Organophosphate and pyrethroid resistances in the tomato leaf miner Tuta absoluta (Lepidoptera: Gelechiidae) from Iran

The tomato leafminer, Tuta absoluta (Meyrich) (Lepidoptera: Gelechiidae), is a serious pest of tomato crops worldwide. The intensive use of chemical pesticides to control it has led to the selection of resistant populations. This study investigated the resistance of T. absoluta populations to pyrethroid and the organophosphate insecticides from ten regions of Iran. The resistance ratios at LC₅₀ for chlorpyrifos and diazinon varied among populations from 4.3 to 12 and from 1.4 to 9.0, respectively. The resistance ratios of the pyrethroids cypermethrin, deltamethrin and permethrin varied from 1.3 to 3.7, 2.7 to 13 and 1.2 to 4.3, respectively. Inclusion of synergists in toxicological bioassays and the variation observed in the activity of esterases, glutathione S‐transferase and cytochrome P450‐dependent monooxygenase suggest the existence of metabolically based resistance. Esterase and P450 biochemical assays were positively correlated with deltamethrin, and cypermethrin tolerance and diazinon tolerance correlated with esterase activity. The genes encoding the organophosphate and pyrethroid target sites acetylcholinesterase (ace1) and sodium channel (kdr) were partly sequenced. The genotyping revealed mutations in high frequencies in all populations leading to an A201S substitution in ace1 and three substitutions in the sodium channel gene L1014F, M918T, T929I. In summary, our results indicate the presence of organophosphate and pyrethroid resistance in Iranian T. absoluta populations with involvement of both detoxification enzymes and target site alterations. Most likely the populations of T. absoluta imported to Iran were resistant upon arrival.     

Resurgence of the cotton bollworm Helicoverpa armigera in northern Greece associated with insecticide resistance

Helicoverpa armigera has been controlled effectively with chemical insecticides in the major cotton crop production areas of northern Greece for many years. However, a resurgence of the pest was observed in 2010, which significantly affected crop production. During a 4‐year survey (2007 – 2010), we examined the insecticide resistance status of H. armigera populations from two major and representative cotton production areas in northern Greece against seven insecticides (chlorpyrifos, diazinon, methomyl, alpha‐cypermethrin, cypermethrin, gamma‐cyhalothrin and endosulfan). Full dose‐response bioassays on third instar larvae were performed by topical application. Lethal doses at 50% were estimated by probit analysis and resistance factors (RF) were calculated, compared to a susceptible laboratory reference strain. Resistance levels were relatively moderate until 2009, with resistance ratios below 10‐fold for organophosphates and carbamates and up to 16‐fold for the pyrethroid alpha‐cypermethrin. However, resistance rose to 46‐ and 81‐fold for chlorpyrifos and alpha‐cypermethrin, respectively in 2010, when the resurgence of the pest was observed. None of the known pyrethroid resistance mutations were found in the pyrethroid‐resistant insects. The possible association between resistance and H. armigera resurgence in Greece is discussed.     

Insecticide resistance,associated mechanisms and fitness aspects in two Brazilian Stegomyia aegypti (= Aedes aegypti) populations

In Brazil, insecticide resistance in Stegomyia aegypti (= Aedes aegypti) (Diptera: Culicidae) populations to pyrethroids and to the organophosphate (OP) temephos is disseminated. Currently, insect growth regulators (IGRs) and the OP malathion are employed against larvae and adults, respectively. Bioassays with mosquitoes from two northeast municipalities, Crato and Aracaju, revealed, in both populations, susceptibility to IGRs and malathion (RR₉₅ ≤ 2.0), confirming the effectiveness of these compounds. By contrast, temephos and deltamethrin (pyrethroid) resistance levels were high (RR₉₅ > 10), which is consistent with the use of intense chemical control. In Crato, RR₉₅ values were > 50 for both compounds. Knock‐down‐resistant (kdr) mutants in the voltage‐gated sodium channel, the pyrethroid target site, were found in 43 and 32%, respectively, of Aracaju and Crato mosquitoes. Biochemical assays revealed higher metabolic resistance activity (esterases, mixed function oxidases and glutathione‐S‐transferases) at Aracaju. With respect to fitness aspects, mating effectiveness was equivalently impaired in both populations, but Aracaju mosquitoes showed more damaging effects in terms of longer larval development, decreased bloodmeal acceptance, reduced engorgement and lower numbers of eggs laid per female. Compared with mosquitoes in Crato, Aracaju mosquitoes exhibited lower OP and pyrethroid RR₉₅, increased activity of detoxifying enzymes and greater effect on fitness. The potential relationship between insecticide resistance mechanisms and mosquito viability is discussed.     

