The biology of Aedes (Ochlerotatus) albifasciatus Macquart, 1838 (Diptera: Culicidae) in Central Argentina

Behavior of Aedes albifasciatus (Diptera: Culicidae) larvae from eggs with different dormancy times and its relationship with parasitism by Strelkovimermis spiculatus (Nematoda: Mermithidae)

Aedine mosquitoes go through unfavorable periods as dormant eggs. However, extended dormancy times lead to a depletion of egg reserves, which might be partially compensated by changes in larval-feeding behavior. Changes in larval behavior may also be induced by parasitism by mermithids probably as a strategy to reduce the impact of the parasite on the host. The most abundant floodwater mosquito in southern South America is Aedes albifasciatus (Diptera: Culicidae), a species naturally parasitized by Strelkovimermis spiculatus (Nematoda: Mermithidae). This study aimed to evaluate the behavior of fourth-instar larvae of Ae. albifasciatus from eggs with 2, 4, and 6 months of dormancy, parasitized or not by S. spiculatus. To achieve this, eight categories of behavior were defined, and then, each individual was observed for 5 min, and its behaviors and their duration were recorded. The behaviors with the highest percentage of observation time were still (50.1%) and wriggle swimming (33.6%), while those with the lowest percentage of observation time were those involving active foraging (less than 8%). A great variability in the behaviors was observed among individuals within the same treatment. The type and duration of each behavior did not vary in relation to egg dormancy time or parasitism, but parasitism affected the level of larval activity. Parasitized larvae performed, on average, fewer behaviors than non-parasitized larvae. This result supports the hypothesis that S. spiculatus parasitism affects the behavior of Ae. albifasciatus larvae by reducing their level of activity.

Susceptibility of the floodwater mosquito Aedes albifasciatus from eggs of different dormancy times to the nematode parasite Strelkovimermis spiculatus

This study aimed to evaluate the effects of egg dormancy times on susceptibility of larvae of the floodwater mosquito Aedes albifasciatus (Diptera: Culicidae) to parasitism by their natural enemy Strelkovimermis spiculatus (Nematoda: Mermithidae) and on their life history traits. Aedes albifasciatus eggs stored for 2, 4, 6, 8 and 10 months were hatched, and the larvae either exposed to S. spiculatus (treatment group) or not exposed (control group). Egg dormancy time had a negative effect on the retention of parasites, but no effect on the prevalence and intensity of parasitism or the melanization of nematodes. The survival to adulthood of control individuals decreased as dormancy time increased, whereas that of exposed individuals that remained uninfected was constant and low. A trend towards increasing development times with longer dormancy times was detected in the control group, but not in the exposed noninfected group. The results suggest nonconsumptive effects of parasites in exposed but not infected larvae from eggs with short dormancy times. In contrast, the relatively low fitness of larvae from eggs with long dormancy times regardless of their contact with the nematodes may be the result of the nutritional deprivation during the egg stage.

A Predation Record of Nabis paranensis (Hemiptera: Heteroptera) Over Aedes albifasciatus

Aedesalbifasciatus is one of the most widely distributed mosquitoes in the Southern Cone of South America, and a known vector of western equine encephalomyelitus. Despite its wide range of distribution and relatively large abundance over its distribution, little is known about the ecological interactions in adults. In this contribution we record the first predation record of the damselbug Nabis paranensis (Nabidae) over an adult male of A. albifasciatus. The interaction record is analyzed and discussed.

Mosquito (Diptera: Culicidae) larval ecology in natural habitats in the cold temperate Patagonia region of Argentina

BACKGROUND

Knowledge of immature habitats is an important focus for investigations of mosquito community ecology, and may improve our understanding of how environmental variables increase risk of mosquito-borne diseases by influencing the distributions and abundances of species. In Patagonia region, where climatic and ecological factors could be only borderline suitable for mosquito development, relatively little is known about larval ecology. The present study focuses on associations of environmental conditions in natural aquatic habitats with abundances of mosquito species that have colonized such habitats in Patagonia.

