Mechanisms responsible for high levels of permethrin resistance in the house fly

Lack of fitness costs associated with resistance to permethrin in Musca domestica

Resistance to permethrin has been reported in Pakistani strains of Musca domestica. The present study explored the performance of biological traits and analyzed life tables to determine whether there is any detrimental effect of permethrin resistance on the fitness of permethrin-resistant strains [an isogenic resistant strain (Perm-R) and a field strain (Perm-F)] compared to a susceptible strain (Perm-S). Perm-R and Perm-F exhibited 233.93- and 6.87-fold resistance to permethrin, respectively. Life table analyses revealed that the Perm-R strain had a significantly shorter preadult duration, longer longevity, shorter preoviposition period, higher fecundity, finite rate of increase, intrinsic rate of increase, net reproductive rate and a shorter mean generation time, followed by the Perm-F strain when compared to the Perm-S strain. Data of the performance of biological traits reveled that permethrin resistance strains had a better fit than that of the Perm-S strain. The enhanced fitness of resistant strains of M. domestica may accelerate resistance development to permethrin and other pyrethroids in Pakistan. Some possible measures to manage M. domestica and permethrin resistance in situations of fitness advantage are discussed.

Selection for, and characterization of, fluralaner resistance in the house fly, Musca domestica

House flies, Musca domestica (L), are the mechanical vector of >100 human and animal pathogens, including those that are antibiotic-resistant. Given that house flies are associated closely with human and livestock activity, they present medical and veterinary health concerns. Although there are numerous strategies for control of house fly populations, chemical control has been favored in many facilities. Products with pyrethroid active ingredients have been used predominantly for >35 years in space sprays. As a result, strong selection for pyrethroid resistance has led to reduced control of many populations. Reliance on a limited number of insecticides for decades has created fly control problems necessitating the discovery and formulation of new control chemistries. Fluralaner is a relatively new insecticide and acaricide (first reported in 2010), belonging to the isoxazoline class. These insecticides target the glutamate- and gamma-aminobutyric acid-gated (GABA) chloride channels, which is a different mode of action from other insecticides used against house flies. Although is it not currently registered for house fly control in the United States, previous work has shown that fluralaner is highly toxic to house flies and that there was limited cross-resistance found in laboratory strains having high levels of resistance to other insecticides. Herein, we characterized the time and age dependency of fluralaner toxicity, detected cross-resistance in populations from across the United States, and selected a highly resistant (>11,000-fold) house fly strain. We found that the fluralaner LD₅₀ of 18-24 h old flies was 2-fold higher than for 5-6 d old flies. This appears to be due to more rapid penetration of fluralaner into the 5-6 d old flies. Fluralaner resistance was inherited as an intermediate to incompletely dominant trait and was mapped to chromosomes 5 and 3. Resistance could be suppressed to 7-fold with piperonyl butoxide, suggesting that cytochrome P450 (CYP)-mediated detoxification was a major mechanism of resistance. Decreased penetration was also demonstrated as a mechanism of resistance. The utility of fluralaner for house fly control is discussed.

Global Pattern of kdr-Type Alleles in Musca domestica (L.)

Purpose of Review

Houseflies, Musca domestica L., are an important sanitary pest that affects human and domesticated animals. They are mechanical carriers of more than 100 human and animal diseases including protozoan, bacterial, helminthic, and viral infections. Recently, it was demonstrated that houseflies acquired, harbored, and transmitted SARS-CoV-2 (COVID-19) for up to 1 day post-exposure. The most widely used control strategy relays on the application of pyrethroid insecticides due to their effectiveness, low mammalian toxicity, low cost, and environmental safety. The main mechanism of action of pyrethroids is to exert their toxic effects through affecting the voltage-sensitive sodium channel (VSSC) modifying the transmission of the nerve impulse and leading to the death of the insects. Target site insensitivity of the VSSC is due to the presence of single nuclear polymorphisms (SNPs) named knockdown mutations (kdr). In this review, we synthetize recent data on the type and distribution of these mutations globally.

Recent Findings

Housefly resistance is reported in several countries. Increased applications of pyrethroids to control housefly populations led to the emergence of multiple evolutionary origins of resistance determined by five amino acid substitutions or specific mutations in the VSSC: kdr (L1014F), kdr-his (L1014H), super-kdr (M918T + L1014F), type N (D600N + M918T + L1014F), and 1B (T929I + L1014F). According to the global map obtained, high levels of resistance to pyrethroids are associated with the L1014F mutation found mostly in North America, Europe, and Asia, while the super-kdr mutation was mostly found in the American continent. The level of protection conferred by these alleles against pyrethroids was generally kdr-his < kdr < Type N ≤ super-kdr ≤ 1B. The relative fitness of the alleles under laboratory conditions was susceptible ≅ kdr-his > kdr > super-kdr suggesting that the fitness cost of an allele was relative to the presence of other alleles in a population and that the reversion of resistance in a free insecticide environment might be quite variable from one region to another.

Summary

An adequate integrated pest management program should consider monitoring susceptibility to pyrethroids to detect early levels of resistance and predict the spread and evolution of resistant phenotypes and genotypes. From this review, the pyrethroid resistance status of housefly population was determined in very few countries and has evolved independently in different areas of the world affecting chemical control programs.

Cypermethrin and ivermectin resistance in field populations of Rhipicephalus sanguineus sensu lato (Latrielle, 1806) collected from dogs in south India

The abuse of acaricides for control of Rhipicephalus sanguineus sensu lato ticks led to a strong selection pressure for acaricide resistance. Data on acaricide resistance in R. sanguineus s.l. populations from India is meager though veterinarians frequently report treatment failures. The present study was aimed to characterize the level of resistance in R. sanguineus s.l. against most commonly used drugs, cypermethrin and ivermectin in Andhra Pradesh, south India. Fourteen adult female tick populations were collected from naturally infested dogs at veterinary clinics, residence and stray dog in nine state municipal corporations/municipalities. Information on the history of dog treatment with acaricides was obtained by interviewing dog owners. The larval packet test (LPT) and larval immersion test (LIT) were used on the larvae of ticks to test for resistance to cypermethrin and ivermectin, respectively. Mortality data of each tick population was analyzed by probit analysis. Corresponding to the most susceptible population, thirteen (92.6%) and six (42.9%) tick populations were regarded as resistant to cypermethrin and ivermectin, respectively. The phenotypic level of cypermethrin (resistance ratios at LC50% varied from 1.55 to 13.29) and ivermectin (resistance ratios at LC50% ranged from 1.16 to 4.79) resistance varied distinctly between the populations. Additionally, all the populations were tested using PCR to demonstrate the frequency of the point mutation in sodium channel gene that corresponds with resistance of R. sanguineus s.l. to cypermethrin. A nucleotide substitution (T2134C) on domain III segment VI of the sodium channel gene was noticed in phenotypically resistant tick populations on DNA sequencing. Ivermectin resistance in the brown dog ticks is reported for the first time from India.

