Environmental factors associated with the distribution of Anopheles arabiensis and Culex quinquefasciatus in a rice agro-ecosystem in Mwea, Kenya

Studies were conducted between May and June, 2006 to investigate the environmental factors affecting the distribution of An. arabiensis Patton and Culex quinquefasciatus Say in Mwea, Kenya. The sampling unit comprised all non-paddy aquatic habitats and ten randomly selected paddies and canals located within a 200 m radius from the periphery of the study site. Thirteen physico-chemical variables were recorded for each sampling site in each sampling occasion and a sample of mosquito larvae and other aquatic invertebrates collected. The non-paddy aquatic habitats identified included pools and marshes. Morphological identification of 1,974 mosquito larvae yielded four species dominated by Cx. quinquefasciatus (73.2%) and An. arabiensis (25.0%). Pools were associated with significantly higher Cx. quinquefasciatus larval abundance and less diversity of other aquatic invertebrates compared with other habitat types. In contrast, the abundance of An. arabiensis did not differ significantly among habitat types. Culex quinquefasciatus habitats had higher water conductivity and exhibited a higher abundance of other aquatic invertebrates than An. arabiensis habitats. Chi-square analysis indicated that the two species were more likely to coexist in the same habitats than would be expected by chance alone. Anopheles arabiensis larvae were positively associated with dissolved oxygen and adults of family Haliplidae and negatively associated with emergent vegetation and Heptageniidae larvae. Culex quinquefasciatus larvae were positively associated with dissolved oxygen, total dissolved solids, Chironomidae larvae, and Microvelidae adults and negatively associated with emergent vegetation. These findings suggest that both biotic and abiotic factors play a significant role in niche partitioning among Cx. quinquefasciatus and An. arabiensis, a factor that should be considered when designing an integrated vector control program.     

The salivary purine nucleosidase of the mosquito,Aedes aegypti

A cDNA clone originating from adult female Aedes aegypti mosquitoes was found with substantial similarity to nucleosidases of the EC 3.2.2.1 enzyme class. Although this type of enzyme is unusual in animals, abundant enzyme activity was found in salivary homogenates of this mosquito, but not in salivary homogenates of the mosquitoes Anopheles gambiae and Culex quinquefasciatus, or the sand fly Lutzomyia longipalpis. Aedes salivary homogenate hydrolyses inosine and guanosine to hypoxanthine and xanthine plus the ribose moiety, but does not hydrolyse the pyrimidines uridine and cytidine, thus characterizing the presence of a purine nucleosidase activity. The enzyme is present in oil-induced saliva, indicating that it is secreted. Male Ae. aegypti salivary gland homogenates (SGH) have very low purine nucleosidase activity, suggesting that the enzyme plays a role in mosquito blood feeding. A novel isocratic HPLC method to separate nucleosides and their bases is described.     

Cell death and regeneration in the midgut of the mosquito, Culex quinquefasciatus

Haematophagy, the utilization of blood as food, has evolved independently among insects such as mosquitoes, bedbugs, fleas, and others. Accordingly, several distinct biological adaptations have occurred in order to facilitate the finding, ingestion and digestion of blood from vertebrate sources. Although blood meals are essential for survival and reproduction of these insects, mechanical and chemical stresses are caused by the ingestion of a sizable meal (frequently twice or more times the weight of the insect) containing large amounts of cytotoxic molecules such as haem. Here we present data showing that the stresses caused by a blood meal induce cell death in the midgut epithelium of Culex quinquefasciatus mosquitoes. The process involves apoptosis, ejection of dead cells to the midgut lumen and differentiation of basal regenerative cells to replace the lost digestive cells. The basal cell differentiation in blood-fed mosquito midguts represents an additional mechanism by which insects cope with the stresses caused by blood meals. C. quinquefasciatus adult females are unable to replace lost cells following a third or fourth blood meal, which may have a significant impact on mosquito longevity, reproduction and vectorial capacity.     