Characterization of mosquito CYP6P7 and CYP6AA3: differences in substrate preference and kinetic properties

Cytochrome P450 monooxygenases are involved in insecticide resistance in insects. We previously observed an increase in CYP6P7 and CYP6AA3 mRNA expression in Anopheles minimus mosquitoes during the selection for deltamethrin resistance in the laboratory. CYP6AA3 has been shown to metabolize deltamethrin, while no information is known for CYP6P7. In this study, CYP6P7 was heterologously expressed in the Spodoptera frugiperda (Sf9) insect cells via baculovirus‐mediated expression system. The expressed CYP6P7 protein was used for exploitation of its enzymatic activity against insecticides after reconstitution with the An. minimus NADPH‐cytochrome P450 reductase enzyme in vitro. The ability of CYP6P7 to metabolize pyrethroids and insecticides in the organophosphate and carbamate groups was compared with CYP6AA3. The results revealed that both CYP6P7 and CYP6AA3 proteins could metabolize permethrin, cypermethrin, and deltamethrin pyrethroid insecticides, but showed the absence of activity against bioallethrin (pyrethroid), chlorpyrifos (organophosphate), and propoxur (carbamate). CYP6P7 had limited capacity in metabolizing λ‐cyhalothrin (pyrethroid), while CYP6AA3 displayed activity toward λ‐cyhalothrin. Kinetic properties suggested that CYP6AA3 had higher efficiency in metabolizing type I than type II pyrethroids, while catalytic efficiency of CYP6P7 toward both types was not significantly different. Their kinetic parameters in insecticide metabolism and preliminary inhibition studies by test compounds in the flavonoid, furanocoumarin, and methylenedioxyphenyl groups elucidated that CYP6P7 had different enzyme properties compared with CYP6AA3. © 2011 Wiley Periodicals, Inc.     

Pyrethroid susceptibility and behavioral avoidance in Anopheles epiroticus,a malaria vector in Thailand

The physiological susceptibility to insecticides and the behavioral responses of four wild‐caught populations of female Anopheles epiroticus to synthetic pyrethroids (deltamethrin, permethrin, and alpha‐cypermethrin) were assessed. Test populations were collected from different localities along the eastern coast, Trat (TR), Songkhla (SK), and Surat Thani (ST) and one population from the western coast, Phang Nga (PN). Results showed that all four populations of An. epiroticus were susceptible to all three synthetic pyrethroids tested. Behavioral responses to test compounds were characterized for all four populations using an excito‐repellency test system. TR displayed the strongest contact excitation (‘irritancy’) escape response (76.8% exposed to deltamethrin, 74.1% permethrin, and 78.4% alpha‐cypermethrin), followed by the PN population (24.4% deltamethrin, 35% permethrin, and 34.4% for alpha‐cypermethrin) by rapidly escaping test chambers after direct contact with surfaces treated with each active ingredient compared with match‐paired untreated controls. Moderate non‐contact repellency responses to all three compounds were observed in the TR population but were comparatively weaker than paired contact tests. Few mosquitoes from the SK and ST populations escaped from test chambers, regardless of insecticide tested or type of trial. We conclude that contact excitation was a major behavioral response in two populations of An. epiroticus, whereas two other populations showed virtually no escape response following exposure to the three pyrethroids. The explanation for these large unexpected differences in avoidance responses between pyrethroid‐susceptible populations of the same species is unclear and warrants further investigation.     

