Genetic and biochemical studies of resistance to permethrin in a pyrethroid-resistant strain of the housefly (Musca domestica L.)

One or more weak factors of resistance on autosome 2, and barely detectable resistance on autosome 3, confer moderate resistance to several pyrethroids (5–13-fold) in the field-collected Ipswich strain of houseflies. In these flies, which unlike other pyrethroid-resistant strains lack kdr or super-kdr, pyrethroid resistance probably developed in response to prolonged treatment of buildings for animals with pyrethrins synergised with piperonyl butoxide. Substrains, isolated genetically from Ipswich flies and with resistance only on autosome 2, degraded permethrin more rapidly than susceptible flies and produced larger amounts of very polar metabolites. In this, they differed from flies with kdr or super-kdr which resembled susceptible flies in their metabolism of permethrin. NIA 16388 (propyl prop-2-ynyl phenylphosphonate) was a better synergist and reduced the metabolism of permethrin more than piperonyl butoxide in both the susceptible and resistant insects. The slight increase in synergism and minimal decrease in metabolism when piperonyl butoxide was applied with NIA 16388 indicated that the latter also inhibited detoxication that was sensitive to piperonyl butoxide.     

Phenobarbital induction of permethrin detoxification and phenobarbital metabolism in susceptible and resistant strains of the beet armyworm Spodoptera exigua (Hübner)

The permethrin resistant strain (TR-strain) of the beet armyworm, Spodoptera exigua (Hübner), has 92.5-fold resistance to permethrin (at LD₅₀ level) compared to the permethrin susceptible strain (TS-strain). Bioassay involving permethrin mixed with piperonyl butoxide, an inhibitor of microsomal cytochrome P450s, significantly reduced the resistance ratio from 92.5- to 7.9-fold. However, S,S,S-tributylphosphorotrithioate and diethylmaleate which are inhibitors of esterases and glutathione S-transferase, respectively, did not affect the resistance level. These results indicate that the detoxification of permethrin in the TR-strain was primarily due to the cytochrome P450 monooxygenases. LD₅₀ for permethrin was increased to 4.5-fold by the pre-treatment of phenobarbital in the TS-strain. The effect of induction by phenobarbital was almost completely overcome by the piperonyl butoxide treatment. However, it was observed that phenobarbital treatment did not cause any change in the toxicity of permethrin to TR strain. Since this result deviated from the expectation that the metabolism of phenobarbital in the TR-strain should be greater than that in the TS-strain, it was deemed necessary to compare the metabolism of phenobarbital between the TS- and TR-strains. Comparison was made based on the concentration of phenobarbital in the hemolymph and whole body. The results showed no significant difference in phenobarbital treatment between the two strains used in this study suggesting the possibility that the induction system in TS-strain is different from the TR-strain.     

Biochemical mechanisms of methoxyfenozide resistance in the cotton leafworm Spodoptera littoralis

BACKGROUND: Methoxyfenozide is a lepidopteran‐specific insecticide that belongs to a new group of insecticides, the non‐steroidal ecdysteroid agonists, also called moulting accelerating compounds (MACs). To investigate the risk of resistance and possible mechanisms conferring resistance to methoxyfenozide, the authors selected in the laboratory for a resistant strain of the cotton leafworm Spodoptera littoralis (Boisd.), which is a representative lepidopteran model and an important pest in cotton and vegetables worldwide, with a high risk for resistance development. RESULTS: After selection with methoxyfenozide during 13 generations, toxicity data showed that the selected strain developed fivefold resistance to methoxyfenozide in comparison with the susceptible strain. Measurement of the detoxification enzymes demonstrated that the monooxygenase (MO) activity was 2.1 times higher in the selected strain, whereas there was no change for esterases and glutathione‐S‐transferases. When the inhibitors piperonyl butoxide (PBO), S,S,S‐tributyl phosphorotrithioate (DEF) and diethyl maleate were tested as synergists, the respective synergistic ratios were 0.97, 0.96 and 1.0 for the susceptible strain, and 2.2, 0.96 and 1.1 for the resistant strain. The significant synergistic effect by PBO concurs with the increased MO activity in the selected strain. CONCLUSION: Taken overall, the present study supports the importance of MO‐mediated metabolism in resistance to methoxyfenozide, directing tactics to fight against resistance development for this novel group of insecticides. Copyright © 2009 Society of Chemical Industry     

