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The effects of minor variations in the malathion molecule upon the resistance of Culex tarsalis have been examined. By replacing the carboethoxy group by carbomethoxy, the 60-fold resistance to malathion is abolished. The results confirm the importance of carboxyesterase action in determining susceptibility to malathion.
The author reviews the growing body of evidence that urban populations of Culex pipiens fatigans have increased extensively during recent years in both Africa and Asia. Two main factors seem to be responsible-the spread of urbanization, providing favourable habitats for this essentially urban mosquito; and the increasing use of residual insecticides, leading to disregard of conventional sanitary measures and a consequent proliferation of suitable culicine breeding-places. In addition, it is possible that the use of insecticides may be bringing about a selective destruction of less resistant competitors of C. fatigans.Present knowledge of this species is too exiguous to allow of a full understanding of the reasons for its increase, to assess its significance in relation to disease transmission, or definitively to adopt economical and practicable control methods. However, a number of possible lines of approach to this problem are suggested.
This paper is concerned mainly with the relationship between microfilarial periodicity and vector periodicity. The so-called "non-periodic" Pacific form of Wuchereria bancrofti in fact shows a well-marked and relatively constant periodicity. The amplitude of this periodicity is low, which may account for the difficulty of detecting it in small clinical samples. The periodicity is well adapted to the biting cycle of Aëdes polynesiensis, though less so than that of the "semi-periodic" Brugia malayi to forest Mansonioides. Microfilarial periodicity is discussed in the light of recent work on circadian rhythms in other animals and it is suggested that certain possibilities have been given insufficient weight. The use of the term "migration" to describe microfilarial translocation within the host may have given rise to misunderstanding but it is in good agreement with current concepts of migration. Recent work suggests some behavioural heterogeneity in Culex fatigans which could render it a useful mosquito for elucidating certain important but little-understood problems.
Observations have been made on Culex pipiens fatigans in the Taveta-Pare area in East Africa from 1954 to 1961, during which time dieldrin was applied to local houses over a 3(1/2)-year period as part of an experiment in malaria control. At the end of the period of residual spraying the numbers of C. p. fatigans had been reduced by two-thirds in the Taveta area, but in the South Pare area, only some 160 km away, this mosquito was twice as numerous as before spraying. The results of susceptibility tests carried out in untreated and dieldrin-treated areas showed that the susceptibility of C. p. fatigans in East Africa to chlorinated hydrocarbon insecticides was low compared with that of Anopheles gambiae, and varied by at least 20 times in some areas relatively short distances apart. C. p. fatigans from all areas sampled were, however, susceptible to fenthion and malathion.
The mechanism of resistance to DDT was investigated in Culex mosquitos to ascertain whether it was associated with detoxication to DDE as in houseflies and Aedes aegypti. Resistant larvae of C. tarsalis were found to effect this dehydrochlorination, whereas the susceptible larvae did not; resistant larvae of C. fatigans completely converted all the absorbed DDT to the metabolite DDE. Enzyme assays in vitro showed that the resistant C. fatigans had 10 times the dehydrochlorinating activity, and resistant C. tarsalis four times the dehydrochlorinating activity, of their susceptible counterparts. The DDT-dehydrochlorinase enzyme of C. fatigans resembled that of Aedes aegypti more than that of the housefly, though differing from it in at least one respect.
The toxic and/or development retarding effects on Culex pipiens mosquito larvae by methanol and ether extracts of Azadirachta indica, Rhazya stricta and Syzygium aromaticum were investigated separately. All were found to show biological activity, however, the methanol extracts showed the most profound effects. R. stricta showed marked acute (2 d) and chronic (10 d) toxic effects, having an LC(50) and 95% CL of 251(209-326) and 140(110-178); 467(416-699) and 211(198-421) ppm, for the methanol and ether extracts, respectively. Only 3.3% of the larvae pupated and no adults emerged even at the lowest concentration (200 ppm) of methanol extract. Both A. indica extracts were toxic to C. pipiens larvae but at higher concentrations, showing an acute and chronic LC(50) and 95% CL of 824(692-980) and 265(111-481); 1620(1380-1892) and 675(514-887) ppm for the methanol and ether extracts, respectively. The methanol extracts of A. indica, at concentrations above 800 ppm, reduced pupation to 3.3% and completely inhibited adult emergence. Both extracts of S. aromaticum were less toxic to the larvae, however their influence on development was remarkable, causing complete inhibition of adult emergence at 200 and 600 ppm concentrations of the methanol and ether extracts, respectively. Future application of these extracts to larval habitats may lead to promising results in mosquito management programmes.
