Insecticide resistance and cross-resistance in the house fly (Diptera: Muscidae)

A house fly strain, ALHF, was collected from a poultry farm in Alabama after a control failure with permethrin, and further selected in the laboratory with permethrin for five generations. The level of resistance to permethrin in ALHF was increased rapidly from an initial 260-fold to 1,800-fold after selection. Incomplete suppression of permethrin resistance by piperonyl butoxide (PBO) and S,S,S,-tributylphosphorotrithioate (DEF) reveals that P450 monooxygenase- and hydrolase-mediated detoxication, and one or more additional mechanisms are involved in resistance to permethrin. The ALHF strain showed a great ability to develop resistance or cross-resistance to different insecticides within and outside the pyrethroid group including some relatively new insecticides. Resistance to beta-cypermethrin, cypermethrin, deltamethrin, and propoxur (2,400-4,200-, 10,000-, and > 290-fold, respectively, compared with a susceptible strain, aabys) in ALHF house flies was partially or mostly suppressed by PBO and DEF, indicating that P450 monooxygenases and hydrolases are involved in resistance to these insecticides. Partial reduction in resistance with PBO and DEF implies that multiresistance mechanisms are responsible for resistance. Fifteen- and more than fourfold resistance and cross-resistance to chlorpyrifos and imidacloprid, respectively, were not effected by PBO or DEF, indicating that P450 monooxygenases and hydrolases are not involved in resistance to these two insecticides. Forty-nine-fold cross-resistance to fipronil was mostly suppressed by PBO and DEF, revealing that monooxygenases are a major mechanism of cross-resistance to fipronil. Multiresistance mechanisms in the ALHF house fly strain, however, do not confer cross-resistance to spinosad, a novel insecticide derived from the bacterium Saccharopolyspora spinosa. Thus, we propose that spinosad be used as a potential insecticide against house fly pests, especially resistant flies.     

Studies on the resistance to various insecticides of a house fly strain (Diptera: Muscidae) selected with azamethiphos.

A colony of azamethiphos-resistant house flies, Musca domestica (L.), was obtained from Denmark and further selected in the laboratory with azamethiphos for four generations. LD50s for various insecticides were determined and compared with those of a susceptible house fly strain. The selected flies showed cross-resistance to all insecticides evaluated. The flies were highly resistant to most organophosphorus, carbamate, and chlorinated hydrocarbon insecticides except prothiophos, p,p'-DDT, and the pyrethroids. We conclude that the main mechanisms responsible for resistance are presumed to be factors other than acetylcholinesterase sensitivity and nerve sensitivity due to knockdown resistance.     

Susceptibility of spinosad in Musca domestica (Diptera: Muscidae) field populations

The toxicity of spinosad was determined in one susceptible and five insecticide-resistant laboratory strains of house fly, Musca domestica L. Spinosad was relatively slow-acting, but highly toxic to house flies. In a feeding bioassay, spinosad LC50 at 72 h was 0.51 microg of spinosad per gram of sugar, making it 6.3- and 3.5-fold more toxic to house flies compared with azamethiphos and methomyl, respectively. In topical application bioassay, the LD50 at 48 h of spinosad in susceptible house flies was 40 ng per 20 mg of house fly, making spinosad less toxic than the pyrethroid bioresmethrin synergized by piperonyl butoxide and the organophosphate dimethoate. The insecticide-resistant laboratory strains had resistance factors to spinosad at LC50 in feeding bioassay from 1.5 to 5.5 and at LD50 in topical application bioassay from 2.5 to 4.7, indicating that in house fly cross-resistance to the major insecticide classes will not initially be of major concern for the use of spinosad for house fly control. The toxicity of spinosad was also evaluated against 31 field populations of house flies collected from livestock farms across Denmark. The field populations were 2.2- to 7.5-fold resistant to spinosad at 72 h in feeding bioassay, but based on steep slopes in the bioassay and the limited variation of spinosad toxicity against the various field populations, we consider the field populations to be spinosad-susceptible. We propose a diagnostic dose of 12 microg of spinosad per gram of sugar in feeding bioassay with impregnated sugar for determination of resistant house flies, which is 10x the LC95 of the susceptible strain WHO and approximately = 2x the LD95 of the field populations. Spinosad showed no substantial cross-resistance to the pyrethroid bioresmethrin synergized by piperonyl butoxide, the anticholinesterases dimethoate, azamethiphos, methomyl, and spinosad in house fly field populations.     

