Effects of deltamethrin and its commercial formulation DECIS on different cell types in vitro: Cytotoxicity,cellular binding,and intracellular localization

To investigate the effect of deltamethrin and other pyrethroids on nonexcitable cells, we tested these molecules on animal (fibroblasts) and plant (algae) cells in culture. The effects of pyrethroids on cellular proliferation vary according to their structure. Deltamethrin (from 5 × 10⁻⁵ M) affected the growth of fibroblasts, but was inactive on other cells. Commercial formulations (DECIS EC and DECIS FLO) were more active to cells, but this effect was due mostly to the matrix. However, these matrices facilitated the penetration of deltamethrin into cells. Studies of [³H]deltamethrin uptake indicated the labeling of macromolecules, but labeling was reduced significantly after treatment with organic solvents. Autoradiographic studies with [³H]deltamethrin showed labeling of cells at the cytpolasmic level.     

Pyrethroid insecticides: Potent,stereospecific enhancers of mouse brain sodium channel activation

Deltamethrin and NRDC 157, pyrethroid insecticides that produce different poisoning syndromes in mammals, enhanced veratridine-dependent, sodium channel-mediated ²²Na⁺ uptake in mouse brain synaptosomes. Concentrations producing half-maximal enhancement were 2.5 × 10^?8M (deltamethrin) and 2.2 × 10^?7M (NRDC 157). This effect was stereospecific: The nontoxic 1S enantiomers had no significant effect on veratridine-dependent activation. At high deltamethrin concentrations, enhancement was maximal at 5 × 10^?5?1 × 10^?4M veratridine. Pyrethroid enhancement was completely blocked by 5 × 10^?6M tetrodotoxin, and neither pyrethroid affected ²²Na⁺ uptake in the absence of veratridine at concentrations up to 1 × 10^?5M. The relative potencies of deltamethrin and NRDC 157 in the synaptosomal sodium channel assay agree well with their relative acute toxicities to mice when administered by intracerebral injection. These findings demonstrate that pyrethroids exemplifying both characteristic poisoning syndromes are potent, stereospecific modifiers of sodium channel function in mammalian brain.     

Binary mixtures of pyrethroids produce differential effects on Ca influx and glutamate release at isolated presynaptic nerve terminals from rat brain

Isolated rat brain synaptosomes were used to evaluate the action of pyrethroid mixtures on Ca²⁺ influx and subsequent glutamate release under depolarizing conditions. In equipotent binary mixtures at their respective and/or estimated EC₅₀s with deltamethrin always as one of the two components, cismethrin, λ-cyhalothrin, cypermethrin, esfenvalerate and permethrin were additive and S-bioallethrin, fenpropathrin and tefluthrin were less-than-additive on Ca²⁺ influx. In binary mixtures with deltamethrin always as one of the two components, esfenvalerate, permethrin and tefluthrin were additive and λ-cyhalothrin was less-than-additive on glutamate release. Binary mixture of S-bioallethrin and cismethrin was additive for both Ca²⁺ influx and glutamate release. Only a subset of pyrethroids (S-bioallethrin, cismethrin, cypermethrin, and fenpropathrin) in binary mixtures with deltamethrin caused a more-than-additive effect on glutamate release. These binary mixtures were, however, only additive (cismethrin and cypermethrin) or less-than-additive (S-bioallethrin and fenpropathrin) on Ca²⁺ influx. Therefore, increased glutamate release evoked by this subset of pyrethroids in binary mixture with deltamethrin is not entirely occurring by Ca²⁺-dependent mechanisms via their action at voltage-sensitive calcium channels. These results suggest that pyrethroids do not share a common mode of toxicity at presynaptic nerve terminals from rat brain and appear to affect multiple target sites, including voltage-sensitive calcium, chloride and sodium channels.     