Immune-Related Genes of Megalurothrips usitatus (Bagrall) Against Beauveria brongniartii and Akanthomyces attenuatus Identified Using RNA Sequencing

Megalurothrips usitatus (Bagrall) is an important pest of legumes worldwide, causing great economic loss every year. Beauveria brongniartii and Akanthomyces attenuatus have shown considerable pathogenicity against M. usitatus in our previous studies. The medial lethal concentration (LC₅₀) and the sublethal lethal concentration (LC₂₅) of B. brongniartii isolate SB010 against M. usitatus were 8.38 × 10⁵ and 1.73 × 10⁵ conidia mL⁻¹, respectively, whereas those of A. attenuatus isolate SCAUDCL-53 against M. usitatus were 4.37 × 10⁵ and 2.97 × 10⁴ conidia mL⁻¹, respectively. This study reports the transcriptome-based explanation of the stress responses of M. usitatus following the application of B. brongniartii and A. attenuatus. The analysis of the transcriptomic data revealed the expression of 254, 207, 195, and 234 immunity-related unigenes by M. usitatus in response to B. brongniartii LC₅₀ (SB1), B. brongniartii LC₂₅ (SB2), A. attenuatus LC₅₀ (V1), and A. attenuatus LC₂₅ (V2), respectively. The biological function and metabolic pathway analyses showed that these unigenes were mainly related to pattern recognition receptors, information transduction factors, and reaction factors, such as scavenger receptor, cytochrome b5, cuticle protein, lysozyme, and serine protease.

Laboratory Selection, Cross-Resistance, Risk Assessment to Lambda-Cyhalothrin Resistance, and Monitoring of Insecticide Resistance for Plant Bug Lygus pratensis (Hemiptera: Miridae) in Farming-Pastoral Ecotones of Northern China

The plant bug Lygus pratensis Linnaeus (Hemiptera: Miridae) is an important insect pest of alfalfa in grassland farming in northern China. A field population of L. pratensis was selected in the laboratory for 14 consecutive generations with lambda-cyhalothrin to generate 42.555-fold resistance. Selection also induced low cross-resistance to imidacloprid and beta-cypermethrin, and medium cross-resistance to deltamethrin. Realized heritability (h2) of lambda-cyhalothrin resistance was 0.339. Susceptible baselines of L. pratensis were established for five insecticides using the glass-vial method, the values of which were 6.849, 3.423, 8.778, 3.559, and 117.553 ng/cm2 for phoxim, methomyl, imidacloprid, lambda-cyhalothrin, and avermectin, respectively, along with the calculated LC99 diagnostic doses. This resistance risk assessment study suggests that a high risk of lambda-cyhalothrin resistance exists in the field. In addition, a 5-year field investigation of resistance monitoring of L. pratensis was conducted in seven alfalfa regions in farming-pastoral ecotones in northern China. The resistance levels of most populations were very low for phoxim, methomyl, and avermectin, with an upward trend for lambda-cyhalothrin resistance in the DK (Dengkou County), TKT (Tuoketuo County), XL (Xilinhot), and LX (Linxi County) populations during 2015-2019, and medium resistance level to imidacloprid in the TKT population in five years we sampled. The study provided information on chemical control, lambda-cyhalothrin resistance development, baseline susceptibility, and the status of resistance to five commonly-used insecticides against L. pratensis. These results could be used to optimize pyrethroid insecticide use as part of a pest integrated resistance management strategy against this key insect pest of alfalfa.

Comparative proteomics reveals mechanisms that underlie insecticide resistance in Culex pipiens pallens Coquillett

Mosquito control based on chemical insecticides is considered as an important element of the current global strategies for the control of mosquito-borne diseases. Unfortunately, the development of insecticide resistance of important vector mosquito species jeopardizes the effectiveness of insecticide-based mosquito control. In contrast to target site resistance, other mechanisms are far from being fully understood. Global protein profiles among cypermethrin-resistant, propoxur-resistant, dimethyl-dichloro-vinyl-phosphate-resistant and susceptible strain of Culex pipiens pallens were obtained and proteomic differences were evaluated by using isobaric tags for relative and absolute quantification labeling coupled with liquid chromatography/tandem mass spectrometric analysis. A susceptible strain of Culex pipiens pallens showed elevated resistance levels after 25 generations of insecticide selection, through iTRAQ data analysis detected 2,502 proteins, of which 1,513 were differentially expressed in insecticide-selected strains compared to the susceptible strain. Finally, midgut differential protein expression profiles were analyzed, and 62 proteins were selected for verification of differential expression using iTRAQ and parallel reaction monitoring strategy, respectively. iTRAQ profiles of adaptation selection to three insecticide strains combined with midgut profiles revealed that multiple insecticide resistance mechanisms operate simultaneously in resistant insects of Culex pipiens pallens. Significant molecular resources were developed for Culex pipiens pallens, potential candidates were involved in metabolic resistance and reducing penetration or sequestering insecticide. Future research that is targeted towards RNA interference of the identified metabolic targets, such as cuticular proteins, cytochrome P450s, glutathione S-transferases and ribosomal proteins proteins and biological pathways (drug metabolism—cytochrome P450, metabolism of xenobiotics by cytochrome P450, oxidative phosphorylation, ribosome) could lay the foundation for a better understanding of the genetic basis of insecticide resistance in Culex pipiens pallens.

Global protein profiles were compared among a susceptible strain of Cx. pipiens pallens and strains that were cypermethrin-resistant, propoxur-resistant, and dimethyl-dichloro-vinyl-phosphate-resistant after 25 generations of selection by distinct chemical insecticide families, multiple mechanisms were found to operate simultaneously in resistant mosquitoes of Cx. pipiens pallens, including mechanisms to lower penetration of or sequester the insecticide or to increase biodegradation of the insecticide via subtle alterations in either the cuticular protein levels or the activities of detoxification enzymes (P450s and glutathione S-transferases).

Characterization of permethrin resistance in a Musca domestica strain: resistance development, cross-resistance potential and realized heritability

BACKGROUND

Permethrin (a pyrethroid) has been recommended for the management of a number of insect pests, including Musca domestica. Recently, resistance to permethrin has been reported from the Punjab province, Pakistan, which necessitates the need to manage the resistance problem. For this reason, a field strain of M. domestica was selected with permethrin for ten consecutive generations (G1-G10) to determine the risk assessment, the rapidity with which permethrin resistance can develop, and the presence or absence of the cross-resistance phenomenon.

RESULTS

The results revealed that permethrin resistance increased very quickly in response to selection experiments. The resistance ratio (RR) value increased by 16.04- to 605.92-fold and the LD₅₀ value increased by 100.22 to 3787.08 ng fly^-1 after selection experiments. Moreover, the increase in LD₅₀ value remained stable when the selected strain (Perm-SEL) was cultured in the absence of permethrin for the next five generations (G11-G15). The results revealed the possibility of minor cross-resistance in the Perm-SEL strain with β-cyfluthrin and deltamethrin, and lack of cross-resistance with imidacloprid and spinosad. The realized heritability (h² ) value for the Perm-SEL strain was 0.23. Assuming a mean slope value of 1.27 and an h² value of 0.23, then ten to four generations would be estimated for a 10-fold increase in the LD₅₀ values at 50-90% selection intensity, respectively.

CONCLUSION

The results of present study revealed high risk for permethrin resistance development under continuous selection pressure. Permethrin resistance developed very quickly under laboratory conditions, which might reflect the likelihood of resistance evolution in field conditions if permethrin selection pressure holds for a long time. © 2019 Society of Chemical Industry.