二斑叶螨对甲氰菊酯的抗性选育及解毒酶活力变化

为了明确二斑叶螨Tetranychus urticae Koch对甲氰菊酯产生抗性的机理,在室内模拟田间药剂的选择压力, 用甲氰菊酯对二斑叶螨敏感品系（S）进行逐代汰选,选育至38代时, 获得了抗性倍数( resistance ratio, RR)为247.35的抗甲氰菊酯品系（Fe-R）。对S和Fe-R解毒酶活性的分析表明,Fe-R₃₈体内羧酸酯酶（carboxylesterase, CarE）、酸性磷酸酯酶(acid phosphatase, ACP)、 碱性磷酸酯酶(alkaline phosphatase, ALP)、谷胱甘肽-S-转移酶(glutathione s-transferase, GSTs)和多功能氧化酶(mixed function oxidase, MFO)较S体内相应酶的活力显著升高（P< 0.05）,其相对比值（R/S）分别为1.822,13.941,3.789,4.262和17.386。此外,筛选至第9,19,25,32代时,除Fe-R₂₅和Fe-R₃₂的MFO活性与S相比有显著性差异（P< 0.05）外,其余解毒酶（CarE,ACP,ALP,GSTs）的活性与S相比均无显著性差异(P>0.05)。筛选至第38代时, 5种解毒酶的活力与S相比均差异显著(P<0.05)。结果说明二斑叶螨Fe-R随着筛选代数的增加（第25代后）,MFO活性的上升可能是二斑叶螨对甲氰菊酯产生抗性的主要原因。     

Dynamics of resistance to the neonicotinoids acetamiprid and thiamethoxam in Bemisia tabaci (Homoptera: Aleyrodidae)

The dynamics of resistance in the sweetpotato whitefly, Bemisia tabaci (Gennadius), to the neonicotinoids acetamiprid and thiamethoxam was studied extensively in cotton fields in Israel during the cotton-growing seasons 1999-2003. Whitefly strains were collected in early and late seasons mainly in three locations in northern, central, and southern Israel. The whiteflies were assayed under laboratory conditions for susceptibility to neonicotinoids, as part of the Israeli cotton insecticide resistance management strategy. Selections to both acetamiprid and thiamethoxam and cross-resistance between them also were conducted in the laboratory. Although no appreciable resistance to acetamiprid was observed up to 2001, a slight increase of approximately five-fold resistance was detected during 2002 and 2003. However, from 2001 to 2003 thiamethoxam resistance increased >100-fold in the Ayalon Valley and Carmel Coast cotton fields. In cross-resistance assays with both neonicotinoids, the strain that had been selected with thiamethoxam for 12 generations demonstrated almost no cross-resistance to acetamiprid, whereas the acetamiprid-selected strain exhibited high cross-resistance of >500-fold to thiamethoxam.     

寄主抗药性对菜蛾绒茧蜂抗药性发展的影响

室内筛选小菜蛾中抗品系（SRP）、高抗品系（RP）、以敏感小菜蛾幼虫（SP）为寄主的菜蛾绒茧蜂SRC品系和以SRP幼虫为寄主的菜蛾绒茧蜂RSC品系对氰戊菊酯的抗性。分别筛选了13、14、14和13代。小菜蛾SRP和RP品系分别获得了68.9和605.8倍的抗性,菜蛾绒茧蜂SRC和RSC品系分别获得了4.3和11.0倍的抗性。上述结果表明通过施药于体内有寄生蜂的小菜蛾幼虫筛选寄生蜂抗性,可以获得具有抗性的寄生蜂。以SRP为寄主的RSC品系的抗性水平高于以SP为寄主的SRC品系的抗性水平,表明和抗性较高的寄主同步筛选,寄生蜂的抗性发展更快。小菜蛾SP、SRP和RP三个品系幼虫的多功能氧化酶（MFO）活性比为1∶1.15∶1.50;菜蛾绒茧蜂SC、SRC和RSC三个品系幼虫的MFO活性比为1∶1.10∶1.49,成蜂的MFO活性比为1∶1.18∶1.54;而每种昆虫不同品系的羧酸酯酶（CarE）、总酯酶（Es）活性水平与其抗性水平变化不一致,表明抗性与MFO活性升高有关,而与CarE和Es的活性无关。     

