Metabolic mechanisms of insecticide resistance in the western flower thrips, Frankliniella occidentalis (Pergande)

The interactions between six insecticides (methiocarb, formetanate, acrinathrin, deltamethrin, methamidophos and endosulfan) and three potential synergists (piperonyl butoxide (PBO), S,S,S-tributyl phosphorotrithioate (DEF) and diethyl maleate (DEM)) were studied by topical exposure in strains selected for resistance to each insecticide, and in a susceptible strain of Frankliniella occidentalis (Pergande). In the susceptible strain PBO produced appreciable synergism only of formetanate, methiocarb and methamidophos. Except for endosulfan, PBO synergized all the insecticides to varying degrees in the resistant strains. A very high level of synergism by PBO was found with acrinathrin, which reduced the resistance level from 3344- to 36-fold. PBO slightly synergized the carbamates formetanate (4.6-fold) and methiocarb (3.3-fold). PBO also produced a high synergism of deltamethrin (12.5-fold) and methamidophos (14.3-fold) and completely restored susceptibility to both insecticides. DEF did not produce synergism with any insecticide in the resistant strains and DEM was slightly synergistic to endosulfan (3-fold). These studies indicate that an enhanced detoxification, mediated by cytochrome P-450 monooxygenases, is the major mechanism imparting resistance to different insecticides in F occidentalis. Implications of different mechanisms in insecticide resistance in F occidentalis are discussed.     

Various mechanisms responsible for permethrin metabolic resistance in seven field-collected strains of the German cockroach from Iran, Blattella germanica (L.) (Dictyoptera: Blattellidae)

Insecticide resistance in the German cockroach can be mediated by a number of mechanisms, the most common being enhanced enzymatic metabolism. Seven field-collected strains of German cockroach, Blattella germanica (L.) with various levels of resistance to pyrethroids, five out of which were also cross-resistant to DDT were used in this study. The investigation of possible mechanisms responsible for permethrin resistance was carried out using the synergists PBO, DEF and DMC and biochemical assays, including general esterases, glutathione S-transeferases and monooxygenases assays, using an automated microtitre plate reader. PBO and DEF, the inhibitors of cytochrome p450 monooxygenases and general esterases, respectively, affected permethrin resistance to varying degrees depending on the strain. DDT resistance in five strains were not completely eliminated by the synergist DMC, an inhibitor of glutathione S-transferase enzymes, suggesting that a further non-metabolic resistance mechanism such as kdr-type may be present. This suggestion was further supported by GST assay data, where a little elevation in GST activity was detected in only two strains. The synergist data supported by biochemical assays implicated that cytochrome p450 monooxygenases or hydrolases are involved in permethrin resistance in some strains. However, these results implicated both enhanced oxidative and hydrolytic metabolism of permethrin as resistance mechanism in the other strains. The results of synergist and biochemical studies implicated that all the field-collected permethrin resistant strains have developed diverse mechanisms of resistance, although these strains have been collected from the same geographic area. The change in resistance ratios of some strains by using PBO or DEF is discussed. It is of interest to note that because resistance to permethrin was not completely eliminated by DEF and PBO, it is likely that one or more additional mechanisms are involved in permethrin resistance in every strain studied.     

Synergism of insecticides provides evidence of metabolic mechanisms of resistance in the obliquebanded leafroller Choristoneura rosaceana (Lepidoptera: Tortricidae)

The interactions between six insecticides (indoxacarb, cypermethrin, chlorpyrifos, azinphosmethyl, tebufenozide and chlorfenapyr) and three potential synergists, (piperonyl butoxide (PBO), S,S,S-tributyl phosphorotrithioate (DEF) and diethyl maleate (DEM)) were studied by dietary exposure in a multi-resistant and a susceptible strain of the obliquebanded leafroller, Choristoneura rosaceana (Harris). The synergists did not produce appreciable synergism with most of the insecticides in the susceptible strain. Except for tebufenozide, PBO synergized all the insecticides to varying degrees in the resistant strain. A very high level of synergism by PBO was found with indoxacarb, which reduced the resistance level from 705- to 20-fold when PBO was administered alone and to around 10-fold when used in combination with DEF. DEF also synergized indoxacarb, cypermethrin, chlorpyrifos, azinphosmethyl and tebufenozide in the resistant strain. DEM produced synergism of indoxacarb, chlorpyrifos, azinphos-methyl and chlorfenapyr in the resistant strain. DEM was highly synergistic to cypermethrin, and to some extent to tebufenozide in both the susceptible and resistant strains equally, implying that detoxification by glutathione S-transferases was not a mechanism of resistance for these insecticides. The high level of synergism seen with DEM in the case of cypermethrin may be due to an increase in oxidative stress resulting from the removal of the antioxidant, glutathione. These studies indicate that enhanced detoxification, often mediated by cytochrome P-450 monooxygenases, but with probable esterase and glutathione S-transferase contributions in some cases, is the major mechanism imparting resistance to different insecticides in C. rosaceana.     

