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     

Monitoring of resistance of insecticides in house fly (Musca domestica) populations in hungary

Samples of 24 house fly (Musca domestica L.) populations were collected from animal farms in Hungary in 1990 and kept in the laboratory to determine their susceptibility to different types of insecticide: organochlorines, organophosphates, carbamates, pyrethroids, macrocyclic lactone and insect growth regulators. The adulticides were tested with topical bioassay in all 24 populations, the larvicides were studied with treated larval medium in 16 populations. The data were expressed as LD₅₀ and LC₅₀ values (ng fly ?1 and mg kg ?1 larval medium respectively). The percentages of populations which had resistance ratios > 10 at LD₅₀ or LC₅₀ were: 63% to DDT, 50% to methoxychlor, 13% to lindane, 83% to malathion, 63% to trichlorfon, 4% to propetamphos, 96% to dioxacarb, 46% to propoxur, 4% to methomyl, 13% to pyrethrum, 96% to bioresmethrin, 63% to permethrin, 58% to cypermethrin, 79% to SK-80, 79% to deltamethrin, 38% to invermectin, 0% to diflubenzuron, 0% to cyromazine. Correlation analysis showed a high degree of positive correlation among the adulticides except for ivermectin, bioresmethrin and SK-80. No cross-resistance was found between the larvicides and the conventional adulticides. Differences of insecticide resistance levels among the populations surveyed were studied by principal component and factor analysis. A fairly good relationship between resistance status and control practices used on farms was revealed. The populations originating from those farms where the application of adulticides had been frequent or regular and where high resistance was shown to most chemicals could be separated from the others.     

The biochemistry of insecticide resistance in Anopheles sacharovi: Comparative studies with a range of insecticide susceptible and resistant Anopheles and Culex species

Fourth instar larvae, the progeny from wild-caught Anopheles sacharovi females, were subjected to a number of biochemical tests and the results were compared to those from similar tests on laboratory insecticide resistant and susceptible strains of anopheline and culicine mosquitoes. DDT resistance in An. sacharovi is associated with the ability to rapidly metabolise DDT to DDE. The organophosphorus and carbamate resistance was not associated with quantitative changes in esterases, multifunction oxidases, or glutathione S-transferase. The acetylcholinesterase was less sensitive to malaoxon and propoxur than laboratory susceptible An. albimanus, and plots of inhibition suggest that the population was polymorphic for more than one form of acetylcholinesterase. Metabolism studies on malathion and pirimiphos methyl did not indicate resistance due to increased metabolism. There was no evidence of penetration barriers contributing to resistance to either DDT or malathion, and there was no indication of any resistance to pirimiphos methyl in our tests.     

Cross-resistance to pyrethroids and other insecticides in Aedes aegypti

Two substrains of Aedes aegypti, already resistant to DDT and pyrethroids, were further selected using either DDT or permethrin by mass exposure of the females only. DDT selection over 14 generations raised the resistance to DDT so far that no accurate LC₅₀ values could be determined. Selection with permethrin raised the tolerance to an irregular plateau 7–10 times the original. DDT selection in the adults raised the DDT resistance of the larvae, but this could be partly overcome using a dehydrochlorinase inhibitor. The resistance to pyrethroids was increased but tolerance of dieldrin, malathion and propoxur compounds was little changed. Permethrin selection of the adults raised resistance to pyrethroids more than DDT selection but also increased DDT resistance. Similar patterns were found for the larval insects. A strain from Demerara in Guyana showed both DDT and pyrethroid resistance, including strong resistance to pyrethrins together with dieldrin and propoxur. It was concluded that two major independent resistance mechanisms existed in the selected strains, a dehydrochlorinase affecting DDT alone, and an unknown mechanism, probably nerve insensitivity (kdr) affecting both DDT and pyrethroids.     

The pyrethroid resistance status and mechanisms in Aedes aegypti from the Guerrero state,Mexico