The cross-resistance to pyrethrins and eight synthetic pyrethroids,of an organophosphorus-resistant strain of the rust-red flour beetle Tribolium castaneum (herbst)

Comparisons with standard susceptible insects showed that a strain of Tribolium castaneum, with a specific resistance to malathion and its carboxylic ester analogues, had no cross-resistance to topical applications of natural pyrethrins. Another strain of T. castaneum, showing resistance to many organophosphorus (OP) insecticides, was cross-resistant to pyrethrins ( × 34) and eight synthetic pyrethroids also applied topically; least cross-resistance occurred with resmethrin ( × 2.2), bioresmethrin ( × 3.3) and phenothrin ( × 4.0). Generally larger resistance factors were recorded with formulations synergised by piperonyl butoxide (PB). The greatest cross-resistance encountered was with unsynergised tetramethrin ( × 338). Apart from tetramethrin, factors of synergism did not exceed 5.7 with either the susceptible or multi-OP resistant strains. PB antagonised six of the nine pyrethroids against the multi-OP resistant strain. Antagonism also occurred with two of these six, permethrin (cis: trans ratio 1:3) and 5-prop-2-ynylfurfuryl ( 1RS)-cis,trans-chrysanthemate (‘Prothrin’), against the susceptible strain. Considering only formulations without the synergist, the most effective compounds against the susceptible strain, relative to pyrethrins, were bioresmethrin (2.7) and permethrin (2.4). Similarly with the multi-OP resistant strain the most effective compounds were bioresmethrin (28), resmethrin (14) and permethrin (6.6). Thus the LD₅₀ (the dose required to kill 50% of the test species) for bioresmethrin against the resistant strain (0.14 μg) only slightly exceeded the LD₅₀ for pyrethrins against the susceptible strain (0.12 μg).     

Genetic and biochemical mechanisms limiting fipronil toxicity in the LPR strain of house fly,Musca domestica

Fipronil is a new insecticide which exerts its toxic action by interacting with the insect GABA-gated chloride channel. Previous studies have shown that cyclodiene-resistant insects have low to moderate levels of cross-resistance to fipronil, while other resistant strains are usually susceptible. In contrast, we recently found a strain (LPR) of house fly (Musca domestica L) with 15-fold cross-resistance to fipronil that was not associated with cyclodiene resistance. Fipronil cross-resistance in LPR was inherited as an intermediately dominant, autosomal, multigenic trait. [¹⁴C]Fipronil was observed to penetrate into LPR flies more slowly than into susceptible flies. S,S,S-tributylphosphorotrithioate and diethyl maleate pretreatment did not reduce the level of fipronil cross-resistance, while piperonyl butoxide resulted in a slight decrease. These results indicate that decreased penetration and monooxygenase-mediated detoxification may be mechanisms contributing to fipronil cross-resistance in the LPR strain. © 1999 Society of Chemical Industry     

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.     

A comparison of permethrin and methomyl insecticides for the control of a multiresistant strain of houseflies (Musca domestica)

A methomyl sugar bait formulation and permethrin residual spray were compared for the control of a multi-insecticide resistant strain of housefly in a UK pig farm. The methomyl was applied as a granular scatter bait at the manufacturer's recommended rate of 25 mg m^?2 active ingredient (a.i.) to the treated floor area. Permethrin was applied at 32, 64 and 128 mg m^?2 a.i. to structural surfaces. The highest deposit rate of permethrin used was four times that recommended by the manufacturer for the control of flying insects. The methomyl bait gave effective control but the permethrin spray failed at all deposit rates tested. The use of permethrin increased resistance to this compound at the KD50 level from x 13 to x 560 within 10 weeks and significantly increased the proportion of flies resistant to natural pyrethrins synergised with piperonyl butoxide (P<0.01).     

Resistance and cross-resistance to insecticides in human head lice from Florida and California

Head lice from Florida (SF-HL) and California (SC-HL) were resistant to permethrin compared with colonized susceptible lice from Panama (PA-HL) (5.5- and 3.4-fold, respectively) and Ecuador (EC-HL) (8.5- and 5.3-fold, respectively). Permethrin-resistant lice were cross-resistant to pyrethrum and DDT. DNA sequencing validated presence of kdr-type mutations (T929I and L932F). Permethrin resistance was synergized by piperonyl butoxide (PBO) in SC-HL. Resistance to malathion in SF-HL (1.4–2.2-fold) and SC-HL (2.1–3.6-fold) was detected. Malathion resistance in SF-HL was synergized by S,S,S-tributylphosphorotrithioate (DEF) and by PBO in SC-HL. Malathion/permethrin-resistant lice from the UK (BR-HL) were synergized by DEF but not synerziged by PBO. PBO protected BR-HL from malathion, indicating suppressed desulfuration. Abamectin resistance in SF-HL (1.7–2.5-fold) and SC-HL (1.8–2.3-fold) was detected. No resistance to lindane was found. Thus, multiple resistance mechanisms against commonly available and widely used pediculicides and insecticides are apparently occurring.     