The effects of two pyrethroids,cismethrin and deltamethrin,on skeletal muscle and the trigeminal reflex system in the rat

The actions of a cyano pyrethroid (deltamethrin) and a non-cyano pyrethroid (cismethrin) upon trigeminal motor reflexes and isolated muscle responses were studied in the rat. Deltamethrin caused a marked facilitation of the muscle response to nerve stimulation in pithed rats at 2.5 μmol kg⁻¹. In intact anaesthetised rats this was associated with abnormal repetitive EMG discharges and, at 4 μmol kg⁻¹ with a suppression of late components of the reflex response to sensory stimuli in the spinal trigeminal nucleus and trigeminal motor nucleus. In contrast cismethrin had no effect on the muscle response to direct nerve stimulation at up to 15 μmol kg⁻¹, but produced abnormal extra responses to sensory stimuli in the trigeminal ganglion, spinal and motor nuclei, and jaw muscles at 9 μmol kg⁻¹. It is concluded that whilst deltamethrin produces reflex excitation within the trigeminal system at a primarily muscular site, cismethrin produces excitation at all stages of the reflex loop. This contrast is consistent with the known difference in duration of sodium current prolongation produced by the two pyrethroids. These findings, together with other known central actions of deltamethrin suggest that it has multiple sites of action in the intact animal, both central and peripheral, whilst most of the simpler symptoms produced by cismethrin may adequately be explained by action at a reflex level.     

Action of cismethrin and deltamethrin on functional attributes of isolated presynaptic nerve terminals from rat brain

Isolated presynaptic nerve terminals (synaptosomes) prepared from rat brain were used to evaluate the actions of a tremor (T)-syndrome (cismethrin) and a choreoathetosis-salivation (CS)-syndrome (deltamethrin) pyrethroid on the functional attributes of synaptosomes by measuring calcium influx and endogenous neurotransmitter (l-glutamate) release with fluorescent assays. Both cismethrin and deltamethrin stimulated calcium influx, however, only deltamethrin enhanced Ca²⁺-dependent neurotransmitter release and its action was stereospecific, concentration-dependent, stimulated by depolarization, unaltered by tetrodotoxin, and blocked by ω-conotoxin GVIA. Our results delineate a separate action of deltamethrin on presynaptic nerve terminals from that elicited by cismethrin and implicate Ca_v2.2 calcium channels as target sites for deltamethrin that is consistent with the observed in vivo release of neurotransmitter at the onset of convulsive symptom caused by CS-syndrome pyrethroids. This information will allow a more complete understanding of the molecular and cellular nature of pyrethroid-induced neurotoxicity and expands our knowledge of the structure–activity relationships of pyrethroids in regards to their action on voltage-sensitive calcium channels.     

Action of pyrethroids on a nerve-muscle preparation of the clawed frog,Xenopus laevis

The effects of a range of pyrethroids on end-plate potentials and muscle action potentials were studied in the pectoralis nerve-muscle preparation of the clawed frog, Xenopus laevis. The noncyano pyrethroids allethrin, cismethrin, bioresmethrin, and IR-cisphenothrin caused moderate presynaptic repetitive activity only, resulting in the occurrence of multiple end-plate potentials (epps). Trains of repetitive muscle action potentials without presynaptic repetitive activity were observed after the α-ethynyl pyrethroid S-5655 and after the α-cyano pyrethroids cypermethrin, deltamethrin, FCR 1272, and FCR 2769. An intermediate group of pyrethroids consisting of the non-cyano compounds 1R-permethrin, des-cyano-deltamethrin, NAK 1901 and NAK 1963, and the α-cyano pyrethroids cyphenothrin and fenvalerate caused both types of effect. The insecticidally inactive S-enantiomers of permethrin had no effect on the nerve-muscle preparation. Trains of repetitive action potentials in pyrethroid-treated muscle fibers were followed by a depolarizing afterpotential which in general decayed more rapidly for the non-cyano pyrethroids than for the α-cyano pyrethroids. The rate of decay of the depolarizing afterpotential decreased gradually as the temperature was lowered, whereas the pre- and postsynaptic repetitive activity remained largely unaffected over a large temperature range. It is concluded that in muscle membrane like in nerve membrane the pyrethroid-induced repetitive activity is due to a prolongation of the sodium current and that a clear distinction between non-cyano pyrethroids on the one hand and α-cyano compounds on the other cannot be made on the basis of the present results.     