METHODS

We described the mosquito community composition within 26 natural temporary pools, and assessed the general relationships between environmental variables (pH, water temperature, conductivity, salinity, dissolved oxygen, aquatic plant cover and main nutrients) and larval abundances using redundancy analysis (RDA). Additionally, we compiled monthly climate data and vegetation indices for each larval habitat, and estimated the probability of presence for two of the most abundant species, describing through generalized linear models (GLM) the environmental, climatic and landscape variables-probability of occurrence relationships.

RESULTS

Seven species belonging to the genera Culex and Aedes were identified, with Culex apicinus, Cx. acharistus and Aedes albifasciatus being the most abundant. Mean larval densities were low (6.8 ± 2.8 larvae/dip), and the highest species richness and larval densities were recorded in northern and central areas. Aedes albifasciatus, a species of sanitary importance, was widely distributed, being the only one collected south of the 45th parallel of S latitude. RDA indicated that aquatic conductivity, pH, water depth, dissolved oxygen, ammonia and soluble reactive phosphorous accounted for the main part of the variation in the species composition. According to GLMs, wind speed was the variable that best described the presence of Ae. albifasciatus, and the probability of finding this species was positively associated with high wind speed values. On the other hand, the EVI vegetation index was the only variable included in the Cx. apicinus model, whereby there was a great probability of presence in arid areas with lower EVI values.

CONCLUSIONS

Our results enhance our knowledge of larval habitat ecology under the extreme environmental conditions of Patagonia and will guide future efforts to understand how multiple effects can affect mosquito ecology and public health at higher latitudes.

Genomic Characterization and Seroprevalence Studies on Alphaviruses in Uruguay

Alphaviruses (Togaviridae) are arboviruses frequently associated with emerging infectious diseases. In this study, we aimed to investigate the presence of alphaviruses in Uruguay by detecting the viral genome in mosquitoes and neutralizing antibodies in equines. A total of 3,575 mosquitoes were analyzed for alphavirus genome detection. Serologic studies were performed on 425 horse sera by plaque reduction neutralization test (PRNT80) against Venezuelan equine encephalitis virus (VEEV) subtype IAB, Pixuna virus (PIXV), Rio Negro virus (RNV), western equine encephalitis virus (WEEV), and Madariaga virus (MADV). Mosquitoes belonging to six genera were captured and 82.9% were identified as Culex pipiens. Two Cx. pipiens pools collected in Fray Bentos and Las Toscas localities were alphavirus positive, and phylogenetic analyses showed that the sequences grouped into two different clusters: the lineage I of eastern equine encephalitis virus and RNV (VEEV complex), respectively. Plaque reduction neutralization test assays showed antibodies against strains of the VEEV complex, MADV, and WEEV. Rio Negro virus was the most geographically widespread virus, showing higher seroprevalences (up to 20%). Seroprevalences against VEEV IAB ranged between 4.6% and 13%; antibodies against PIXV, WEEV, and MADV were less frequent (3-4%). In conclusion, RNV exhibited the highest seroprevalence in horses, a wide geographical distribution, and viral genome was detected in Cx. pipiens mosquitoes. Madariaga virus had a low seroprevalence in equines, but an epizootic lineage typical of North America was detected in Cx. pipiens mosquitoes. Taken together, our results show that alphaviruses are present in Uruguay with variable occurrence and geographical distribution being a potential threat for human and equine health.

First Record of Aedes albifasciatus in Magallanes Region, Chile

Aedes albifasciatus is known for transmitting western equine encephalomyelitis (WEE), a virus that can affect humans and animals. In this work, we record for the first time Ae. albifasciatus from the Magallanes Region in Chile, as the only culicid species known from this southernmost region. Although WEE poses a serious problem to cattle production in neighboring Argentina, no WEE has been recorded in Chile. The need for surveillance programs of WEE in Magallanes is also discussed here.

Wing morphometrics of Aedes (Ochlerotatus) albifasciatus (Macquart, 1838) (Diptera: Culicidae) from different climatic regions of Argentina

BACKGROUND

Gene flow restrictions between populations of Aedes albifasciatus, the vector of Western equine encephalitis and Dirophilaria immitis, have been described in the central region of Argentina. Genetic and eco-physiological variations usually result in local forms reflecting the climatic regions. Mosquito wings and their different parts have ecological functions in flight and communication. Therefore, wing shape could be considered an aspect of sexual dimorphism, and its eco-physiological responses can be expressed as morphological changes induced by the environment.