Insecticide resistance monitoring in whitefly (Bemisia tabaci) (Hemiptera: Aleyrodidae) in Oman

The sweet potato whitefly, Bemisia tabaci Gennadius, is an important insect pest of many crops including vegetables through direct feeding damage and as a vector of several plant viruses. Intensive use of insecticides has led to the development of insecticide resistance in global B. tabaci populations. This study was conducted to establish susceptibility levels to deltamethrin, thiamethoxam and pyriproxyfen in seven geographically different populations of B. tabaci MEAM1 adults in Oman. All B. tabaci populations showed very low to low level of resistance (2.1–12.3 fold) to deltamethrin. All B. tabaci populations showed no resistance to very low level of resistance to thiamethoxam (2.2–6.2 fold) and pyriproxyfen (2.4–3.5 fold). A likelihood analysis showed the possibility for control failure in two populations (Barka and Salalah) to deltamethrin, however, no possible failure was detected in all populations for thiamethoxam and pyriproxyfen. An insecticide resistance dynamics study in one population (SQU-1) showed a loss in susceptibility to deltamethrin with increase in the LC₅₀ value from 25.1 mg L⁻¹ to 84.5 mg L⁻¹ between 2017 and 2019 resulting in 5.3 fold increase in RF. The study results determined that several B. tabaci populations are at the initial stages of resistance development to deltamethrin and cross-resistance with thiamethoxam and pyriproxyfen. Vegetable farmers in Oman, the Barka and Salalah regions in particular, should be cautious in the repeated use of one class of insecticide alone.     

Resistance status of house flies (Diptera: Muscidae) from southeastern Nebraska beef cattle feedlots to selected insecticides

The status of resistance to three insecticides (permethrin, stirofos, and methoxychlor), relative to a laboratory-susceptible colony, was evaluated in field populations of house flies, Musca domestica L., collected from two beef cattle feedlots in southeastern Nebraska. Topical application and residual exposure to treated glass surfaces were suitable methods for determining the resistance status of house flies to permethrin, stirofos, or methoxychlor. However, in most cases, residual exposure was more sensitive in resistance detection (i.e., higher resistance ratios). The field populations tested were moderately resistant to permethrin (RR = 4.9-fold and RR = 7.3-fold, for topical application and residual exposure, respectively) and extremely resistant to stirofos and methoxychlor (not accurately quantifiable because of low mortality at the highest possible concentrations or doses). Probable explanations for the resistance status of these house fly populations and implications for global feedlot fly management are discussed.     

拟除虫菊酯抗性家蝇的交互抗药性研究

拟除虫菊酯的广泛使用使家蝇普遍产生了抗药性,为有效的控制家蝇的危害,需要了解家蝇对轮换或新杀虫剂的交互抗性状况。作者用点滴法测定了两个实验室汰选的拟除虫菊酯抗性家蝇品系对几种杀虫剂的交互抗性,结果表明:二氯苯醚菊酯和溴氰菊酯之间存在较高程度的交互抗药性;拟除虫菊酯抗性较高的家蝇对作用机制不同的新农药(多杀菌素、氟虫腈)表现较低程度的交互抗性。     

Cross-Resistance between Cry1 Proteins in Fall Armyworm (Spodoptera frugiperda) May Affect the Durability of Current Pyramided Bt Maize Hybrids in Brazil