The action of pyrethroids on the voltage-sensitive calcium channel of Paramecium tetraurelia

Calcium regulation is an important event in synaptic transmission and neuronal function, which is governed by a very intricate signal transduction system which is not completely understood. Using a variety of pharmacological assays, we have characterized the action of deltamethrin on the ciliary voltage-sensitive calcium channel and on phospholipase C activity of Paramecium tetraurelia Sonneborn, an organism that does not possess a voltage-sensitive sodium channel. In fura-2 fluorometric assays, which examined whole cells and ciliary membrane vesicles enriched with calcium channels, deltamethrin stimulated Ca²⁺ uptake. We also determined that the phospholipase C activity of the ciliary membrane vesicles is regulated by the βγ-subunit from heterotrimeric G-proteins. Subsequent treatment with deltamethrin resulted in a substantial and highly significant increase in phospholipase C activity. These results provide evidence that the molecular mode of action of pyrethroids on the voltage-sensitive calcium channel is distinct from the action of this insecticide on the voltage-sensitive sodium channel and may be dependent, in part, upon an interaction with the βγ-subunit of heterotrimeric G-protein.     

Promotion of norepinephrine release and inhibition of calcium uptake by pyrethroids in rat brain synaptosomes

A series of 25 pyrethroids were assessed for their effects on Na⁺-dependent norepinephrine release and on Ca²⁺ uptake in vitro using a crude rat brain synaptosomal preparation. The most effective pyrethroids required a concentration of 3–10 μM to promote norepinephrine release. Plotting release data versus lipophilicity (as log P) for each compound resulted in a parabolic curve with log P_opt being 5.4 for maximal release. The release promoted by most of the compounds assessed at 30 μM could not be or was only partially reversed by either tetrodotoxin or substituting choline for Na⁺ conditions which readily reversed the release promoting effects of veratridine. Thus, many pyrethroids, particularly those without the α-cyano group, did not display their expected effects on the Na⁺ channel in rat brain. When assessed at 5 μM, pyrethroids inhibited, had no effect, or caused increases in the amount of Ca²⁺ incorporated in the presence of ATP. The effectiveness of the various pyrethroids to inhibit Ca²⁺ uptake again displayed a parabolic relationship with log P_opt being 6.4. It was concluded that the variations in pyrethroid effects on norepinephrine release and Ca²⁺ uptake are not solely related to their particular chemical structures, but to lipophilicity. The effects of many pyrethroids on Ca²⁺ metabolism, particularly displacement of bound Ca²⁺, better explain the transmitter release promoting properties in vitro rather than a direct effect on the Na⁺ channel. No direct relationship between known toxicity to mammals and Ca²⁺ inhibition by pyrethroids was established.     

Depolarization of motor nerve terminals by pyrethroids in susceptible and kdr-resistant house flies

When applied at concentrations of one nM or higher to a house fly larval neuromuscular preparation, deltamethrin (DM) and fenvalerate (FV) greatly increased miniature excitatory postsynaptic potential (mepsp) rate and blocked neuromuscular transmission. The DM-induced mepsp discharge was abolished by tetrodotoxin (TTX), removal of Ca²⁺ from the saline, or by application of hyperpolarizing stimuli to the nerve, indicating that it was due to depolarization of the presynaptic terminals. Also, in the presence of TTX, K⁺ depolarization increased mepsp rate at the same external K⁺ concentration before and after DM treatment, confirming that DM released transmitter by depolarizing the nerve terminals rather than by altering the voltage dependence of transmitter release. The potassium channel blocker tetraethylammonium (TEA) increased mepsp rate somewhat, while aconitine (20 μM), which keeps sodium channels open, increased mepsp rate consistently. Pretreatment of nerves with a subthreshold dose of TEA greatly increased the mepsp rate-increasing activity of DM and aconitine, while a subthreshold level of aconitine did not synergize DM. These observations suggest that DM, like aconitine, depolarized nerves by modifying the sodium channels. Knockdown resistant (kdr) larvae were resistant to the depolarizing action of DM and aconitine but not to that of TEA, indicating that the kdr gene produced a modified sodium channel which was less sensitive to the action of pyrethroids and aconitine. During sustained transmitter release by DM, evoked release gradually declined, resulting in a condition called early block in which spontaneous release was high and release could be evoked by electrotonic depolarization of the nerve terminals, but not by a nerve action potential. Early block was probably due to conduction block in the nerve terminals. Early block eventually gave way to late block, characterized by the decline of spontaneous release to subnormal levels and complete failure of evoked release. After late block, the calcium ionophore X-537A could not release transmitter, suggesting that late block was due to depletion of available transmitter. DM did not have a direct effect upon extrasynaptic muscle membrane. However, after late block, muscles were left insensitive to the putative transmitters glutamate and aspartate when these were bath or iontophoretically applied. A low rate of mepsps persisted after late block, indicating that the muscles were still sensitive to the natural transmitters.     

