In vitro metabolism of EPN and EPNO in susceptible and resistant strains of house flies

EPN is twice as toxic as EPNO to house flies from both the Diazinon-resistant strain and the susceptible strain. EPN and EPNO are also eight times more toxic to the susceptible than the resistant strain. This is due to the ability of the resistant strain to metabolize these compounds to a greater extent. Metabolism by the glutathione S-transferases present in the 100,000g supernatant is more extensive than that by the NADPH-dependent microsomal mixed-function oxidases. The glutathione S-transferases are the major route of metabolism for EPN and appear to be the principal mechanism conferring resistance. EPN was metabolized by the microsomal fraction via oxidative desulfuration to the oxygen analog, EPNO, and by oxidative dearylation to p-nitrophenol. EPNO was metabolized by the same system to p-nitrophenol and desethyl EPNO as well as to an unknown metabolite. The soluble fraction metabolized EPN to p-nitrophenol, S-(p-nitrophenyl)glutathione, O-ethyl phenylphosphonothioic acid, and S-(O-ethyl phenylphosphonothionyl)glutathione. The identification of the latter conjugate demonstrates a new type of metabolite of organophosphorus compounds. EPNO was metabolized by the soluble fraction to p-nitrophenol and S-(p-nitrophenyl)glutathione.     

Studies on the metabolism of vamidothion and its thioanalog in insecticide-resistant and susceptible house fly strains

The in vivo and in vitro metabolism of vamidothion [O,O-dimethyl S-[2-(1-methylcarbamoyl)-ethylthio] ethylphosphorothiolate] as well as the in vitro metabolism of thiovamidothion [O,O-dimethyl S-[2-(1-methylcarbamoyl)ethylthio] ethylphosphorodithioate] was investigated in insecticide-resistant and susceptible house fly strains. Vamidothion was converted in vivo to the sulfoxide, the principle metabolite, and subsequently to the sulfone at a slower rate. Vamidothion and vamidothion sulfoxide were hydrolyzed at the PS and SC bond. The resulting primary alcohol metabolite was further oxidized to a carboxylic acid followed by decarboxylation. No metabolism of vamidothion or thiovamidothion occurred in vitro without the addition of NADPH. The addition of NADPH resulted in rapid conversion of vamidothion to the sulfoxide, and thiovamidothion was oxidatively metabolized to six metabolic products. No qualitative differences were found between resistant and susceptible strains, but there were signficant quantitative differences. The metabolism was highest in the Rutgers strain followed by Cornell-R, Hirokawa, and then CSMA strain. The route of vamidothion and thiovamidothion metabolism was via the cytochrome P-450-dependent monooxygenase system, and none of the resistant strains showed glutathione S-transferase activity toward vamidothion or thiovamidothion. No further oxidation of vamidothion sulfoxide to the sulfone was observed and also no hydrolysis products were formed, in vitro.     

Metabolic resistance mechanisms to phosalone in the common pistachio psyllid, Agonoscena pistaciae (Hem.: Psyllidae)

The common pistachio psyllid, Agonoscena pistaciae, is the most damaging pest of pistachio in Iran, and is generally controlled by insecticides belonging to various classes especially, phosalone. The toxicity of phosalone in nine populations of the pest was assayed using the residual contact vial and insect-dip methods. The bioassay results showed significant discrepancy in susceptibility to phosalone among the populations. Resistance ratio of the populations to the susceptible population ranged from 3.3 to 11.3. The synergistic effects of TPP, PBO and DEM were evaluated on the susceptible and the most resistant population to determine the involvement of esterases, mixed function oxidases and glutathione S-transferases in resistance mechanisms, respectively. The level of resistance to phosalone in the resistant population was suppressed by TPP, PBO and DEM, suggesting that the resistance to phosalone is mainly caused by esterase detoxification. Biochemical enzyme assays revealed that esterase, glutathione S-transferase and cytochrome P450 monooxygenase activities in the resistant population was higher than that in the susceptible. Glutathione-S-transferases play a minor role in the resistance of the pest to phosalone.     

