Levels Of Permethrin Resistance Research Articles

Fitness costs of individual and combined pyrethroid resistance mechanisms, kdr and CYP-mediated detoxification, in Aedes aegypti.

BackgroundAedes aegypti is an important vector of many human diseases and a serious threat to human health due to its wide geographic distribution and preference for human hosts. A. aegypti also has evolved widespread resistance to pyrethroids due to the extensive use of this insecticide class over the past decades. Mutations that cause insecticide resistance result in fitness costs in the absence of insecticides. The fitness costs of pyrethroid resistance mutations in A. aegypti are still poorly understood despite their implications for arbovirus transmission.Methodology/Principle findingsWe evaluated fitness based both on allele-competition and by measuring specific fitness components (i.e. life table and mating competition) to determine the costs of the different resistance mechanisms individually and in combination. We used four congenic A. aegypti strains: Rockefeller (ROCK) is susceptible to insecticides; KDR:ROCK (KR) contains only voltage-sensitive sodium channel (Vssc) mutations S989P+V1016G (kdr); CYP:ROCK (CR) contains only CYP-mediated resistance; and CYP+KDR:ROCK (CKR) contains both CYP-mediated resistance and kdr. The kdr allele frequency decreased over nine generations in the allele-competition study regardless of the presence of CYP-mediated resistance. Specific fitness costs were variable by strain and component measured. CR and CKR had a lower net reproductive rate (R0) than ROCK or KR, and KR was not different than ROCK. There was no correlation between the level of permethrin resistance conferred by the different mechanisms and their fitness cost ratio. We also found that CKR males had a reduced mating success relative to ROCK males when attempting to mate with ROCK females.Conclusions/SignificanceBoth kdr and CYP-mediated resistance have a fitness cost affecting different physiological aspects of the mosquito. CYP-mediated resistance negatively affected adult longevity and mating competition, whereas the specific fitness costs of kdr remains elusive. Understanding fitness costs helps us determine whether and how quickly resistance will be lost after pesticide application has ceased.

Permethrin Resistance Status and Associated Mechanisms in Aedes albopictus (Diptera: Culicidae) From Chiapas, Mexico.

There are major public health concerns regarding the spread of mosquito-borne diseases such as dengue, Zika, and chikungunya, which are mainly controlled by using insecticides against the vectors, Aedes aegypti (Linnaeus) and Aedes albopictus (Skuse). Pyrethroids are the primary class of insecticides used for vector control, due to their rapid knockdown effect and low toxicity to vertebrates. Unfortunately, continued use of pyrethroids has led to widespread insecticide resistance in Ae. aegypti; however, we lack information for Ae. albopictus-a sympatric species in Chiapas since 2002. In this study, we evaluated the permethrin resistance status of Ae. albopictus collected from Mexico and Texas. We also selected for permethrin resistance in the laboratory and investigated the potential mechanisms conferring resistance in this species. Knockdown resistance mutations, specifically F1534C, in the voltage-gated sodium channel gene, and increased activity of detoxifying enzymes were evaluated. Low levels of permethrin resistance (<2.4-fold) were observed in our field populations of Ae. albopictus and the F1534C mutation was not detected in any of the sites. Low levels of resistance were also observed in the artificially selected strain. There was significantly higher cytochrome P450 activity in our permethrin-selected and nonselected strains from Mexico compared to the control strain. Our results suggest the Ae. albopictus sampled from 2016 are mostly susceptible to pyrethroids. These results contrast with the high levels of permethrin resistance (>58-fold) found in Ae. aegypti from the same sites in Mexico. This research indicates the importance of continued monitoring of Ae. albopictus populations to prevent resistance from developing in the future.