Bacteria-mediated modification of insecticide toxicity in the yellow fever mosquito, Aedes aegypti

The incidence of mosquito-borne disease poses a significant threat to human and animal health throughout the world, with effective chemical control interventions limited by widespread insecticide resistance. Recent evidence suggests that gut bacteria of mosquitoes, known to be essential in nutritional homeostasis and pathogen defense, may also play a significant role in facilitating insecticide resistance. This study investigated the extent to which bacteria contribute to the general esterase and cytochrome P450 monooxygenase (P450)-mediated detoxification of the insecticides propoxur and naled, as well as the insecticidal activity of these chemistries to the yellow fever mosquito, Aedes aegypti. Experiments conducted using insecticide synergists that reduce general esterase and P450 activity demonstrate a role for both groups of enzymes in the metabolic detoxification of propoxur and naled. Furthermore, reduction of bacteria in mosquito larvae using broad-spectrum antibiotics was found to decrease the metabolic detoxification of propoxur and naled, suggesting that the bacteria themselves may be contributing to the in vivo metabolic detoxification of these insecticides. This was supported by in vitro assays using culturable gut bacteria isolated from mosquito larvae which demonstrated that the bacteria were capable of reducing insecticide toxicity. More work is needed, however, to fully elucidate the contribution of bacteria in Ae. aegypti larvae to the metabolic detoxification of insecticides.

CYP-mediated resistance and cross-resistance to pyrethroids and organophosphates in Aedes aegypti in the presence and absence of kdr

Aedes aegypti thrives in urban environments and transmits several debilitating human viral diseases. Thus, our ability to control this mosquito species in endemic areas is of utmost importance. The use of insecticides, mostly pyrethroids and organophosphates (OPs), has long been the primary means of controlling A. aegypti, but widespread insecticide resistance has emerged. The two main mechanisms of pyrethroid resistance in A. aegypti are CYP-mediated detoxification and mutations in the target site, voltage-sensitive sodium channel (Vssc), referred to as knockdown resistance (kdr). Knowledge about the contributions and interactions of these mechanisms to resistance is important for the understanding of the molecular and evolutionary basis of insecticide resistance, and to determine the effectiveness of insecticides. In this study, we address two aims: 1) determine the patterns of CYP-mediated cross-resistance to pyrethroid and OP insecticides, both in the presence and absence of kdr (S989P + V1016G), and 2) determine whether the interaction between the two mechanisms yields a greater than, less than, or additive effect on resistance. We tested seven pyrethroids and four OPs against three congenic strains of A. aegypti: ROCK (susceptible), CYP:ROCK (CR) (resistant due to CYP-mediated detoxification without kdr), and CYP + KDR:ROCK (CKR) (resistant due to both CYPs and kdr), and compared these to the congenic KDR:ROCK strain that was previously reported. We found that resistance ratios (RRs) were variable between pyrethroids and strains, ranging from 6.2- to 42-fold for CR, and 70- to 261-fold for CKR. In general, we found that CYP-mediated resistance alone contributes less to resistance than kdr. The effect of the combined mechanisms on resistance was significantly greater than additive for all pyrethroids except (1R)-trans-fenfluthrin. CYP-mediated pyrethroid resistance conferred cross-resistance to both methyl paraoxon and fenitrothion, and negative cross-resistance to methyl parathion and naled. Based on our results, we recommend that etofenprox and cyfluthrin be avoided for A. aegypti control in areas where these two resistance mechanisms are prevalent.

Effect of Synergists on Deltamethrin Resistance in the Common Bed Bug (Hemiptera: Cimicidae)

The common bed bug, Cimex lectularius L. (Hemiptera: Cimicidae), is an obligate hematophagous insect that has resurged worldwide since the early 2000s. Bed bug control is largely based on the widespread, intensive application of pyrethroid-based insecticide formulations, resulting in the emergence of insecticide-resistant bed bug populations. Insecticide resistance is frequently linked to metabolic detoxification enzymes such as cytochrome monooxygenase (P450s), esterases, glutathione S-tranferase, and carboxylesterase. Therefore, one way to overcome insecticide resistance could be the formulation of insecticides with synergists that counteract metabolic resistance. To test this hypothesis, we evaluated the impact of four synergists-piperonyl butoxide (PBO), diethyl maleate (DEM), S,S,S-tributyl phosphorotrithioate (DEF), and triphenyl phosphate (TPP)-on deltamethrin efficacy in two pyrethroid-resistant bed bug strains. A statistically significant difference in synergism ratios (SR) of a highly resistant field-derived strain (Jersey City, resistance ratio [RR] = 20,000) was noted when any of the four synergists (PBO SR = 20.5; DEM SR = 11.7; DEF SR = 102.3; and TPP SR = 9.7) were used with deltamethrin. In a less deltamethrin-resistant strain, Cincinnati (RR = 3,333), pretreatment with PBO and DEM significantly synergized deltamethrin (PBO SR = 158.8; DEM = 58.8), whereas application of DEF and TPP had no synergistic effect. The synergism data collected strongly suggest that detoxification enzymes play a significant role in the metabolic mechanisms that mediate deltamethrin resistance in bed bugs. The development and use of safe metabolic synergists that suppress detoxification enzymes offers an interesting avenue for the management of insecticide-resistant field populations.

CYP-mediated permethrin resistance in Aedes aegypti and evidence for trans-regulation

Aedes aegypti poses a serious risk to human health due to its wide global distribution, high vector competence for several arboviruses, frequent human biting, and ability to thrive in urban environments. Pyrethroid insecticides remain the primary means of controlling adult A. aegypti populations during disease outbreaks. As a result of decades of use, pyrethroid resistance is a global problem. Cytochrome P450 monooxygenase (CYP)-mediated detoxification is one of the primary mechanisms of pyrethroid resistance. However, the specific CYP(s) responsible for resistance have not been unequivocally determined. We introgressed the resistance alleles from the resistant A. aegypti strain, Singapore (SP), into the genetic background of the susceptible ROCK strain. The resulting strain (CKR) was congenic to ROCK. Our primary goal was to determine which CYPs in SP are linked to resistance. To do this, we first determined which CYPs overexpressed in SP are also overexpressed in CKR, with the assumption that only the CYPs linked to resistance will be overexpressed in CKR relative to ROCK. Next, we determined whether any of the overexpressed CYPs were genetically linked to resistance (cis-regulated) or not (trans-regulated). We found that CYP6BB2, CYP6Z8, CYP9M5 and CYP9M6 were overexpressed in SP as well as in CKR. Based on the genomic sequences and polymorphisms of five single copy CYPs (CYP4C50, 6BB2, 6F2, 6F3 and 6Z8) in each strain, none of these genes were linked to resistance, except for CYP6BB2, which was partially linked to the resistance locus. Hence, overexpression of these four CYPs is due to a trans-regulatory factor(s). Knowledge on the specific CYPs and their regulators involved in resistance is critical for resistance management strategies because it aids in the development of new control chemicals, provides information on potential environmental modulators of resistance, and allows for the detection of resistance markers before resistance becomes fixed in the population.

Using genomic data to study insecticide resistance in the house fly, Musca domestica

The house fly, Musca domestica, is a major pest at livestock facilities throughout the world. Insecticides have been the most common control strategy for flies, but many populations have evolved resistance. The speed by which we are able to identify the mutations responsible for resistance has been a major challenge for the development of high throughput resistance monitoring assays as new insecticides are introduced for control. This is particularly true for mutations that cause trans regulation of a gene, which then results in resistance. In this paper we take advantage of the conserved homology of dipteran chromosomes to assign 3069 genes to chromosomes. Of these, 234 were of toxicological interest (CYPs, esterases/hydrolases, glutathione S-transferases (GSTs) and target sites). The chromosomal location of genes known from linkage analysis studies matched the location predicted by homology mapping in ten out of ten cases, indicating a high reliability of our approach. The CYPs, esterases/hydrolases and GSTs were not randomly distributed throughout the genome. They clustered on chromosomes, but the pattern was different between the CYPs, esterases/hydrolases and GSTs. Examples are provided for how the availability of the house fly genome, combined with an ability to assign genes to chromosomes, will help to accelerate research in house flies.