Genetically modified plants expressing insecticidal proteins from Bacillus thuringiensis (Bt) offer valuable options for managing insect pests with considerable environmental and economic benefits. Despite the benefits provided by Bt crops, the continuous expression of these insecticidal proteins imposes strong selection for resistance in target pest populations. Bt maize (Zea mays) hybrids have been successful in controlling fall armyworm (Spodoptera frugiperda), the main maize pest in Brazil since 2008; however, field-evolved resistance to the protein Cry1F has recently been reported. Therefore it is important to assess the possibility of cross-resistance between Cry1F and other Cry proteins expressed in Bt maize hybrids. In this study, an F₂ screen followed by subsequent selection on MON 89034 maize was used to select an S. frugiperda strain (RR) able to survive on the Bt maize event MON 89034, which expresses the Cry1A.105 and Cry2Ab2 proteins. Field-collected insects from maize expressing the Cry1F protein (event TC1507) represented most of the positive (resistance allele-containing) (iso)families found. The RR strain showed high levels of resistance to Cry1F, which apparently also conferred high levels of cross resistance to Cry1A.105 and Cry1Ab, but had only low-level (10-fold) resistance to Cry2Ab2. Life history studies to investigate fitness costs associated with the resistance in RR strain revealed only small reductions in reproductive rate when compared to susceptible and heterozygous strains, but the RR strain produced 32.2% and 28.4% fewer females from each female relative to the SS and RS (pooled) strains, respectively. Consistent with the lack of significant resistance to Cry2Ab2, MON 89034 maize in combination with appropriate management practices continues to provide effective control of S. frugiperda in Brazil. Nevertheless, the occurrence of Cry1F resistance in S. frugiperda across Brazil, and the cross-resistance to Cry1Ab and Cry1A.105, indicates that current Cry1-based maize hybrids face a challenge in managing S. frugiperda in Brazil and highlights the importance of effective insect resistance management for these technologies.     

Establishing the role of detoxifying enzymes in field‐evolved resistance to various insecticides in the brown planthopper (Nilaparvata lugens) in South India

The brown planthopper (BPH), Nilaparvata lugens (Stål), is one of the major pests of rice throughout Asia. Extensive use of insecticides for suppressing N. lugens has resulted in the development of insecticide resistance leading to frequent control failures in the field. The aim of the present study was to evaluate resistance in the field populations of N. lugens from major rice growing states of South India to various insecticides. We also determined the activity of detoxifying enzymes (esterases [ESTs], glutathione S‐transferases [GSTs], and mixed‐function oxidases [MFOs]). Moderate levels of resistance were detected in the field populations to acephate, thiamethoxam and buprofezin (resistance factors 1.05–20.92 fold, 4.52–14.99 fold, and 1.00–18.09 fold, respectively) as compared with susceptible strain while there were low levels of resistance to imidacloprid (resistance factor 1.23–6.70 fold) and complete sensitivity to etofenoprox (resistance factor 1.05–1.66 fold). EST activities in the field populations were 1.06 to 3.09 times higher than the susceptible strain while for GST and MFO the ratios varied from 1.29 to 3.41 and 1.03 to 1.76, respectively. The EST activity was found to be correlated to acephate resistance (r = 0.999, P ≥ 0.001). The high selection pressure of organophosphate, neonicotinoid, and insect growth regulator (IGR) in the field is likely to be contributing for resistance in BPH to multiple insecticides, leading to control failures. The results obtained will be beneficial to IPM recommendations for the use of effective insecticides against BPH.     

Biochemical genetics of altered acetylcholinesterase resistance to insecticides in the house fly

Resistance to the organophosphate insecticide tetrachlorvinphos was examined in a house fly (Musca domestica L.) strain with an altered acetylcholinesterase (AChE) of decreased sensitivity to inhibition by the insecticide. Genetic tests showed that both resistance and the altered AChE were controlled by semidominant gene(s) on chromosome II. The gene for resistance was five crossover units from the mutant marker stubby wing (stw). A house fly strain was prepared in which resistance was introduced in to a susceptible stw strain by recombination. Biochemical assays revealed that the altered AChE was introduced along with resistance. Assays of the AChE of resistant and susceptible stw strains by two independent methods showed that the enzyme from resistant flies was 30 times more slowly inhibited by tetrachlorvinphos than the enzyme from susceptible flies.This work was supported in part by NIH Grant ES 00901.Technical Article 13340, Texas Agricultural Experiment Station.     