Toxicity of spinosad to susceptible and resistant strains of house flies,Musca domestica

The toxicity of spinosad, a new insecticide derived from the bacterium Saccharopolyspora spinosa, was evaluated against susceptible and resistant strains of house fly (Musca domestica L.). Spinosad was highly toxic to house flies based on 72-h LD₅₀ values and the symptoms of poisoning were consistent with a neurotoxic mechanism of action. Spinosad was relatively slow acting, with the maximum toxicity noted at 72 h. Piperonyl butoxide and S,S,S,-tribu-tylphosphorotrithioate synergized the toxicity of spinosad by 3·0- and 1·8-fold, respectively, while diethyl maleate had no significant effect. These results suggest that there is a small degree of monooxygenase-mediated spinosad detoxification in house flies, while hydrolases may be only minimally important and glutathione transferases may have no role. There were no substantial levels of cross-resistance detected, except in the LPR strain where a low 4·3-fold cross-resistance was observed. The cyclodiene-resistant OCR strain was 2·7-fold more sensitive to spinosad than the susceptible strain (CS). These results suggest that cross-resistance may not be a limiting factor for the use of spinosad against house flies. © 1998 Society of Chemical Industry     

Pyrethroid tolerance in Culex pipiens pipiens var molestus from Marin County, California

In May 2001 a sample of Culex pipiens pipiens variety molestus Forskål from Marin County, California, collected as larvae and reared to adults, was found to show reduced resmethrin and permethrin knock‐down responses in bottle bioassays relative to a standard susceptible Cx pipiens quinquefasciatus Say colony (CQ1). Larval susceptibility tests, using CQ1 as standard susceptible, indicated that the Marin mosquitoes had LC₅₀ resistance ratios of 18.3 for permethrin, 12 for deltamethrin and 3.3 for pyrethrum. A colony of Marin was established and rapidly developed higher levels of resistance in a few generations after exposure to permethrin as larvae. These selected larvae were shown to cross‐resist to lambda‐cyhalothrin as well as to DDT. However, adult knock‐down time in the presence of permethrin, resmethrin and pyrethrum was not increased after increase in tolerance to pyrethroids as larvae. Partial and almost complete reversion to susceptibility as larvae was achieved with S, S, S‐tributylphosphorotrithioate and piperonyl butoxide (PBO), respectively, suggesting the presence of carboxylesterase and P450 monooxygenase mediated resistance. Insensitive target site resistance (kdr) was also detected in some Marin mosquitoes by use of an existing PCR‐based diagnostic assay designed for Cx p pipiens L mosquitoes. Carboxylesterase mediated resistance was supported by use of newly synthesized novel pyrethroid‐selective substrates in activity assays. Bottle bioassays gave underestimates of the levels of tolerance to pyrethroids of Marin mosquitoes when compared with mortality rates in field trials using registered pyrethroid adulticides with and without PBO. This study represents the first report of resistance to pyrethroids in a feral population of a mosquito species in the USA. Copyright © 2003 Society of Chemical Industry     

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.     

Organophosphorothioates and synergised synthetic pyrethroids as grain protectants on bulk wheat