Behavioural and insecticidal effects of organophosphate‐, carbamate‐ and pyrethroid‐treated mosquito nets against African malaria vectors

Three insecticides – the pyrethroid deltamethrin, the carbamate carbosulfan and the organophosphate chlorpyrifos‐methyl – were tested on mosquito nets in experimental huts to determine their potential for introduction as malaria control measures. Their behavioural effects and efficacy were examined in Anopheles gambiae Giles s.s. (Diptera: Culicidae) and Anopheles funestus Giles s.s. in Muheza, Tanzania, and in Anopheles arabiensis Patton and Culex quinquefasciatus Say in Moshi, Tanzania. A standardized dosage of 25 mg/m² plus high dosages of carbosulfan (50 mg/m², 100 mg/m² and 200 mg/m²) and chlorpyrifos‐methyl (100 mg/m²) were used to compare the three types of insecticide. At 25 mg/m², the rank order of the insecticides for insecticide‐induced mortality in wild An. gambiae and An. funestus was, respectively, carbosulfan (88%, 86%) > deltamethrin (79%, 78%) > chlorpyrifos‐methyl (35%, 53%). The rank order of the insecticides for blood‐feeding inhibition (reduction in the number of blood‐fed mosquitoes compared with control) in wild An. gambiae and An. funestus was deltamethrin > chlorpyrifos‐methyl > carbosulfan. Carbosulfan was particularly toxic to endophilic anophelines at 200 mg/m², killing 100% of An. gambiae and 98% of An. funestus that entered the huts. It was less effective against the more exophilic An. arabiensis (67% mortality) and carbamate‐resistant Cx quinquefasciatus (36% mortality). Carbosulfan deterred anophelines from entering huts, but did not deter carbamate‐resistant Cx quinquefasciatus. Deltamethrin reduced the proportion of insects engaged in blood‐feeding, probably as a consequence of contact irritancy, whereas carbosulfan seemed to provide personal protection through deterred entry or perhaps a spatial repellent action. Any deployment of carbosulfan as an individual treatment on nets should be carried out on a large scale to reduce the risk of diverting mosquitoes to unprotected individuals. Chlorpyrifos‐methyl was inferior to deltamethrin in terms of mortality and blood‐feeding inhibition and would be better deployed on a net in combination with a pyrethroid to control insecticide‐resistant mosquitoes.     

Insecticide Susceptibility of Field‐collected Blattella germanica (Blattaria: Blattalidae) in Busan,Republic of Korea during 2014

The susceptibility values for field‐collected (BG‐BG) and laboratory (KS‐BG) strains of German cockroaches (BS‐BG), Blattella germanica (L.) to five synthetic pyrethroid and three organophosphate insecticides were assessed by topical application. Both KS‐BG and BS‐BG strains demonstrated the highest levels of susceptibility to deltamethrin with LD₅₀ values of 0.0035 μg/female and 1.5744 μg/female, respectively. The KS‐BG strain demonstrated the lowest susceptibility level to fenthion with an LD₅₀ value of 0.0698 μg/female, while the BS‐BG field population showed the lowest susceptibility to bifenthrin with 4.6811 μg/female. The BS‐BG field population showed resistance levels of 50 to 640 fold higher against the eight pesticides evaluated compared to the KS‐BG strain. The resistance ratios (RR) of the eight pesticides were compared using populations of B. germanica that were collected from the same locality during 2007 and 2014. The field population of B. germanica collected during 2014 demonstrated significantly high resistance to an organophosphates and all pyrethroids when compared with those collected during 2007, but RR values were <10 except for chlorpyrifos‐methyl and chlorpyrifos with RR values of 74 and 104, respectively.     