METHODS

To compare the geographical and sexual variations with respect to wing shape and size in two Ae. albifasciatus populations from contrasting climates of Argentina (temperate: Buenos Aires, and the arid steppe of Patagonia: Sarmiento), the wings of adults reared in thermal trays at different constant temperatures (10-29 °C) were analyzed.

RESULTS

The wing size of Ae. albifasciatus showed inverse linear relationships with the rearing thermal condition and higher slope for Buenos Aires. In the cool range (10-17 °C), geographical size variations responded to the converse Bergmann's rule, where Buenos Aires individuals were larger than those from Sarmiento. Sexual shape dimorphism occurred in both populations while geographical variation in shape was observed in both sexes.

CONCLUSIONS

Buenos Aires individuals showed greater response sensitivity with respect to the size-temperature relation than those from Sarmiento. The converse Bergmann's rule in size variation could be due to a higher development rate in Sarmiento to produce more cohorts in the limited favorable season. The shape could be more relevant with respect to the size in the study of population structures due to the size being more liable to vary due to changes in the environment. The geographical variations with respect to morphology could be favored by the isolation between populations and adaptations to the environmental conditions. Our results demonstrate that the shape and size of wing provide useful phenotypic information for studies related to sexual and environmental adaptations.

Life Table Analysis of Culex pipiens Under Simulated Weather Conditions In Egypt

The effect of fluctuating ambient air and water temperatures and relative humidity (RH) on the population parameters of Culex pipiens was studied using life table data during the winter-spring and summer-autumn seasons in Giza Province, Egypt. In winter-spring (20.46 ± 1.8°C and 51.68 ± 3.63% RH), the preadult survival rate for Cx. pipiens was 80.65%, which was higher than the 50.00% observed in the summer-autumn (30.19 ± 0.67°C and 52.68 ± 8.04% RH). During the winter-spring period, the developmental time of egg, larvae, and pupae of Cx. pipiens was 3.52, 12.69, and 5.92 days, respectively. There was a significantly shorter developmental period for summer-autumn (1.0, 8.59, and 2.04 days, respectively). Both female and male adults survived longer during winter-spring (61.83 days for female and 42.62 days for male adults) than during summer-autumn (24.38 days for female and 20.83 days for male adults). The fecundity (F) of female Cx. pipiens (524.7 eggs/female) and the net reproductive rate (R₀) (245.4 offspring) were significantly higher during winter-spring seasons than in the summer-autumn (163.3 eggs/female and 38.10 offspring). Despite the higher F and R₀ values in winter-spring, the intrinsic rate of increase (r_m = 0.1181/day) and the finite rate of increase (λ = 1.1253/day) of the Cx. pipiens population were significantly lower compared to summer-autumn (r_m = 0.1648/day; λ = 1.1792/day). Consequently, the mean generation time during winter-spring was significantly longer (46.6 days) than during summer-autumn (22.09 days). The data suggest that Cx. pipiens is an r-strategist.

Culicidae (Diptera) community structure, spatial and temporal distribution in three environments of the province of Chaco, Argentina

The aim of this work was to study the composition of the Culicidae community in three environments with different degrees of anthropic disturbance in the province of Chaco. The nonparametric richness estimator ACE was calculated to measure the completeness of the sampling in each environment, and the diversity of each environment (α-diversity) were estimated. The composition, abundance and uniformity of species from the different environments were compared by means of range-species curves. β-diversity was measured as species complementarity, which allowed us to know the degree of dissimilarity among the environments. The synanthropic index was estimated, identifying urban environment (synanthropic) species, eusynanthropic species, and wild species. The influence of climatic factors (mean temperature, relative humidity and rainfall) on the monthly and annual variations of the identified mosquito species was analysed using multilevel Poisson models with over-dispersion. The wild environment showed higher diversity and the semi-urban environment higher species richness. Despite having lower S and abundance, the urban environment showed greater diversity than semi-urban environment, although it also showed lower completeness, which means S could increase. Anthropogenic disturbance can lead to the elimination and/or modification or substitution of habitats, with the subsequent loss of richness in the more sensitive species of Culicidae fauna, although the conditions are also favourable for the settling of opportunistic and exotic species well-adjusted to disturbed environments. Most of the species abundances were positively influenced by the analysed climatic variables. The study area showed a rich Culicidae fauna of public health significance, with a risk of pathogen transmission, suggesting the need of further studies and the febrile and entomological surveillance.