Effects of the pyrethroids bioallethrin,deltamethrin and RU-15525 on the electrical activity of a cuticular mechanoreceptor of the cockroach Periplaneta americana

A study has been made of the effects of bioallethrin, RU-15525 [5-benzyl-3-furylmethyl (1R)-cis-2,2-dimethyl-3-(tetrahydro-2-oxo-3-thienylidenemethyl)-cyclopropanecarboxylate, ‘Kadethrin’], and deltamethrin on the electrical activity, measured in vivo, of a cuticular mechanoreceptor of Periplaneta americana. The modifications induced by these pyrethroids on the membrane excitability can be classified into two groups: Type I effects (bioallethrin) are characterised by a substantial increase in the number of action potentials triggered at the initiation site by a given mechanical stimulation, by an electrical activity persisting after mechanical stimulus has been stopped (repetitive activity), and possibly, by an inhibition of excitability of the cell membrane. Type II effects (RU-15525 and deltamethrin), are characterised by an inhibition of the excitability of the initiation site. In the case of RU-15525, there was a transient spontaneous electrical activity. Both types of effects have been linked to an action on the sodium channel, particularly at the initiation site. The preparation studied, which possessed no synapses, was shown to be more sensitive to deltamethrin (which is also the most insecticidal of the three pyrethroids) than to either allethrin or RU-15525. These results suggest that it is unnecessary to envisage a main target (sodium channel) that is different for the two types of pyrethroid.     

Interaction of tralomethrin,tralocythrin, and related pyrethroids in Na+ channels of insect and mammalian neuronal cells

The neurotoxic activity of the tetrahalogenated pyrethroids, tralomethrin and tralocythrin, which are dibromo adducts of deltamethrin and RU 24501, respectively, and RU 27218, which is a dichloro adduct of RU 24501, have been studied on two different excitable membranes using in vitro methods: insect axonal membranes and mammalian neuroblastoma cells in culture. The results show that molecules whose chrysanthemic acid chain at C₃ is saturated by halogen addition have intrinsic toxicity on the sodium channels of nerve membranes. They increase the amplitude of negative after-potential, depolarize the cockroach axonal membrane, and stabilize an open conformation of voltage-dependent Na⁺ channels in mouse neuroblastoma cells, thereby producing a massive uptakeof Na⁺ by the cells. All effects were observed in the absence of metabolic transformation of the compounds tested. In light of these experiments, it is concluded that tralomethrin and tralocythrin cannot be considered as propyrethroids.     

Evidence for different mechanisms of action of the three pyrethroids,deltamethrin, cypermethrin,and fenvalerate,on the excitation threshold of myelinated nerve

The effects of three α-cyano pyrethroids (deltamethrin, fenvalerate, and cypermethrin) on the electrophysiological function of single myelinated nerve fibers from Rana esculenta were investigated. The time course of pyrethroid-induced changes on the threshold interval, V_s − V_m (V_s: threshold voltage; V_m: membrane potential), as well as stationary membrane parameters determining this interval was measured on the same nerve preparation (membrane potential V_m, stationary transition voltage V_Tr, stationary sodium conductance). The results suggest that the mechanisms of changing the threshold interval are different for the three pyrethroids. Deltamethrin and cypermethrin increase this interval until inexcitability, deltamethrin by increasing the stationary sodium conductance and cypermethrin by blocking the sodium conductance. Fenvalerate, however, insignificantly affects the threshold interval because both V_m and V_s are shifted parallel by about the same amount in the same direction (depolarization). These qualitatively different effects of chemically related substances differentiate the pyrethroids from other classes of substances which are effective on the nerve function and suggests that the molecular mechanisms underlying the pyrethroid effects might have a unique quality.     