Characterization of a Sodium Channel Mutation in Permethrin-Resistant Rhipicephalus sanguineus (Acari: Ixodidae)

The brown dog tick, Rhipicephalus sanguineus (Latrielle) sensu lato, is an important ectoparasite of dogs and occasionally humans, capable of transmitting several pathogens, such as Rickettsia and Ehrlichia, which are of veterinary and medical importance. The brown dog tick is distributed worldwide and has an affinity for human habitations in much of its range. In some populations, lack of integrated pest management plans and overuse of pyrethroid pesticides and other sodium channel inhibitors has resulted in high levels of resistance to permethrin. Recently, a highly conserved region of the R. sanguineus sodium channel was sequenced, indicating that a single nucleotide polymorphism of thymine to cytosine on domain III segment VI of the sodium channel could confer resistance. A molecular assay targeting a point mutation in the sodium channel was developed and optimized to separate ticks expressing permethrin resistance from those from a susceptible colony. Thereafter, multiple field-collected phenotypically permethrin-resistant populations were evaluated using this molecular assay to determine genotype. As confirmed by DNA sequencing, a point mutation was present at a high rate in phenotypically resistant tick populations that was not present in the susceptible strain. These data suggest an additional permethrin resistance mechanism to metabolic resistance, which has been reported for this tick species, and confirm its association with phenotypic resistance. The results of this study further emphasize the need to preserve acaricide chemistry through rotation of active ingredients used to control ectoparasites.

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.

Evolution of resistance to pyrethroid insecticides in Musca domestica

Houseflies, Musca domestica L., are a significant pest because of the numerous diseases they transmit. Control of housefly populations, particularly at animal production facilities, is frequently done using pyrethroid insecticides which kill insects by prolonging the open time of the voltage-sensitive sodium channel (VSSC). Houseflies have evolved resistance to pyrethroids owing to mutations in Vssc and by cytochrome-P450-mediated detoxification. Three Vssc mutations are known: kdr (L1014F), kdr-his (L1014H) and super-kdr (M918T + L1014F). Generally, the levels of resistance conferred by these mutations are kdr-his < kdr < super-kdr, but this pattern does not hold for multihalogenated benzyl pyrethroids, for which super-kdr confers less resistance than kdr. P450-mediated resistance can result from overexpression of CYP6D1 or another P450 (unidentified) whose overexpression is linked to autosomes II or V. The initial use of field-stable pyrethroids resulted in different patterns of evolution across the globe, but with time these mutations have become more widespread in their distribution. What is known about the fitness costs of the resistance alleles in the absence of insecticide is discussed, particularly with respect to the current and future utility of pyrethroid insecticides. © 2016 Society of Chemical Industry.

Pyrethroid resistance in Culex pipiens mosquitoes

Mosquitoes within the Culex pipiens complex are widely distributed and important in the transmission of many human diseases. Insecticides, pyrethroids in particular, remain a mainstay for control of these important vectors. In this paper we review what is known about the levels, mechanisms and fitness costs of pyrethroid resistance in Cx. pipiens. Pyrethroid resistance in Cx. pipiens is a global problem, and resistance ratios of up to 7000-fold have been found in larvae of field collected mosquitoes. However, there is considerable variation between populations, indicating significant geographic heterogeneity of the resistance. The two major mechanisms of resistance to pyrethroids in Culex are mutations in Vssc (target site insensitivity) and overexpression of cytochrome P450(s) (increased detoxification). The most frequently reported Vssc mutation is L1014F (i.e. kdr), which has been found throughout the world. The L1014S mutation has been found in Cx. p. pallens from Japan and China, and in Cx. p. pipiens from China. The L1014C mutation has only been reported for Cx. p. pipens molestus from China and the V1016G mutation has only been reported from Saudi Arabia. Studies on the P450s of Cx. pipiens have identified several that are overexpressed (measured as transcript levels) in pyrethroid resistant strains. CYP9M10 is consistently overexpressed in pyrethroid resistant Cx. pipiens from at least seven countries, suggesting this P450 might be of global importance in resistance. Both CYP9M10-mediated pyrethroid resistance and kdr have fitness costs in the absence of insecticides under certain environmental conditions. Research needs and future directions are discussed.

A de novo transcriptome of European pollen beetle populations and its analysis, with special reference to insecticide action and resistance

The pollen beetle Meligethes aeneus is the most important coleopteran pest in European oilseed rape cultivation, annually infesting millions of hectares and responsible for substantial yield losses if not kept under economic damage thresholds. This species is primarily controlled with insecticides but has recently developed high levels of resistance to the pyrethroid class. The aim of the present study was to provide a transcriptomic resource to investigate mechanisms of resistance. cDNA was sequenced on both Roche (Indianapolis, IN, USA) and Illumina (LGC Genomics, Berlin, Germany) platforms, resulting in a total of ∼53 m reads which assembled into 43 396 expressed sequence tags (ESTs). Manual annotation revealed good coverage of genes encoding insecticide target sites and detoxification enzymes. A total of 77 nonredundant cytochrome P450 genes were identified. Mapping of Illumina RNAseq sequences (from susceptible and pyrethroid-resistant strains) against the reference transcriptome identified a cytochrome P450 (CYP6BQ23) as highly overexpressed in pyrethroid resistance strains. Single-nucleotide polymorphism analysis confirmed the presence of a target-site resistance mutation (L1014F) in the voltage-gated sodium channel of one resistant strain. Our results provide new insights into the important genes associated with pyrethroid resistance in M. aeneus. Furthermore, a comprehensive EST resource is provided for future studies on insecticide modes of action and resistance mechanisms in pollen beetle.

Resistance of the house fly Musca domestica (Diptera: Muscidae) to lambda-cyhalothrin: mode of inheritance, realized heritability, and cross-resistance to other insecticides

Lambda-cyhalothrin, a pyrethroid insecticide, has been used frequently for the control of house flies, Musca domestica L., worldwide including Pakistan. To assess the resistance risk and design a resistance management strategy, a house fly population was exposed to lambda-cyhalothrin in the laboratory to assess inheritance and heritability, and cross-resistance to other insecticides, including different chemical classes. After 11 generations of selection, the population developed 113.57-fold resistance to lambda-cyhalothrin compared to the susceptible population. There was no cross-resistance to bifenthrin and methomyl, but very low cross-resistance to abamectin and indoxacarb in the lambda-cyhalothrin selected population compared to the field population. Synergism bioassay with piperonyl butoxide and S,S,S-tributylphosphorotrithioate indicated that lambda-cyhalothrin resistance was associated with microsomal oxidases and esterases. The LC50 values of F1 (Lambda-SEL ♀ × Susceptible ♂) and F'1 (Lambda-SEL ♂ × Susceptible ♀) populations were not significantly different and dominance (DLC) values were 0.68 and 0.62. The resistance to lambda-cyhalothrin was completely recessive (DML = 0.00) at highest dose and completely dominant at lowest dose (DML = 0.95). The monogenic model of inheritance showed that lambda-cyhalothrin resistance was controlled by multiple factors. The heritability values were 0.20, 0.04, 0.003, 0.07 and 0.08 for lambda-cyhalothrin, bifenthrin, methomyl, indoxacarb and abamectin resistance, respectively. It was concluded that lambda-cyhalothrin resistance in house flies was autosomally inherited, incompletely dominant and controlled by multiple factors. These findings would be helpful to improve the management of house flies.

