Weather as an effective predictor for occurrence of dengue fever in Taiwan

We evaluated the impacts of weather variability on the occurrence of dengue fever in a major metropolitan city, Kaohsiung, in southern Taiwan using time-series analysis. Autoregressive integrated moving average (ARIMA) models showed that the incidence of dengue fever was negatively associated with monthly temperature deviation (beta=-0.126, p=0.044), and a reverse association was also found with relative humidity (beta=-0.025, p=0.048). Both factors were observed to present their most prominent effects at a time lag of 2 months. Meanwhile, vector density record, a conventional approach often applied as a predictor for outbreak, did not appear to be a good one for diseases occurrence. Weather variability was identified as a meaningful and significant indicator for the increasing occurrence of dengue fever in this study, and it might be feasible to be adopted for predicting the influences of rising average temperature on the occurrence of infectious diseases of such kind at a city level. Further studies should take into account variations of socio-ecological changes and disease transmission patterns to better propose the increasing risk for infectious disease outbreak by applying the conveniently accumulated information of weather variability.     

Stormwater drains and catch basins as sources for production of Aedes aegypti and Culex quinquefasciatus

We present data showing that structures serving as drains and catch basins for stormwater are important sources for production of the mosquito arbovirus vectors Aedes aegypti and Culex quinquefasciatus in Mérida City, México. We examined 1761 stormwater drains – located in 45 different neighborhoods spread across the city – over dry and wet seasons from March 2012 to March 2013. Of the examined stormwater drains, 262 (14.9%) held water at the time they were examined and 123 yielded mosquito immatures. In total, we collected 64,560 immatures representing nine species. The most commonly encountered species were Cx. quinquefasciatus (n = 39,269) and Ae. aegypti (n = 23,313). Ae. aegypti and Cx. quinquefasciatus were collected during all 11 months when we found water-filled stormwater drains, and both were found in stormwater drains located throughout Mérida City. We also present data for associations between structural characteristics of stormwater drains or water-related characteristics and the abundance of mosquito immatures. In conclusion, stormwater drains produce massive numbers of Ae. aegypti and Cx. quinquefasciatus across Mérida City, both in the wet and dry seasons, and represent non-residential development sites that should be strongly considered for inclusion in the local mosquito surveillance and control program.     

Spatial analysis of Aedes aegypti immatures in Northern Argentina: Clusters and temporal instability

The objective of this study was to analyze the spatio-temporal patterns of Aedes aegypti immatures based on four entomological surveys that inspected over 6000 households in a large neighborhood of the city of Clorinda between 2007 and 2008. Global and local spatial point pattern analyses of immature presence or absence, habitat quality (estimated using a previously obtained statistical model) and pupal production were performed. Global analyses showed aggregation of both infestation and habitat quality up to 10 times bigger than previously described, ranging from 150 to 400 m between surveys. Pupal production was also clustered but at smaller scales than infestation presence/absence. The location of the clusters was temporally unstable between surveys. There was no spatial structure related to the control strategy; lots treated with temephos and lots uninspected (i.e., closed or refusing) were randomly distributed. These results suggest a combination of exogenous (the aggregation of better quality habitats) and endogenous (dispersal) processes explaining the observed patterns of larger-scale infestation. A spatial targeting strategy at the neighborhood scale would not be as cost-effective in Clorinda as in other sites where stable smaller-scale clusters permit the identification of key premises.     

Spatio-temporal dynamics of dengue 2009 outbreak in Córdoba City,Argentina

During 2009 the biggest dengue epidemic to date occurred in Argentina, affecting almost half the country. We studied the spatio-temporal dynamics of the outbreak in the second most populated city of the country, Córdoba city. Confirmed cases and the results of an Aedes aegypti monitoring during the outbreak were geolocated. The imported cases began in January, and the autochthonous in March. Thirty-three percent of the 130 confirmed cases were imported, and occurred mainly at the center of the city. The autochthonous cases were more frequent in the outskirts, specially in the NE and SE. Aedes aegypti infestation showed no difference between neighborhoods with or without autochthonous cases, neither between neighborhoods with autochthonous vs. imported cases. The neighborhoods with imported cases presented higher population densities. The majority of autochthonous cases occurred at ages between 25 and 44 years old. Cases formed a spatio-temporal cluster of up to 20 days and 12 km. According to a mathematical model that estimates the required number of days needed for transmission according to daily temperature, the number of cases begun to fall when more than 15.5 days were needed. This may be a coarse estimation of mean mosquito survival in the area, provided that the study area is close to the global distribution limit of the vector, and that cases prevalence was very low.     