Prevalence of parasitism and adult survival time of Aedes albifasciatus (Diptera: Culicidae) parasitized by Strelkovimermis spiculatus (Nematoda: Mermithidae)

We described the carryover of Strelkovimermis spiculatus (Poinar and Camino) (Nematoda: Mermithidae) from mosquito larvae, the primary site of maturation, to adults. We analyzed the survival time of male and female Aedes albifasciatus (Macquart) (Diptera: Culicidae) parasitized by S. spiculatus, the time of emergence of nematodes from adult mosquitoes, and the state of parasitism in the same mosquito cohorts during the immature stages. Mosquito larvae with single and multiple parasitism (up to 11 parasites) were observed. The mortality of mosquito larvae and adults was produced in all cases where at least one mermithid emerged. The mortality of S. spiculatus showed an increasing trend in mosquito larvae with larger numbers of nematodes and was higher in larvae parasitized by eight or more nematodes. Maximum survival of parasitized adult females of Ae. albifasciatus was 38 days, while non-parasitized adult males and females survived 39 and 41 days, respectively. Strelkovimermis spiculatus mortality was observed in Ae. albifasciatus larvae with single or multiple parasitisms. The spread of mermithid parasitism in adult mosquito populations is discussed.

Seasonal pattern of daily activity of Aedes caspius, Aedes detritus, Culex modestus, and Culex pipiens in the Po Delta of northern Italy and significance for vector-borne disease risk assessment

The increasing concern about vector-borne diseases such as West Nile disease in northern Italy motivated our analysis of data on the mosquito fauna and the seasonal and daily flight patterns collected in 1998 in the Po Valley. Collections were performed once a week from May to November, with human landing collections and CO(2) traps. Culex pipiens was present from July to October and showed a clearly unimodal nocturnal flight habit. Culex modestus appeared in July-August and showed a bimodal flight pattern, (main peak during the evening and a minor one in the morning). Aedes caspius was present from May to November (highest densities in July-August) and showed a bimodal flight pattern with a major crepuscular peak and a minor dawn peak in the morning. Aedes detritus was the most abundant species in May, with a crepuscular sharply bimodal flight pattern, particularly according to human landing collections. Sunset and sunrise time, in combination with the solar phase (that determines daylight duration and its trend of changing) were the main factors affecting flight behavior. Temperature, relative humidity, and wind speed differently affected the flight behavior of mosquito females according to the species.

Models for predicting Aedes aegypti larval indices based on satellite images and climatic variables

Forecasting models were developed for predicting Aedes aegypti larval indices in an endemic area for dengue (cities of Tartagal and Orán, northwestern Argentina), based on the Breteau and House indices and environmental variables considered with and without time lags. Descriptive models were first developed for each city and each index by multiple linear regressions, followed by a regional model including both cities together. Finally, two forecasting regional models (FRM) were developed and evaluated. FRM2 for the Breteau index and House index fit the data significantly better than FRMI. An evaluation of these models showed a higher correlation FRM1 than for FRM2 for the Breteau index (r = 0.83 and 0.62 for 3 months; r = 0.86 and 0.67 for 45 days) and the House index (r = 0.85 and 0.79 for 3 months; r = 0.79 and 0.74 for 45 days). Early warning based on these forecasting models can assist health authorities to improve vector control.