Biochemical characteristics of insecticide resistance in the fall armyworm, Spodoptera frugiperda (J.E. Smith)

A strain of the fall armyworm, Spodoptera frugiperda (J.E. Smith), collected from corn in Citra, Florida, showed high resistance to carbaryl (562-fold) and methyl parathion (354-fold). Biochemical studies revealed that various detoxification enzyme activities were higher in the field strain than in the susceptible strain. In larval midguts, activities of microsomal oxidases (epoxidases, hydroxylase, sulfoxidase, N-demethylase, and O-demethylase) and hydrolases (general esterase, carboxylesterase, β-glucosidase) were 1.2- to 1.9-fold higher in the field strain than in the susceptible strain. In larval fat bodies, various activities of microsomal oxidases (epoxidases, hydroxylase, N-demethylase, O-demethylases, and S-demethylase), glutathione S-transferases (CDNB, DCNB, and p-nitrophenyl acetate conjugation), hydrolases (general esterase, carboxylesterase, β-glucosidase, and carboxylamidase) and reductases (juglone reductase and cytochrome c reductase) were 1.3- to 7.7-fold higher in the field strain than in the susceptible strain. Cytochrome P450 level was 2.5-fold higher in the field strain than in the susceptible strain. In adult abdomens, their detoxification enzyme activities were generally lower than those in larval midguts or fat bodies; this is especially true when microsomal oxidases are considered. However, activities of microsomal oxidases (S-demethylase), hydrolases (general esterase and permethrin esterase) and reductases (juglone reductase and cytochrome c reductase) were 1.5- to 3.0-fold higher in the field strain than in the susceptible strain. Levels of cytochrome P450 and cytochrome b₅ were 2.1 and 1.9-fold higher, respectively, in the field strain than in the susceptible strain. In addition, acetylcholinesterase from the field strain was 2- to 85-fold less sensitive than that from the susceptible strain to inhibition by carbamates (carbaryl, propoxur, carbofuran, bendiocarb, thiodicarb) and organophosphates (methyl paraoxon, paraoxon, dichlorvos), insensitivity being highest toward carbaryl. Kinetics studies showed that the apparent K_m value for acetylcholinesterase from the field strain was 56% of that from the susceptible strain. The results indicated that the insecticide resistance observed in the field strain was due to multiple resistance mechanisms, including increased detoxification of these insecticides by microsomal oxidases, glutathione S-transferases, hydrolases and reductases, and target site insensitivity such as insensitive acetylcholinesterase. Resistance appeared to be correlated better with detoxification enzyme activities in larval fat bodies than in larval midguts, suggesting that the larval fat body is an ideal tissue source for comparing detoxification capability between insecticide-susceptible and -resistant insects.     

Interactions of allelochemicals with detoxication enzymes of insecticide-susceptible and resistant fall armyworms

The induction of glutathione S-transferases and microsomal oxidases by host plants and allelochemicals was examined in sixth-instar larvae of insecticide-susceptible and resistant strains of the fall armyworm, Spodoptera frugiperda (J. E. Smith). Among 11 host plants studied, parsnip and parsley were the best inducers of glutathione S-transferase, resulting in increases of 39- and 19-fold, respectively, compared with the artificial diet. The inducer in parsnip leaves was identified by mass spectrometry, high-pressure liquid chromatography, gas chromatography, and thin-layer chromatography as xanthotoxin, a furanocoumarin. Xanthotoxin also showed a bimodal effect on the microsomal oxidase systems, increasing cytochrome P-450 content and heptachlor epoxidase activity but inhibiting aldrin epoxidase, biphenyl 4-hydroxylase, and p-chloro-N-methylaniline N-demethylase. Using indole 3-acetonitrile, indole 3-carbinol, and flavone as inducers, the inducing pattern of glutathione S-transferases was the same toward 3,4-dichloronitrobenzene, 1-chloro-2,4-dinitrobenzene, and methyl iodide. Microsomal oxidase and glutathione S-transferase were also inducible by host plants and allelochemicals in larvae of a carbaryl-resistant strain.     

Metabolism of methyl bromide by susceptible and resistant strains of the granary weevil, Sitophilus granarius (L.)

