrin (RDL) γ-aminobutyric acid (GABA) receptors in insects, thereby impairing inhibitory neurotransmission. Some insect species, such as the diamondback moth (Plutella xylostella), possess more than one Rdl gene. The involvement of multiple Rdls in fipronil toxicity and resistance remains largely unknown. In this study, we investigated the roles of two Rdl genes, PxRdl1 and PxRdl2, in P. xylostella fipronil action. In Xenopus oocytes, PxRDL2 receptors were 40 times less sensitive to fipronil than PxRDL1. PxRDL2 receptors were also less sensitive to GABA compared with PxRDL1. Knockout of the fipronil-sensitive PxRdl1 reduced the fipronil potency 10-fold, whereas knockout of the fipronil-resistant PxRdl2 enhanced the fipronil potency 4.4-fold. Furthermore, in two fipronil-resistant diamondback moth field populations, PxRdl2 expression was elevated 3.7- and 4.1-fold compared with a susceptible strain, whereas PxRdl1 expression was comparable among the resistant and susceptible strains. Collectively, our results indicate antagonistic effects of PxRDL1 and PxRDL2 on fipronil action in vivo and suggest that enhanced expression of fipronil-resistant PxRdl2 is potentially a new mechanism of fipronil resistance in insects.

Resistance to Fipronil in the Common Bed Bug (Hemiptera: Cimicidae)

Cimex lectularius L. populations have been documented worldwide to be resistant to pyrethroids and neonicotinoids, insecticides that have been widely used to control bed bugs. There is an urgent need to discover new active ingredients with different modes of action to control bed bug populations. Fipronil, a phenylpyrazole that targets the GABA receptor, has been shown to be highly effective on bed bugs. However, because fipronil shares the same target site with dieldrin, we investigated the potential of fipronil resistance in bed bugs. Resistance ratios in eight North American populations and one European population ranged from 1.4- to >985-fold, with highly resistant populations on both continents. We evaluated metabolic resistance mechanisms mediated by cytochrome P450s, esterases, carboxylesterases, and glutathione S-transferases using synergists and a combination of synergists. All four detoxification enzyme classes play significant but variable roles in bed bug resistance to fipronil. Suppression of P450s and esterases with synergists eliminated resistance to fipronil in highly resistant bed bugs. Target-site insensitivity was evaluated by sequencing a fragment of the Rdl gene to detect the A302S mutation, known to confer resistance to dieldrin and fipronil in other species. All nine populations were homozygous for the wild-type genotype (susceptible phenotype). Highly resistant populations were also highly resistant to deltamethrin, suggesting that metabolic enzymes that are responsible for pyrethroid detoxification might also metabolize fipronil. It is imperative to understand the origins of fipronil resistance in the development or adoption of new active ingredients and implementation of integrated pest management programs.

The Use of Insecticides to Manage the Western Corn Rootworm, Diabrotica virgifera virgifera, LeConte: History, Field-Evolved Resistance, and Associated Mechanisms

The western corn rootworm, Diabrotica virgifera virgifera LeConte (Dvv) is a significant insect pest of maize in the United States (U.S.). This paper reviews the history of insecticide use in Dvv management programs, Dvv adaptation to insecticides, i.e., field-evolved resistance and associated mechanisms of resistance, plus the current role of insecticides in the transgenic era. In the western U.S. Corn Belt where continuous maize is commonly grown in large irrigated monocultures, broadcast-applied soil or foliar insecticides have been extensively used over time to manage annual densities of Dvv and other secondary insect pests. This has contributed to the sequential occurrence of Dvv resistance evolution to cyclodiene, organophosphate, carbamate, and pyrethroid insecticides since the 1950s. Mechanisms of resistance are complex, but both oxidative and hydrolytic metabolism contribute to organophosphate, carbamate, and pyrethroid resistance facilitating cross-resistance between insecticide classes. History shows that Dvv insecticide resistance can evolve quickly and may persist in field populations even in the absence of selection. This suggests minimal fitness costs associated with Dvv resistance. In the transgenic era, insecticides function primarily as complementary tools with other Dvv management tactics to manage annual Dvv densities/crop injury and resistance over time.