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.     

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.     

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

A field population of brown planthoppers (Nilaparvata lugens St?l) 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.     

抗阿维菌素小菜蛾的细胞色素P450酶系研究

通过对不同发育时期敏感和抗阿维菌素小菜蛾品系细胞色素P450含量的测定,以及使用不同模式底物对P450单加氧酶活性的比较研究发现:除成虫期外,不同发育时期抗性品系小菜蛾中P450和细胞色素b5的含量都高于敏感品系;抗性品系还原型辅酶Ⅱ(NADPH)-细胞色素P450还原酶活性是敏感品系的1.97倍;同时发现抗性品系中甲氧试卤灵-O-脱甲基酶(MROD)、乙氧试卤灵-O-脱乙基酶(EROD)、乙氧基香豆素-O-脱乙基酶(ECOD)以及对硝基苯甲醚-O-脱甲基酶(PNOD)的活性均明显高于敏感品系,分别为敏感品系的9.41、4.15、1.67和2.94倍。研究结果表明,细胞色素P450含量和单加氧酶活性的增高是小菜蛾对阿维菌素产生抗性的一个重要机制。     

Characterization of imidacloprid resistance in the housefly Musca domestica (Diptera: Muscidae)

In recent years, imidacloprid was introduced to control the housefly in China and it was documented that the housefly indeed showed signs of resistance to imidacloprid somewhere but not in China. Therefore, a housefly population collected from filed (IFS) was selected continuously with imidacloprid to establish the resistant strain (IRS) and the basic characteristics were investigated in this study. After continuous selection over 21 generations, the resistance ratio increased from 9.01 to 140, and different levels of cross-resistance were developed to beta-cypermethrin, chlorpyrifos, chlorfenapyr, acetamiprid and azamethiphos in the IRS strain. The realized heritability of resistance was 0.10. The synergistic ratios for IRS pretreated with DEF, DEM and PBO were 1.68, 1.52 and 2.53, and the corresponding ones for IFS were 3.17, 1.87 and 2.67, respectively. Synergistic and biochemical assays suggested that the cytochrome P450 may play an important role in the imidacloprid resistance comparing with GSTs- and carboxylesterases-mediated detoxification in the IRS strain, and there might be additional mechanisms (e.g. reduced target-site sensitivity) contributed to imidacloprid resistance in the IRS strain.     

Enhancement of thiazopyr bioefficacy by inhibitors of monooxygenases

The preceding paper described inhibition of thiazopyr metabolism in plant seedlings by inhibitors of cytochrome P450 monooxygenases, and the lack of inhibition by esterase inhibitors. We now describe greenhouse evaluation of the effects of these metabolic inhibitors on the bioefficacy of thiazopyr. Inhibitors of cytochrome P450 monooxygenases, piperonyl butoxide (PBO), 1-aminobenzotriazole (ABT), metyrapone (MET) and tetcyclacis (TET), all enhanced the bioefficacy of thiazopyr against pigweed and other plant species. In contrast, inhibitors of esterases, tributyl phosphate (TBP) and triphenyl phosphate (TPP), produced only slight enhancement of thiazopyr activity. The effect of PBO was dose-dependent and was demonstrated against barnyardgrass, grain sorghum, redroot pigweed, seedling johnsongrass, and giant foxtail. PBO demonstrated no enhancement of thiazopyr activity in velvetleaf, tall morningglory, cotton, or soybeans. Bioefficacy was most enhanced via exposure of seedling shoots to PBO and thiazopyr. The combination of results from the present and the preceding papers suggests that PBO enhances thiazopyr bioefficacy by effectively inhibiting thiazopyr metabolism in plants.     