Resistance to the organophosphate insecticide, malathion, in a strain of Culex tarsalis mosquitoes is due to increased activity of a malathion carboxylesterase (MCE). To determine whether resistance was due to a qualitative or quantitative change in the MCE, the enzyme was purified from both malathion-resistant and -susceptible mosquitoes. Enzyme kinetic measurements revealed that the two strains have one MCE in common, but resistant mosquitoes also have a unique MCE which hydrolyses malathion 18 times faster. Interestingly, this MCE does not hydrolyse alpha-naphthyl acetate, a substrate commonly used to detect increased levels of esterases in other organophosphate-resistant insects. Unlike the over-produced esterase of some related mosquito species, each MCE in C. tarsalis accounts for only a small fraction (0.015%) of the total extractable protein in either strain. Therefore, resistance in these insects is due to the presence of a qualitatively different enzyme, and not to a quantitative increase of a non-specific esterase. This study therefore demonstrates that the underlying biochemical mechanisms of insecticide resistance in one insect cannot necessarily be predicted from those of another, even closely related species.
When the Culex tigripes spermatid begins to elongate, the nucleus exhibits on its surface invaginations of the nuclear envelope. These invaginations have a uniform diameter of 0.3 microns. They separate from the envelope of the nucleus and form spherical intranuclear vesicles. In the old spermatids these vesicles are imprisoned in the condensed chromatin. The spermatozoon also possesses these vesicles which are then ovoid in shape. This process of vesiculation permits the diminution of the surface of the nucleus when it decreases in volume during spermiogenesis.
Two mosquito strains of Culex quinquefasciatus (Say), MAmCq(G0) and HAmCq(G0), were collected from Mobile and Huntsville, Alabama, respectively. MAmCq(G0) and HAmCq(G0) were further selected in the laboratory with permethrin for one and three generations, respectively. The levels of resistance to permethrin in MAmCq(G1) (after one-generation selection) and HAmCq(G3) (after three-generation selection) increased rapidly. Resistance to permethrin in MAmCq(G1) and HAmCq(G3) was partially suppressed by piperonyl butoxide (PBO), S,S,S-tributylphosphorotrithioate (DEF) and diethyl maleate (DEM), inhibitors of cytochrome P450 monooxygenases, hydrolases and glutathione S-transferases (GST), respectively, suggesting these three enzyme families are important in conferring permethrin resistance in both strains. A substitution of leucine to phenylalanine (L to F) resulting from a single nucleotide polymorphism (SNP), termed the kdr mutation, in the para-homologous sodium channel gene has been reported as a very common mutation associated with pyrethroid resistance of insects. A 341-bp sodium channel gene fragment, where the kdr mutation resides, was generated by PCR from genomic DNAs of Cx. quinquefasciatus strains. We found that the kdr mutation was present in both permethrin-selected and unselected HAmCq and MAmCq mosquito populations, suggesting that the kdr mutation plays the role in permethrin resistance. There was no significant change in the frequency and heterozygosity of the A to T SNP for the kdr allele between permethrin-selected and unselected MAmCq and HAmCq mosquitoes, indicating that other mechanisms are involved in the evolution of resistance in mosquitoes selected by permethrin in the laboratory.
A malathion-resistant (RM) strain of Culex pipiens pallens Coq was obtained by successively selecting a field population with malathion in the laboratory. The synergistic effect of iprobenfos on malathion toxicity and alpha-naphthyl acetate (alpha-NA) esterase assay revealed that malathion resistance in the RM strain was associated with increased alpha-NA esterase activity and the synergism was mainly due to the inhibition by iprobenfos of this activity. There was no difference in alpha-NA esterase activity between the larvae and female adults in the susceptible (S) strain, but the activity in the adults was 13-fold higher than in the larvae of the RM strain. To understand the effect of the application of a mixture of iprobenfos and malathion on the evolution of malathion resistance, an artificial strain (Syn) was generated by mixing the RM and S strains with 0.1 frequency of the malathion-resistant individuals. The offspring of the Syn strain were divided into two sub-strains, Rm and Rm+ibp, which were successively treated with, respectively, malathion alone and malathion + iprobenfos (1:2) at LC70. In the mixture, the fungicide iprobenfos acted as a synergist of malathion. After treatment for 10 generations, the resistance level to malathion was 317.4-fold for the Rm sub-strain, whereas for the Rm+ibp sub-strain it was only 38.9-fold, compared with the Syn strain. Similar results were obtained by measurement of alpha-NA esterase activity from both larvae and female adults. The alpha-NA esterase activities in larvae and female adults at F10 generation were 2.6- and 10.9-fold from the Rm+ibp sub-strain and 5.7- and 98.5-fold from the Rm sub-strain, respectively, compared with the Syn strain. The above results suggested that iprobenfos, although it cannot completely stop or prevent the onset of malathion resistance, could dramatically delay its evolution.