Horizontal Transfer of the Tetracycline Resistance Gene <Emphasis Type="Italic">tetM</Emphasis> Mediated by pCF10 Among <Emphasis Type="Italic">Enterococcus faecalis</Emphasis> in the House Fly (<Emphasis Type="Italic">Musca domestica</Emphasis> L.) Alimentary Canal

The house fly (Musca domestica L.) alimentary canal was evaluated for the potential of horizontal transfer of tetM on plasmid pCF10 among Enterococcus faecalis. Two sets of experiments were conducted: (1) house flies without surface sterilization and (2) surface-sterilized flies. Both sets of flies were exposed to E. faecalis OG1RF:pCF10 as donor for 12 h and then E. faecalis OG1SSp as recipient for 1 h. Another group of flies received the recipient first for 12 h followed by exposure to the donor strain for 1 h. House flies were screened daily to determine the donor, recipient, and transconjugant bacterial load for up to 5 days. In addition, the sponge-like mouth parts used for food uptake (labellum) of surface-sterilized house flies were removed and analyzed for donors, recipients, and transconjugants, separately. In both groups of flies (n = 90 flies/group), transfer occurred within 24 h after exposure with a transconjugant/donor rate from 8.6 × 10⁻⁵ to 4.5 × 10¹. Transconjugants were also isolated from the house fly labellum. Our data suggest that the house fly digestive tract provides a suitable environment for horizontal transfer of conjugative plasmids and antibiotic resistance genes among enterococci. Our results emphasize the importance of this insect as a potential vector of antibiotic-resistant bacterial strains.     

杀虫双和杀螟丹选育对小菜蛾抗药性的形成及其抗性机制

用杀虫双和杀螟丹在实验室以点滴法处理小菜蛾Plutella xylostella L.四龄幼虫,以连续继代药剂淘汰选育其抗药性。至35代,药剂汰选的小菜蛾对杀虫双和杀螟丹的抗药性较选育前正常品系分别提高了51倍和25倍。其抗药性的形成发展均呈S形,可认为已成为抗性品系。以有机磷、氨基甲酸酯、拟除虫菊酯及有机氮等11种杀虫剂测试抗杀虫双小菜蛾品系和抗杀螟丹小菜蛾品系对常用药剂的敏感度结果表明：对杀虫双、杀螟丹和杀虫环之间有较严重的正交互抗性;对敌敌艮、马拉硫磷和杀螟松有轻微交互抗性产生;对溴氰菊酯、氯氰菊酯、氯菊酯和灭多威、久效威等药剂更加敏感,呈负交互抗性。用聚丙烯酰胺凝胶电泳(PAGE)法测定表明,抗药性产生与特异性酯酶的形成有一定关系。用比色法和酸度法测定,抗性品系的乙酰胆碱酯酶(AchE)活性降低,羧酸酯酶(CarE)活性无差异。加增效剂Pb和SV₁：于四龄幼虫表皮,对抗杀虫双小菜蛾晶系分别有6.28及1.45倍的增效作用;对抗杀螟丹小菜蛾品系分别有4.85及1.39倍的增效作用,可见多功能氧化酶(MFO)为小菜蛾抗杀虫双和抗杀螟丹的重要因子。     

Permethrin resistance in Aedes aegypti (Linnaeus) collected from Kuala Lumpur,Malaysia