RDL A301S alone does not confer high levels of resistance to cyclodiene organochlorine or phenyl pyrazole insecticides in Plutella xylostella

Mutations in the GABA-gated chloride channel are associated with resistance to cyclodiene organochlorine and phenyl pyrazole insecticides. The best characterised of these is A301S, which was initially identified in a Dieldrin resistant strain of Drosophila melanogaster. The orthologous mutation has been found in a variety of different crop pests including the diamond back moth Plutella xylostella. However, the contribution of this mutation to resistance in this species remains unclear. We have used the CRISPR/Cas9 system in order to edit Plutella xylostella PxGABARalpha1 to Serine at the 301 orthologous position (282 in PxGABARalpha1) in an insecticide sensitive strain isolated from Vero Beach (VB) USA. In this edited line, no high level of resistance is conferred to Dieldrin, Endosulfan or Fipronil, rather only a subtle shift in sensitivity which could not confer commercially important resistance. We conclude that the high level of commercial resistance to cyclodiene organochlorine and phenyl pyrazole insecticides observed in some field isolates of Plutella xylostella cannot arise from A282S in PxGABARalpha1 alone.

Genotype to phenotype, the molecular and physiological dimensions of resistance in arthropods

The recent accumulation of molecular studies on mutations in insects, ticks and mites conferring resistance to insecticides, acaricides and biopesticides is reviewed. Resistance is traditionally classified by physiological and biochemical criteria, such as target-site insensitivity and metabolic resistance. However, mutations are discrete molecular changes that differ in their intrinsic frequency, effects on gene dosage and fitness consequences. These attributes in turn impact the population genetics of resistance and resistance management strategies, thus calling for a molecular genetic classification. Mutations in structural genes remain the most abundantly described, mostly in genes coding for target proteins. These provide the most compelling examples of parallel mutations in response to selection. Mutations causing upregulation and downregulation of genes, both in cis (in the gene itself) and in trans (in regulatory processes) remain difficult to characterize precisely. Gene duplications and gene disruption are increasingly reported. Gene disruption appears prevalent in the case of multiple, hetero-oligomeric or redundant targets.

Knockdown of the ionotropic γ-aminobutyric acid receptor (GABAR) RDL gene decreases fipronil susceptibility of the small brown planthopper, Laodelphax striatellus (Hemiptera: Delphacidae)

Insect γ-aminobutyric acid receptors (GABARs) are important molecular targets of cyclodiene and phenylpyrazole insecticides. Previously GABARs encoding rdl (resistant to dieldrin) genes responsible for dieldrin and fipronil resistance were identified in various economically important insect pests. In this study, we cloned the open reading frame cDNA sequence of rdl gene from fipronil-susceptible and fipronil-resistant strains of Laodelphax striatellus (Lsrdl). Sequence analysis confirmed the presence of a previously identified resistance-conferring mutation. Different alternative splicing variants of Lsrdl were noted. Injection of dsLsrdl reduced the mRNA abundance of Lsrdl by 27-82%, and greatly decreased fipronil-induced mortality of individuals from both susceptible and resistant strains. These data indicate that Lsrdl encodes a functional RDL subunit that mediates susceptibility to fipronil. Additionally, temporal and spatial expression analysis showed that Lsrdl was expressed at higher levels in eggs, fifth-instar nymphs, and female adults than in third-instar and fourth-instar nymphs. Lsrdl was predominantly expressed in the heads of 2-day-old female adults. All these results provide useful background knowledge for better understanding of fipronil resistance related ionotropic GABA receptor rdl gene expressed variants and potential functional differences in insects.

Genetics and mechanisms of imidacloprid resistance in house flies

Imidacloprid is the most widely used neonicotinoid insecticide against house flies, which are major pests at animal production facilities worldwide. However, cases of both physiological and behavior resistance have been reported. Recently, physiological resistance to imidacloprid was found in the United States (California and Florida). However, no studies have been undertaken to characterize this resistance in house flies from the United States. Three imidacloprid selections of a strain originally collected from Florida increased the level of resistance, ultimately resulting in a strain that had 2300-fold resistance in females and 130-fold in males. Imidacloprid resistance was not overcome with piperonyl butoxide (PBO) suggesting that resistance is not due to detoxification by cytochrome P450s. Resistance was mapped to autosomes 3 and 4. There was⩾100-fold cross-resistance to acetamiprid and dinotefuran, but no cross-resistance to spinosad. The resistance in this imidacloprid selected population was unstable and declined over a period of several months. The significance of these results to management of imidacloprid resistance in the field, and potential mechanisms of resistance involved, are discussed.