Evaluation of resistance to permethrin, cypermethrin and deltamethrin in different populations of Musca domestica (L.), collected from the Iranian dairy cattle farms

Abstract The house fly, Musca domestica (Linnaeus) (Diptera: Muscidae), is a major pest for human and livestock health and is also resistant to different insecticides. Herein, six M. domestica populations were collected, five of them from industrial cattle farms and the Koohrang population from a remote area as a susceptible population. The resistance/susceptibility of populations to three pyrethroids was evaluated. High levels of permethrin resistance were observed in all field populations and the resistance ratios (RRs) were estimated to vary from 52- to 129-fold. Resistant populations also exhibited resistance to other pyrethroids (cypermethrin and deltamethrin), with RRs ranging between 45- and 180-fold. According to synergistic (piperonyl butoxide, diethyl maleate and triphenyl phosphate) and enzymatic assays, resistant populations exhibited multiple resistance phenotypes. Cytochrome P450 monooxygenases (P450s), glutathione S-transferases (GSTs), and carboxylesterases (CarEs) were found to be involved in pyrethroid resistance in Isfahan population, P450s and GSTs in Mobarake population and CarEs detoxified pyrethroids in Natanz and Alavijeh populations. As substitution of Leucine (CTT) with Phenylalanine (TTT) at position 1014 of the voltage sensitive sodium channel (VSSC) gene is the most common mutation conferring resistance to pyrethroids in M. domestica, we sequenced a partial fragment of IIS6 and L1014F mutation was detected in all resistant populations. The present study provides valuable information for early detection of pyrethroid resistance and developing resistance management strategies in the house fly populations.

Knockdown Resistance Mutations in Aedes aegypti (Diptera: Culicidae) From Puerto Rico.

Permethrin resistance is widespread in Aedes aegypti (L.), the main dengue, zika, and chikungunya virus vector in Latin America and the Caribbean. A common mechanism of resistance to pyrethroids-knockdown resistance (kdr)-is conferred through mutations in the insect's voltage-dependent sodium channel. In this mosquito, around 10 replacement substitutions in the voltage-gated sodium channel gene (vgsc) have been reported in pyrethroid-resistant strains. Two of these mutations, named Ile1,016 and Cys1,534, are widespread in mosquito populations from Latin America and the Caribbean. This study assessed the levels of permethrin resistance and the frequency of two kdr mutations in eight Ae. aegypti populations collected in Puerto Rico in 2013. Permethrin resistance factors ranged from 33-214-fold relative to the New Orleans reference strain. The frequency of kdr mutation Ile1,016 ranged from 0.65 to fixation (1.0), and for Cys1,534 frequencies varied from 0.8 to fixation. Alarmingly, two populations-Carolina and Caguas-reached fixation at both loci. Our results suggest that permethrin effectiveness for Ae. aegypti control is compromised in these collections from Puerto Rico.

Multiple mutations and mutation combinations in the sodium channel of permethrin resistant mosquitoes, Culex quinquefasciatus

A previous study identified 3 nonsynonymous and 6 synonymous mutations in the entire mosquito sodium channel of Culex quinquefasciatus, the prevalence of which were strongly correlated with levels of resistance and increased dramatically following insecticide selection. However, it is unclear whether this is unique to this specific resistant population or is a common mechanism in field mosquito populations in response to insecticide pressure. The current study therefore further characterized these mutations and their combinations in other field and permethrin selected Culex mosquitoes, finding that the co-existence of all 9 mutations was indeed correlated with the high levels of permethrin resistance in mosquitoes. Comparison of mutation combinations revealed several common mutation combinations presented across different field and permethrin selected populations in response to high levels of insecticide resistance, demonstrating that the co-existence of multiple mutations is a common event in response to insecticide resistance across different Cx. quinquefasciatus mosquito populations.

Identification of a Mutation Associated With Permethrin Resistance in the &lt;I&gt;Para&lt;/I&gt;-Type Sodium Channel of the Stable Fly (Diptera: Muscidae)