Methyl bromide was metabolized by susceptible and resistant strains of adult granary weevil, Sitophilus granarius (L.), mainly by conjugation with glutathione. S-Methyl glutathione and S-methyl cysteine were produced by both strains and S-methyl glutathione sulfoxide was identified as a metabolite in the resistant strain. In the untreated insects, no significant difference was observed in glutathione S-transferase activity but the resistant contained approximately twice as much glutathione per insect as the susceptible strain. When the insects were treated with methyl bromide, the glutathione content of both strains was lowered; proportionally, however, the decrease was considerably higher in the susceptible than in the resistant strain. These results indicate that conjugation of methyl bromide with glutathione is a major detoxication pathway and tolerance to this fumigant is related, in part at least, to the level of glutathione in the granary weevil.     

Resistance to insecticides in winter- and summer-forms of pear psylla,Psylla pyricola

Summer-form pear psylla, Psylla pyricola Foerster, from sprayed pear were resistant to endosulfan (2·4-fold), methiocarb (2·5-fold), ethylan (5·8-fold), azinphos-methyl (7·7-fold), and fenvalerate (40·1-fold). Esterase (3·8-fold), glutathione transferase (1·8-fold), and cytochrome P-450 monooxygenase (1·6-fold) detoxification enzyme activity was higher in resistant than in susceptible summer forms. Synergism by piperonyl butoxide and S,S,S-tributylphosphorotrithioate (DEF) was added evidence for cytochrome P-450 monooxygenases and esterases as resistance mechanisms. Reduced penetration may also have contributed to resistance, as indicated by a 1·6-fold slower penetration of azinphos-methyl in resistant than susceptible summer-forms. Similar differences in insecticide toxicity and esterase and glutathione transferase activities were observed between winter-forms of resistant and susceptible pear psylla. Winter-forms of P. pyricola were up to three times more tolerant to insecticides than summer-forms. Higher cytochrome P-450 monooxygenase activity (1·7-fold) and slower azinphosmethyl penetration (2·1-fold) in winter-forms may have contributed to their greater insecticide tolerance; however, sequestration may also have been involved.     

Characterization of acephate resistance in the diamondback moth Plutella xylostella

Decreased acetylcholinesterase (AChE) sensitivity and metabolic detoxification mediated by glutathione S-transferases (GSTs) were examined for their involvement in resistance to acephate in the diamondback moth, Plutella xylostella. The resistant strain showed 47.5-fold higher acephate resistance than the susceptible strain had. However, the resistant strain was only 2.3-fold more resistant to prothiofos than the susceptible strain. The resistant strain included insects having the A298S and G324A mutations in AChE1, which are reportedly involved in prothiofos resistance in P. xylostella, showing reduced AChE sensitivity to inhibition by methamidophos, suggesting that decreased AChE1 sensitivity is one factor conferring acephate resistance. However, allele frequencies at both mutation sites in the resistant strain were low (only 26%). These results suggest that other factors such as GSTs are involved in acephate resistance. Expression of GST genes available in P. xylostella to date was examined using the resistant and susceptible strains, revealing no significant correlation between the expression and resistance levels.     

Resistance selection and biochemical mechanism of resistance to two Acaricides in Tetranychus cinnabarinus (Boiduval)

A Tetranychus cinnabarinus strain was collected from Chongqing, China. After 42 generations of selection with abamectin and 20 generations of selection with fenpropathrin in the laboratory, this T. cinnabarinus strain developed 8.7- and 28.7-fold resistance, respectively. Resistance to abamectin in AbR (abamectin resistant strain) and to fenpropathrin in FeR (fenpropathrin resistant strain) was partially suppressed by piperonyl butoxide (PBO), diethyl maleate (DEM) and triphenyl phosphate (TPP), inhibitors of mixed function oxidase (MFO), glutathione S-transferases (GST), and hydrolases, respectively, suggesting that these three enzyme families are important in conferring abamectin and fenpropathrin resistance in T. cinnabarinus. The major resistant mechanism to abamectin was the increasing activities of carboxylesterases (CarE), glutathione-S-transferase (GST) and mixed function oxidase (MFO), and the activity in resistant strain developed 2.7-, 3.4- and 1.4-fold contrasted to that in susceptible strain, respectively. The activity of glutathione-S-transferase (GST) in the FeR strain developed 2.8-fold when compared with the susceptible strain, which meant the resistance to fenpropathrin was related with the activity increase of glutathione-S-transferase (GST) in T. cinnabarinus. The result of the kinetic mensuration of carboxylesterases (CarE) showed that the structure of CarE in the AbR has been changed.     