Pyrethroid resistance and cross‐resistance in the German cockroach,Blattella germanica (L)

A German cockroach (Blatella germanica (L)) strain, Apyr‐R, was collected from Opelika, Alabama after control failures with pyrethroid insecticides. Levels of resistance to permethrin and deltamethrin in Apyr‐R (97‐ and 480‐fold, respectively, compared with a susceptible strain, ACY) were partially or mostly suppressed by piperonyl butoxide (PBO) and S,S,S,‐tributylphosphorotrithioate (DEF), suggesting that P450 monooxygenases and hydrolases are involved in resistance to these two pyrethroids in Apyr‐R. However, incomplete suppression of pyrethroid resistance with PBO and DEF implies that one or more additional mechanisms are involved in resistance. Injection, compared with topical application, resulted in 43‐ and 48‐fold increases in toxicity of permethrin in ACY and Apyr‐R, respectively. Similarly, injection increased the toxicity of deltamethrin 27‐fold in ACY and 28‐fold in Apyr‐R. These data indicate that cuticular penetration is one of the obstacles for the effectiveness of pyrethroids against German cockroaches. However, injection did not change the levels of resistance to either permethrin or deltamethrin, suggesting that a decrease in the rate of cuticular penetration may not play an important role in pyrethroid resistance in Apyr‐R. Apyr‐R showed cross‐resistance to imidacloprid, with a resistance ratio of 10. PBO treatment resulted in no significant change in the toxicity of imidacloprid, implying that P450 monooxygenase‐mediated detoxication is not the mechanism responsible for cross‐resistance. Apyr‐R showed no cross‐resistance to spinosad, although spinosad had relatively low toxicity to German cockroaches compared with other insecticides tested in this study. This result further confirmed that the mode of action of spinosad to insects is unique. Fipronil, a relatively new insecticide, was highly toxic to German cockroaches, and the multi‐resistance mechanisms in Apyr‐R did not confer significant cross‐resistance to this compound. Thus, we propose that fipronil could be a valuable tool in integrated resistance management of German cockroaches. © 2001 Society of Chemical Industry     

Temporal synergism can enhance carbamate and neonicotinoid insecticidal activity against resistant crop pests

BACKGROUND: Piperonyl butoxide (PBO) effectively synergises synthetic pyrethroids, rendering even very resistant insect pests susceptible, provided a temporal element is included between exposure to synergist and insecticide. This concept is now applied to carbamates and neonicotinoids. RESULTS: A microencapsulated formulation of PBO and pirimicarb reduced the resistance factor in a clone of Myzus persicae (Sulzer) from >19 000- to 100-fold and in Aphis gossypii (Glover) from >48 000- to 30-fold. Similar results were obtained for a strain of Bemisia tabaci Gennadius resistant to imidacloprid and acetamiprid, although a second resistant strain did not exhibit such a dramatic reduction, presumably owing to the presence of target-site insensitivity and the absence of metabolic resistance. Synergism was also observed in laboratory susceptible insects, suggesting that, even when detoxification is not enhanced, there is degradation of insecticides by the background enzymes. Use of an analogue of PBO, which inhibits esterases but has reduced potency against microsomal oxidases, suggests that acetamiprid resistance in whiteflies is largely oxidase based. CONCLUSION: Temporal synergism can effectively enhance the activity of carbamates and neonicotinoids against resistant insect pests. Although the extent of this enhancement is dependent upon the resistance mechanisms present, inhibition of background enzymes can confer increased sensitivity against target-site resistance as well as increased metabolism. .     

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.     

Cross‐resistance study and biochemical mechanisms of thiamethoxam resistance in B‐biotype Bemisia tabaci (Hemiptera: Aleyrodidae)

BACKGROUND: B‐biotype Bemisia tabaci (Gennadius) has invaded China over the past two decades. To understand the risks and to determine possible mechanisms of resistance to thiamethoxam in B. tabaci, a resistant strain was selected in the laboratory. Cross‐resistance and the biochemical mechanisms of thiamethoxam resistance were investigated in the present study. RESULTS: A 66.3‐fold thiamethoxam‐resistant B. tabaci strain (TH‐R) was established after selection for 36 generations. Compared with the susceptible strain (TH‐S), the selected TH‐R strain showed obvious cross‐resistance to imidacloprid (47.3‐fold), acetamiprid (35.8‐fold), nitenpyram (9.99‐fold), abamectin (5.33‐fold) and carbosulfan (4.43‐fold). No cross‐resistance to fipronil, chlorpyrifos or deltamethrin was seen. Piperonyl butoxide (PBO) and triphenyl phosphate (TPP) exhibited significant synergism on thiamethoxam effects in the TH‐R strain (3.14‐ and 2.37‐fold respectively). However, diethyl maleate (DEM) did not act synergistically with thiamethoxam. Biochemical assays showed that cytochrome P450 monooxygenase activities increased 1.21‐ and 1.68‐fold respectively, and carboxylesterase activity increased 2.96‐fold in the TH‐R strain. However, no difference was observed for glutathione S‐transferase between the two strains. CONCLUSION: B‐biotype B. tabaci develops resistance to thiamethoxam. Cytochrome P450 monooxygenase and carboxylesterase appear to be responsible for the resistance. Reasonable resistance management that avoids the use of cross‐resistance insecticides may delay the development of resistance to thiamethoxam in this species. Copyright © 2009 Society of Chemical Industry     