Dengue is one of the most important vector-borne diseases worldwide and is a public health problem in Mexico. Most programs in dengue endemic countries rely on insecticides for Aedes control. In Mexico, pyrethroid insecticides (mainly permethrin and deltamethrin) have been extensively used over a decade as adulticides and represented a strong selection for insecticide resistance for dengue vectors in several parts of the country. We studied the type, frequency and distribution of insecticide resistance mechanisms in Aedes aegypti from six municipalities in the state of Guerrero selected on the basis of historically intense chemical control and a high risk for dengue transmission. Ae. aegypti eggs were collected from October 2009 to January 2010 using ovitraps. F₁ adults, emerged from these collections, were exposed to permethrin, deltamethrin and DDT in WHO diagnostic tests and showed high resistance levels to both pyrethroids and DDT. This was consistent with the presence of increased metabolic enzyme activities and target site insensitivity due to kdr mutations. Biochemical assays showed elevated esterase and glutathione S-transferase activities in the six municipalities. The V1016I kdr mutation on the IIS6 domain of the sodium channel gene was present in an overall frequency of 0.80. A second mutation, F1534C on the IIIS6 domain of the same gene was also detected, being the first report of this mutation in Guerrero. The multiple resistance mechanisms present in Ae. aegypti from Guerrero state represent a warning for the efficacy of the pyrethroid usage and consequently for the success of the dengue control program.     

Esterase-mediated malathion resistance in the human head louse, Pediculus capitis (Anoplura: Pediculidae)

Resistance in a dual malathion- and permethrin-resistant head louse strain (BR-HL) was studied. BR-HL was 3.6- and 3.7-fold more resistant to malathion and permethrin, respectively, compared to insecticide-susceptible EC-HL. S,S,S-Tributylphosphorotrithioate synergized malathion toxicity by 2.1-fold but not permethrin toxicity in BR-HL. Piperonyl butoxide did not synergize malathion or permethrin toxicity. Malathion carboxylesterase (MCE) activity was 13.3-fold and general esterase activity was 3.9-fold higher in BR-HL versus EC-HL. There were no significant differences in phosphotriesterase, glutathione S-transferase, and acetylcholinesterase activities between strains. There was no differential sensitivity in acetylcholinesterase inhibition by malaoxon. Esterases from BR-HL had higher affinities and hydrolysis efficiencies versus EC-HL using various naphthyl-substituted esters. Protein content of BR-HL females and males was 1.6- and 1.3-fold higher, respectively, versus EC-HL adults. Electrophoresis revealed two esterases with increased intensity and a unique esterase associated with BR-HL. Thus, increased MCE activity and over-expressed esterases appear to be involved in malathion resistance in the head louse.     

Control failure likelihood and spatial dependence of insecticide resistance in the tomato pinworm, Tuta absoluta

BACKGROUND: Insecticide resistance is a likely cause of field control failures of Tuta absoluta, but the subject has been little studied. Therefore, resistance to ten insecticides was surveyed in seven representative field populations of this species. The likelihood of control failures was assessed, as well as weather influence and the spatial dependence of insecticide resistance. RESULTS: No resistance or only low resistance levels were observed for pyrethroids (bifenthrin and permethrin), abamectin, spinosad, Bacillus thuringiensis and the mixture deltamethrin + triazophos (<12.5‐fold). In contrast, indoxacarb exhibited moderate levels of resistance (up to 27.5‐fold), and chitin synthesis inhibitors exhibited moderate to high levels of resistance (up to 222.3‐fold). Evidence of control failures was obtained for bifenthrin, permethrin, diflubenzuron, teflubenzuron, triflumuron and B. thuringiensis. Weather conditions favour resistance to some insecticides, and spatial dependence was observed only for bifenthrin and permethrin. CONCLUSION: Insecticide resistance in field populations of the tomato pinworm prevails for the insecticides nowadays most frequently used against them—the chitin synthesis inhibitors (diflubenzuron, triflumuron and teflubenzuron). Local selection favoured by weather conditions and dispersal seem important for pyrethroid resistance evolution among Brazilian populations of T. absoluta and should be considered in designing pest management programmes. Copyright © 2011 Society of Chemical Industry     

Thermal behaviour and biological activity against Aedes aegypti (Diptera: Culicidae) of permethrin and pyriproxyfen in a smoke‐generating formulation