Cytochrome P450 monooxygenase-mediated permethrin resistance confers limited and larval specific cross-resistance in the southern house mosquito, Culex pipiens quinquefasciatus

The cytochrome P450-dependent monooxygenases (P450s) are an important enzymatic system that metabolizes xenobiotics (e.g., pesticides), as well as endogenous compounds (e.g., hormones). P450-mediated metabolism can result in detoxification of insecticides such as pyrethroids, or can be involved in the bioactivation and detoxification of insecticides such as organophosphates. We isolated (from the JPAL strain) a permethrin resistant strain (ISOP450) of Culex pipiens quinquefasciatus, having 1300-fold permethrin resistance using standard backcrossing procedures. ISOP450 is highly related to the susceptible lab strain (SLAB) and the high resistance to permethrin is due solely to P450-mediated detoxification. This is the first time in mosquitoes that P450 monooxygenase involvement in pyrethroid resistance has been isolated and studied without the confounding effects of kdr. Resistance in ISOP450 is incompletely dominant (D = +0.3), autosomally linked, and monofactorally inherited. It is expressed in the larvae, but not in adults. Cross-resistance to pyrethroids lacking a 3-phenoxybenzyl moiety (tetramethrin, fenfluthrin, bioallethrin, and bifenthrin) ranged from 1.5- to 12-fold. ISOP450 had only limited (6.6- and 11-fold) cross-resistance to 3-phenoxybenzyl pyrethroids with an α-cyano group (cypermethrin and deltamethrin, respectively). Examination of cross-resistance patterns to organophosphate insecticides in ISOP450 showed an 8-fold resistance to fenitrothion, while low, but significant, levels of negative cross-resistance were found for malathion (RR = 0.84), temephos (RR = 0.73), and methyl-parathion (RR = 0.55). The importance and uniqueness of this P450 mechanism in insecticide resistance is discussed.     

Metabolic resistance mechanisms of the housefly (Musca domestica) resistant to pyraclofos

In vivo and in vitro metabolism of pyraclofos labeled with ¹⁴C on benzene ring was studied in the pyraclofos-resistant and -susceptible female houseflies. In vivo metabolism studies, the metabolic rate of pyraclofos was the same in both strains. Pyraclofos primarily undergoes metabolic detoxification by cleavage of P-S-alkyl bond, and cleavage of the P-O-aryl bond followed by CHP [1-(4-chlorophenyl)-4-hydroxypyrazole]]-glucose conjugation. Cleavage of P-O-aryl bond and CHP-glucose conjugation is more predominant in the resistant strain whereas the cleavage of P-S-propyl bond resulting in EHP-CHP [O-1-(4-chlorophenyl)pyrazol-4-yl ethyl hydrogen phosphate] is more preferred in the susceptible strain. CHP production by P-O-aryl bond cleavage was controlled by P450 monooxygenase and esterase. UDP-glucosyltransferase appeared to play an important role in the pyraclofos metabolism of the resistant strain. Production of CHP-glucose conjugate was largely reduced by piperonyl butoxide and S,S,S-tributylphosphorotrithioate in both strains. EHP-CHP production seemed to be controlled by P450 monooxygenase and stimulated by UDP-glucose.     