Synergism of teflubenzuron and chlorfluazuron in an acylurea-resistant field strain of Plutella xylostella L. (lepidoptera: Yponomeutidae)

Topical application of the synergists piperonyl butoxide (PB) and S,S,S-tributyl phosphorotrithioate (DEF) to second-instar larvae of a standard laboratory strain (FS) and an unselected Malaysian field strain (CH) of the diamondback moth Plutella xylostella had no significant effect on the toxicity of the acylurea insecticides, chlorfluazuron and teflubenzuron, in a subsequent leafdip bioassay. In contrast, pre-treatment with PB or DEF in acylurea-selected subpopulations of the CH strain with varying levels of cross-resistance to chlorfluazuron and teflubenzuron significantly increased (up to 34-fold and 28-fold, respectively) the toxicity of both compounds, suggesting that microsomal monooxygenases and esterases may be involved in resistance. The addition of a mineral oil, ‘Sunspray 6E’, to topically-applied chlorfluazuron consistently reduced its LD₅₀ value, and the effect of the oil appeared to be greatest on the most resistant population of P. xylostella. However, the effects of the oil were not significant (P > 0·05) and further studies are necessary to determine whether a penetration factor is present in the CH strain.     

西花蓟马抗甲氨基阿维菌素苯甲酸盐种群的交互抗性与生化抗性机制

为了解西花蓟马Frankliniella occidentalis(Pergande)对甲氨基阿维菌素苯甲酸盐的抗性风险,采用生物和生化测定方法研究了西花蓟马甲维盐抗性种群与其它杀虫剂的交互抗性和生化抗性机制。西花蓟马甲维盐抗性种群对阿维菌素有高水平交互抗性,抗性倍数为31.656,对啶虫脒有中等水平交互抗性,为12.182,对吡虫啉、溴虫腈、氯氟氰菊酯、毒死蜱和灭多威有低水平交互抗性,为5.517~8.568,而对多杀菌素无明显交互抗性。增效剂胡椒基丁醚(PBO)、马来酸二乙酯(DEM)、三丁基三硫磷酸酯(DEF)和磷酸三苯酯(TPP)对甲维盐抗性种群和田间种群均有显著增效作用。甲维盐抗性种群多功能氧化酶细胞色素P₄₅₀和b₅含量、O-脱甲基酶、谷胱甘肽S-转移酶和羧酸酯酶活性均显著提高,分别为敏感种群的3.89、3.61、5.32、4.42和1.30倍,表明多功能氧化酶、谷胱甘肽S-转移酶和羧酸酯酶等解毒代谢酶活性的提高是西花蓟马对甲维盐产生抗性的重要机制。     

The effect of pretreatment with S,S,S-tributyl phosphorotrithioate on deltamethrin resistance and carboxylesterase activity in Aphis gossypii (Glover) (Homoptera: Aphididae)

The synergism of S,S,S-tributyl phosphorotrithioate (DEF) and its effect on carboxylesterase activity were investigated in deltamethrin-selected resistant (DRR) and susceptible (DSS) strains of cotton aphids, Aphis gossypii (Glover). Compared to the DSS strain, the DRR strain showed 23,900-fold resistance to deltamethrin, and 7560- and 99-fold cross-resistance to bifenthrin and ethofenprox, respectively. The synergist, DEF, increased the toxicity of both deltamethrin and bifenthrin, but not of ethofenprox when DEF was pretreated of 15 h. DEF exhibited significant inhibition on the carboxylesterase activity in the DRR strain, but no significant effect on that of the DSS strain in vitro. After the cotton aphids exposing to DEF, the carboxylesterase activity decreased gradually until 15 h and then gradually recovered until 24 h in the DRR strain, which fluctuated according to the effect of DEF on the deltamethrin toxicity detected using DEF pretreatment in the DRR strain. Therefore, our studies suggested that the effect of DEF on carboxylesterase was associated with deltamethrin resistance in the DRR strain.     