Long-term storage and viability of Ochlerotatus albifasciatus (Diptera: Culicidae)

The viability of Ochlerotatus albifasciatus (Macquart) eggs stored at room temperature and at 5 degrees C was studied over 31 months. After 12, 18 and 31 months of storage, eggs were acclimatized at 22 degrees C for ten days, and then inundated twice every seven days. The effect of the storage period on the percentage of hatching was analyzed by one way ANOVA. Differences on the hatching response between the first and second flooding were analyzed by paired t-test. Differences on the hatching response between the two storage conditions were analyzed by Mann-Whitney rank test. Results showed that (1) Oc. albifasciatus eggs were able to survive and hatch over 31 months; (2) the percent hatching of eggs stored at 5 degrees C was higher than that of eggs stored at room temperature; and (3) low temperatures and long periods without water favor installment hatching.

Predicting the spatial distribution of Ochlerotatus albifasciatus (Diptera: Culicidae) abundance with NOAA imagery

Ochlerotatus albifasciatus is a vector of western equine encephalomyelitis in Argentina and a nuisance mosquito affecting beef and dairy production. The objective of this study was to analyze whether environmental proxy data derived from 1 km resolution NOAA-AVHRR images could be useful as a rapid tool for locating areas with higher potential for Oc. albifasciatus activity at a regional scale. Training sites for mosquito abundance categories were 3.3x3.3 km polygons over sampling sites. Abundance was classified into two categories according to a proposed threshold for economic losses. Data of channels 1, 2, 4 and 5 were used to calculate five biophysical variables: normalized differences vegetation index (NDVI), land surface temperature, total precipitable water, dew point and vapour saturation deficit. A discriminant analysis correctly classified 100% of the areas predicted to be above or below the economic threshold of 2500 mosquitoes per night of capture, respectively. Components of the NDVI, the total precipitable water and the dew point temperature contributed most to the function value. The results suggest that environmental data derived from AVHRR-NOAA could be useful for rapidly identifying adequate areas for mosquito development or activity.

Mortality in immatures of the floodwater mosquito Ochlerotatus albifasciatus (Diptera: Culicidae) and effects of parasitism by Strelkovimermis spiculatus (Nematoda: Mermithidae) in Buenos Aires Province, Argentina

Life tables were constructed for six cohorts of immature stages of the floodwater mosquito Ochlerotatus albifasciatus (Macquart) in a park in Buenos Aires, highlighting the mortality attributable to the parasitic nematode, Strelkovimermis spiculatus Poinar & Camino. Two cohorts were selected to compare parasite incidence in all mosquito stages when low and high parasitism occurred. Development time of Oc. albifasciatus from first instar to adult was 7.7-10 days in the spring, 6 days in the summer, and 10.9-21.9 days in the fall. Survival was estimated as 0-1.4% in the spring, 2% in the summer and 0.2-4.4% in the fall. The highest "K" value (Killing power) occurred during a fall cohort when prevalence of the parasite was 86.9%, and the lowest in a spring cohort. Parasitism occurred during all seasons, but S. spiculatus persisted to adult only in the summer and fall, when adult mosquitoes developed from parasitized third and fourth instars larvae. The abundance of S. spiculatus differed between old and young larvae only when parasite prevalence was the highest. Although pupae and adults of Oc. albifasciatus were parasitized, no pupal mortality attributable to parasitism was recorded. The proportion of parasitized adults ranged from 14.2% and 5.7% in the two cohorts compared. Pupal wet weight and adult wing lengths did not differ between parasitized and unparasitized individuals.

Spatial pattern of abundance of the mosquito, Ochlerotatus albifasciatus, in relation to habitat characteristics

Ochlerotatus albifasciatus (Macquart) (Diptera: Culicidae) is the main vector of the western equine encephalomyelitis (WEE) virus and potentially of other arboviruses in Argentina. Surges of adult population abundance during the rainy season are a nuisance, affecting milk and beef production. Larvae develop in short periods in shallow temporary ground pools on fresh or brackish water. Although adults seem to disperse long distances from larval habitats, little is known about their habitat preferences. This work studied factors affecting the spatial pattern of adult Oc. albifasciatus abundance. Adult mosquitoes were collected using CDC miniature light traps baited with CO2 at 28 sites located to the south of Mar Chiquita Lagoon, from November 1997 to April 1998. Each site was typified according to its predominating vegetation cover, potential breeding site occurrence, land slope and cattle density. The spatial and temporal patterns of abundance suggested that Oc. albifasciatus prefers prairies and natural grasslands subject to periodic flooding vs. woodland and farm land. A discriminant function based on the proximity to potential larval habitats, distance to woodland and land slope accurately classified 95% of the data categorized as having an average high (>500 mosquitoes) or low (<500 mosquitoes) abundance, and was validated using six sites located away from the study area. An analysis of the temporal variation of mosquito abundance highlighted the influence of the dynamics of the larval habitats on adult mosquito abundance.