Insecticide resistance status of Aedes aegypti (L.) from Colombia

BACKGROUND: To evaluate the insecticide susceptibility status of Aedes aegypti (L.) in Colombia, and as part of the National Network of Insecticide Resistance Surveillance, 12 mosquito populations were assessed for resistance to pyrethroids, organophosphates and DDT. Bioassays were performed using WHO and CDC methodologies. The underlying resistance mechanisms were investigated through biochemical assays and RT‐PCR. RESULTS: All mosquito populations were susceptible to malathion, deltamethrin and cyfluthrin, and highly resistant to DDT and etofenprox. Resistance to lambda‐cyhalothrin, permethrin and fenitrothion ranged from moderate to high in some populations from Chocó and Putumayo states. In Antioquia state, the Santa Fe population was resistant to fenitrothion. Biochemical assays showed high levels of both cytochrome P450 monooxygenases (CYP) and non‐specific esterases (NSE) in some of the fenitrothion‐ and pyrethroid‐resistant populations. All populations showed high levels of glutathione‐S‐transferase (GST) activity. GSTe2 gene was found overexpressed in DDT‐resistant populations compared with Rockefeller susceptible strain. CONCLUSIONS: Differences in insecticide resistance status were observed between insecticides and localities. Although the biochemical assay results suggest that CYP and NSE could play an important role in the pyrethroid and fenitrothion resistance detected, other mechanisms remain to be investigated, including knockdown resistance. Resistance to DDT was high in all populations, and GST activity is probably the main enzymatic mechanism associated with this resistance. The results of this study provide baseline data on insecticide resistance in Colombian A. aegypti populations, and will allow comparison of changes in susceptibility status in this vector over time. Copyright © 2011 Society of Chemical Industry     

Characterization of malathion resistance in a Mexican population of Rhizopertha dominica

Malathion resistance of a field-collected population of Rhizopertha dominica (Coleoptera: Bostrichidae) from Mexico was evaluated and the resistance mechanisms were characterized both in vivo and in vitro. The Mexican population showed a resistance level of 50-fold at LC₅₀ as compared with that of a susceptible laboratory population. Malathion bioassays with the synergists triphenyl phosphate, piperonyl butoxide and diethyl maleate suggested that esterases were likely to contribute to the resistance whereas cytochrome P450 monooxygenases and glutathione S-transferases were not. In-vitro assays of esterases indicated that the general esterase activity was 1·3-fold higher in the Mexican population than in the susceptible population. However, the phosphotriesterase activity in the resistant population was 3·7-fold higher than in the susceptible population. Significantly higher phosphotriesterase activity in the resistant population was further indicated by 3·4-fold increase of V_max in enzyme kinetics and higher frequency of individuals with high phosphotriesterase activity in this population. All these findings suggested that phosphotriesterases play a role in malathion resistance in the Mexican population of lesser grain borer. © 1998 SCI     

Cross‐resistance and possible mechanisms of chlorpyrifos resistance in Laodelphax striatellus (Fallén)

BACKGROUND: Laodelphax striatellus (Fallén) is a major pest of cultivated rice and is commonly controlled in China with the organophosphate insecticides. To develop a better resistance management strategy, a chlorpyrifos‐resistant strain of L. striatellus was selected in the laboratory, and its cross‐resistance to other insecticides and possible mechanisms of the chlorpyrifos resistance were investigated. RESULTS: After 25 generations of selection with chlorpyrifos, the selected strain of L. striatellus developed 188‐fold resistance to chlorpyrifos in comparison with the susceptible strain, and showed 14‐ and 1.6‐fold cross‐resistance to dichlorvos and thiamethoxam respectively. There was no apparent cross‐resistance to abamectin. Chlorpyrifos was synergised by the inhibitor triphenyl phosphate; the carboxylesterase synergistic ratio was 3.8 for the selected strain, but only 0.92 for the susceptible strain. The carboxylesterase activity of the selected strain was approximately 4 times that of the susceptible strain, whereas there was no significant change in the activities of alkaline phosphatase, acid phosphatase, glutathione S‐transferase and cytochrome P450 monooxygenase between the strains. The Michaelis constant of acetylcholinesterase, maximum velocity of acetylcholinesterase and median inhibitory concentration of chlorpyrifos‐oxon on acetylcholinesterase were 1.7, 2.5 and 5 times higher respectively in the selected strain. CONCLUSION: The high cross‐resistance to the organophosphate dichlorvos in the chlorpyrifos‐resistant strain suggests that other non‐organophosphate insecticides would be necessary to counter resistance, should it arise in the field. Enhanced activities of carboxylesterase and the acetylcholinesterase insensitivity appear to be important mechanisms for chlorpyrifos resistance in L. striatellus. Copyright © 2010 Society of Chemical Industry     

The metabolism of R-20458 [(E)-6,7-epoxy-1-(4-ethylphenoxy)-3,7-dimethyl-2-octene] in rat hepatocyte suspensions