Potentiation/Antagonism of pyrethroids with organophosphate insecticides in Bemisia tabaci (Homoptera: Aleyrodidae)

The binary mixtures of pyrethroids cypermethrin, alpha-cypermethrin, zeta-cypermethrin, bifenthrin, fenpropathrin, lambda-cyhalothrin, and deltamethrin plus organophosphates ethion, profenofos, chlorpyrifos, quinalphos, acephate, methamidophos, methyl parathion, and triazophos were evaluated on putatively resistant field populations of Bemisia tabaci (Gennadius) (Homoptera: Aleyrodidae) from Pakistan using a leaf-dip bioassay. Ethion exhibited good potentiation with all the pyrethroids. Quinalphos potentiated cypermethrin, fenpropathrin, and lambda-cyhalothrin but not bifenthrin. Acephate was potentiating with bifenthrin and fenpropathrin but antagonistic with zeta-cypermethrin. A potentiation effect was also found when methamidophos was mixed with bifenthrin and fenpropathrin. However, profenofos was antagonistic with cypermethrin, bifenthrin, and lambda-cyhalothrin. Similarly, bifenthrin + methyl parathion and deltamethrin + triazophos mixtures were antagonistic when tested on several populations of B. tabaci. Chlorpyrifos was antagonistic with cypermethrin but had an additive effect with fenpropathrin.     

First report of amitraz and cypermethrin resistance in Rhipicephalus sanguineus sensu lato infesting dogs in Mexico

Engorged female Rhipicephalus sanguineus sensu lato (Ixodida: Ixodidae) were collected from dogs in the state of Yucatán, Mexico. Fourteen tick populations were collected from dogs at seven veterinary clinics, four residential homes and three cattle farms. The larval immersion test was used in the progeny of collected adult females to test susceptibility to amitraz and cypermethrin. Dose–mortality regressions, 50% lethal concentrations (LC₅₀), confidence intervals and slope were estimated by probit analysis. For amitraz, 12 tick populations (85.7%) were classified as resistant and low inter‐population variation in the phenotypic level of resistance was evident [resistance ratios (RRs) at LC₅₀: 1.0–13.0]. For cypermethrin, 12 tick populations (85.7%) were classified as resistant and substantial inter‐population variation in the phenotypic level of resistance was evident (RRs at LC₅₀: 1.0–104.0). Thus, amitraz resistance in R. sanguineus s.l. is common, but generally occurs at low levels; however, alarmingly high levels of cypermethrin resistance are present in R. sanguineus s.l. populations in dogs in Yucatán, Mexico. The intensive use of both acaricides to control ectoparasites on dogs is likely to lead to more serious resistance problems that may cause high levels of control failure in the future.     

Insecticide susceptibility status and major detoxifying enzymes activity in Anopheles subpictus from Kasur,Pakistan

Anopheles subpictus s.l. Grassi (Diptera: Culicidae) is a malaria vector in South Asia, where insecticides are the mainstay for vector control interventions. Information on any variation in metabolic enzyme levels in mosquitoes is helpful with respect to adapting alternative strategies for vector control. The scarce data on the biochemical basis of insecticide resistance in malaria vectors of Pakistan limit the available information for vector control interventions within the country. The insecticide susceptibility status and its biochemical basis against dichlorodiphenyltrichloroethane (DDT) (4%), deltamethrin (0.05%) and permethrin (0.75%) in An. subpictus s.l. collected from all Tehsils of district Kasur were evaluated. For this purpose, a World Health Organization susceptibility bioassay was performed followed by the detection of altered metabolic enzyme activity using biochemical assays. Similarly, a significant difference in knock‐down effect was observed among field collected and susceptible strain against all insecticides 24 h post exposure. The overall mean mortality rates of DDT, deltamethrin and permethrin were 27.86% [95% confidence interval (CI) = 29.65–26.06], 44.89% (95% CI = 46.23–43.54) and 78.82% (95% CI = 80.16–77.47), respectively. The biochemical assays revealed an elevated level of metabolic enzymes in the field population. The results provide evidence of resistance against organochlorine and pyrethroid groups in a field population of An. subpictus s.l. from district Kasur mediated by multiple metabolic mechanisms, including acetylcholinesterases, esterases, cytochrome P450 and glutathione S‐transferases.     