Efficacy of etofenprox against insecticide susceptible and resistant mosquito strains containing characterized resistance mechanisms

tofenprox is a non-ester pyrethroid insecticide with comparable toxicity and a similar mode of action to other pyrethroids. Cross-resistance studies on mosquitoes showed no effect of carboxylesterase, elevated esterase, altered acetylcholinesterase or glutathione S -transferase-based resistance mechanisms on etofenprox toxicity, when compared to standard susceptible strains of Anopheles and Culex . Cross-resistance to etofenprox occurred in a pyrethroid-resistant strain of Culex quinquefasciatus with both oxidase and 'kdr'-like resistance mechanisms.
Dose–response data for susceptible mosquito strains suggest that, in standard W.H.O. susceptibility tests of adult mosquitoes, appropriate discriminating concentrations of etofenprox for detection of resistance would be 0.1% for Culex and 0.25% for Anopheles .     

Malathion and pyrethroid resistance in Culex quinquefasciatus from Cuba: efficacy of pirimiphos-methyl in the presence of at least three resistance mechanisms

Use of malathion for mosquito control in Cuba for 7 years up to 1986 has selected for elevated non-specific esterase and altered acetylcholinesterase (AChE) resistance mechanisms in populations of the pest mosquito Culex quinquefasciatus Say. These mechanisms are still present in relatively high frequencies in the Havana area, despite the replacement of malathion by pyrethroid insecticides for the last 3 years in the mosquito control programme. Samples of Culex quinquefasciatus populations from within a 100 km radius of Havana had high levels of resistance to malathion and lower levels of resistance to propoxur, but there was little or no cross-resistance to the organophosphorus insecticide pirimiphos-methyl. Selection with malathion for twenty-two consecutive generations in the laboratory increased the level of malathion resistance to 1208-fold and propoxur level to 1002-fold, but the maximum level of pirimiphos-methyl resistance was only 11-fold. Pirimiphos-methyl is still operationally effective, despite the resistance mechanisms segregating, so this insecticide if used for control is unlikely to select either of the known resistance factors directly in the field population. Since 1986, pyrethroids have been used extensively, and low levels of pyrethroid resistance were detected in two of five field population samples tested. Malathion selection did not increase the level of pyrethroid resistance, which indicates that one or more distinct pyrethroid resistance factors are now being selected in the field populations of Culex quinquefasciatus.     

Relationship between mosquito (Diptera: Culicidae) landing rates on a human subject and numbers captured using CO2-baited light traps

Capture rates of insectary-reared female Aedes albopictus (Skuse), Anopheles quadrimaculatus Say, Culex nigripalpus Theobald, Culex quinquefasciatus Say and Aedes triseriatus (Say) in CDC-type light traps (LT) supplemented with CO2 and using the human landing (HL) collection method were observed in matched-pair experiments in outdoor screened enclosures. Mosquito responses were compared on a catch-per-unit-effort basis using regression analysis with LT and HL as the dependent and independent variables, respectively. The average number of mosquitoes captured in 1?min by LT over a 24-h period was significantly related to the average number captured in 1?min by HL only for Cx. nigripalpus and Cx. quinquefasciatus. Patterns of diel activity indicated by a comparison of the mean response to LT and HL at eight different times in a 24-h period were not superposable for any species. The capture rate efficiency of LT when compared with HL was ≤15% for all mosquitoes except Cx. quinquefasciatus (43%). Statistical models of the relationship between mosquito responses to each collection method indicate that, except for Ae. albopictus, LT and HL capture rates are significantly related only during certain times of the diel period. Estimates of mosquito activity based on observations made between sunset and sunrise were most precise in this regard for An. quadrimaculatus and Cx. nigripalpus, as were those between sunrise and sunset for Cx. quinquefasciatus and Ae. triseriatus.     