The insect sodium channel is of particular interest for evaluating resistance to pyrethroids because it is the target molecule for this major class of neurotoxic insecticides. The stable fly, Stomoxys calcitrans (L.) (Diptera: Muscidae), sodium channel coding sequence representing domains IS6 through IVS6 was isolated, and the sequence encoding domain II was compared among individuals of a laboratory strain selected for resistance to permethrin and the unselected, parental generation. A point mutation resulting in a leucine-to-histidine amino acid change was identified (Leul014His), and its location corresponded with that observed for knockdown resistance (kdr) mutations in other insects. As a result, the allele was designated kdr-his. A molecular assay was developed to assess the frequency of this mutation in genomic DNA of individual stable flies from the laboratory selections, which provided further evidence that the kdr-his allele accounts for the observed level ofpermethrin resistance in the selected strain. The assay was then used to evaluate the frequency of the mutation from five field-collected populations originating from three horse farms near Ocala, FL; one horse farm near Gainesville, FL; and one dairy farm near Hague, FL. Frequency of the kdr-his allele ranged from 0.46 to 0.78, supporting further investigation of allele prevalence throughout the stable fly season and in response to field insecticide application.

Larvicidal Activity of Mosquito Sterol Carrier Protein-2 Inhibitors to the Insecticide-Resistant MosquitoCulex quinquefasciatus(Diptera: Culicidae)

AeSCP-2 inhibitors (SCPIs) compete with cholesterol for binding to a mosquito sterol carrier protein-2 (AeSCP-2) known to aid in the uptake of cholesterol in mosquito cells. The larvicidal activities of AeSCP-2 inhibitor-1 (SCPI-1) and inhibitor-2 (SCPI-2) against Culex quinquefasciatus Say (Diptera: Culicidae) were therefore examined in insecticide-resistant Culex mosquitoes HAmCq(G9), MAmCq(G2), and BAmCq(G0). All of the resistant Culex mosquito strains exhibited similar sensitivity to SCPI-1 and SCPI-2 inhibitors compared with a susceptible S-Lab strain. When an AeSCP-2 inhibitor was applied simultaneously with permethrin, the toxicity of permethrin to the second-instar larvae of all four strains of Culex mosquitoes increased, suggesting a synergistic effect of AeSCP-2 inhibitors on the toxicity of permethrin against Culex mosquitoes. Both SCPI-1 and SCPI-2 inhibitors caused a 2.4- to 3-fold reduction in the level of permethrin resistance in the highly resistant strain HAmCq(G9). This result suggests that the mode of action of the AeSCP-2 inhibitors, which reduces the uptake of cholesterol by inhibiting the function of AeSCP-2 in mosquito cells, may interfere with the mechanisms or ability that govern permethrin resistance in the HAmCq(G9) mosquito strain.

Dynamics and mechanisms of permethrin resistance in a field population of the horn fly, Haematobia irritans irritans

AbstractA study was conducted at the Pressler ranch, near Kerrville, Texas, USA between 2002 and 2006 to determine the dynamics and mechanisms of resistance to permethrin in a field population of the horn fly, Haematobia irritans irritans (L.). Changes of resistance to pyrethroid insecticide associated with use of a pour‐on formulation of cyfluthrin in 2002 and use of diazinon ear tags in subsequent years were studied using a filter paper bioassay technique and a polymerase chain reaction assay that detects two sodium channel mutations, kdr and super‐kdr resistance alleles. A maximum of 294‐fold resistance to permethrin was observed in the summer of 2002. A significant decrease in the resistance level was observed in spring 2003, and resistance continued to decline after animals were treated with diazinon ear tags. In response to pyrethroid treatments, the allelic kdr and super‐kdr frequency increased from 56.3% to 93.8% and from 7.5% to 43.8%, respectively in 2002, and decreased significantly in 2003 when the pyrethroid insecticide was no longer used to treat animals. Females were found to have a higher allelic super‐kdr frequency than males in 2002, while no difference was detected between males and females in the allelic kdr frequency. There was a significant positive correlation between frequencies of the sodium channel mutations and levels of permethrin resistance, suggesting that the sodium channel mutations, kdr and super‐kdr, are the major mechanisms of resistance to pyrethroids in this horn fly population. Results of synergist bioassays also indicated possible contributions of two metabolic detoxification mechanisms, the mixed function oxidases (MFO) and glutathione S‐transferases (GST). Compared to a horn fly infestation of an untreated herd, treatments with the pyrethroid pour‐on formulation failed to control horn flies at the Pressler ranch in 2002. Sustained control of horn flies was achieved with the use of diazinon ear tags in 2003 and subsequent years.