High potential risk of dengue transmission during the hot-dry season in Nha Trang City, Vietnam

We visited houses and inspected water-holding containers to determine the potential risks of dengue transmission during different seasons. This survey was conducted in two neighbourhoods of Nha Trang City in July and December 2006, which correspond to the middle of the hot-dry season and the beginning of the cool-wet season, respectively. We inspected a total of 1438 wet containers in 196 premises during both survey periods; 20% of the containers were positive for Aedes aegypti larvae and 8% for A. aegypti pupae. Indoor water-holding containers, which were sparsely distributed, exhibited high pupal productivity and efficiency (pupal productivity of a type of container/prevalence of that type of container) in either the first survey conducted in July, or the second, conducted in December. Although rainfall may not influence the number and distribution of water-holding containers in the city, the high average temperature in the first survey period resulted in a higher potential risk of dengue transmission. Our analysis suggests that if intensive source reduction is conducted in summer and containers with high pupal productivity and efficiency are targeted, the risk of dengue transmission in the city could be effectively reduced.     

Infection of adult Aedes aegypti and Ae. albopictus mosquitoes with the entomopathogenic fungus Metarhizium anisopliae

This study describes a laboratory investigation on the use of the insect-pathogenic fungus Metarhizium anisopliae against adult Aedes aegypti and Ae. albopictus mosquitoes. At a dosage of 1.6 x 10(10)conidia/m(2), applied on material that served as a mosquito resting site, an average of 87.1+/-2.65% of Ae. aegypti and 89.3+/-2.2% of Ae. albopictus became infected with the fungus. The life span of fungus-contaminated mosquitoes of both species was significantly reduced compared to uninfected mosquitoes. LT(50)-values of fungus-contaminated mosquitoes ranged between 3.1+/-0.2 days (male Ae. aegypti) and 4.1+/-0.3 days (female Ae. aegypti). LT(50)-values of uncontaminated mosquitoes ranged from 17.7+/-0.4 days (female Ae. albopictus) to 19.7+/-0.6 days (male Ae. albopictus). These results indicate that both mosquito species are highly susceptible to infection with this entomopathogen. Requirements for developing and incorporating this biological control method into current strategies to control major diseases vectored by these species, such as dengue fever, are discussed.     

Effect of temperature on life history traits during immature development of Aedes aegypti and Culex quinquefasciatus (Diptera: Culicidae) from Córdoba city,Argentina

We investigated how ambient temperature under fluctuating conditions affects the larval–pupal immature traits of Aedes aegypti and Culex quinquefasciatus mosquitoes from Córdoba city, Argentina, and established each species development threshold and physiological time. Based on life tables, three cohorts of each mosquito species were reared in the laboratory under small fluctuating temperatures conditions of 15.2 ± 1.7 °C, 17.9 ± 1.6 °C, 21.6 ± 0.7 °C and 25.3 ± 0.4 °C for Ae. aegypti, and 16.6 ± 1.7 °C, 18.7 ± 1.7 °C and 25.2 ± 0.3 °C for Cx. quinquefasciatus. Immature development time and survival values, and also thermal development threshold and physiological time were estimated. Development times of all larval and pupal stages of Ae. aegypti and Cx. quinquefasciatus were significantly affected by the rearing temperatures, decreasing when temperature increased. Mean Ae. aegypti total (larva + pupa) development time ranged from 21.9 to 8.6 days, at 15.2 and 25.3 °C, whereas, for Cx. quinquefasciatus varied between 23.5 to 9.2 days at 16.6 and 25.2 °C, respectively. Larval and pupal survival of both species was affected by different rearing temperatures, increasing in general as temperature increased. For Ae. aegypti the total immature survival ranged from 26% at 15.2 °C to 92% at 21.6 °C; however, temperature did not have significant effect on this variable. The total immature survival of Cx. quinquefasciatus was significantly and positively affected by temperatures, ranging from 32 to 88%, at 16.6 and 25.2 °C. The temperature development threshold and the physiological time estimated for Ae. aegypti and Cx. quinquefasciatus were 11.11 °C and 93.74 degree-days, and 10.96 °C and 136.87 degree-days, respectively. The results of the present study showed that temperature significantly affects the larval–pupal immature traits of these mosquito species of sanitary importance, from the central region of Argentina. All the parameters recorded are useful for the development of mosquito management models.     