Determination of the resistance to organophosphate, carbamate, and pyrethroid insecticides in Panamanian Anopheles albimanus (Diptera: Culicidae) mosquitoes

Introduction. The susceptibility of Anopheles albimanus to organophosphates, carbamates and pyrethroid insecticides was unknown in the Panama communities of Aguas Claras, Pintupo and Puente Bayano, located in the Amerindian Reservation of Madungandi. This region is considered a malaria transmission area, where An. albimanus is the main vector. Objective. The resistance to organophosphate insecticides, carbamates and pyrethroids was evaluated in field populations of the Anopheles albimanus in Panama. Materials and methods. Progeny of An. albimanus collected in three localities in the indigenous Madugandi region were exposed to bioassays of susceptibility to organophosphate insecticides (fenitrothion, malathion and chlorpyrifos), the carbamate (propoxur) and pyrethroids (deltamethrin, lambdacyhalothrin, cyfluthrin and cypermethrin). The protocols were in accordance with those established for adult mosquitoes by World Health Organization. Results. The three strains of the An. albimanus were resistant to the pyrethroid insecticides deltamethrin, lambdacyhalothrin, cyfluthrin and cypermethrin. Susceptibility remained for the organophosphate insecticides fenitrothion, malathion, chlorpyrifos, and the carbamate insecticide propoxur. Conclusion. The results provided important information to the vector control program, contributing to the application of new strategies on the use of insecticides, and thereby lengthening the life of the insecticide in use.     

Insecticide Mixtures Could Enhance the Toxicity of Insecticides in a Resistant Dairy Population of Musca domestica L

House flies, Musca domestica L., are important pests of dairy operations worldwide, with the ability to adapt wide range of environmental conditions. There are a number of insecticides used for their management, but development of resistance is a serious problem. Insecticide mixtures could enhance the toxicity of insecticides in resistant insect pests, thus resulting as a potential resistance management tool. The toxicity of bifenthrin, cypermethrin, deltamethrin, chlorpyrifos, profenofos, emamectin benzoate and fipronil were assessed separately, and in mixtures against house flies. A field-collected population was significantly resistant to all the insecticides under investigation when compared with a laboratory susceptible strain. Most of the insecticide mixtures like one pyrethroid with other compounds evaluated under two conditions (1∶1-“A” and LC₅₀: LC₅₀-“B”) significantly increased the toxicity of pyrethroids in the field population. Under both conditions, the combination indices of pyrethroids with other compounds, in most of the cases, were significantly below 1, suggesting synergism. The enzyme inhibitors, PBO and DEF, when used in combination with insecticides against the resistant population, toxicities of bifenthrin, cypermethrin, deltamethrin and emamectin were significantly increased, suggesting esterase and monooxygenase based resistance mechanism. The toxicities of bifenthrin, cypermethrin and deltamethrin in the resistant population of house flies could be enhanced by the combination with chlorpyrifos, profenofos, emamectin and fipronil. The findings of the present study might have practical significance for resistance management in house flies.     