Expression of the cytochrome P450s, CYP6P3 and CYP6M2 are significantly elevated in multiple pyrethroid resistant populations of Anopheles gambiae s.s. from Southern Benin and Nigeria.

BackgroundInsecticide resistance in Anopheles mosquitoes is threatening the success of malaria control programmes. This is particularly true in Benin where pyrethroid resistance has been linked to the failure of insecticide treated bed nets. The role of mutations in the insecticide target sites in conferring resistance has been clearly established. In this study, the contribution of other potential resistance mechanisms was investigated in Anopheles gambiae s.s. from a number of localities in Southern Benin and Nigeria. The mosquitoes were sampled from a variety of breeding sites in a preliminary attempt to investigate the role of contamination of mosquito breeding sites in selecting for resistance in adult mosquitoes.ResultsAll mosquitoes sampled belonged to the M form of An. gambiae s.s. There were high levels of permethrin resistance in an agricultural area (Akron) and an urban area (Gbedjromede), low levels of resistance in mosquito samples from an oil contaminated site (Ojoo) and complete susceptibility in the rural Orogun location. The target site mutation kdrW was detected at high levels in two of the populations (Akron f = 0.86 and Gbedjromede f = 0.84) but was not detected in Ojoo or Orogun. Microarray analysis using the Anopheles gambiae detox chip identified two P450s, CYP6P3 and CYP6M2 up regulated in all three populations, the former was expressed at particularly high levels in the Akron (12.4-fold) and Ojoo (7.4-fold) populations compared to the susceptible population. Additional detoxification and redox genes were also over expressed in one or more populations including two cuticular pre-cursor genes which were elevated in two of the three resistant populations.ConclusionMultiple resistance mechanisms incurred in the different breeding sites contribute to resistance to permethrin in Benin. The cytochrome P450 genes, CYP6P3 and CYP6M2 are upregulated in all three resistant populations analysed. Several additional potential resistance mechanisms were also identified that warrant further investigation. Metabolic genes were over expressed irrespective of the presence of kdr, the latter resistance mechanism being absent in one resistant population. The discovery that mosquitoes collected from different types of breeding sites display differing profiles of metabolic genes at the adult stage may reflect the influence of a range of xenobiotics on selecting for resistance in mosquitoes.

Evidence of Pyrethroid Resistance in Eggs ofPediculus humanus capitis(Phthiraptera: Pediculidae) from Argentina

Insecticide resistance in Pediculus humanus capitis De Geer 1778 (Phthiraptera: Pediculidae) from different countries has been well documented in the last years. Otherwise, little research has been reported about insecticide resistance in insect embryos and none in human louse embryos. In this work, we studied insecticide resistance in eggs of three head lice populations whose pyrethroid resistance was shown in adults and nymphs compared with a susceptible laboratory body louse strain. All head louse populations showed high permethrin resistance in eggs. Levels of permethrin resistance (LCRs) assessed in eggs by immersion technique were higher than those previously reported for the corresponding populations of adults by topical application. Comparison of LCR values for different populations showed that there was a direct relationship between the resistance levels assessed in eggs and those in adults. All permethrin-resistant eggs showed high resistance to d-phenothrin and dichloro-diphenyl-trichloroethane (DDT) and low resistance to carbaryl, which is in common with the resistance profile established for adults and nymphs. The results concerning the high resistance to pyrethroid (permethrin and d-phenothrin) and the cross-resistance to DDT and carbaryl detected in head louse eggs and adults suggested similar resistance mechanisms in eggs and adults of head louse populations from adults.