BACKGROUND: The most common ways to control dengue vector Aedes aegypti (L.) are larval source reduction in domestic habitats and ground application of small quantities of aerosol insecticide (ultralow volume). Nevertheless, these actions have been shown repeatedly to be ineffective in controlling Ae. aegypti populations. RESULTS: The efficacy of a new smoke‐generating formulation containing pyriproxyfen and permethrin was evaluated in the laboratory. Smoke‐generating tablets containing each insecticide individually or combined were prepared, and the recovery of the insecticides from the smoke was determined. Recovery values of over 90% were obtained for pyriproxyfen, and around 50% for permethrin. The biological efficacy of pyriproxyfen released in the smoke was evaluated in the laboratory, on late third‐instar or early fourth‐instar Ae. aegypti larvae, using different concentrations of pyriproxyfen and exposure times. Adult emergence inhibition (EI) values of 100% were obtained at 30 min, and a dose‐dependent effect was observed at 5 min. The effect of pyriproxyfen released in the smoke was due to direct contact with the larvicide in the water rather than by inhalation of the fumes. The efficacy of permethrin released in the fumes was also evaluated as knockdown effect (KT₅₀) on adults for a tablet containing permethrin alone or permethrin plus pyriproxyfen. There was no significant difference in KT₅₀ values obtained for permethrin (KT₅₀ = 19.9 min) and permethrin plus pyriproxyfen (KT₅₀ = 19.4 min). CONCLUSION: The excellent laboratory performance of this new formulation on immature stages and adults indicates that a smoke‐generating tablet containing pyriproxyfen and permethrin could be a new tool for controlling mosquitoes. Copyright © 2009 Society of Chemical Industry     

A comparative study of insecticide resistance assays with the German cockroach

Females of a diazinon-resistant strain of German cockroach exhibiting cross resistance to propoxur were evaluated for resistance to diazinon and propoxur by six assays: LD₅₀s for topical application and injection; LC₅₀, KT₅₀ and LT₅₀ for deposits on wood and KT₅₀ for deposits on glass. Also investigated were certain variables in the topical application technique and the experimental conditions for topical synergism studies. Among all assays, diazinon resistance ratios (resistant:susceptible) ranged from 3- to 145-fold; propoxur varied from 1- to 14-fold. Resistance ratios based on KT₅₀ data were consistently low (<4x) for both insecticides. Resistance to diazinon was 13x when applied topically and 6x when injected. Resistance to propoxur was 14x by injection compared to 8x for topical application. The LC₅₀ assay exhibited the greatest difference in resistance to the two insecticides: namely, 145x to diazinon and 1x(no resistance) to propoxur. By LT₅₀ analysis, resistance to propoxur was 10x compared with 100x resistance to diazinon. Thus, the method used had a significant effect on the final value of the resistance ratio. Even though resistance ratios may vary widely among different techniques, the final choice of a method depends upon many factors. The LT₅₀ method may provide a more realistic appraisal of resistance in wild populations of German cockroaches especially when experimental residues are similar to those used in control programmes.     

Mutated sodium channel genes and elevated monooxygenases are found in pyrethroid resistant populations of Sri Lankan malaria vectors

The present status of pyrethroid resistance in vectors of malaria; Anopheles culicifacies and Anopheles subpictus, was tested in two malarious Districts, Anuradhapura and Trincomalee, of Sri Lanka. Both species were resistant to permethrin and susceptible to cypermethrin and cyfluthrin. An. subpictus were resistant to deltamethrin. λ-Cyhalothrin and etofenprox resistance was shown only by Anuradhapura An. subpictus. Although there were no differences among the populations for esterase and glutathione S-transferase activities, increased monooxygenase levels were found among Trincomalee populations. The voltage-gated sodium channel gene, the target site gene of pyrethroids, was partially sequenced to screen for mutations previously associated with insecticide resistance. The classic leucine to phenylalanine substitution, TTA to TTT, was detected in An. subpictus. It appears that both kdr type and monooxygenase resistance underlie pyrethroid resistance in these two malaria vectors of Sri Lanka.     

Diversity and frequencies of genetic mutations involved in insecticide resistance in field populations of the house fly (Musca domestica L.) from China

Insecticides have been extensively used for house fly control in China, with dichlorvos and deltamethrin being widely used. Knowledge about the current status of insecticide resistance and the underlying genetic changes is crucial for developing effective fly control strategies. The susceptibility to dichlorvos and deltamethrin, and the frequencies of genetic mutations involved in insecticide resistance were studied in five field populations of the house fly collected across China. Bioassay results show that flies exhibit 14- to 28-fold resistance to dichlorvos and 41- to 94-fold resistance to deltamethrin, indicating that dichlorvos and deltamethrin resistance are common in house fly populations in China. Molecular analysis reveals that flies from the five various locations carry resistance alleles at multiple loci and have diverse allelic types, different relative frequencies and combinations of each allele. Four non-synonymous single nucleotide polymorphisms (SNPs) (i.e. V260L, G342A/V, F407Y) in acetylcholinesterase (Ace) and two mutations (W251L/S) in a carboxylesterase (MdαE7) were commonly present in the field house flies. The L1014H rather than L1014F mutation in the voltage sensitive sodium channel gene (Vssc) was widely distributed in Chinese house flies. CYP6D1v1, which confers pyrethroid resistance, was found in all the five tested populations in China, although its frequency in house fly from Shandong province was very low. Our results suggest that resistance monitoring and management of house flies should be customized for a given location.     