Indirect effects of cigarette butt waste on the dengue vector Aedes aegypti (Diptera: Culicidae)

Despite major insecticide-based vector control programs, dengue continues to be a major threat to public health in urban areas. The reasons for this failure include the emergence of insecticide resistance and the narrowing of the spectrum of efficient products. Cigarette butts (CBs), the most commonly discarded piece of waste, also represent a major health hazard to human and animal life. CBs are impregnated with thousands of chemical compounds, many of which are highly toxic and none of which has history of resistance in mosquitoes. This study was performed to examine whether exposure to CB alters various biological parameters of parents and their progeny. We examined whether the mosquito changes its ovipositional behaviors, egg hatching, reproductive capacity, longevity and fecundity in response to CB exposure at three different concentrations. Females tended to prefer microcosms containing CBs for egg deposition than those with water only. There were equivalent rates of eclosion success among larvae from eggs that matured in CB and water environments. We also observed decreased life span among adults that survived CB exposure. Extracts of CB waste have detrimental effects on the fecundity and longevity of its offspring, while being attractive to its gravid females. These results altogether indicate that CB waste indirectly affect key adult life traits of Aedes aegypti and could conceivably be developed as a novel dengue vector control strategy, referring to previously documented direct toxicity on the larval stage. But this will require further research on CB waste effects on non-target organisms including humans.     

Effect of niloticin,a protolimonoid isolated from Limonia acidissima L. (Rutaceae) on the immature stages of dengue vector Aedes aegypti L. (Diptera: Culicidae)

The aim of the present study was to evaluate the mosquitocidal activity of fractions and a compound niloticin from the hexane extract of Limonia acidissima L. leaves on eggs, larvae and pupae of Aedes aegypti L. (Diptera: Culicidae). In these bioassays, the eggs, larvae and pupae were exposed to concentrations of 2.5, 5.0, 7.5 and 10.0 ppm for fractions and 0.5, 1.0, 1.5 and 2.0 ppm for compound. After 24 h, the mortality was assessed and the LC₅₀ and LC₉₀ values were calculated for larvae and pupae. Per cent ovicidal activity was calculated for eggs after 120 h post treatment. Among the sixteen fractions screened, fraction 8 from the hexane extract of L. acidissima generated good mosquitocidal activity against Ae. aegypti. The LC₅₀ and LC₉₀ values of fraction 8 were 4.11, 8.04 ppm against Ae. aegypti larvae and 4.19, 8.10 ppm against Ae. aegypti pupae, respectively. Further, the isolated compound, niloticin recorded strong larvicidal and pupicidal activities. The 2 ppm concentration of niloticin showed 100% larvicidal and pupicidal activities in 24 h. The LC₅₀ and LC₉₀ values of niloticin on Ae. aegypti larvae were 0.44, 1.17 ppm and on pupae were 0.62, 1.45 ppm, respectively. Niloticin presented 83.2% ovicidal activity at 2 ppm concentration after 120 h post treatment and niloticin exhibited significant growth disruption and morphological deformities at sub lethal concentrations against Ae. aegypti. The structure of the isolated compound was identified on the basis of single XRD and spectral data (¹H NMR and ¹³C NMR) and compared with literature spectral data. The results indicate that niloticin could be used as a potential natural mosquitocide.     

Characterization of insecticide resistance in Trinidadian strains of Aedes aegypti mosquitoes