Evidence of Pyrethroid Resistance in Eggs of &lt;I&gt;Pediculus humanus capitis&lt;/I&gt; (Phthiraptera: Pediculidae) from Argentina

Insecticide resistance in Pediculus humanus capitis De Geer 1778 (Phthiraptera: Pediculidae) from different countries has been well documented in the last years. Otherwise, little research has been reported about insecticide resistance in insect embryos and none in human louse embryos. In this work, we studied insecticide resistance in eggs of three head lice populations whose pyrethroid resistance was shown in adults and nymphs compared with a susceptible laboratory body louse strain. All head louse populations showed high permethrin resistance in eggs. Levels of permethrin resistance (LCRs) assessed in eggs by immersion technique were higher than those previously reported for the corresponding populations of adults by topical application. Comparison of LCR values for different populations showed that there was a direct relationship between the resistance levels assessed in eggs and those in adults. All permethrin-resistant eggs showed high resistance to d-phenothrin and dichloro-diphenyl-trichloroethane (DDT) and low resistance to carbaryl, which is in common with the resistance profile established for adults and nymphs. The results concerning the high resistance to pyrethroid (permethrin and d-phenothrin) and the cross-resistance to DDT and carbaryl detected in head louse eggs and adults suggested similar resistance mechanisms in eggs and adults of head louse populations from adults.

Isolation and Identification of an Esterase from a Mexican Strain of Boophilus microplus (Acari: Ixodidae)

A strain of Mexican Boophilus microplus (Cz) collected near Coatzacoalcos, Veracruz, Mexico, exhibits a moderate, but significant, level of permethrin resistance. Unlike other highly permethrin resistant strains, the Cz strain does not have a mutation within the sodium channel gene that results in target-site insensitivity. However, the Cz strain possesses a substantial increase in general and permethrin esterase activity relative to highly permethrin resistant and control strains suggesting the involvement of a metabolic esterase(s) in the expression of permethrin resistance. We report the isolation of a 62.8 kDa protein from Cz strain larvae that we think is the esterase previously reported as Cz EST9. In addition, internal amino acid sequence data obtained from the 62.8 kDa protein suggest that it is the gene product of a previously reported B. microplus carboxylesterase cDNA. We propose that the 62.8 kDa protein (Cz EST9) has permethrin hydrolytic activity and as a result plays an important role in Cz strain resistance to permethrin.

Isolation and identification of an esterase from a Mexican strain of Boophilus microplus (Acari: Ixodidae).

A strain of Mexican Boophilus microplus (Cz) collected near Coatzacoalcos, Veracruz, Mexico, exhibits a moderate, but significant, level of permethrin resistance. Unlike other highly permethrin resistant strains, the Cz strain does not have a mutation within the sodium channel gene that results in target-site insensitivity. However, the Cz strain possesses a substantial increase in general and permethrin esterase activity relative to highly permethrin resistant and control strains suggesting the involvement of a metabolic esterase(s) in the expression of permethrin resistance. We report the isolation of a 62.8 kDa protein from Cz strain larvae that we think is the esterase previously reported as Cz EST9. In addition, internal amino acid sequence data obtained from the 62.8 kDa protein suggest that it is the gene product of a previously reported B. microplus carboxylesterase cDNA. We propose that the 62.8 kDa protein (Cz EST9) has permethrin hydrolytic activity and as a result plays an important role in Cz strain resistance to permethrin.

Genetics and mechanisms of permethrin resistance in the YPER strain of house fly

A strain of house fly was collected from the Third Yumenoshima Island in Japan and selected with the pyrethroid insecticide permethrin. Resistance to permethrin evolved to >18,400-fold in the selected (YPER) strain, and the mechanisms and genetics of resistance in this strain were examined. Permethrin resistance was decreased by pretreatment with piperonyl butoxide or 2-propynyl 2,3,6-trichlorophenyl ether, but not S, S, S-tributylphosphorotrithioate or diethylmaleate. The level of total cytochromes P450 was 2.7-fold increased, and the level of cytochrome b 5 was 1.5-fold increased, in YPER compared to the susceptible CS strain. These results suggest P450 monooxygenases, but not hydrolases or glutathione S-transferase, are a mechanism of resistance in the YPER strain. Analysis of the para-homologous sodium channel α-subunit gene in YPER indicates this strain has the super-kdr allele. Permethrin resistance in the YPER strain was inherited as a multigenic and incompletely recessive trait. A factorial analysis of resistance in the YPER strain indicated the relative contribution to resistance by the different autosomes was 2>3>5⩾1. Comparison of YPER with three other strains of house fly having high levels of permethrin resistance (LPR, NG98, and ALHF) indicates the genetic basis of this trait is variable between populations. This indicates there may be a greater difficulty for the development of diagnostic tools that could be used reliably over large areas to monitor pyrethroid resistance in house flies than was previously thought.