Association between insecticide use and greenhouse whitefly (Trialeurodes vaporariorum westwood) resistance to insecticides in hawaii

Susceptibility to acephate, methomyl, and permethrin was determined with laboratory bioassays for adults of greenhouse whitefly, Trialeurodes vaporariorum Westwood, from 12 to 14 sites in Hawaii. Comparisons at LC₅₀ showed up to 42-fold resistance to acephate, 36-fold resistance to methomyl, and 8-fold resistance to permethrin. Higher levels of resistance to acephate and methomyl than to permethrin are consistent with greater use of organophosphates and carbamates than pyrethroids by growers. Insecticide use varied from 1 to 98 insecticide sprays per site per season. Significant positive associations between LC₅₀ for each insecticide and frequency of application of the same insecticide were found across sites. This finding suggests that local variation in insecticide use was an important cause of variation in susceptibility.     

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.     

Present status and countermeasures of insecticide resistance in agricultural pests in China

Of the 23 species of agricultural pest known to resist insecticides in China, 4 are cotton pests, 4 rice pests and 5 are pests of brassicae. In the green rice leafhopper, malathion resistance is caused by increased carboxylesterase (CarE) activity, which plays a more important role in the resistance to dimethoate than the mixed-function oxidases (mfos). The in-vitro and in-vivo results are in agreement with studies of synergism of malathion and dimethoate by TPP and EBP. These synergists delay the development of resistance, and EBP when added to malathion has limited the development of resistance to malathion in the green rice leafhopper. In the cotton aphid, resistance to organophosphates involves several factors: acetylcholinesterase (AChE) insensitivity, high CarE activity, slight (× 2) increase in glutathione S-transferases (GSH-ases), mfo activity as well as reduced penetration. In vitro, the I₅₀ of the insensitive AChE is × 14 that of S aphids, and anaphthyl-acetate CarE hydrolysing activity is 70 times greater in R than in S aphids. Insecticide mixtures, alternation or rotation can delay build-up of resistance; resistance to malathion and trichlorfon was delayed in Culex pipiens pallens when the two insecticides were used together. Used singly each insecticide selected for high resistance within 25 generations. Mosaic rotation of dimethoate and fenvalerate delayed the onset of insecticide resistance in Lipaphis erysimi pseudobrassicae.     

Selection of Anopheles stephensi with DDT and dieldrin and cross-resistance spectrum to pyrethroids and fipronil

The field strain of Anopheles stephensi, the main malaria vector in south of Iran, was colonized in laboratory and selected with DDT and dieldrin in two separate lines for 3 generations to a level of 19.5- and 14-fold for DDT and dieldrin resistance, respectively. Synergist tests with chlorofenethol (DMC) and piperonyl butoxide (PBO) on the selected strains indicated that dehydrochlorination and oxidative detoxification might be the underlying mechanisms involved in the resistance to dieldrin and DDT in selected strains. DDT selection decreased susceptibility to DDT and pyrethroids including lambdacyhalothrin, permethrin deltamethrin and cyfluthrin. The result also showed that selection with dieldrin caused negative and positive cross-resistance to pyrethroid and fipronil, respectively. Based on these results, it can be concluded that besides metabolic resistance mechanisms, other factors such as mutation in γ aminobutyric acid (GABA) and voltage-gated sodium channels (Kdr) might be involved.     

The epoxidation of aldrin by microsomes from the Fc strain of housefly: A genetic survey