The crayfish stretch receptor organ: A useful model system for investigating the effects of neuroactive substances: II. A pharmacological investigation of pyrethroid mode of action

The crayfish stretch receptor neuron is very sensitive to disruption by pyrethroids. Experiments were therefore carried out on this preparation to investigate whether all observed symptoms of pyrethroid poisoning could be explained in terms of a single action of pyrethroids on the gating mechanism of sodium channels. Other possible mechanisms of pyrethroid action, including effects on potassium channels, inhibitory synapses, and inhibition of calcium ATPases, were also investigated. An effect on sodium channels was clearly demonstrated, but the contribution of other mechanisms was less clear. It appears that the action on the sodium channel could be responsible for the majority, if not all, of the observed effects of pyrethroids on sensory cells.     

Effects of topical and bath applications of the insecticide deltamethrin on electrical activity in a leg mechanoreceptor of the cockroach,Periplaneta americana

Two different methods were used to analyze the effects of topical and bath applications of low concentrations of deltamethrin. The first method enabled the analysis of the receptor potential as well as that of the spike activity generated by mechanical or electrical stimulation of the mechano-receptor. The second method enabled recording of the conducted spike activity originating from a mechanical stimulation of the receptor and propagated along the sensory nerve. Topical application of small amounts of deltamethrin (10⁻⁹ to 10⁻⁷ g) had little effect on the receptor potential induced by mechanical stimulation of the sensory hair but blocked the action potentials within a few minutes. Electrical stimulation of the receptor cell revealed that conduction in the dendrite was affected first by the insecticide. The effects of topical application on conducted activity were compared with those of bath applications (2 × 10⁻⁸ to 2 × 10⁻⁷ M) and no significant difference was found, suggesting a rapid penetration of the insecticide through the cuticle. These effects were not reversible and this absence of reversibility was not correlated with the integrity of the barrier which protects the receptor cell from rapid changes in the ionic composition of the hemolymph. Deltamethrin was never found to induce bursts of activity in the mechanoreceptor cell under investigation either at rest or following mechanical or electrical stimulation. There are, however, some indications that other receptor cells may respond differently to this insecticide.     

Characterization of 11 commercial pyrethroids on the functional attributes of rat brain synaptosomes

The action of 11 commercial pyrethroids on Ca²⁺ influx and glutamate release was assessed using high-throughput functional assays with rat brain synaptosomes to better understand the mechanistic nature of pyrethroid-induced neurotoxicity and aid in the reassessment of pyrethroids in vivo. Concentration-dependent response curves for each of the non-cyano and α-cyano containing pyrethroids were determined and the data used in a cluster analysis. The previously characterized α-cyano pyrethroids that induce the CS-syndrome (cypermethrin, deltamethrin, and esfenvalerate) increased Ca²⁺ influx and glutamate release, and clustered with two other α-cyano pyrethroids (β-cyfluthrin and λ-cyhalothrin) that shared these same actions. Previously characterized T-syndrome pyrethroids (bioallethrin, cismethrin, and fenpropathrin) did not share these actions and clustered with two other non-cyano pyrethroids (tefluthrin and bifenthrin) that likewise did not elicit these actions. Our current findings indicate that pyrethroids that have an α-cyano group (with the exception of fenpropathrin) were more potent enhancers of Ca²⁺ influx and glutamate release under depolarizing conditions than pyrethroids that did not possess this functional group. The collective data set does not support the hypothesis that pyrethroids, as a class, act in a similar fashion at presynaptic nerve terminals.     

State-dependent modification of voltage-gated sodium channels by pyrethroids

Pyrethroids disrupt nerve function by altering the rapid kinetic transitions between conducting and nonconducting states of voltage-gated sodium channels that underlie the generation of nerve action potentials. Recent studies of pyrethroid action on cloned insect and mammalian sodium channel isoforms expressed in Xenopus laevis oocytes show that in some cases pyrethroid modification is either absolutely dependent on or significantly enhanced by repeated channel activation. These use-dependent effects have been interpreted as evidence of preferential binding of at least some pyrethroids to the open, rather than resting, state of the sodium channel. This paper reviews the evidence for state-dependent modification of insect and mammalian sodium channels expressed in oocytes by pyrethroids and considers the implications of state-dependent effects for understanding the molecular mechanism of pyrethroid action and the development and testing of models of the pyrethroid receptor.     