Antibody Capture Immunoassay for the Detection of Permethrin Carboxylesterase in Colorado Potato Beetle, Leptinotarsa decemlineata Say

Polyclonal antibodies were generated against the three denatured forms (30-, 48-, and 59-kDa proteins) of permethrin carboxylesterase from Colorado potato beetle. Each antiserum showed cross-reactivities to all the denatured forms of permethrin carboxylesterase as judged by antibody capture immunoassay. An antibody capture immunoassay was developed using the immunoglobulin purified from the 30-kDa antiserum in conjunction with denatured Colorado potato beetle hemolymph as the antigen source. In this antibody capture immunoassay, the hemolymph from the permethrin-resistant strain produced 1.1–5.3 times higher signal levels than that from the susceptible strain depending on assay conditions, suggesting that the hemolymph from the resistant strain contains more permethrin carboxylesterase. These results corroborate our original findings that permethrin carboxylesterase is overexpressed in the hemolymph of the permethrin-resistant strain of Colorado potato beetle and contributes to overall permethrin resistance. The relative quantities of permethrin carboxylesterase in four different strains of Colorado potato beetle with different levels of permethrin resistance were assessed with this immunoassay and with a carboxylesterase activity assay using α-naphthyl caproate as substrate. The average levels of permethrin carboxylesterase determined by the antibody capture immunoassay were better correlated with the levels of permethrin resistance than those determined by the carboxylesterase activity. Thus, the antibody capture immunoassay can be utilized as a sensitive means to detect the levels of permethrin carboxylesterase as a resistance marker in field populations of Colorado potato beetle.

Insecticide Resistance in Field Populations of the Tarnished Plant Bug (Heteroptera: Miridae) in Cotton in the Mississippi Delta

Adult tarnished plant bugs, Lygus lineolaris (Palisot de Beauvois), collected in July and August from 4 cotton fields in the Mississippi Delta with plant bug control problems were tested with a glass vial bioassay for resistance to insecticides. Resistance levels determined for the different field populations were compared with those found by using the glass vial bioassay and susceptible plant bugs collected from weeds near Crossett, AR. Significantly higher levels of pyrethroid resistance were found in plant bugs from all 4 cotton fields as compared with pyrethroid resistance in bugs from Crossett. Plant bugs from one cotton field near Schlater, MS, were reared through 9 generations in the laboratory and tested in various generations for resistance to 4 pyrethroid, 5 organophosphorus, and 1 cyclodiene insecticide. Plant bugs in this resistant strain had multiple insecticide resistance to all 3 classes of insecticides. The highest levels of insecticide resistance were to pyrethroids, suggesting that the resistance resulted from selection with these insecticides in cotton. High levels of permethrin resistance were unstable and declined in plant bugs reared in the laboratory in the absence of any insecticide selection pressure. When reared in the laboratory, resistant bugs from Schlater had lower fecundity than a susceptible laboratory strain, and survival from egg hatch to adults was lower than in the susceptible strain. The distribution of insecticide resistance in tarnished plant bug populations from the Mississippi Delta is unknown, but control of populations with multiple insecticide resistance will be difficult. Compliance with the insecticide resistance management plan currently recommended for control of the tobacco budworm, Heliothis virescens (F.), in Arkansas, Louisiana, and Mississippi (by not using pyrethroids in cotton until July) is probably the best strategy for preventing the development of plant bug populations with multiple insecticide resistance in July and August.