The DDT-resistant housefly strain, Fe, known to resist DDT by biochemical oxidation, is also resistant to carbamate insecticides and has a high in vitro microsomal epoxidase activity. The purpose of this investigation was to determine whether the DDT resistance, associated with chromosome V, is also responsible for the resistance to carbamates and for the high epoxidase levels. Genetic procedures for segregating the R factors were employed using a multimarker insecticide susceptible strain designated acbco. The technique involved backcrossing the F₁ hybrid of the resistant and susceptible parents to the susceptible parent. The genotypes with a single R chromosome from the Fc parent were retained for further development as substrains and for toxicological and biochemical studies.These studies revealed that both resistance to the carbamate insecticide, propoxur, and the high in vitro microsomal epoxidation of aldrin were lost during the genetic isolation of the R factors. However, the resistance to DDT, associated with chromosome V, was present in the substrain carrying this chromosome from the Fc parent. All of the substrains were induced five- to seven-fold, by feeding phenobarbital at 1% in the diet for 3 days.Additional substrains synthesized from the substrains carrying chromosomes II and V or III and V from the Fc parent did not possess sufficient propoxur resistance or aldrin epoxidase activity to account for that present in the R parent.The interpretation of these rseults is that neither the carbamate resistance nor the microsomal epoxidase of the Fc strain is due to the factor which oxidizes DDT. Furthermore, the factor responsible for the high microsomal epoxidase activity is not due to a single chromosome such as chromosome II which is the case in other housefly strains with high oxidase activities.     

Carbofuran triggers flight motor output in pyrethroid-blocked reflex pathways of the house fly

The initiation of flight from loss of tarsal contact (flight reflex responses) and the tergotrochanteral muscle (TTM) responses evoked by brain stimulation were analyzed during carbofuran, permethrin, deltamethrin, and DDT poisoning in the house fly. Blockage of the flight reflex by LD₅₀ doses of permethrin or deltamethrin was rapid, but the effects of DDT on the flight reflex took hours to develop. In addition, carbofuran treatment induced spontaneous flight in blocked preparations by an action in the central nervous system. This result suggests that pyrethroid blockage of the flight reflex was due to an action on sensory nerves, since the central flight program and its associated efferent systems were functionally intact. The relevance of this finding in terms of pyrethroid knockdown is discussed. The TTM response was unaffected by permethrin or deltamethrin both early and late in the poisoning process, possibly because the evoked TTM response does not involve peripheral sensory nerves, which seem to be important sites of pyrethroid action early in poisoning. Carbofuran induced repetitive firing and blockage of the TTM response within 1 hr, but normal responses were observed late in poisoning, which is consistent with the reversibility of carbamate inhibition of acetylcholinesterase. DDT caused no change in the evoked TTM response until bursts were recorded about 15 hr after treatment; this was another example of a slowly developing DDT effect. The protracted development of various DDT actions was concordant with a hypothesis of reduced efficacy at a proposed target site, viz., the sodium channels of nerve membranes.     

Pesticide resistance in glasshouse whitefly (Trialeurodes vaporariorum west

A survey in SE England showed that seven out of 17 populations of glasshouse whitefly were resistant to DDT. Resistance to malathion occurred in 15 populations and to resmethrin in eight. Mecarbam and methiocarb gave good kills in screening tests but some insect growth regulators gave variable results. The synthetic pyrethroid permethrin was slightly more effective against a population of whitefly resistant to both DDT and malathion than against a susceptible population. Parthenogenetic females from a population susceptible to DDT and malathion produced almost entirely male progeny. An unsuccessful attempt was made to lower the resistance level of a DDT/malathion resistant strain by introducing these males to resistant females.     

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.     

Variability in resistance-related enzyme activities in field populations of the tarnished plant bug, Lygus lineolaris

Widespread use of Bt crops for control of lepidopterous pests has reduced insecticide use and provided the tarnished plant bug the opportunity to become a serious pest on mid-South cotton. Organophosphate insecticides have predominantly been used against plant bugs in recent years due to the reduced efficacy of other insecticides. In this study, a biochemical approach was developed to survey enzymatic levels associated with organophosphate resistance levels in field populations of the tarnished plant bug. Forty-three populations were collected from the delta areas of Arkansas, Louisiana, and Mississippi. Three esterase substrates and one substrate each of glutathione S-transferase (GST) and acetylcholinesterase (AChE) were used to determine corresponding detoxification enzyme activities in different populations. Compared to a laboratory susceptible colony, increases up to 5.29-fold for esterase, 1.96-fold for GST, and 1.97-fold for AChE activities were detected in the field populations. In addition to the survey of enzyme activities among the populations, we also examined the susceptibility of major detoxification enzymes to several inhibitors which could be used in formulations to synergize insecticide toxicity against the target pests. As much as 52-76% of esterase, 72-98% of GST, and 93% of AChE activities were inhibited in vitro. Revealing variable esterase and GST activities among field populations may lead to a better understanding of resistance mechanisms in the tarnished plant bug. This study also reports effective suppression of detoxification enzymes which may be useful in future insecticide resistance management program for the tarnished plant bug and other Heteropteran pests on Bt crops.