The effects of pyrethroids on the electrical activity of neurosecretory cells from the brain of Rhodnius prolixus

The effects of pyrethroids on the on-going electrical activity of the axons of neurosecretory cells from the brain of fifth instar Rhodnius prolixus have been studied using extracellular electrodes. Low concentrations of the pyrethroids decamethrin, bioresmethrin, permethrin, and bioallethrin all produce dramatic increases in the overall frequency and dramatic changes in the pattern of electrical activity when applied directly to the exposed brain and corpora cardiaca in an otherwise intact insect. This change in activity was brought about by a recruitment in active units and the production of phasic acivity. A doubling of frequency over that of controls was brought about by low doses of the pyrethroids, namely decamethrin, 1 × 10^?10M; bioresmethrin, 2 × 10^?10M; permethrin, 1 × 10^?9M; and bioallethrin, 2 × 10^?7M. Similar hyperactivity of this system occurred during intoxication of intact insects following topical application of LD₉₅ bioresmethrin. The enhanced sensitivity shown by neurosecretory cells over that of other cell types is discussed, as is the possibility that the increases in electrical activity of neurosecretory axons may result in massive neurohormonal release and thereby contribute to the eventual poisoning of the insect.     

Actions of pyrethroid insecticides on insect axonal sodium channels

The actions of pyrethroid insecticides were tested on isolated giant axons of the cockroach Periplaneta americana, using oil-gap, single-fibre recording techniques. Current-clamp and voltage-clamp experiments were used to determine the actions of pyrethroids on axonal membrane potentials and ionic currents. Treatment with deltamethrin at micromolar concentrations caused gradual depolarisation of the axon accompanied by a reduction in amplitude of the action potential. This depolarisation was enhanced by an increase in stimulation frequency. Other synthetic pyrethroids: 3,4,5,6-tetrahydrophthalimidomethyl (1RS)-cis-3-[(RS)-2,2-dimethylcyclopropyl]-2,2-dimethylcyclopropanecarboxylate, biopermethrin and its (1S)-enantiomer, (1R)-tetramethrin, S-bioallethrin, bioresmethrin and its (1S)-enantiomer, cismethrin, and 5-benzyl-3-furylmethyl (E)-(1R)-cis-2,2-dimethyl-3-(2-oxothiolan-3-ylidenemethyl)cyclopropanecarboxylate (RU-15525, ‘Kadethrin’) were investigated. The (1S)-enantiomers were inactive, but all the other pyrethroids tested, apart from deltamethrin, induced prolonged negative (depolarising) after-potentials. All the treatments with the active pyrethroids resulted in the appearance of a voltage and time-dependent ‘maintained’ sodium conductance. The duration of this ‘slow’ conductance varied considerably depending on the pyrethroid under test. Clearly, the effectiveness of pyrethroids on whole insects is not determined only by the degree to which they directly modify the properties of sodium channels. Nevertheless, voltage-clamp experiments on isolated axons readily permit direct comparison of the actions of different pyrethroids on the sodium channels of insect neurones.     

Action of avermectin B1a on the leg muscles and the nervous system of the American cockroach

The action of avermectin was studied in the leg muscle and the central nervous system of the American cockroach, Periplanata americana L. Avermectin at a low concentration (10^?7M) causes a failure of the leg muscles to respond to external stimuli within 30 min without affecting the magnitude of contraction. Avermectin was found to stimulate Cl^? uptake by the leg muscles within 4 min at 10^?7M. The threshold concentration to cause such stimulation was on the order of 10^?8M. This stimulatory action could be antagonized by picrotoxinin (10^?4M) and to a lesser extent by bicuculline methiodide (10^?4M). The phenomenon is observable under both Na⁺-free and K⁺-free conditions. It was concluded that the action of avermectin is to open the chloride channel on the plasma membrane. This action of avermectin does not seem to be mediated through GABA, GABA receptors, diazepine receptors, or picrotoxinin receptor in this insect species, and therefore suggests that avermectin directly attacks the chloride channel proper both in the central nervous and the neuromuscular systems.     

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.