Role of a γ-aminobutryic acid (GABA) receptor mutation in the evolution and spread of Diabrotica virgifera virgifera resistance to cyclodiene insecticides

The western corn rootworm, Diabrotica virgifera virgifera, is a damaging pest of cultivated corn that was controlled by applications of cyclodiene insecticides from the late 1940s until resistance evolved ∼10 years later. Range expansion from the western plains into eastern USA coincides with resistance development. An alanine to serine amino acid substitution within the Rdl subunit of the gamma-aminobutyric acid (GABA) receptor confers resistance to cyclodiene insecticides in many species. We found that the non-synonymous single nucleotide polymorphism (SNP) G/T at the GABA receptor cDNA position 838 (G/T(838)) of D. v. virgifera resulted in the alanine to serine change, and the codominant SNP allele T(838) was genetically linked to survival of beetles in aldrin bioassays. A phenotypic gradient of decreasing susceptibility from west to east was correlated with higher frequencies of the resistance-conferring T(838) allele in the eastern-most populations. This pattern exists in opposition to perceived selective pressures since the more eastern and most resistant populations probably experienced reduced exposure. The reasons for the observed distribution are uncertain, but historical records of the range expansion combined with the distribution of susceptible and resistant phenotypes and genotypes provide an opportunity to better understand factors affecting the species' range expansion.

The cys-loop ligand-gated ion channel gene family of Tetranychus urticae: implications for acaricide toxicology and a novel mutation associated with abamectin resistance

The cys-loop ligand-gated ion channel (cysLGIC) super family of Tetranychus urticae, the two-spotted spider mite, represents the largest arthropod cysLGIC super family described to date and the first characterised one within the group of chelicerates. Genome annotation, phylogenetic analysis and comparison of the cysLGIC subunits with their counterparts in insects reveals that the T. urticae genome encodes for a high number of glutamate- and histamine-gated chloride channel genes (GluCl and HisCl) compared to insects. Three orthologues of the insect γ-aminobutyric acid (GABA)-gated chloride channel gene Rdl were detected. Other cysLGIC groups, such as the nAChR subunits, are more conserved and have clear insect orthologues. Members of cysLGIC family mediate endogenous chemical neurotransmission and they are prime targets of insecticides. Implications for toxicology associated with the identity and specific features of T. urticae family members are discussed. We further reveal the accumulation of known and novel mutations in different GluCl channel subunits (Tu_GluCl1 and Tu_GluCl3) associated with abamectin resistance in T. urticae, and provide genetic evidence for their causality. Our study provides useful toxicological insights for the exploration of the T. urticae cysLGIC subunits as putative molecular targets for current and future chemical control strategies.

Mechanisms underlying fipronil resistance in a multiresistant field strain of the German cockroach (Blattodea: Blattellidae)

German cockroaches (Blattella germanica L.) have significant impacts on human health, most notably they are implicated as causes of childhood asthma. Gel bait formulations of fipronil, a phenylpyrazole insecticide, have been in use for German cockroach control in the United States since 1998. Previously, dieldrin resistant German cockroach strains were shown to have 7- to 17-fold cross-resistance to fipronil. More recently, a field-collected strain (GNV-R) displayed approximately 36-fold resistance to topically applied fipronil at the LD50 level, which is the highest level of fipronil resistance reported to date in the German cockroach. The aim of the current research was to identify mechanism(s) responsible for high-level fipronil resistance in the GNV-R strain. Synergist bioassays conducted using topical and injection application methods implicated cytochrome P450-mediated detoxification in resistance. Electrophysiological recordings using the suction-electrode technique revealed the nervous system of the GNV-R strain is insensitive to fipronil. In agreement with electrophysiology results, the alanine to serine (A302S) mutation encoded by the gamma-amino butyric acid-gated chloride channel subunit gene resistance to dieldrin, which confers limited cross-resistance to fipronil, was detected in 95% of GNV-R strain individuals. Logistic regression analysis showed that A302S mutation frequency correlates with neurological insensitivity as shown by electrophysiology data. Overall, results of synergism bioassays, electrophysiological recordings, and A302S mutation frequency measurements suggest that fipronil resistance in the GNV-R strain is caused by the combined effects of enhanced metabolism by cytochrome P450s and target-site insensitivity caused by the A302S-encoding mutation in the resistance to dieldrin gene.