The metabolism of R-20458 [(E)-6,7-epoxy-1-(4-ethylphenoxy)-3,7-dimethyl-2-octene] by rat hepatocytes has been analyzed and compared with that of juvenile hormone I [methyl-(E,E)-cis-10,11-epoxy-7-ethyl-3,11-dimethyl-2,6-tridecadienoate] under identical conditions. The metabolism of R-20458 is characterized by the predominance of NADPH-dependent cytochrome P-450 and epoxide hydrolase reactions; whereas, JH I is metabolized mainly by carboxylesterase, epoxide hydrolase, and glutathione S-transferases. The metabolites of R-20458 have been shown to correspond to (E)-6,7-epoxy-1-(4-hydroxyethylphenoxy)-3,7-dimethyl-2-octene; (E)-6,7-epoxy-1-(4-acetylphenoxy)-3,7-dimethyl-2-octene; (E)-6,7-dihydroxy-1-(4-ethylphenoxy)-3,7-dimethyl-2-octene; and, (E)-6,7-dihydroxy-1-(4-acetylphenoxy)-3,7-dimethyl-2-octene. The production of the α-hydroxyethyl, p-acetylphenoxy, and acetylphenoxy-6,7-diol metabolites is markedly inhibited by SKF 525-A. No dramatic effects are produced by diethylmaleate and 1,2-epoxy-3,3,3-trichloropropane.     

in vitro metabolism of azinphosmethyl in susceptible and resistant houseflies

The in vitro metabolism of [¹⁴C-methoxy] or [³²P]azinphosmethyl by subcellular fractions of abdomens from a resistant and a susceptible strain of houseflies was studied. The degradative activity in both strains was associated with the microsomal and soluble fractions and required NADPH and glutathione, respectively. The resistant strain possessed higher activity for both the mixed-function oxidases and the glutathione transferase than the susceptible strain, and both systems appear to be important in the resistance mechanism. The mixed-function oxidases were involved in the oxidative desulfuration as well as the dearylation of azinphosmethyl. A glutathione transferase located in the soluble fraction catalyzed the formation of desmethyl azinphosmethyl and methyl glutathione. This enzyme also demethylated azinphosmethyl oxygen analog. Although the soluble fraction exhibited both glutathione S-alkyltransferase and S-aryltransferase activity against noninsecticidal substrates, no evidence of the transfer of the benzazimide moiety from azinphosmethyl to glutathione was obtained. Sephadex G-100 chromatography of the soluble enzymes revealed a common eluting fraction responsible for both types of transferase activity.     

Fipronil metabolism,oxidative sulfone formation and toxicity among organophosphate‐ and carbamate‐resistant and susceptible western corn rootworm populations

Fipronil toxicity and metabolism were studied in two insecticide‐resistant, and one susceptible western corn rootworm (Diabrotica virgifera virgifera, LeConte) populations. Toxicity was evaluated by exposure to surface residues and by topical application. Surface residue bioassays indicated no differences in fipronil susceptibility among the three populations. Topical bioassays were used to study the relative toxicity of fipronil, fipronil + the mono‐oxygenase inhibitor piperonyl butoxide, and fipronil's oxidative sulfone metabolite in two populations (one resistant with elevated mono‐oxygenase activity). Fipronil and fipronil‐sulfone exhibited similar toxicity and application of piperonyl butoxide prior to fipronil resulted in marginal effects on toxicity. Metabolism of [¹⁴C]fipronil was evaluated in vivo and in vitro in the three rootworm populations. In vivo studies indicated the dominant pathway in all populations to be formation of the oxidative sulfone metabolite. Much lower quantities of polar metabolites were also identified. In vitro studies were performed using sub‐cellular protein fractions (microsomal and cytosolic), and glutathione‐agarose purified glutathione‐S‐transferase. Oxidative sulfone formation occurred almost exclusively in in vitro microsomal reactions and was increased in the resistant populations. Highly polar metabolites were formed exclusively in in vitro cytosolic reactions. In vitro reactions performed with purified, cytosolic glutathione‐S‐transferase (MW = 27 kDa) did not result in sulfone formation, although three additional polar metabolites not initially detectable in crude cytosolic reactions were detected. Metabolism results indicate both cytochromes P450 and glutathione‐S‐transferases are important to fipronil metabolism in the western corn rootworm and that toxic sulfone formation by P450 does not affect net toxicity. © 2000 Society of Chemical Industry     