Differential effects of Bacillus sphaericus strain 2362 on Culex quinquefasciatus and its competitor Culex cinereus in West Africa

The larval susceptibility to Bacillus sphaericus strain 2362 of the non-man-biting mosquito Culex cinereus and the urban filariasis vector Culex quinquefasciatus, two competitor mosquitoes in polluted habitats, was compared. In the laboratory, both species ingested a similar amount of B. sphaericus spores when fed c. 2 x 10(5) spores per ml for 30 min. However, in the same experiment, third-instar larvae of Cx quinquefasciatus were reduced by 98% at 24 h exposure while Cx cinereus larvae were only reduced by 6% at 72 h. In the field, preimaginal populations of Cx cinereus ingested, within a week, more than 99% of the applied spores, but showed no significant decrease through 14 days in cesspools treated at 10 g/m2 of a flowable concentrate of B. sphaericus 2362, containing 2 x 10(10) spores/g. It is proposed that specific biological control of Cx quinquefasciatus could result from appropriate treatment of breeding-sites with larvicidal B. sphaericus and competitive displacement by Cx cinereus or other mosquitoes with larvae that are more tolerant of B. sphaericus.     

Differential modulation of murine host immune response by salivary gland extracts from the mosquitoes Aedes aegypti and Culex quinquefasciatus

Mosquitoes (Diptera: Culicidae) are major vectors of numerous infectious agents. Compounds in mosquito saliva not only facilitate blood-feeding, but may also have an impact upon the immune system of vertebrate hosts. Consequently, the exposure to mosquito saliva may influence pathogen transmission, establishment and disease development. Using two medically important vector mosquitoes, Aedes aegypti (L.) and Culex quinquefasciatus Say, we examined the effects of mosquito saliva on immune cells of host mice. After antigen-specific or non-specific stimulation, murine splenocyte proliferation and production of both Th1 and Th2 cytokines were significantly reduced in the presence of salivary gland extract (SGE) from Ae. aegypti, but not SGE from Cx. quinquefasciatus. T cell populations were highly susceptible to this suppression, showing increased mortality and reduced division rates - judged by flow cytometric analyses. Evidently these two culicine mosquitoes differ in their host immunomodulatory activities.     

Predation on Mosquito Larvae by Mesocyclops thermocyclopoides (Copepoda: Cyclopoida) in the Presence of Alternate Prey

The cyclopoid copepod Mesocyclops thermocyclopoides, a dominant invertebrate predator in many shallow ponds and temporary water bodies in northern India, feeds on cladocerans, rotifers, ciliates and when present, on mosquito larvae also. We studied in the laboratory the prey consumption rates of the copepod on first and fourth instar larvae of two species of mosquito (Anopheles stephensi and Culex quinquefasciatus) in relation to their density. We also studied its prey selectivity with mosquito larvae in the presence of an alternate prey (the cladocerans‐either Moina macrocopa or Ceriodaphnia cornuta) in different proportions. With either mosquito species, the copepod actively selected Instar‐I larvae, avoiding the Instar‐IV larvae, and with either instar, selected Anopheles stephensi over Culex quinquefasciatus. When prey choice included the cladoceran as an alternate prey, the copepod selected the cladoceran only when the other prey was Instar‐IV mosquito larvae. Our results point to the potential and promise of M. thermocyclopoides as a biological agent for controlling larval populations of vectorially important mosquito species.     

The characterization of ion regulation in Amazonian mosquito larvae: evidence of phenotypic plasticity,population-based disparity,and novel mechanisms of ion uptake