Development of strategies for monitoring indoxacarb and gel bait susceptibility in the German cockroach (Blattodea: Blattellidae)

BACKGROUND

Advion(®) cockroach gel bait (6 g kg(-1) indoxacarb) is in widespread use for Blattella germanica (L.) control in the United States. However, baseline susceptibility levels to indoxacarb in German cockroach field populations are not known. Hence, this research sought to develop monitoring strategies to estimate the susceptibility of German cockroach populations to indoxacarb.

RESULTS

Four bioassays were evaluated: topical lethal dose (LD), formulated gel bait lethal time (LT), vial lethal concentration (LC) and gel bait matrix LD. Of these methods, the vial LC and gel bait matrix LD bioassays were the most economical and relevant assessment strategies. For indoxacarb susceptibility monitoring, a two-tiered approach was developed that utilizes diagnostic concentrations and doses in vial LC (30 and 60 µg vial(-1) ) and gel bait matrix LD (1.0, 1.5 and 2.5 µg insect(-1) ) formats.

CONCLUSIONS

A two-tiered susceptibility monitoring strategy was developed that includes testing field populations at diagnostic concentrations and doses in first-tier vial LC bioassays and second-tier gel bait matrix LD bioassays. The vial method facilitates rapid identification of field strains with reduced susceptibility. The feeding bioassay effectively simulates field exposure to Advion(®) and therefore has utility for secondary confirmation of susceptibility shifts and identification of behavioral resistance (i.e. bait aversion).

Affordable assays for genotyping single nucleotide polymorphisms in insects

Insect genome projects and DNA sequence databases are providing unprecedented amounts of information about variation at specific nucleotides in protein- and RNA-coding genes. Single nucleotide polymorphisms (SNPs) are abundant in all insect species so far examined and are proving useful in population genetics, linkage mapping and marker-assisted selection. A number of studies has already identified SNPs associated with insecticide resistance, especially mutations conferring reduced target site sensitivity. Unfortunately, most modern, high-throughput, automated SNP detection technologies are expensive or require the use of expensive equipment and are therefore not accessible to laboratories on a limited budget or to our colleagues in developing countries. In this review, we provide a chronological and comprehensive list of all SNP methods. We emphasize and explain those techniques in which genotypes can be identified by eye or that only require agarose gel electrophoresis. We provide examples where these techniques have or are currently being applied to insects.

Towards genetic markers in animal populations as biomonitors for human-induced environmental change

Genetic markers provide potentially sensitive indicators of changes in environmental conditions because the genetic constitution of populations is normally altered well before populations become extinct. Genetic indicators in populations include overall genetic diversity, genetic changes in traits measured at the phenotypic level, and evolution at specific loci under selection. While overall genetic diversity has rarely been successfully related to environmental conditions, genetically based changes in traits have now been linked to the presence of toxins and both local and global temperature shifts. Candidate loci for monitoring stressors are emerging from information on how specific genes influence traits, and from screens of random loci across environmental gradients. Drosophila research suggests that chromosomal regions under recent intense selection can be identified from patterns of molecular variation and a high frequency of transposable element insertions. Allele frequency changes at candidate loci have been linked to pesticides, pollutants and climate change. Nevertheless, there are challenges in interpreting allele frequencies in populations, particularly when a large number of loci control a trait and when interactions between alleles influence trait expression. To meet these challenges, population samples should be collected for longitudinal studies, and experimental programmes should be undertaken to link variation at candidate genes to ecological processes.