Mechanisms of pyrethroid resistance in the dengue mosquito vector, Aedes aegypti: target site insensitivity, penetration, and metabolism

Aedes aegypti is the major vector of yellow and dengue fevers. After 10 generations of adult selection, an A. aegypti strain (SP) developed 1650-fold resistance to permethrin, which is one of the most widely used pyrethroid insecticides for mosquito control. SP larvae also developed 8790-fold resistance following selection of the adults. Prior to the selections, the frequencies of V1016G and F1534C mutations in domains II and III, respectively, of voltage-sensitive sodium channel (Vssc, the target site of pyrethroid insecticide) were 0.44 and 0.56, respectively. In contrast, only G1016 alleles were present after two permethrin selections, indicating that G1016 can more contribute to the insensitivity of Vssc than C1534. In vivo metabolism studies showed that the SP strain excreted permethrin metabolites more rapidly than a susceptible SMK strain. Pretreatment with piperonyl butoxide caused strong inhibition of excretion of permethrin metabolites, suggesting that cytochrome P450 monooxygenases (P450s) play an important role in resistance development. In vitro metabolism studies also indicated an association of P450s with resistance. Microarray analysis showed that multiple P450 genes were over expressed during the larval and adult stages in the SP strain. Following quantitative real time PCR, we focused on two P450 isoforms, CYP9M6 and CYP6BB2. Transcription levels of these P450s were well correlated with the rate of permethrin excretion and they were certainly capable of detoxifying permethrin to 4'-HO-permethrin. Over expression of CYP9M6 was partially due to gene amplification. There was no significant difference in the rate of permethrin reduction from cuticle between SP and SMK strains.

Drug-efflux and target-site gene expression patterns in Haemonchus contortus larvae able to survive increasing concentrations of levamisole in vitro

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A Haemonchus contortus isolate subdivided according to level of resistance to levamisole in vitro.

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Increased P-glycoprotein gene expression in larvae showing low level resistance.

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Decreased expression of nAChR subunit and receptor assembly genes in larvae showing higher levels of resistance.

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Results suggest drug efflux mediated low level resistance, with target site changes conferring higher level resistance.

While there is some evidence that changes in nicotinic acetylcholine receptor (nAChR) subunits confer resistance to levamisole in gastrointestinal helminth parasites, the exact nature of the resistance mechanism(s) is unclear. We utilised the presence of a resistant fraction within the Wallangra 2003 isolate of Haemonchus contortus larvae in order to subdivide the population into three subpopulations of larvae able to survive increasing concentrations of the drug. We then measured gene expression levels in the subpopulations and the larval population as a whole, focusing on genes encoding the subunit components of levamisole-sensitive receptors, genes encoding ancillary proteins involved in receptor assembly, and P-glycoprotein (P-gp) genes. The subpopulation surviving the lowest levamisole concentration showed increases of 1.5- to 3-fold in a number of P-gp genes (Hco-pgp-3, -4, -10, and -14) alongside unchanged receptor genes, compared to the whole Wallangra larval population. On the other hand, the subpopulation surviving the intermediate levamisole concentration showed an increase in only a single P-gp (Hco-pgp-14), alongside decreases in some receptor subunit (Hco-unc-63a) and ancillary protein genes (Hco-unc-50, Hco-ric-3.1 and 3.1). The subpopulation surviving the highest levamisole concentration showed further decreases in receptor subunit genes (Hco-unc-63a and Hco-unc-29 paralogs) as well as genes involved in receptor assembly (Hco-unc-74, Hco-unc-50, Hco-ric-3.1 and 3.1), alongside no increased P-gp gene levels. This suggests a biphasic pattern of drug resistance in the larvae of this worm isolate, in which a non-specific P-gp-mediated mechanism confers low levels of resistance, while higher level resistance is due to altered receptor subunit composition as a result of changes in both subunit composition and in the levels of proteins involved in receptor assembly.

Insecticide resistance in house flies from the United States: resistance levels and frequency of pyrethroid resistance alleles

Although insecticide resistance is a widespread problem for most insect pests, frequently the assessment of resistance occurs over a limited geographic range. Herein, we report the first widespread survey of insecticide resistance in the USA ever undertaken for the house fly, Musca domestica, a major pest in animal production facilities. The levels of resistance to six different insecticides were determined (using discriminating concentration bioassays) in 10 collections of house flies from dairies in nine different states. In addition, the frequencies of Vssc and CYP6D1 alleles that confer resistance to pyrethroid insecticides were determined for each fly population. Levels of resistance to the six insecticides varied among states and insecticides. Resistance to permethrin was highest overall and most consistent across the states. Resistance to methomyl was relatively consistent, with 65-91% survival in nine of the ten collections. In contrast, resistance to cyfluthrin and pyrethrins + piperonyl butoxide varied considerably (2.9-76% survival). Resistance to imidacloprid was overall modest and showed no signs of increasing relative to collections made in 2004, despite increasing use of this insecticide. The frequency of Vssc alleles that confer pyrethroid resistance was variable between locations. The highest frequencies of kdr, kdr-his and super-kdr were found in Minnesota, North Carolina and Kansas, respectively. In contrast, the New Mexico population had the highest frequency (0.67) of the susceptible allele. The implications of these results to resistance management and to the understanding of the evolution of insecticide resistance are discussed.

Bed bugs evolved unique adaptive strategy to resist pyrethroid insecticides

Recent advances in genomic and post-genomic technologies have facilitated a genome-wide analysis of the insecticide resistance-associated genes in insects. Through bed bug, Cimex lectularius transcriptome analysis, we identified 14 molecular markers associated with pyrethroid resistance. Our studies revealed that most of the resistance-associated genes functioning in diverse mechanisms are expressed in the epidermal layer of the integument, which could prevent or slow down the toxin from reaching the target sites on nerve cells, where an additional layer of resistance (kdr) is possible. This strategy evolved in bed bugs is based on their unique morphological, physiological and behavioral characteristics and has not been reported in any other insect species. RNA interference-aided knockdown of resistance associated genes showed the relative contribution of each mechanism towards overall resistance development. Understanding the complexity of adaptive strategies employed by bed bugs will help in designing the most effective and sustainable bed bug control methods.

Mechanisms of pyrethroid resistance in Haematobia irritans (Muscidae) from Mato Grosso do Sul state, Brazil

Horn fly resistance to pyrethroid insecticides occurs throughout Brazil, but knowledge about the involved mechanisms is still in an incipient stage. This survey was aimed to identify the mechanisms of horn fly resistance to cypermethrin in Mato Grosso do Sul state, Brazil. Impregnated filter paper bioassays using cypermethrin, synergized or not with piperonyl butoxide (PBO) and triphenyl phosphate (TPP), were conducted from March 2004 to June 2005 in horn fly populations (n = 33) from all over the state. All populations were highly resistant to cypermethrin, with resistance factors (RF) ranging from 89.4 to 1,020.6. Polymerase chain reaction (PCR) assays to detect the knockdown resistance (kdr) mutation also were performed in 16 samples. The kdr mutation was found in 75% of the tested populations, mostly with relatively low frequencies (<20%), and was absent in some highly resistant populations. Addition of TPP did not significantly reduce the LC50 in any population. However, PBO reduced LC50s above 40-fold in all tested populations, resulting in RFs ≤ 10 in most cases. Horn fly resistance to cypermethrin is widespread in the state, being primarily caused by an enhanced activity of P450 mono-oxygenases and secondarily by reduced target site sensitivity.