Resistance to selected organochlorin, organophosphate, carbamate and pyrethroid, in Spodoptera litura (Lepidoptera: Noctuidae) from Pakistan

The toxicity of the most commonly used insecticides of organochlorine, organophosphate, pyrethroid, and carbamate groups were investigated against Spodoptera litura (F.) (Lepidoptera: Noctuidae) populations collected for three consecutive years (2004-2006). For a chlorocyclodiene and pyrethroids tested, the resistance ratios compared with Lab-PK were in the range of 10- to 92-fold for endosulfan, 5- to 111-fold for cypermethrin, 2- to 98-fold for deltamethrin, and 7- to 86-fold for beta-cyfluthrin. For organophosphates and carbamates, resistance ratios were in the range of 3- to 169-fold for profenofos, 18- to 421-fold for chlorpyrifos, 3- to 160-fold for quinalphos, 6- to 126-fold for phoxim, 7- to 463-fold for triazophos, and 10- to 389-fold for methomyl and 16- to 200-fold for thiodicarb. Resistance ratios were generally low to medium for deltamethrin and beta-cyfluthrin and high to very high for endosulfan, cypermethrin, profenofos, chlorpyrifos, quinalphos, phoxim, triazophos, methomyl, or thiodicarb. Pairwise comparisons of the log LC50 values of insecticides tested for all the populations showed correlations among several insecticides, suggesting a cross-resistance mechanism. Integration of timely judgment of pest problem, delimiting growing of alternate crops such as arum, rotation of insecticides with new chemicals, and insect growth regulators in relation to integrated pest management could help in manageable control of this important pest.     

Organophosphorus insecticides synergize pyrethroids in the resistant strain of cotton bollworm, Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) from West Africa

Helicoverpa armigera (Hübner) populations from West Africa recently developed resistance to pyrethroid insecticides through enhanced metabolism by mixed-function oxidases. The combination index method was used to study the synergism of pyrethroids by organophosphorus insecticides. Several mixtures of insecticides currently registered to control cotton pest complex in West Africa were tested, including: cypermethrin/ethion, cypermethrin/profenofos, deltamethrin/ triazophos, deltamethrin/chlorpyriphos, cyfluthrin/chlorpyriphos, and betacyfluthrin/chlorpyriphos. In the resistant strain, the organophosphorus insecticides significantly increased the toxicity of pyrethroids suppressing the resistance effect, either by additive or synergistic effects. Significant synergism was shown for the following mixtures: cypermethrin/ethion, deltamethrin/triazophos, and deltamethrin/chlorpyriphos. The use of synergism from these insecticide mixtures should prove to be an additional tool in the overall resistance management strategy because the pyrethroid resistance in H. armigera from West Africa is not yet stable, decreasing between cotton seasons and increasing with treatments. In absence of selection, the susceptibility of H. armigera to insecticides should be restored.     

Evaluation of resistance to permethrin,cypermethrin and deltamethrin in different populations of Musca domestica (L.), collected from the Iranian dairy cattle farms

The house fly, Musca domestica (Linnaeus) (Diptera: Muscidae), is a major pest for human and livestock health and is also resistant to different insecticides. Herein, six M. domestica populations were collected, five of them from industrial cattle farms and the Koohrang population from a remote area as a susceptible population. The resistance/susceptibility of populations to three pyrethroids was evaluated. High levels of permethrin resistance were observed in all field populations and the resistance ratios (RRs) were estimated to vary from 52- to 129-fold. Resistant populations also exhibited resistance to other pyrethroids (cypermethrin and deltamethrin), with RRs ranging between 45- and 180-fold. According to synergistic (piperonyl butoxide, diethyl maleate and triphenyl phosphate) and enzymatic assays, resistant populations exhibited multiple resistance phenotypes. Cytochrome P450 monooxygenases (P450s), glutathione S-transferases (GSTs), and carboxylesterases (CarEs) were found to be involved in pyrethroid resistance in Isfahan population, P450s and GSTs in Mobarake population and CarEs detoxified pyrethroids in Natanz and Alavijeh populations. As substitution of Leucine (CTT) with Phenylalanine (TTT) at position 1014 of the voltage sensitive sodium channel (VSSC) gene is the most common mutation conferring resistance to pyrethroids in M. domestica, we sequenced a partial fragment of IIS6 and L1014F mutation was detected in all resistant populations. The present study provides valuable information for early detection of pyrethroid resistance and developing resistance management strategies in the house fly populations.     