Organophosphorothioates and synergised synthetic pyrethroids were used in duplicate field trials carried out on bulk wheat in commercial silos in Queensland and New South Wales. Laboratory bioassays using malathion-resistant strains of insects were carried out on samples of treated grain at intervals over 9 months. These established that all treatments were generally effective. Deltamethrin (2 mg kg^?1)+ piperonyl butoxide (8 mg kg^?1), fenitrothion (12 mg kg^?1)+ fenvalerate (1 mg kg^?1)+ piperonyl butoxide (8 mg kg^?1), fenitrothion (12 mg kg^?1)+ phenothrin (2 mg kg^?1)+ piperonyl butoxide (8 mg kg^?1) and pirimiphos-methyl (4 mg kg^?1)+ permethrin (1 mg kg^?1)+ piperonyl butoxide (8 mg kg^?1) controlled common field strains of Sitophilus oryzae (L.) and Rhyzopertha dominica (F.). Against a highly resistant strain of S. oryzae, deltamethrin (2 mg kg^?1)+ piperonyl butoxide (8 mg kg^?1) was superior to the remaining treatments. All treatment combinations completely prevented progeny production in Tribolium castaneum (Herbst), T. confusum Jacquelin du Val and in Ephestia cautella (Walker). Residues of deltamethrin, fenvalerate, permethrin and phenothrin were determined and shown to be highly persistent on stored wheat. During milling, residues accumulated in the bran fractions and were reduced in white flour. They were not significantly reduced during baking.     

Indoxacarb resistance in the house fly, Musca domestica

Indoxacarb (DPX-MP062) is a recently introduced oxadiazine insecticide with activity against a wide range of pests, including house flies. It is metabolically decarbomethoxylated to DCJW. Selection of field collected house flies with indoxacarb produced a New York indoxacarb-resistant (NYINDR) strain with >118-fold resistance after three generations. Resistance in NYINDR could be partially overcome with the P450 inhibitor piperonyl butoxide (PBO), but the synergists diethyl maleate and S,S,S-tributyl phosphorothioate did not alter expression of the resistance, suggesting P450 monooxygenases, but not esterases or glutathione S-transferases are involved in the indoxacarb resistance. Conversely, the NYINDR strain showed only 3.2-fold resistance to DCJW, and this resistance could be suppressed with PBO. Only limited levels of cross-resistance were detected to pyrethroid, organophosphate, carbamate or chlorinated hydrocarbon insecticides in NYINDR. Indoxacarb resistance in the NYINDR strain was inherited primarily as a completely recessive trait. Analysis of the phenotypes vs. mortality data revealed that the major factor for indoxacarb resistance is located on autosome 4 with a minor factor on autosome 3. It appears these genes have not previously been associated with insecticide resistance.     

Development of resistance to pyrethroids in insects resistant to other insecticides

Resistance to pyrethroids in insects is rare, but its recent rapid development in the field suggests that this resistance may be facilitated by previous exposure to or by resistance to insecticides of unrelated groups. To test this houseflies of strain 49_r2b, originally resistant to dimethoate in the field, were selected eight times during ten generations with either pyrethrum extract or bioresmethrin with or without piperonyl butoxide or with dimethoate. Selecting with any of the pyrethroids led to resistance to these insecticides and in particular to pyrethrum/piperonyl butoxide. Selecting with pyrethrum/piperonyl butoxide resulted in strongest resistance to the pyrethroids tested, whereas selecting with bioresmethrin/piperonyl butoxide resulted in least resistance. These results show that dimethoate-resistant flies selected with pyrethroids can readily develop resistance to these insecticides, but development of resistance can be minimised by using bioresmethrin/piperonyl butoxide. The implications of these findings on the sequential use of insecticides are discussed.     

Absence of enhanced detoxication of permethrin in pyrethroid-resistant house flies

The mechanisms of resistance to pyrethroids were studied in a permethrin-selected (147-R) strain of the house fly, Musca domestica L. Approximately 12-fold synergism was obtained with a mixture of (1R)-trans-permethrin:piperonyl butoxide (1:5) so that the resistance decreased from 97-fold to 22-fold. Tests with the esterase inhibitor S,S,S-tributyl phosphorotrithioate produced very little synergism in either the resistant (R) strain (1.6-fold) or the susceptible (S) strain (1.9-fold). An investigation of the microsomal components revealed that compared to the S strain, the R strain demonstrated twice as much cytochrome P-450 and cytochrome b₅ and double the rate of NADPH-cytochrome c reductase activity. In addition, the rate of p-nitroanisole O-demethylation was found to be six times greater in the R strain. An in vivo accumulation study showed that the R strain displayed a decreased rate of penetration of trans-[¹⁴C]permethrin. When treated at equitoxic doses the R strain was found to tolerate 50-fold more internal permethrin than the S strain. An in vitro metabolism study indicated that there was no difference between strains in the overall rate of metabolism of trans-[¹⁴C]permethrin. The evidence obtained supports the conclusion that several resistance factors are involved but that decreased sensitivity of the nervous system to the action of pyrethroids is the principal mechanism of resistance in the 147-R strain.     