The toxicities of Mtx1 toxin against dipteran and lepidopteran species have been evaluated in this study. It was shown that Mtx1 has little or no toxicity to the tested lepidopteran species, but has moderate-level toxicity to Aedes albopictus Skuse (Diptera: Culicidae) and high-level toxicity to both susceptible and binary toxin-resistant Culex quinquefasciatus Say (Diptera: Culicidae). The LC(50) values of Mtx1 against a susceptible C. quinquefasciatus colony SLCq and two resistant colonies RLCq1/C3-41 and RLCq2/IAB59 selected in the laboratory with Bacillus sphaericus (Mayer & Neide) strains C3-41 and IAB59 respectively were 0.508, 0.854 and 0.675 mg L(-1) respectively. The data indicate that Mtx1 has a different mode of action from the binary toxin, and that it could be an alternative toxin to delay or overcome resistance development to binary toxin in C. quinquefasciatus.
The insecticidal activities of essential oil extracts from leaves and flowers of aromatic plants against fourth-instar larvae of the mosquito Culex pipiens molestus Forskal were determined. Extracts of Myrtus communis L were found to be the most toxic, followed by those of Origanum syriacum L, Mentha microcorphylla Koch, Pistacia lentiscus L and Lavandula stoechas L with LC50 values of 16, 36, 39, 70 and 89 mg litre-1, respectively. Over 20 major components were identified in extracts from each plant species. Eight pure components (1,8-cineole, menthone, linalool, terpineol, carvacrol, thymol, (1S)-(-)-alpha-pinene and (1R)-(+)-alpha-pinene) were tested against the larvae. Thymol, carvacrol, (1R)-(+)-alpha-pinene and (1S)-(-)-alpha-pinene were the most toxic (LC50 = 36-49 mg litre-1), while menthone, 1,8-cineole, linalool and terpineol (LC50 = 156-194 mg litre-1) were less toxic.
In May 2001 a sample of Culex pipiens pipiens variety molestus Forskål from Marin County, California, collected as larvae and reared to adults, was found to show reduced resmethrin and permethrin knock-down responses in bottle bioassays relative to a standard susceptible Cx. pipiens quinquefasciatus Say colony (CQ1). Larval susceptibility tests, using CQ1 as standard susceptible, indicated that the Marin mosquitoes had LC50 resistance ratios of 18.3 for permethrin, 12 for deltamethrin and 3.3 for pyrethrum. A colony of Marin was established and rapidly developed higher levels of resistance in a few generations after exposure to permethrin as larvae. These selected larvae were shown to cross-resist to lambda-cyhalothrin as well as to DDT. However, adult knock-down time in the presence of permethrin, resmethrin and pyrethrum was not increased after increase in tolerance to pyrethroids as larvae. Partial and almost complete reversion to susceptibility as larvae was achieved with S, S, S-tributylphosphorotrithioate and piperonyl butoxide (PBO), respectively, suggesting the presence of carboxylesterase and P450 monooxygenase mediated resistance. Insensitive target site resistance (kdr) was also detected in some Marin mosquitoes by use of an existing PCR-based diagnostic assay designed for Cx. p. pipiens L mosquitoes. Carboxylesterase mediated resistance was supported by use of newly synthesized novel pyrethroid-selective substrates in activity assays. Bottle bioassays gave underestimates of the levels of tolerance to pyrethroids of Marin mosquitoes when compared with mortality rates in field trials using registered pyrethroid adulticides with and without PBO. This study represents the first report of resistance to pyrethroids in a feral population of a mosquito species in the USA.
The efficacy of insecticide mixtures of permethrin (pyrethroid) and propoxur (carbamate) was tested by larval bioassays on two strains of Culex quinquefasciatus (Say), one resistant to pyrethroids and the other resistant to carbamates. The method consisted in combining one insecticide at the highest concentration causing no mortality (LC0) with increasing concentrations of the second one. The concentration-mortality regression lines were determined for permethrin and propoxur alone and in combination, and synergism ratios (SR) were calculated in order to determine the magnitude of an increase or decrease in efficacy with use of the mixtures. With the pyrethroid-resistant strain (BK-PER), the results showed that propoxur at LC0 significantly enhanced the insecticidal activity of permethrin (SR50 = 1.54), especially on the upper range of the concentration-mortality regression. Conversely, when permethrin at LC0 was tested with propoxur against the carbamate resistant strain (R-LAB), an antagonistic effect was observed (SR50 = 0.67). With the BK-PER strain, an increased oxidative detoxification (MFO) appeared to be the main mechanism responsible for the synergistic interaction. Nevertheless, antagonism in the R-LAB strain is probably due to a physiological perturbation implying different target sites for pyrethroid (ie sodium channel) and carbamate insecticides [ie acetylcholinesterase (EC 3.3.3.7) and choline acetyltransferase (EC 2.3.1.6)].