This study is the first step in characterizing ion uptake mechanisms of mosquito larvae from the Amazon region of Brazil. Hemolymph NaCl levels and rates of unidirectional Na(+) and Cl(-) uptake were measured in larvae of Aedes aegypti and Culex quinquefasciatus in a series of environmental manipulations that are known to challenge ion regulation in other aquatic animals. Despite being reared for numerous generations in dilute media (20 micromol L(-1) NaCl), both species were able to maintain high hemolymph NaCl concentrations, a departure from previous studies. Exposure to distilled water or high-NaCl media did not affect hemolymph ion levels, but pH 3 caused significant decreases in hemolymph Na(+) and Cl(-) levels in both species. Exposure to water from Rio Negro (pH 5.5), an organically rich but ion-poor body of water, did not disturb hemolymph Na(+) and Cl(-) levels or the uptake of these ions. Acute exposure to control media or Rio Negro water titrated to pH 3.5 caused inhibition of Na(+) uptake and stimulation of Cl(-) uptake in C. quinquefasciatus, but A. aegypti larvae experienced only a significant reduction of Na(+) uptake in Rio Negro/pH 3.5 treatment. The stimulation of Cl(-) uptake at low pH has been documented only in aquatic insects and differs from all other invertebrate and vertebrate species. A similar pattern of Na(+) uptake inhibition and Cl(-) uptake stimulation was observed in A. aegypti larvae exposed to bafilomycin A(1), a blocker of V-type H(+) ATPase. Culex quinquefasciatus larvae were unaffected by this drug. Both Na(+) and Cl(-) uptake were reduced when C. quinquefasciatus larvae were exposed to acetazolamide, indicating that H(+) and HCO(3)(-), derived from hydration of CO(2), are involved with Na(+) and Cl(-) uptake. Kinetic analysis of Na(+) and Cl(-) uptake in C. quinquefasciatus, A. aegypti, and Anopheles nuneztovari larvae indicate that these Amazonian species share similar high-capacity and high-affinity mechanisms. Comparison of the Amazonian C. quinquefasciatus with a Californian population provided evidence of both phenotypic plasticity and population disparity in Na(+) and Cl(-) uptake, respectively. When the California population of C. quinquefasciatus was reared in a medium similar to that of the Amazonian group (60 micromol L(-1) NaCl) instead of 4,000 micromol L(-1) NaCl, larvae increased both Na(+) uptake capacity (J(max)) and affinity (i.e., reduced K(m)), yet Cl(-) uptake did not change from its nonsaturating, low-capacity pattern. In the reverse experiment, Amazonian C. quinquefasciatus demonstrated plasticity in both Na(+) and Cl(-) uptake by significantly reducing rates when held in 4,000 micromol L(-1) NaCl for 3 d.     

A polymerase chain reaction based method for the detection of Culex quinquefasciatus (Diptera: culicidae)

Culex quinquefasciatus Say is the major vector of the filarial parasite Wuchereria bancrofti (Cobbold) which causes lymphatic filariasis in humans. A repetitive DNA sequence from the genome of C. quinquefasciatus has been cloned and completely sequenced. The 693 bp cloned fragment had an A+T content of 72%. Dot matrix analysis of the fragment did not reveal any direct or inverted repeats within it. Southern blot analysis using a variety of restriction enzymes appeared to indicate that the cloned fragment was interspersed within the genome with a copy number of approximately 30,000. A search of the GenBank database did not reveal significant homologies to any previously cloned sequences. Although the probe was sensitive enough to detect picogram quantities of DNA, it was not specific for C. quinquefasciatus, as it hybridized with DNA from other mosquito species, Culex pseudovishnui Colless, Culex gelidus Theobald, Culex tritaeniorhynchus Giles, Anopheles vagus D?nitz and Mansonia uniformis (Theobald). However PCR primers derived from the cloned sequence, IpC, were found to be specific and amplified only C. quinquefasciatus DNA. The optimized PCR assay was found to be very sensitive and was capable of detecting DNA from all stages of C. quinquefasciatus thus making it an ideal diagnostic tool.     