Assessment of physiological sublethal effects of imidacloprid on the mirid bug Apolygus lucorum (Meyer-Dür)

Apolygus lucorum (Meyer-Dür) (Hemiptera: Miridae) is currently one of major mirid pests in the Yangtze River and the Yellow River regions in China. Imidacloprid (neonicotinoid) is widely used against pierce-sucking pest insects, including against A. lucorum. In addition to its direct lethal effect, multiple negative sublethal effects may also occur in exposed insects. We assessed potential sublethal effects of imidacloprid on some biological characteristics of A. lucorum with the aim of increasing rational use of imidacloprid against that cotton pest. The lethal toxicity of imidacloprid on adults of A. lucorum was determined in laboratory conditions by a topical application bioassay (LD(50) = 6.70 ng a.i. [active ingredient]/A. lucorum adult). We also estimated a sublethal dose, LD(5) (0.38 ng a.i./adult), a low lethal dose, LD(25) (1.96 ng a.i./adult), and moderate lethal dose, LD(40) (3.97 ng a.i./adult). The sublethal dose of imidacloprid (LD(5)) shortened the pre-oviposition period of females but increased the time required for eggs to develop (i.e. longer embryogenesis). The low lethal dose (LD(25)) also reduced the pre-oviposition period. Females exposed to the LD(40) laid eggs that developed faster but overall percentage of eggs hatching was reduced. LD(25) and LD(40) reduced longevity of males but not of females. In addition, the susceptibility to seven insecticides generally used on Chinese crops was not modified in A. lucorum previously exposed to the LD(25) of imidacloprid. Our results demonstrate sublethal effects of low doses of imidacloprid on A. lucorum (notably on pre-oviposition period and egg development) which may have an impact on population dynamics of this pest.

Pyriproxyfen and house flies (Diptera: Muscidae): effects of direct exposure and autodissemination to larval habitats

Pyriproxyfen is an insect growth regulator with juvenile hormone-like activity that has potential uses for dipterans that are difficult to manage with conventional insecticides, such as house flies (Musca domestica L.). The objectives of this study were to determine the efficacy of this insect growth regulator against house flies using variety of delivery systems and target life stages, including an evaluation of the potential for autodissemination by female flies to larval development sites. Adult female house flies exposed to filter paper (3.75% active ingredient) or sugar treated with pyriproxyfen (0.01-0.1%) produced significantly fewer F1 pupae than untreated flies. Adult emergence from pupae was unaffected. In contrast, treatment of larval rearing medium with 0.35 ml/cm2 of a 12 mg pyriproxyfen/liter preparation had no effect on the number of pupae developing from eggs but markedly inhibited adult emergence from those pupae. There was little difference in susceptibility between an insecticide-susceptible and a wild strain of house fly. The LC50 for inhibiting fly emergence of dust formulations in diatomaceous earth incorporating commercial pyriproxyfen products ranged from 8 to 26 mg/liter, with little difference among products. Compared with untreated flies, significantly fewer pupae were produced at concentrations > 0.5% and no adults were produced at concentrations > 0.05% pyriproxyfen. When gravid females were exposed for 1 h to treated fabric (6 mg pyriproxyfen/cm2) and allowed to oviposit in rearing media containing eggs, sufficient pyriproxyfen was autodisseminated to reduce adult emergence from those eggs by > 99%. Intermittent contact with treated fabric over 2 d reduced adult emergence by 63-76%.

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.

Frequencies of pyrethroid resistance-associated mutations of Vssc1 and CYP6D1 in field populations of Musca domestica L. in Turkey

House flies were collected from 16 different provinces in the Aegean and Mediterranean regions of Turkey, and the frequencies of pyrethroid resistance-associated mutations in Vssc1 and CYP6D1 in these field-collected populations were studied. Although there is no organized resistance management program for house fly control in Turkey, it is known that different groups of insecticides, including pyrethroids, are used. The frequencies of both Vssc1 and CYP6D1 alleles were weighted toward the susceptibles, with Vssc1-susceptible alleles having higher frequencies in both regions (0.75 in Aegean and 0.69 in Mediterranean populations) than CYP6D1-susceptible alleles (0.65 in Aegean and 0.56 in Mediterranean populations). The frequencies of kdr-his alleles were higher than the frequencies of kdr alleles in these populations. While the frequencies of kdr-his alleles were close to each other in the Aegean (0.23) and Mediterranean (0.17) populations, the frequencies of kdr alleles remarkably differed in these two regions, with values of 0.02 and 0.14, respectively. In contrast to Europe, Asia, and the U.S.A., no super-kdr allele was detected in the samples from both regions. We identified six and eight different Vssc1+CYP6D1 genotype classes in the Aegean and Mediterranean regions, respectively. The three most common genotype classes in the regions were susceptible Vssc1 with heterozygous CYP6D1v1 (29%), sus/kdr-his1 with heterozygous CYP6D1v1 (23%), and susceptible Vssc1 with CYP6D1 (22%). The total frequencies of these three most common genotype classes (approximately 75%) obtained in our study were very close to the value obtained in Florida in a previous study, which was related by the similarity of temperature patterns between Florida and the corresponding regions of Turkey. This may reflect the lack of overwintering fitness cost associated with resistance alleles in both climates.

Molecular cloning of Bombyx mori cytochrome P450 gene and its involvement in fluoride resistance

To investigate the effects of fluorosis on development and gene expression profiles of silkworm, highly resistant silkworm strain 441, and highly susceptible silkworm strain 440 were treated with 200ppm fluoride (designated as 440F and 441F) and water (designated as 440DZ and 441DZ). Fluorotic silkworm showed body color and behavior changes. Statistical analysis indicated that growth index of 440F was lower than 440DZ, 441DZ, and 441F. The mortality of 440F was higher than others. Fluorescent differential display enabled us to obtain a differentially expressed cDNA. Bioinformatics analyses indicated that it belonged to cytochrome P450 family, denoted Bmcyp306a1, which contained seven exons and six introns. Phylogenetic tree showed BmCYP306A1 had high homology with Manduca sexta' P450 protein. Expression analysis indicated that Bmcyp306a1 was exclusively expressed in 441DZ and 441F and was down-regulated under fluoride treatment. The tissue-specific expression indicated Bmcyp306a1 had high-expression level in midgut and ovary in 441F. The data revealed that there was obvious dose-effect and times relationship with the pathological changes and gene expression. Expression profiles of Bmcyp306a1 suggested that P450 gene was crucial to physiological modification and might be involved in fluoride resistance.

Efficacy of various pyrethroid structures against a highly metabolically resistant isogenic strain of Helicoverpa armigera (Lepidoptera: Noctuidae) from China

BACKGROUND

Resistance to pyrethroids and other types of insecticides in Helicoverpa armigera (Hübner) has been documented in many countries. The isolation of specific resistance mechanisms in isogenic strains is an optimal approach to investigate cross-resistance pattern, and to validate resistance breaking pyrethroids. In this study an isogenic metabolic resistance CMR strain was successfully isolated from a field pyrethroid-resistant population of H. armigera. With this strain, cross-resistance among 19 pyrethroid insecticides with varying chemical structures was analysed.