Permethrin resistance status of a laboratory strain, a permethrin-selected strain and three field strains of Aedes aegypti collected in Kuala Lumpur, Malaysia were evaluated using three standard laboratory bioassays: WHO larval bioassay, WHO adult mosquito bioassay, and mixed function oxidase (MFO) enzyme microassay. The LC₅₀ values of field strains from the WHO larval bioassay did not differ significantly. The highest LC₅₀ value was from the Taman Melati field strain (0.39 mg/L). The resistance ratio for the permethrin-selected strain and the field strains ranged from 1.86 fold to 5.57 fold. Pre-exposure to piperonyl butoxide (PBO) in the WHO adult bioassay and MFOs enzyme microassay reduced the LT₅₀ values and reduced the mean optical density of elevated oxidase activity (0.28–0.42) at 630 nm. The LC₅₀ or LT₅₀ values and the level of oxidases were significantly correlated (r = 0.825; p< 0.05). This study confirmed the presence of permethrin resistance in these mosquito populations.     

Disruption of a Cadherin Gene Associated with Resistance to Cry1Ac δ-Endotoxin of Bacillus thuringiensis in Helicoverpa armigera

A laboratory strain (GY) of Helicoverpa armigera (Hübner) was established from surviving larvae collected from transgenic cotton expressing a Bacillus thuringiensis var. kurstaki insecticidal protein (Bt cotton) in Gaoyang County, Hebei Province, People's Republic of China, in 2001. The GYBT strain was derived from the GY strain through 28 generations of selection with activated Cry1Ac delivered by diet surface contamination. When resistance to Cry1Ac in the GYBT strain increased to 564-fold after selection, we detected high levels of cross-resistance to Cry1Aa (103-fold) and Cry1Ab (>46-fold) in the GYBT strain with reference to those in the GY strain. The GYBT strain had a low level of cross-resistance to B. thuringiensis var. kurstaki formulation (Btk) (5-fold) and no cross-resistance to Cry2Aa (1.4-fold). Genetic analysis showed that Cry1Ac resistance in the GYBT strain was controlled by one autosomal and incompletely recessive gene. The cross-resistance pattern and inheritance mode suggest that the Cry1Ac resistance in the GYBT strain of H. armigera belongs to “mode 1,” the most common type of lepidopteran resistance to B. thuringiensis toxins. A cadherin gene was cloned and sequenced from both the GY and GYBT strains. Disruption of the cadherin gene by a premature stop codon was associated with a high level of Cry1Ac resistance in H. armigera. Tight linkage between Cry1Ac resistance and the cadherin locus was observed in a backcross analysis. Together with previous evidence found with Heliothis virescens and Pectinophora gossypiella, our results confirmed that the cadherin gene is a preferred target for developing DNA-based monitoring of B. thuringiensis resistance in field populations of lepidopteran pests.     

Knockdown and mortality comparisons among spinosad-, imidacloprid-, and methomyl-containing baits against susceptible Musca domestica (Diptera: Muscidae) under laboratory conditions

The activity of spinosad, imidacloprid, and methomyl baits and technical actives were assessed against susceptible house flies, Musca domestica L. (Diptera: Muscidae). In a feeding assay, imidacloprid affected flies more rapidly than methomyl or spinosad, but spinosad was 2.7 times more potent than methomyl and 8 times more potent than imidacloprid. The profile of technical actives correlated with their respective fly bait formulations in laboratory assays. Although having the most rapid onset of activity in laboratory tests, up to 50% of flies remained alive after exposure to imidacloprid bait. In contrast, <5% of flies survived 24-h exposure to spinosad or methomyl baits. High temperature reduced the knockdown activity of imidacloprid bait and slowed the speed of kill for spinosad and methomyl baits over a 24-h exposure period. Spinosad and methomyl baits were also superior to imidacloprid when applied to the floors of environmentally controlled rooms at label recommended rates, providing good fly control for up to 21 d. The fact that a significant percentage of flies exposed to imidacloprid were rapidly knocked down but subsequently remained alive in all of the assays suggested that flies were recovering from initial exposure to this compound. Given its favorable safety profile, a high degree of initial and residual activity comparable with methomyl and lack of cross-resistance to other chemistries, spinosad bait may be a valuable component of house fly control programs to help control or delay the emergence of resistant populations.     