Synergism of Insecticides in Tests with Resistant Soybean Looper Larvae (Lepidoptera: Noctuidae) in the Laboratory and Field

The F1 generation of several field-collected strains of soybean looper larvae, Pseudoplusia includens (Walker), were tested for synergism of permethrin by piperonyl butoxide (PB) and S, S, S tributylphosphorotrithioate (DEF) in 1992. The synergism of permethrin and methomyl by PB was tested in 1993. Permethrin was completely synergized by 10 µg PB per larva in strains from Louisiana; these strains were 2.7-14.1-fold resistant. Permethrin toxicity was increased 8.6-fold by 10 µg PB per larva in a strain from Texas, which was 14.8-fold resistant. Toxicity was increased only 4.6- and 7.5-fold by 10 and 20 g µPB per larva, respectively, in a Puerto Rico strain; this strain had the highest reported level of permethrin resistance (426.6-fold). We observed no significant synergism of permethrin in any of the strains tested with DEF. Methomyl was synergized to a lesser degree than permethrin by PB in a strain from Louisiana. Field trials were conducted in 1992 and 1993 in soybean in Louisiana with 0.11 kg AI/ha permethrin and several rates of PB. Soybean looper control with permethrin + 1.12 kg Al/ha PB was comparable to that achieved with the current recommended standard insecticide, thiodicarb, at 0.50 kg(AI)/ha, and significantly greater than that achieved with permethrin alone.

Activity of two novel insecticides against permethrin-resistantPseudoplusia includens

Two experimental insecticides, AC 303,630 and MK 244, were tested against a laboratory colony and three field strains of Pseudoplusia includens (Walker). Topical application bioassays indicated that permethrin resistance in the field strains ranged from 3.9 to 489.0-fold. In leaf dip bioassays, LC50 and LC90 values for AC 303,630 ranged from 6.7 to 15.1 mg litre −1 and 8.7 to 28.2 mg litre −1, respectively, for third-instar larvae. The Louisiana 1991 field strain was significantly more susceptible to AC 303,630 than the laboratory and other field strains. The LC50 (but not LC90) for the Louisiana 1992 field strain was significantly higher than that of the laboratory strain. However, there was no difference in toxicity of AC 303,630 between the field strain with the highest level of permethrin resistance and the laboratory strain. LC50 and LC90 values for MK 244 in leaf dip bioassays ranged from 0.014 to 0.023 mg litre −1 and 0.079 to 0.174 mg litre −1, respectively. There were no significant differences in LC 50 or LC 90 among any of the strains for MK 244. Field trials in soybean were also conducted in 1991 and 1992 in an area of Louisiana where permethrin efficacy against P. includens has declined. In both years, AC 303,630 at 0.11–0.22 kg ha −1, and MK 244 at 0.0042–0.0084 kg ha −1, provided significantly better control than permethrin at 0.11 kg ha −1, and control equal to the recommended standard, thiodicarb. These studies indicate no cross-resistance exists between the experimental insecticides and permethrin.

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

AbstractThe mechanisms responsible for &gt; 6000‐fold permethrin resistance in a pyrethroid‐selected strain of house fly, Learn‐PyR, were investigated. Through electrophysiological, in vitro metabolism, in vivo penetration and synergism studies it was demonstrated that the resistance mechanisms consisted of enhanced metabolic detoxification via the mixed‐function oxidase (MFO) system, target‐site insensitivity and decreased cuticular penetration. The major resistance mechanism was the MFO‐mediated detoxification. The elevated MFO activity was correlated with higher levels of cytochrome P‐450, cytochrome b5 and NADPH‐cytochrome c reductase activity. The kinetics of the latter showed similar Km but greater Vmax values in the Learn‐PyR than in the susceptible strain, suggesting that the elevated activity was due to an altered amount, but not an altered form, of the enzyme. The Learn‐PyR strain showed widely varying levels of resistance to the pyrethroids tested. Comparison of the pyrethroid structures with the resistance ratios revealed that resistance was highest in the presence of an unsubstituted phenoxybenzyl alcohol moiety. Substitution or certain modifications of the alcohol moiety reduced the level of resistance. Structure of the acid moiety or the presence or absence of an a‐CN group did not affect the resistance level. These results are discussed with reference to the resistance mechanisms present.