Ochlerotatus albifasciatus in rain pools of Buenos Aires: seasonal dynamics and relation to environmental variables

The immature stages of Ochlerotatus albifasciatus develop in temporary pools. The present study aims at evaluating the seasonal dynamics of the aquatic stages of this mosquito, also analyzing the relationship among their presence and breeding success to some relevant climatic and environmental variables in the ephemeral rain pools of an urban park. Nineteen cohorts of O. albifasciatus that developed synchronously after rain events were recorded in all seasons. The proportions of mosquito-positive pools were significantly higher during the fall-winter period than in the spring-summer months (p < 0.001). The presence of this mosquito species was positively related to the amount of rain (p < 0.001), whereas negatively correlated to air temperature (p < 0.05) within a 5.2 to 29.7 degrees C range. The distribution of the number of cohorts per pool throughout the year was grouped (variance/mean: 3.96), indicating that these habitats were not equally suitable as breeding sites. The immature stages of O. albifasciatus were detected in pools belonging to all of the categories of surface area, depth, duration, vegetation cover, and insolation. However, the proportion of pools where immature mosquitoes were detected was positively and significantly related to surface, depth, duration, and vegetation cover. On the other hand, the proportion of mosquito-positive pools was higher at an intermediate insolation degree. Our results suggest that although preimaginal stages were present in all seasons, high temperatures may be unfavorable to larval development, and substrate vegetation may regulate water temperature. The positive relationship between the proportion of mosquito-positive pools and pool size and duration might reflect a strategy of O. albifasciatus to accomplish immature development.

Winter biology of Aedes albifasciatus (Diptera: Culicidae) from Córdoba, Argentina

Host-seeking females of Aedes albifasciatus (Macquart) were collected from April to September 1997, kept under seminatural conditions, and offered sugar solution and blood. Daily survival of females ranged from 0.91 to 0.96, with blood fed females living longer than sugar fed females. Overall, 43% of engorged females completed a gonotrophic cycle, and 15% of them refed and completed a second gonotrophic cycle. The life expectancy of females emerging at the end of summer was longer than those that emerged during winter. Immature developmental time and the developmental threshold were estimated by regression. Embryo development was recorded during autumn, winter, and spring, with a duration of 5-9 d. The developmental threshold for eggs was estimated to be 2.28 degrees C. Egg mortality varied from 0.51 to 0.74. The development time for larva and pupa was between 16 and 29 d and was significantly correlated with temperature. The developmental threshold for larvae and pupae was estimated to be 4.75 degrees C. A greater proportion of females than males emerged when temperatures averaged < or = 18 degrees C. Larval and pupal mortality was high at temperatures below the developmental threshold. Aedes albifasciatus females remained gonotrophically active and immature development continued during winter in Córdoba (10 degrees C isotherm).

Epizootiological studies of Amblyospora albifasciati (Microsporidiida: Amblyosporidae) in natural populations of Aedes albifasciatus (Diptera: Culicidae) and Mesocyclops annulatus (Copepoda: Cyclopidae) in a transient floodwater habitat