Identification of mutations in the para sodium channel of Bemisia tabaci from Crete, associated with resistance to pyrethroids

We investigated the mechanisms of resistance to α-cypermethrin in a Q biotype, highly resistant Bemisia tabaci strain (GRMAL-RP) isolated from Crete. Cytochrome P450-dependent monoxygenase activity with the substrate ethoxycoumarin, and carboxylesterase activity with the substrates α-naphthyl-acetate, β-naphthyl-acetate, and para-nitrophenol acetate were substantially elevated in the GRMAL-RP, compared to the susceptible SUD-S strain, while glutathione-S-transferase activity with the substrate 1-chloro-2,4-dinitrobenzene was not different. The metabolic inhibitors piperonyl butoxide and S,S,S-tributyl phosphorotrithioate synergised cypermethrin toxicity in the GRMAL-RP strain, however, mortality was still lower than that of the susceptible strain, indicating the presence of an additional resistance mechanism. Analysis of the sequence of the IIS4-IIS6 region of the para sodium channel gene of the GRMAL-RP strain revealed two amino acid replacements compared to that of the SUD-S susceptible strain. One is the leucine to isoleucine substitution at position 925 (L925I) previously implicated in B. tabaci pyrethroid resistance and the other is a novel kdr resistant mutation for B. tabaci, a threonine to valine substitution at position 929 (T929V). Genotype analysis showed that the L925I and T929V were present in all GRMAL-RP males tested, at an approximately 1:1 frequency, but never in combination in the same haplotype.     

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.     

Potentiation of Super-kdr resistance to deltamethrin and other pyrethroids by an intensifier (factor 161) on autosome 2 in the housefly (Musca domestica L.)

An intensifier (factor 161) identified on the second autosome in a pyrethroid-resistant strain of houseflies (Musca domestica L.) was isolated and introduced into a strain with super-kdr. Unlike E_0.39, which on its own also confers very weak (< × 3) resistance to pyrethroids, factor 161 very strongly intensified super-kdr resistance to pyrethroids. Together, factor 161 and super-kdr conferred immunity to deltamethrin in female houseflies (LD₅₀ > 20 μg fly^?1) but produced much less intensification of resistance to WL 48281, the (1R)cis (αS) isomer of cypermethrin, which differs from deltamethrin only in having chlorine instead of bromine substituents in the acid side-chain. Intensification was strongly decreased by piperonyl butoxide and propyl prop-2-ynylphenylphosphonate (NIA) but was unaffected by S,S,S-tributyl phosphorotrithioate (DEF). This synergism suggests involvement of oxidative rather than esteratic metabolism in the intensification of super-kdr by factor 161.     

An Example of Cross-resistance to Pyrethroids in DDT-resistant Aedes aegypti

Comparisons of the susceptibility of several strains of adult Aedes aegypti were made. Mosquitoes from Bangkok and Jakarta were found to be highly resistant to DDT and resistant to pyrethroids relative to a laboratory strain. A strain from Singapore, where less DDT has been used, was susceptible to DDT and pyrethroids. Two strains from the Caribbean had LC₅₀ values to DDT 3 times that of the reference strain while the LC₅₀ values against bioresmethrin synergised with piperonyl butoxide were 1 1/2 times raised. Another two strains from central Africa were 2 times tolerant of DDT and 1 1/2 times tolerant of bioresmethrin plus piperonyl butoxide. Agents which block DDT-dehydrochlorinase, esterases and oxidases each caused small increases in the mortality of the Bangkok strain due to DDT and bioresmethrin as well as augmenting toxicity to the susceptible reference strain. It is tentatively suggested that resistance in the Bangkok strain is due to a combination of the actions of these and perhaps other resistance mechanisms.     

Molecular analysis of resistance in a deltamethrin-resistant strain of Musca domestica from China

We investigated the molecular basis of resistance in a strain of house fly (BJD) from Beijing, China. This strain showed 567-fold resistance to commonly used deltamethrin. Flies were 64-fold resistant to deltamethrin synergized by piperonyl butoxide (PBO). The 5′-flanking sequence of the cytochrome P450 gene CYP6D1 in BJD strain had a 15-bp insert as in the LPR strain. Two mutations (kdr, super-kdr) in the voltage sensitive sodium channel (VSSC) were also detected in the BJD strain. Our results showed that a combination of resistance alleles for CYP6D1 and VSSC existed in deltamethrin resistant house flies in China.