Weather and landscape factors associated with adult mosquito abundance in southwestern Georgia, U.S.A

Mosquito community composition and population dynamics were compared to weather variables and land use/cover data during 2008 to determine which variables affected population dynamics at the J.W. Jones Ecological Research Center in southwestern Georgia. Models relating adult mosquito distributions to weather variables and time of year were compared using Akaike's Information Criterion (AIC) model selection. Precipitation, temperature, humidity, and Keetch-Byram Drought Index were important factors correlated with mosquito abundance or presence/absence for the species considered. A cluster analysis, which grouped eight sites based on the percentages of land use/cover and hydric soils located in a 1-km radius surrounding collection sites, and an indicator species analysis were used to investigate the associations among 11 mosquito species and sites with similar land use/cover. Aedes albopictus (Skuse), Culex coronator Dyar & Knab, Culex quinquefasciatus Say, and Culex salinarius Coquillett were associated with sites that had the most anthropogenic influence, while Coquillettidia perturbans (Walker) and Psorophora ferox (von Humboldt) were associated with natural land cover such as wetlands and forested land. This study demonstrates that regional climate and land use/cover data can be predictive of the population dynamics of certain mosquito populations and is the first to examine how the distribution of Cx. coronator adults relate to land use/cover in the southeastern United States.     

Reproductive aspects of the mosquito Culex quinquefasciatus (Diptera:Culicidae) infected with Wuchereria bancrofti (Spirurida: Onchocercidae)

This study reports on the relationship between Wuchereria bancrofti infection and female body size, intake of blood and fecundity in the mosquito Culex quinquefasciatus, vector of this filarial parasite in Recife (Brazil). Adults from field collected larvae were infected via a membrane feeding procedure, using blood with parasitaemia ranging from 724-6,000 mf/ml. A positive correlation was observed between mosquito size (measured by wing length) and egg production in uninfected females. However, this relationship did not exist in W. bancrofti infected mosquitoes. This change is unlikely to be the result of changes in blood ingestion as no significant difference was found when infected and uninfected females were compared. Variation in egg production observed between trials could not be associated with parasite density in the blood. These results suggest infection with W. bancrofti may disrupt the relationship between mosquito size and egg production during the first gonotrophic cycle of C. quinquefasciatus such that fecundity is sometimes reduced. However, this overall affect is variable and many groups of mosquitoes do not respond in this way.     

Epoxide hydrolase activities and epoxy fatty acids in the mosquito Culex quinquefasciatus

Culex mosquitoes have emerged as important model organisms for mosquito biology, and are disease vectors for multiple mosquito-borne pathogens, including West Nile virus. We characterized epoxide hydrolase activities in the mosquito Culex quinquefasciatus, which suggested multiple forms of epoxide hydrolases were present. We found EH activities on epoxy eicosatrienoic acids (EETs). EETs and other eicosanoids are well-established lipid signaling molecules in vertebrates. We showed EETs can be synthesized in vitro from arachidonic acids by mosquito lysate, and EETs were also detected in vivo both in larvae and adult mosquitoes by LC-MS/MS. The EH activities on EETs can be induced by blood feeding, and the highest activity was observed in the midgut of female mosquitoes. The enzyme activities on EETs can be inhibited by urea-based inhibitors designed for mammalian soluble epoxide hydrolases (sEH). The sEH inhibitors have been shown to play diverse biological roles in mammalian systems, and they can be useful tools to study the function of EETs in mosquitoes. Besides juvenile hormone metabolism and detoxification, insect epoxide hydrolases may also play a role in regulating lipid signaling molecules, such as EETs and other epoxy fatty acids, synthesized in vivo or obtained from blood feeding by female mosquitoes.     

Isolation of a fragment homologous to the rp49 constitutive gene of Drosophila in the Neotropical malaria vector Anopheles aquasalis (Diptera: Culicidae)

The constitutive ribosomal gene rp49 is frequently used as an endogenous control in Drosophila gene expression experiments. Using the degenerate primer PCR technique we have cloned a fragment homologous to this gene in Anopheles aquasalis Curry, a Neotropical vector of malaria. In addition, based on this first sequence, a new primer was designed, which allowed the isolation of fragments of rp49 in two other species, Aedes aegypti (Linnaeus) and Culex quinquefasciatus Say, suggesting that it could be used to clone fragments of this gene in a number of other mosquito species. Primers were also designed to specifically amplify rp49 cDNA fragments in An. aquasalis and Ae. aegypti, showing that rp49 could be used as a good constitutive control in gene expression studies of these and other vectorially important mosquito species.     