Bioassays and biochemical assays were conducted on eight Trinidadian strains of Aedes aegypti larvae to determine the involvement of biochemical mechanisms in resistance to insecticides. Larval strains were assayed to dichlorodiphenyltrichloroethane (DDT), bendiocarb, temephos and permethrin, using the Centers for Disease Control and Prevention (CDC) time-mortality bioassay method. A Resistance Threshold (RT) was calculated for each insecticide in relation to the CAREC reference susceptible Ae. aegypti strain and larval strains with <80% mortality were considered to be resistant. Biochemical assays were performed to determine the activities of nonspecific esterases (α- and β-), PNPA-esterases, mixed function oxidases (MFO), glutathione-S-transferases (GST) and acetylcholinesterase (AChE) enzymes which are involved in insecticide resistance in mosquitoes. Enzyme profiles of each strain were compared with those of the CAREC reference susceptible strain by Kruskal–Wallis and Dunn's multiple comparison tests (p < 0.05). The CAREC 99th percentile was calculated for each enzyme and the percentage of individuals with enzyme activities above that of the CAREC 99th percentile was calculated. Activities were classified as unaltered (<50%), incipiently altered (15–50%) or altered (>50%) for each strain.The established RTs for permethrin and bendiocarb were 30 and 75 min, respectively; and 120 min for DDT and temephos. All strains were resistant to DDT (1.00–40.25% mortality) and temephos (11.50–74.50% mortality) while six strains were resistant to bendiocarb (51.50–78.50% mortality) and five to permethrin (6.50–42.50% mortality). Biochemical assays revealed that the median activity levels for all enzymes varied significantly (p < 0.05). The Curepe strain had incipiently altered levels of α-esterase while the other seven strains had altered activity with five of them registering 100%. The St Clair strain showed altered activity levels of β-esterase while three strains had incipiently altered levels. The majority of strains had altered activity of MFO enzymes but only the St Clair strain showed altered activity of GST. PNPA-esterases activity was unaltered in all strains and only the Haleland Park strain showed altered remaining AChE activity in the presence of propoxur.Elevated levels of enzymes (incipiently altered or altered), except in the case of PNPA-esterases, show that biochemical resistance may play an important role in the manifestation of insecticide resistance in Trinidadian populations of Ae. aegypti. It is therefore important for insecticide resistance surveillance to be ongoing as the detection of resistance before it spreads throughout an entire population makes it possible for early intervention.     

Reduction in dengue cases observed during mass control of Aedes (Stegomyia) in street catch basins in an endemic urban area in Colombia

Dengue incidence continues to increase globally and, in the absence of an efficacious vaccine, prevention strategies are limited to vector control. It has been suggested that targeting the most productive breeding sites instead of all water-holding containers could be a cost-effective vector control strategy. We sought to identify and continuously control the most productive Aedes (Stegomyia) breeding site in an endemic urban area in Colombia and followed the subsequent incidence of dengue. In the urban area of Guadalajara de Buga, southwestern Colombia, potential breeding sites inside and outside houses were first characterized, and local personnel trained to assess their productivity based on the pupae/person index. Simultaneously, training and monitoring were implemented to improve the dengue case surveillance system. Entomological data and insecticide resistance studies were used to define the targeted intervention. Then, a quasi-experimental design was used to assess the efficacy of the intervention in terms of the positivity index of the targeted and non- targeted breeding sites, and the impact on dengue cases. Street catch basins (storm drains) were the potential breeding site most frequently found containing Aedes immature stages in the baseline (58.3% of 108). Due to the high resistance to temephos (0% mortality after 24 h), the intervention consisted of monthly application of pyriproxyfen in all the street catch basins (n = 4800). A significant decrease in catch basins positivity for Aedes larvae was observed after each monthly treatment (p < 0.001). Over the intervention period, a reduction in the dengue incidence in Buga was observed (rate ratio 0.19, 95% CI 0.12–0.30, p < 0.0001) after adjusting for autocorrelation and controlling with a neighboring town, Palmira, This study highlights the importance of street catch basins as Aedes breeding sites and suggests that their targeted control could help to decrease dengue transmission in such areas.     

Dynamics of midgut microflora and dengue virus impact on life history traits in Aedes aegypti

Significant morbidity and potential mortality following dengue virus infection is a re-emerging global health problem. Due to the limited effectiveness of current disease control methods, mosquito biologists have been searching for new methods of controlling dengue transmission. While much effort has concentrated on determining genetic aspects to vector competence, paratransgenetic approaches could also uncover novel vector control strategies. The interactions of mosquito midgut microflora and pathogens may play significant roles in vector biology. However, little work has been done to see how the microbiome influences the host's fitness and ultimately vector competence. Here we investigated the effects of the midgut microbial environment and dengue infection on several fitness characteristics among three strains of the primary dengue virus vector mosquito Aedes aegypti. This included comparisons of dengue infection rates of females with and without their normal midgut flora. According to our findings, few effects on fitness characteristics were evident following microbial clearance or with dengue virus infection. Adult survivorship significantly varied due to strain and in one strain varied due to antibiotic treatment. Fecundity varied in one strain due to microbial clearance by antibiotics but no variation was observed in fertility due to either treatment. We show here that fitness characteristics of Ae. aegypti vary largely between strains, including varying response to microflora presence or absence, but did not vary in response to dengue virus infection.     