Does behaviour play a role in house fly resistance to imidacloprid‐containing baits?

The objective of this research was to examine the role and type of behavioural mechanisms that function in house fly, Musca domestica L. (Diptera: Muscidae), resistance to an imidacloprid‐containing commercial fly bait, QuickBayt^®, using an insecticide‐susceptible and an imidacloprid‐resistant strain. Mortality and feeding behaviour were observed through choice bioassays of three post‐imidacloprid selected house fly generations to determine whether flies would consume the bait in the presence of an alternative food source. Mortality rates in choice containers progressively decreased in post‐selection flies as QuickBayt^® no‐choice selections proceeded. There were no differences between the proportions of flies observed contacting QuickBayt^® and sugar, respectively, a finding that eliminates repellency as a mechanism of stimulus‐dependent behavioural resistance. However, differences in QuickBayt^® consumption and subsequent mortality between choice and no‐choice containers provided strong support for the evolution of consumption irritancy‐ or taste aversion‐related behavioural resistance. The results of this study support the responsible rotation of insecticide bait formulations for house fly control.     

Dieldrin-resistant cabbage root fly in England

The existence of dieldrin resistance in Erioischia brassicae has been investigated in the main areas in England and Wales where brassicas are grown for human consumption. Dieldrin-susceptible and dieldrin-resistant flies were collected and bred as separate strains in the laboratory. In toxicity tests adult flies were treated with topically applied doses of insecticide in microdrops of cellosolve, and larvae were dipped in solutions of insecticide in 70% acetone and 30% water. Adults of the laboratory resistant strain showed a high level of resistance to dieldrin and aldrin and a lower level of resistance to γ-BHC. There was no cross-resistance with chlorfenvinphos or diazinon. Larvae of the resistant strain were shown to be resistant to dieldrin. Dieldrin resistance has been found in four areas in southern England, but not in a fifth major area, and resistant flies did not increase as rapidly in some areas as others. Sufficient migration of flies appeared to take place to produce similar percentages of resistant flies over areas of several square miles, but considerable differences were found in fly populations about 10 miles apart. It is suggested that the speed of development of insecticide resistance is affected by the proportion of brassica crops which are treated in an area, the amounts of insecticide applied to individual crops, and hence the insecticidal pressure applied to the E. brassicae population.     

Molecular mechanisms conferring asymmetrical cross-resistance between tebufenozide and abamectin in Plutella xylostella

Based on the confirmation of asymmetrical cross-resistance between abamectin and tebufenozide in Plutella xylostella, the present work proved that the cytochrome P450 monooxygenase plays a decisive role in cross-resistance, and the expression of various cytochrome P450 (CYP450) genes in different strains was surveyed to elucidate the molecular basis of the underlying mechanisms. Enzyme analysis showed the activity of cytochrome P450 monooxygenase was notable enhanced in the strains resistant to both tebufenozide (3.07-fold) and abamectin (3.37-fold), suggesting that the enhancement of cytochrome P450 monooxygenase is the main detoxification mechanism responsible for the cross-resistance. CYP4M7 (64.58-fold) and CYP6K1 (41.97-fold) had extremely high expression levels in the Teb-R strain, selected using tebufenozide, which was highly resistant to tebufenozide (RR 185.5) and moderately cross-resistant to abamectin (RR 41.0). When this strain was subjected to further selection using abamectin, the resultant Aba-R strain showed a higher expression of CYP6K1 (60.32-fold). However, the expression of CYP4M7 was reduced (10.62-fold). Correspondingly, the Aba-R strain became more resistant to abamectin (RR 593.8) and less resistant to tebufenozide (RR 28.0). Therefore, we concluded that the over expression of CYP4M7 was the main cause for tebufenozide resistance, and that CYP6K1 mainly conferred abamectin resistance. The asymmetrical cross-resistance occurred because tebufenozide selection not only enhanced the expression of CYP4M7, but also that of CYP6K1. This is the first report on the molecular mechanism of asymmetrical cross-resistance between insecticides.