Beta-cypermethrin resistance associated with high carboxylesterase activities in a strain of house fly, Musca domestica (Diptera: Muscidae)

A housefly strain, originally collected in 1998 from a dump in Beijing, was selected with beta-cypermethrin to generate a resistant strain (CRR) in order to characterize the resistance and identify the possible mechanisms involved in the pyrethroid resistance. The resistance was increased from 2.56- to 4419.07-fold in the CRR strain after 25 consecutive generations of selection compared to a laboratory susceptible strain (CSS). The CRR strain also developed different levels of cross-resistance to various insecticides within and outside the pyrethroid group such as abamectin. Synergists, piperonyl butoxide (PBO) and S,S,S-tributyl phosphorotrithioate (DEF), increased beta-cypermethrin toxicity 21.88- and 364.29-fold in the CRR strain as compared to 15.33- and 2.35-fold in the CSS strain, respectively. Results of biochemical assays revealed that carboxylesterase activities and maximal velocities to five naphthyl-substituted substrates in the CRR strain were significantly higher than that in the CSS strain, however, there was no significant difference in glutathione S-transferase activity and the level of total cytochrome P450 between the CRR and CSS strains. Therefore, our studies suggested that carboxylesterase play an important role in beta-cypermethrin resistance in the CRR strain.     

Cross-resistance and biochemical mechanisms of abamectin resistance in the western flower thrips, Frankliniella occidentalis

Abamectin resistance was selected in the western flower thrips [Frankliniella occidentalis (Pergande)] under the laboratory conditions, and cross-resistance patterns and possible resistance mechanisms in the abamectin-resistant strain (ABA-R) were investigated. Compared with the susceptible strain (ABA-S), the ABA-R strain displayed 45.5-fold resistance to abamectin after 15 selection cycles during 18 generations. Rapid reversion of abamectin resistance was observed in the ABA-R strain in the absence of the insecticide selection pressure. Moderate and low levels of cross-resistance to chlorpyrifos (RR 11.4) and lambda-cyhalothrin (3.98) were observed in the ABA-R strain, but no significant cross-resistance was found to spinosad (2.00), acetamiprid (1.47) and chlorfenapyr (0.26). Our studies also showed that the esterase inhibitor S,S,S-tributyl phosphorotrithioate (DEF) and glutathione S-transferase inhibitor diethyl maleate (DEM) were not able to synergize the toxicity of abamectin, whereas the oxidase inhibitor piperonyl butoxide (PBO) conferred a significant synergism on abamectin in the ABA-R strain (SR 3.00). Biochemical analysis showed that cytochrome P450 monooxygenase activity of the ABA-R strain was 6.66-fold higher than that of the ABA-S strain. It appears that enhanced oxidative metabolism mediated by cytochrome P450 monooxygenases was a major mechanism for abamectin resistance in the western flower thrips.     

The mechanisms of carbaryl resistance in the fall armyworm,Spodoptera frugiperda (J. E. Smith)

The physiological mechanisms of resistance to carbaryl were investigated in a carbaryl-resistant strain of the fall armyworm, Spodoptera frugiperda (J. E. Smith). Piperonyl butoxide greatly reduced the resistance level from 90- to 6-fold, indicating that microsomal cytochrome P-450-dependent monooxygenases may play a major role in resistance. This finding is consistent with metabolic data in which the oxidative metabolism of carbaryl by midgut homogenates was five times more active in the resistant strain than in the susceptible strain. In addition, the resistant strain showed increased activities of microsomal hydroxylation and epoxidation compared to the susceptible strain. Cuticular penetration studies using [¹⁴C]carbaryl revealed that 55% of the applied radioactivity remained on the cuticle of resistant larvae while 32% remained on susceptible larvae 24 hr after topical treatment. The resistance appeared to be unrelated to target site insensitivity. It is concluded that the high level of resistance to carbaryl in this insect was mainly due to enhanced oxidative metabolism of the insecticide (via hydroxylation and epoxidation) with reduced cuticular penetration playing a very minor role, if any.