Esterase inhibition by synergists in the western flower thrips Frankliniella occidentalis

BACKGROUND: Western flower thrips (WFT), Frankliniella occidentalis (Pergande), is among the most important crop pests in the south‐eastern region of Spain. Its increasing resistance to insecticides constitutes a serious problem, and understanding the mechanisms involved is therefore of great interest. Use of synergists to inhibit the enzymes involved in insecticide detoxification is widely used to determine their responsibility for insecticide resistance. However, they do not always act as intended or expected, and caution must be exercised when interpreting synergist results. RESULTS: Laboratory‐selected strains of WFT were used to analyse the effects of the synergists piperonyl butoxide (PBO), S,S,S‐tributyl phosphorotrithioate (DEF) and methiocarb on total esterase activity. Significant differences were found, indicating esterase activity inhibition by DEF, a lower effect for methiocarb and a small inhibition of the activity by PBO. Esterase isoenzyme inhibition by these compounds showed a similar result; this assay revealed an extreme sensitivity of Triplet A (resistance‐associated esterases) to DEF. In an in vivo assay carried out with these compounds at different incubation times, only DEF caused posterior in vitro esterase activity inhibition, with a maximum effect 1 h after treatment. CONCLUSION: In this work, only DEF shows true synergistic inhibition of WFT esterases. Copyright © 2011 Society of Chemical Industry     

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     

Resistance in the mosquito, Culex quinquefasciatus, and possible mechanisms for resistance

Two mosquito strains of Culex quinquefasciatus (Say), MAmCq(G0) and HAmCq(G0), were collected from Mobile and Huntsville, Alabama, respectively. MAmCq(G0) and HAmCq(G0) were further selected in the laboratory with permethrin for one and three generations, respectively. The levels of resistance to permethrin in MAmCq(G1) (after one-generation selection) and HAmCq(G3) (after three-generation selection) increased rapidly. Resistance to permethrin in MAmCq(G1) and HAmCq(G3) was partially suppressed by piperonyl butoxide (PBO), S,S,S-tributylphosphorotrithioate (DEF) and diethyl maleate (DEM), inhibitors of cytochrome P450 monooxygenases, hydrolases and glutathione S-transferases (GST), respectively, suggesting these three enzyme families are important in conferring permethrin resistance in both strains. A substitution of leucine to phenylalanine (L to F) resulting from a single nucleotide polymorphism (SNP), termed the kdr mutation, in the para-homologous sodium channel gene has been reported as a very common mutation associated with pyrethroid resistance of insects. A 341-bp sodium channel gene fragment, where the kdr mutation resides, was generated by PCR from genomic DNAs of Cx. quinquefasciatus strains. We found that the kdr mutation was present in both permethrin-selected and unselected HAmCq and MAmCq mosquito populations, suggesting that the kdr mutation plays the role in permethrin resistance. There was no significant change in the frequency and heterozygosity of the A to T SNP for the kdr allele between permethrin-selected and unselected MAmCq and HAmCq mosquitoes, indicating that other mechanisms are involved in the evolution of resistance in mosquitoes selected by permethrin in the laboratory.     

Esterase-mediated bifenthrin resistance in a multiresistant strain of the two-spotted spider mite, Tetranychus urticae

A field-collected multiresistant strain of Tetranychus urticae Koch exhibiting high resistance to bifenthrin was investigated in comparison with a susceptible laboratory strain. The esterase inhibitor S,S,S-tributyl-phosphorotrithioate (DEF) was able strongly to synergise bifenthrin toxicity in the resistant strain. Optimal conditions for determining esterase activities in T. urticae were determined, and a higher esterase activity towards several artificial substrates was found in this resistant strain, which had a preference for hydrolysing 4-nitrophenyl butyrate. Bifenthrin was able to bind the active centres of T. urticae esterases in vitro, as was determined after competition experiments by a Dixon plot, revealing a higher affinity of bifenthrin in the resistant strain. Bifenthrin-hydrolysing activity in the resistant and susceptible strains was examined in vitro and quantified with gas chromatography. A 7.2-fold higher metabolising rate was found in the resistant strain.