RESULTS

Resistance to pyrethroids in the CMR strain was likely to be due to enhanced oxidative metabolism. The most significant cross-resistance in the CMR strain was between pyrethroids such as fenvalerate, tau-fluvalinate and flumethrin characterised by having both phenoxybenzyl and aromatic acid moieties. Substitution of the phenoxybenzyl group with a polyfluorobenzyl group, as in tefluthrin, benfluthrin and transfluthrin, overcame most of this resistance.

CONCLUSION

The findings in this study support the assertion that it is possible to find pyrethroids that are active against resistant populations. Such pyrethroids could be considered as possible partners or resistance breaking pyrethroids in a pyrethroid resistance management programme for H. armigera in China and in other Asian countries where the oxidative metabolism resistance is a dominant mechanism.

Dynamics of insecticide resistance alleles in house fly populations from New York and Florida

The frequency of insecticide-resistance alleles for two genes (Vssc1 and CYP6D1) was studied in field collected populations of house flies from two different climates. While the frequency of these resistance alleles in flies at dairies from four states has recently been reported, there is no information on the relative change of these allele frequencies over time. House flies were collected during the 2003-2004 season from New York and Florida before the first application of permethrin, during the middle of the field season, after the final application, and again the following spring (following months without permethrin use). Bioassay results indicated that homozygous susceptible and extremely resistant flies were rare, while moderately and highly resistant individuals were relatively common at all times in both states. The frequency of resistance alleles at the New York dairy rose during the season and declined over the winter, suggesting an overwintering fitness cost associated with these alleles. The super-kdr allele was detected for the first time in North America at the end of 2003. In Florida the frequency of the resistance alleles did not increase during the spray season or decrease during the winter, suggesting there is substantial immigration of susceptible alleles to the Florida dairy and no overwintering fitness cost associated with resistance alleles in this climate. Resistance to permethrin correlated well with the frequency of the Vssc1 and CYP6D1 resistance alleles in flies from New York, but not as well in the population from Florida. This suggests there may be a new resistance mechanism or allele evolving in Florida.

Role of the transcriptional repressor mdGfi-1 in CYP6D1v1-mediated insecticide resistance in the house fly, Musca domestica

Gfi-1 is a C(2)H(2)-type zinc finger protein that is a transcriptional repressor in vertebrates and has been implicated in control of CYP6D1 expression in house flies (Musca domestica). A 15 bp insert, which disrupts a putative mdGfi-1 binding site in the CYP6D1v1 promoter has been implicated as a cause of increased expression of CYP6D1, and thus insecticide resistance. Using electrophoretic mobility shift assays we demonstrate that the CYP6D1 promoter from susceptible strains binds mdGfi-1. The 15 bp insert that interrupts the mdGfi-1-binding site in insecticide-resistant strains reduces the amount of mdGfi-1 binding by 9- to 20-fold, consistent with the role of mdGfi-1 in resistance. Partial sequences of mdGfi-1 (spanning the first intron) from individual houseflies from 11 different strains revealed the presence of 23 alleles. There was no consistent difference in the mdGfi-1 alleles between susceptible and CYP6D1-mediated insecticide-resistant strains, indicating that mdGfi-1 alleles were not likely involved in resistance. Polymorphisms were used to map mdGfi-1 to autosome 1. Quantitative real time PCR (qRT-PCR) revealed Gfi-1 expression was higher in the thorax compared to the head and abdomen, and varied between life stages and between strains. However, similar levels of mdGfi-1 were detected in susceptible and resistant adults suggesting that altered levels of mdGfi-1 were not likely a cause of insecticide resistance. The significance of these results to understanding insecticide resistance is discussed.

Frequencies of the pyrethroid resistance alleles of Vssc1 and CYP6D1 in house flies from the eastern United States

House flies were collected from four dairies in Maine, New York, North Carolina, and Florida, where high levels of resistance to permethrin have been documented. Regions of two genes, CYP6D1 and Vssc1, having alleles that confer resistance to permethrin (and other pyrethroids) were analysed from individuals at each collection site. The combinations of resistance alleles for Vssc1 and CYP6D1 were highly variable between each state. The resistance allele CYP6D1v1 was found at a high frequency (0.63-0.91) at all sites. Individuals homozygous susceptible for CYP6D1 were very rare and detected only at the dairy in Maine. In addition to the typical Vssc1 mutation responsible for resistance, kdr (L1014F), we also identified individuals with a L1014H mutation. Although house flies homozygous for the L1014H mutation had a lower level of resistance to permethrin, compared to L1014F, the H1014 resistance allele was frequently detected. No individuals with the super-kdr allele (M918T + L1014F) were detected from the field collections. The intron 3 bp downstream of the kdr mutation was found to be extremely variable, providing an opportunity to reconstruct a phylogeny of Vssc1 alleles. Based on this analysis it appears the kdr-his mutation had multiple evolutionary origins, but that the kdr mutation may have had a single origin. The impacts of these findings on resistance management are discussed.

Determination of oxidative metabolism in Collembolan Proisotoma minuta (Tullberg)

An oxidative metabolism of Collembolan Proisotoma minuta was determined with a model compound of aldrin and dieldrin in this paper. The seven-day LD(50) values for aldrin, dieldrin, and piperonyl butoxide in salt solution were 0.496, 0.367, and 8.346 mg L(-1), respectively. When P. minuta were exposed to aldrin, dieldrin was the sole metabolite. The conversion of aldrin to dieldrin was known to be catalyzed by P450 monooxygenases system. It has been shown that the synergist piperonyl butoxide inhibited the metabolism of aldrin in P. minuta.

Selection for imidacloprid resistance in Nilaparvata lugens: cross-resistance patterns and possible mechanisms

A field population of brown planthoppers (Nilaparvata lugens Stål) was collected and selected for imidacloprid resistance in the laboratory. The resistance increased by 11.35 times in 25 generations and the resistance ratio reached 72.83 compared with a laboratory susceptible strain. The selected resistant strain showed obvious cross-resistance to all the acetylcholine receptor targeting insecticides tested (monosultap 1.44-fold, acetamiprid 1.61-fold, imidacloprid homologues JS599 2.46-fold and JS598 3.17-fold), but not to others. Further study demonstrated that TPP and DEM had no synergism on imidacloprid. However, PBO displayed significant synergism in some different strains, and the synergism increased with resistance (S strain 1.20, field population 1.43 and R strain 2.93). PBO synergism to cross-resistant insecticides was also found in the resistant strain (monosultap 1.25, acetamiprid 1.39, JS598 1.94 and JS599 2.02). We concluded that esterase and glutathione S-transferase play little role in imidacloprid detoxification. The increase of the P450-monooxygenases detoxification is an important mechanism for imidacloprid resistance and target resistance may also exist in this species.