Salivary gland proteins of the mosquito Culex quinquefasciatus

Several properties of the salivary glands of Culex quinquefasciatus mosquitoes were analysed. The amount of protein in female salivary glands increased from 0.26 microg on day one after emergence to about 1.4 microg on day seven. The major polypeptides found in the female salivary glands had molecular weights of 35.7, 28.3, and 20.5 kDa. Antibodies produced by mice immunized by bites of Culex quinquefasciatus female mosquitoes reacted with the 35.7 and 28.3 kDa polypeptides, showing that these molecules were secreted by mosquitoes during blood feeding. The salivary glands of C. Quinquefasciatus females displayed the same morphological and biochemical organization as that of Aedes aegypti mosquitoes, accumulating apyrase in the distal portions and alpha-glucosidase in the proximal portions of the gland. Arch.     

Changes in the haemocyte population of the mosquito, Culex quinquefasciatus, following infection with the filarial parasite, Wuchereria bancrofti

The mosquito Culex quinquefasciatus Say (Diptera: Culicidae) is the vector of the filarial parasite Wuchereria bancrofti (Cobbold) (Spirurida: Onchocercidae), which causes human bancroftian filariasis. Information on the mosquito humoral response against the filarial parasite during the process of its infection and development is important, as it decides the vector competence of the mosquito. Visible changes in the haemocyte population of mosquito, if any, will be an indicator of the possible humoral factors. The present study was aimed at investigating changes in the populations of various types of haemocytes of Cx. quinquefasciatus following infection with W. bancrofti. On day 2 post-feeding on microfilaraemic blood, the haemolymph perfusate of infected mosquitoes with L1 stage of the parasite showed 44.1% granulocytes, 42% prohaemocytes and 13.9% plasmatocytes, whereas that of the control mosquitoes fed on amicrofilaraemic blood showed 63.4% plasmatocytes, 22.2% prohaemocytes and 14.4% granulocytes. Differences in the population numbers of haemocyte types between the infected and control were significant (P > 0.05). However, the mosquitoes examined on day 6 post-feeding, when the parasite was in L2 stage, did not show any such changes. But, similar changes reappeared on day 12 in mosquitoes with L3 stage of the parasite. The observed haemocyte population changes indicate the possibility of some amount of humoral immune response, through the production of certain immune molecules, in Cx. quinquefasciatus infected with W. bancrofti. The nature and exact role of such a response on the filarial parasite development need further investigation.     

The effect of predatory fish exudates on the ovipostional behaviour of three mosquito species: Culex quinquefasciatus, Aedes aegypti and Culex tarsalis

Abstract.  Three mosquito species, Culex tarsalis Coquillett, Culex quinquefasciatus Say and Aedes aegypti L. (Diptera: Culicidae), were examined in laboratory binary choice experiments to investigate whether fish exudates from the mosquitofish, Gambusia affinis (Baird & Girard) (Cyprinodontiformes: Poecilliidae), deter oviposition and whether the responses of these mosquito species to fish exudates in oviposition sites are consistent with the risk of predation from fish experienced by each species in their respective natural breeding habitats. Culex tarsalis was deterred significantly from egg laying by the presence of fish exudates in oviposition cups, consistent with high levels of predation by fish in natural breeding sites. Egg laying by Cx quinquefasciatus was slightly reduced in water with fish exudates, but was not consistently deterred by water conditioned by mosquitofish, consistent with the species' relatively low risk of fish predation in natural habitats. Oviposition by container-breeding Ae. aegypti was not deterred by the presence of fish exudates in oviposition cups, consistent with a low risk of predation by fish in natural habitats.