Methanolic extracts of aerial parts of the medicinal plant Argel (Solenostemma argel (Del.) Hayne), incorporated into rearing media of Culex pipiens L. showed effect on oviposition, egg hatchability and larval viability. Acute toxicity of S. argel extract was found to possess LC(50) of 0.037, 0.031, 0.009 and 0.007 ppm and the LC(95) was found 0.394, 0.293, 0.065 and 0.030 ppm, after 1, 2, 4 and 7 days against the larva of C. pipiens under laboratory conditions. The ovicidal effect of S. argel was relatively less pronounced, however, the 0.1% concentration reduced egg hatch by 33.7%. The mortality of the newly hatched larvae, reared from eggs in this media, reached 100% at concentrations as low as 0.025%, 2 days post hatching. Complete suppression of oviposition within the first 2 days was observed, however its action was gradually lost thereafter. Bioactive effects were mainly attributed to the presence of a variety of bioactive organic substances mainly terpenes, pergenine glucosides, alkaloids and sterols.
Within developing ovaries of many insects, some developing follicles or oocytes usually degenerate (follicular atresia or oosorption), while the others may continue to grow to maturity, thus maintaining the balance between the number of eggs and reproductive circumstances such as available nutrients. To help clarify the phenomenon of follicular atresia during ovarian development, we examined cysteine proteinases stored in mosquito Culex pipiens pallens ovaries. First, analysis using synthesized substrates showed that cathepsin B- and L-like proteinases gradually accumulated in the developing ovaries after a blood meal, which required more than 10 min of preincubation under acidic conditions to reach their maximum activities. However, homogenates of degenerating follicles 3 days after feeding showed proteolytic activities without acid treatment, suggesting that the proteinases had already been activated, while the extract of normally developing follicles collected from the same ovaries required more than 10 min of acid preincubation to reach the optimum activities, suggesting that the enzymes remained as inactive forms. Chemical and immunohistochemical analyses showed that more proteinases are located in the cytoplasm, rather than being associated with yolk granules. Ovarian proteinases, which are believed to become activated at the onset of embryogenesis, should also be activated during oogenesis, presumably to enhance oosorption.
Vector control programmes in many countries face the dual problems of parasite drug resistance and insecticide resistance in the insect vectors of the disease. Here we report for the first time a new esterase-based insecticide resistance mechanism in the filariasis vector Culex quinquefasciatus. The field collected COL strain of C. quinquefasciatus from Columbia was heterogeneous for organophosphorus insecticide resistance. On native polyacrylamide gels it had an elevated beta-naphthyl acetate specific esterase with the same Rf as that for the Est beta 1s involved in insecticide resistance in other strains of this mosquito species. After five generations of temephos insecticide selection, both the esterase specific activity with p-nitrophenyl acetate and the temephos LC50 values were increased, suggesting that elevation of esterase activity was the underlying mechanism of resistance. Western blots with antisera raised to Est alpha 2(1) and Est beta 2(1) from C. quinquefasciatus indicated that the COL strain had an elevated Est alpha 3 enzyme which co-migrated on native gels with Est beta 1. Southern blots indicated that an est alpha 3 gene was amplified in the COL strain and a Cuban mosquito strain (MRes), although the restriction digest patterns of the est beta 1 genes in these two strains are different. In contrast, the Californian TEMR strain, with the amplified est beta 1(1) gene, had no associated elevated Est alpha. Restriction digest patterns for COL and TEMR DNA suggest that they contain an identical est beta 1(1) gene, but our data suggest that the est alpha 3 gene occurs on the same amplicon as an est beta 1 gene although the genes are probably > 10 kb apart. Hence, either the COL strain has two est beta 1 genes or the est beta 1(1) amplicon in TEMR has been disrupted at some stage during the long colonisation of this strain and the amplified est alpha has been lost.
Effects of methoprene and 20-hydroxyecdysone on the development and hemagglutination activity (HA) were studied in both sexes of two members of the Culex pipiens complex-anautogenous C. p. quinquefasciatus and autogenous C. p. molestus. Juvenile hormone analogue (methoprene) and 20-hydroxyecdysone caused developmental changes in both mosquito strains. High larval mortality, prolongation of intermolt period in each larval instar and in the pupal stage, and morphological changes in the larval-pupal and pupal-adult transformations were also observed. Developmental changes were accompanied with some differences in the HA. HA was found in both sexes of both experimental mosquito strains. The juvenile hormone analogue used in the larval stage caused significant decrease of HA in the gut of adults of both sexes. On the other hand, 20-hydroxyecdysone decreased HA only in the female gut. Results obtained indicate that HA depends on the sex, the studied organ, and the level of hormones.