An adult-specific CYP6 P450 gene is overexpressed in a pyrethroid-resistant strain of the malaria vector, Anopheles gambiae

Many malaria control programmes are based on insecticide application as adulticides, often in the form of pyrethroid-impregnated bed nets. However, the efficacy of this control measure can be reduced by genetic changes in vector insecticide susceptibility. Pyrethroid resistance has been detected in the major African malaria vector, Anopheles gambiae, and has been attributed to a combination of target site insensitivity and increased oxidative metabolism of the insecticide, catalysed by cytochrome P450s. An adult-specific cytochrome P450 monooxygenase 6 (CYP6) P450 gene, CYP6Z1, located within a large cluster of cytochrome P450 genes in chromosome arm 3R of An. gambiae, is expressed approximately 11-fold higher in males and 4.5-fold in females from a pyrethroid-resistant strain than in a susceptible strain from the same geographical area. In both strains, CYP6Z1 expression is higher in males than females. Southern blot analysis discounted gene amplification as a cause of this overexpression. The isolation of An. gambiae cDNAs encoding cytochrome b(5) and nicotinamide adenine dinucleotide phosphate (reduced form) (NADPH)-cytochrome P450 reductase cDNAs is also reported.

The house fly aliesterase gene (MdalphaE7) is not associated with insecticide resistance or P450 expression in three strains of house fly

It was recently proposed that a mutation (G137D) in the MdalphaE7 gene was responsible for increasing transcription of a P450 (CYP6A1) resulting in resistance to diazinon. To examine if MdalphaE7 had a role in resistance in other strains we sequenced a fragment (approximately 700 bp) of the MdalphaE7 gene from individual flies of two insecticide susceptible and three insecticide resistant (due to increased monooxygenase-mediated detoxification) strains. Five unique alleles were discovered. While all of the susceptible strains had Gly137, so did the resistant LPR and NG98 strains. Of the two alleles in the YPER strain one had the G137D substitution and the other did not. Based on the lack of correlation between the presence of the 'mutant' MdalphaE7 and resistance (or P450 levels), we conclude that the G137D mutation in MdalphaE7 is not involved in transcriptional control of the P450s involved in resistance in the LPR, NG98 or YPER strains. The relationship between MdalphaE7 alleles and insecticide resistance is discussed in light of these findings.

Cytochromes P450 of insects: the tip of the iceberg

The cytochrome P450-dependent monooxygenases are an extremely important metabolic system involved in the metabolism of endogenous compounds and xenobiotics. Collectively, P450 monooxygenases can metabolize numerous substrates and carry out multiple oxidative reactions. The large number of substrates metabolized is due to the plethora of P450 isoforms and to the broad substrate specificity of some isoforms. Monooxygenases of insects have several functional roles, including growth, development, feeding and protection against xenobiotics, including resistance to pesticides and tolerance to plant toxins. This review begins with background information about P450s and their evolution, followed by a discussion of the extraordinary diversity of insect P450s. Given the enormous interest in studying individual P450s, we then provide a synopsis of the different methods that have been used in their isolation and the substrates that are known to be metabolized. We conclude by summarizing the lessons we have learned from the study of individual insect P450s, including their roles in insecticide resistance, plant-insect interactions and insect physiology. However, these studies are just the 'tip of the iceberg'. Our knowledge continues to expand at a rapid pace, suggesting that the next decade will outpace the last in terms of improving our understanding of the cytochromes P450 of insects.

Effects of the synergists piperonyl butoxide and S,S,S-tributyl phosphorotrithioate on propoxur pharmacokinetics in Blattella germanica (Blattodea: Blattellidae)

Effects of the synergists piperonyl butoxide (PBO) and S,S,S-tributyl phosphorotrithioate (DEF) on propoxur pharmacokinetics were examined in the German cockroach, Blattella germanica (L.). Treatment of adult male German cockroaches with the cytochrome P450 monooxygenase inhibitor, PBO, or the esterase inhibitor, DEF, increased propoxur toxicity by 2- and 6.8-fold, respectively, implicating hydrolysis as a major detoxification route of propoxur in the German cockroach. However, significant hydrolytic metabolism could not be demonstrated conclusively in vitro resulting in a conflict between in situ bioassay data and in vitro metabolic studies. In vitro propoxur metabolism with NADPH-fortified microsomes produced at least nine metabolites. Formation of metabolites was NADPH-dependent; no quantifiable metabolism was detected with cytosolic fractions. However, microsomal fractions lacking an NADPH source did produce a low, but detectable, quantity of metabolites (1.6 pmol). PBO inhibited NADPH-dependent propoxur metabolism in a dose-dependent fashion, implicating cytochrome P450 monooxygenases as the enzyme system responsible for the metabolism. Interestingly, DEF also inhibited the NADPH-dependent metabolism of propoxur, albeit to a lower extent. Treatment with PBO or DEF also caused a significant reduction in the cuticular penetration rate of propoxur. The data demonstrate that unanticipated effects are possible with synergists and that caution must be exercised when interpreting synergist results.

Expression and activity of a house-fly cytochrome P450, CYP6D1, in Drosophila melanogaster

The cytochrome P450 system of animals comprises many individual cytochromes P450 in addition to a single cytochrome P450 reductase and cytochrome b5. Although individual genes of the cytochrome P450 superfamily are highly diverged, the P450 reductase and cytochrome b(5) remain more conserved across taxa. Here, we describe the transformation of Drosophila melanogaster with a house-fly-specific cytochrome P450, CYP6D1. Functional activity of ectopically expressed cytochromes P450 requires successful interaction between the transgenic P450 and the requisite coenzymes of the host organism. Transformed Drosophila, but not controls, contained CYP6D1 protein as identified by protein immunoblotting, elevated total P450 and elevated CYP6D1 enzymatic activity. These data demonstrate that house-fly CYP6D1 can interact with low to moderate efficiency with Drosophila P450 reductase and cytochrome b(5).

Cytochromes P450 and insecticide resistance

The cytochrome P450-dependent monooxygenases (monooxygenases) are an extremely important metabolic system involved in the catabolism and anabolism of xenobiotics and endogenous compounds. Monooxygenase-mediated metabolism is a common mechanism by which insects become resistant to insecticides as evidenced by the numerous insect species and insecticides affected. This review begins by presenting background information about P450s, the role of monooxygenases in insects, and the different techniques that have been used to isolate individual insect P450s. Next, insecticide resistance is briefly described, and then historical information about monooxygenase-mediated insecticide resistance is reviewed. For any case of monooxygenase-mediated resistance, identification of the P450(s) involved, out of the dozens that are present in an insect, has proven very challenging. Therefore, the next section of the review focuses on the minimal criteria for establishing that a P450 is involved in resistance. This is followed by a comprehensive examination of the literature concerning the individual P450s that have been isolated from insecticide resistant strains. In each case, the history of the strain and the evidence for monooxygenase-mediated resistance are reviewed. The isolation and characterization of the P450(s) from the strain are then described, and the evidence of whether or not the isolated P450(s) is involved in resistance is summarized. The remainder of the review summarizes our current knowledge of the molecular basis of monooxygenase-mediated resistance and the implications for the future. The importance of these studies for development of effective insecticide resistance management strategies is discussed.

A new family of cytochrome P450 genes (CYP41) from the cattle tick, Boophilus microplus

We isolated and sequenced a cytochrome P450 (CYP) gene that is sufficiently different from other CYP genes that a new CYP family, CYP41 was created. CYP41 encodes a protein of 518 residues and is most similar to genes from the family CYP3; it is 36% identical to CYP3A2 and 34% identical to CYP3A28. We hypothesise that CYP41 encodes an enzyme that metabolizes xenobiotic compounds i.e. compounds that are foreign to the cattle tick. The phylogenetic position of CYP41 could not be resolved because of the high level of sequence divergence at both the nucleotide and amino acid levels.