Deltamethrin‐induced feeding plasticity in pyrethroid‐susceptible and ‐resistant strains of the maize weevil,Sitophilus zeamais

Phenotypic plasticity contributes to the adaptative evolution of populations exposed to new or altered environments. Feeding plasticity is a component of phenotypic plasticity not usually considered in insect strains adapted to insecticide‐altered environments, but which may either accentuate or mitigate insecticide resistance. This is a concern in the pyrethroid‐resistant strains of the maize weevil Sitophilus zeamais Motsch. (Col., Curculionidae), and the reason for this study. A pyrethroid‐susceptible and two pyrethroid‐resistant strains of maize weevil were subjected to free‐choice and no‐choice tests with maize grains sprayed with increasing doses of the pyrethroid, deltamethrin. The insects from the pyrethroid‐resistant strains exhibited higher feeding avoidance with increased deltamethrin doses than insects from the susceptible strain when subjected to free‐choice tests. The strains of maize weevil physiologically resistant to pyrethroids were also behaviourally resistant to deltamethrin – an additional management concern. The resistant strains avoid deltamethrin‐sprayed grains and are less nutritionally affected by this compound, with divergent responses from the susceptible strain with increased doses of deltamethrin. Furthermore, the higher relative growth rate and consequently higher efficiency of food conversion observed in the insecticide‐resistant strains were significant even without insecticide exposure, indicating that these traits are stimulus‐independent and may persist even without further insecticide selection, potentially limiting the options available for their management.     

High resistance of field populations of the cotton aphid Aphis gossypii Glover (Homoptera: Aphididae) to pyrethroid insecticides in Pakistan

Samples of the cotton aphid, Aphis gossypii Glover, populations collected from the vicinity of Multan in central Pakistan from 1997 to 2000 were evaluated for pyrethroid resistance in comparison with a susceptible laboratory colony using a leaf-dip bioassay. Resistance to seven pyrethroid insecticides viz. cypermethrin, alphacypermethrin, zetacypermethrin, cyfluthrin, fenpropathrin, bifenthrin, and lambdacyhalothrin was generally very high. However, A. gossypii consistently showed lower resistance to deltamethrin than to other pyrethroids. The lower deltamethrin resistance implies that deltamethrin might be less affected by the resistance mechanism(s) present, a feature that could potentially be exploited in strategies for managing A. gossypii. The influence of insecticide use on cotton on the extent and dynamics of resistance in A. gossypii is discussed.     

Changes in malathion and pyrethroid resistance after cypermethrin selection of Culex quinquefasciatus field populations of Cuba

Abstract. Use. of the organophosphorus insecticide malathion for mosquito control in Cuba, for 7 years up to 1986, selected elevated non-specific esterase and altered acetylcholinesterase (AChE) resistance mechanisms in Culex quinquefasciatus. In central Havana space-spraying of malathion was replaced by the pyrethroid cypermethrin in 1987: alternate cycles of malathion and cypermethrin were applied in some of the more rural areas of Havana district during 1987-91. Consequently, populations of Cx quinquefasciatus in the central area of Havana developed resistance to cypermethrin, but there is no evidence of pyrethroid resistance in the outlying areas. Malathion resistance levels declined significantly after 1986, measured both by bioassay and the frequency of the elevated esterase resistance mechanism, and then stabilized with no measurable decline during 1990 in any of the populations tested. These populations had less than 10% frequency of susceptible homozygotes for both the esterase and AChE resistance mechanisms, indicating that organophosphate resistance is still prevalent in Cuban Cx quinquefasciatus. These two mechanisms appear to be in linkage equilibrium, suggesting that current selection for double resistance is not strong. In the central Havana region, pirimiphos-methyl, an organophosphorus insecticide unaffected by the two common malathion resistance mechanisms, is now being used in a resistance management strategy designed to avoid pyrethroid resistance spreading.