Bionomics of Aedes aegypti subpopulations (Diptera: Culicidae) from Misiones Province, northeastern Argentina

Life statistics of four Aedes aegypti subpopulations from the subtropical province of Misiones were studied during autumn and winter, under semi-natural conditions, coming from the localities of Posadas (SW), San Javier (SE), Bernardo de Irigoyen (NE) and Puerto Libertad (NW). The eastern subpopulations are geographically separated by the central mountain system of the province from the western subpopulations. High percentages of larval and pupal survival (97-100%) were recorded, and no significant differences were detected among the four subpopulations. Larvae and pupae lasted approximately 8 days to complete their development, no significant differences being detected among the four subpopulations studied. Sex ratio recorded did not differ significantly from 1:1. Male longevity did not show difference among the different subpopulations, but female longevity was remarkably different among the four subpopulations (F = 16.27; d.f. = (3;8); P = 0.0009), ranging among 11.45 days for San Javier and 57.87 days for Posadas. Fecundity also varied considerably among subpopulations, the greatest number (307.44 eggs/female) being recorded for Posadas (F = 4.13; d.f. = (3;8); P = 0.04). Ae. aegypti females of the western subpopulations lived longer than the eastern subpopulations studied, therefore, the risk of dengue outbreak would be greater on the Misiones Province border with Paraguay.     

Detailed assessment of microhabitat suitability for Aedes aegypti (Diptera: Culicidae) in Buenos Aires, Argentina

Little information is available on the ecology of Aedes aegypti Linnaeus at the southern extreme of its distribution (Buenos Aires, Argentina), particularly on microhabitat suitability. The aim of our study was to identify at a detailed scale, microhabitat factors that correlate with the presence of preimaginal stages of the mosquito. In March 2001, we performed a spatial census of all containers located in a 1 ha patch within a cemetery in Buenos Aires City. On a reference map (1:700) we plotted the position of graves and surrounding corridors, the location of containers, the shade projected by each plant between 10:00 and 16:00 h and vegetation cover. We classified vegetation by height, substrate by composition and shadow by level of exposure to sunlight. We performed univariate and multivariate logistic regression analyses with nine constructed independent variables, some of them at scales of 0.5, 1, 2, 3, and 10 m. Of 850 receptacles examined, 101 contained preimaginal stages of Ae. aegypti. Level of exposure to sunlight, type of substratum, vegetation height and distance of containers to vegetation were significantly associated with the presence of breeding sites at the studied scales. Final multivariate models were significant at scales of 2 m (chi(3)2=25.693, p<0.001) and 3m (chi(3)2=26.440, p<0.001), and 65.9 and 66.8% of our data were correctly classified, respectively, for each scale. Our results suggest that sites less exposed to sunlight, with taller and closer vegetation, and in shaded and vegetated neighbourhoods were the most favourable microhabitats for Ae. aegypti breeding.     

Surveillance of dengue fever cases using a novel Aedes aegypti population sampling method in Trinidad, West Indies: the cardinal points approach

A novel dengue surveillance method is described and used to evaluate 100 suspected dengue fever (DF) cases in county St. Patrick, Trinidad, West Indies. From the 30 confirmed DF cases fully investigated within 48 h of diagnosis, 63% (19/30 houses) of their homes were found harboring Aedes aegypti immature stages. Only houses at the four cardinal points of the index case rather than the entire neighborhood were investigated. The results showed significantly (P<0.001) more Ae. aegypti positive houses were observed to the east (P<0.04) and west (P<0.01) than to the north and south (P>0.9). In addition, from the 150 houses inspected a total of 474 artificial containers were inspected and treated, of which 20.8% (99) were infested with Ae. aegypti immature stages. More than 49% of the containers inspected were small miscellaneous containers, but they only produced 4.0% of the Ae. aegypti immatures, of which only 0.4% were pupae. Water tanks (41.7%), drums (40.4%) and buckets (24.2%) produced over 98% of the pupae. The results of this study imply that dengue vector control programs in Trinidad could increase their efficiency by applying the cardinal points surveillance approach during DF case investigations and concentrating their vector control measures on the most productive containers located at the east and west of the index cases.     

Development and application of a simple colorimetric assay reveals widespread distribution of sodium channel mutations in Thai populations of Aedes aegypti

Dengue fever and its more serious complications dengue haemorrhagic fever and dengue shock syndrome are growing public health problems in tropical and subtropical countries. In the absence of a vaccine, most dengue control programmes rely heavily on the use of insecticides to target the Aedes mosquito vectors. As a limited number of insecticides are routinely used in control, monitoring for the presence of resistance is an essential component of dengue prevention programmes. The pyrethroid insecticides target the voltage-gated sodium channel on the insects’ neurons. Substitutions at residue 1016 of this protein have been associated with pyrethroid and DDT resistance in Aedes aegypti populations from Latin America and Asia. Here we report on the development of a simple colorimetric assay to detect these mutations in individual mosquitoes. Evaluation of this diagnostic assay on 180 Ae. aegypti individuals from Thailand revealed the presence of high frequencies of the Val1016Gly mutation throughout the country. The assay requires no specialised equipment and will enable monitoring for insecticide resistance associated alleles to be routinely incorporated into dengue surveillance operations.     

Efficacy of Thai herbal essential oils as green repellent against mosquito vectors

Repellency activity of Thai essential oils derived from ylang ylang (Cananga odorata (Lamk.) Hook.f. & Thomson: Annonaceae) and lemongrass (Cymbopogon citratus (DC.) Stapf: Poaceae) were tested against two mosquito vectors, Aedes aegypti (L.) and Culex quinquefasciatus (Say). There were compared with two chemical repellents (DEET 20% w/w; Sketolene Shield^® and IR3535, ethyl butylacetylaminopropionate 12.5% w/w; Johnson's Baby Clear Lotion Anti-Mosquito^®). Each herbal repellent was applied in three diluents; coconut oil, soybean oil and olive oil at 0.33 μl/cm² on the forearm of volunteers. All herbal repellent exhibited higher repellent activity than IR3535 12.5% w/w, but lower repellent activity than DEET 20% w/w. The C. odorata oil in coconut oil exhibited excellent activity with 98.9% protection from bites of A. aegypti for 88.7 ± 10.4 min. In addition, C. citratus in olive oil showed excellent activity with 98.8% protection from bites of C. quinquefasciatus for 170.0 ± 9.0 min. While, DEET 20% w/w gave protection for 155.0 ± 7.1–182.0 ± 12.2 min and 98.5% protection from bites of two mosquito species. However, all herbal repellent provided lower repellency activity (97.4–98.9% protection for 10.5–88.7 min) against A. aegypti than C. quinquefasciatus (98.3–99.2% protection for 60–170 min). Our data exhibited that C. odorata oil and C. citratus oil are suitable to be used as green repellents for mosquito control, which are safe for humans, domestic animals and environmental friendly.     

Interaction of a pesticide and larval competition on life history traits of Culex pipiens

Mosquito larval development occurs in aquatic habitats that are directly or indirectly exposed to chemical contaminants. Little is known about how interaction of these chemicals with other biotic and abiotic stressors impact mosquito populations. We used two levels of nutrient (low and high) and four larval densities (10, 20, 30, 40) to examine the effects of low concentrations of insecticide malathion, on Culex pipiens L. mosquitoes experiencing stress from larval competition. Addition of malathion at the high nutrient condition enhanced survival with increasing larval densities, but this effect was not observed at low nutrient condition. Males exposed to malathion were significantly larger than those from control treatments while the effect of malathion on size of females varied with larval density and the level of nutrients. Larval exposure to malathion and low nutrient resulted in significantly larger females with increases in larval densities compared with other treatments. The effect of malathion on male longevity varied with larval density and amount of nutrients. At higher densities, male longevity was consistently higher in low nutrient than in high nutrient conditions and addition of malathion in high nutrient treatment increased male longevity at the highest density. These effects are most likely attributable to release from competition among survivors after mortality from malathion and density-dependent effects. We conclude that biotic conditions of the larval habitat can alter the impact of low concentrations of chemical contaminants on mosquito populations in ways that may influence the pattern of disease transmission and the outcome of vector control efforts.