The epizootiology of the microsporidium Amblyospora albifasciati was studied in natural populations of its definitive host, a multivoltine, neotropical, floodwater mosquito, Aedes albifasciatus, and its intermediate copepod host, Mesocyclops annulatus, in an ephemeral floodwater habitat during a 12-month period. A. albifasciati was enzootic in mosquitoes. Vertically (transovarially) transmitted meiospore infections occurred regularly and were detected in five of eight larval broods but the prevalence of infection was always low, ranging from 0.5 to 6.9% with an overall average of 0.7%. Horizontal transmission of A. albifasciati infection from copepods to mosquitoes was nominal and limited. It was detected at levels of 6.4 to 20% in larval Ae. albifasciatus populations on two occasions, the month of August and late September through early October. The low levels of horizontal transmission of infection to mosquito larvae appeared to be the principal limiting factor that prevented the proliferation of A. albifasciati in Ae. albifasciatus populations. Copepod populations were abundant from May through September and weekly prevalence rates of A. albifasciati averaged over 50% (range = 5.8 to 100%). The moderately high infection rates in M. annulatus copepods were inconsistent with the low prevalence of meiospore infection in Ae. albifasciatus mosquito larvae. Results suggest that either meiospores of A. albifasciati produced in the mosquito host are highly infectious to copepods or they are long-lived and remain viable within the pool as long as some standing water is present. Observations further indicate that A. albifasciati has a significant detrimental impact on M. annulatus copepod populations but minimal impact on larval populations of Ae. albifasciatus at this site.

Effects of flooding and temperature on Aedes albifasciatus development time and larval density in two rain pools at Buenos Aires University City

Aedes albifasciatus is a floodwater mosquito that breeds in temporary waters. This semi-domestic species, widely distributed in Argentina, is a competent vector of the western equine encephalitis. The present study was carried out in two rain pools of the city of Buenos Aires, from April 1998 through March 1999. Samples were taken twice a week during the cold season and daily during the warmer months, starting from October. Immature mosquitoes were collected with a dipper, being the number of dippers proportional to the flooded area. The estimated rainfall thresholds to initiate cohorts of Ae. albifasciatus were: 16-17 mm in the fall-winter period, 25 mm in the spring, and 30 mm in the summer. The development time of the different cohorts and the mean air temperature of their respective periods were estimated in all seasons, ranging from six days (at 24 degress C) to 32 days (at 13 degrees C). The equation that best expresses the relationship between development time and mean air temperature is dt =166,27.e(-0,1435.T) (R(2)=0,92). Significantly shorter development times were recorded for larvae of the first three stages as compared to the fourth larval stage and pupae.

Body size variation of the floodwater mosquito Aedes albifasciatus in Central Argentina

An inverse relationship between larval density and adult body size has been reported for several mosquito species, affecting their survival and vector competence, response to repellents and other factors. Larvae of the floodwater mosquito Aedes (Ochlerotatus) albifasciatus (Macquart) (Diptera: Culicidae) develop quickly in temporary pools, so intraspecific competition (for food or space) might regulate population abundance and affect the size of adult mosquitoes. We investigated the temporal variation of adult female wing-length (an index of body-size) in natural populations of Ae. albifasciatus, using adults collected during each phase of the rainy season. The relationships between adult mosquito abundance, female wing-length, rainfall and temperature were analysed through simple regressions. Skewness of the frequency distribution of wing-lengths showed a strong negative relationship with mean wing-length. The distribution of wing-lengths varied seasonally and was correlated with rainfall 7-15 days previously as the major consequence of breeding site volume. Thus temporal variation of body size in natural populations of Ae. albifasciatus reflected density-dependent changes in the aquatic habitat where immature stages develop, influenced more by rainfall than by temperature or other environmental variables.

Population dynamics of Aedes albifasciatus (Diptera: Culicidae) south of Mar Chiquita Lake, central Argentina

Spatial and temporal changes in the abundance of adult female Aedes (Ochlerotatus) albifasciatus (Macquart) were studied using CDC miniature light traps at 7 sites along an 80-km transect parallel to the southern shore of Mar Chiquita Lake (central Argentina) during the rainy seasons between 1994 and 1997. Abundance was greatest and most variable at sites near larval habitats. Rainfall and an index combining rainfall and temperature predicted adult female abundance near larval habitats 7-15 d in advance. The spatial distribution of population abundance seemed to be influenced strongly by local factors, because temporal change at sampling sites located >10 km apart was asynchronous.

Relative abundance and gonotrophic status of Aedes albifasciatus (Diptera: Culicidae) during the autumn-winter period in Córdoba Province, Argentina

Variations in the abundance of larvae and females of Aedes albifasciatus (Macquart) were monitored biweekly during the autumn-winter periods of 1996 and 1997 on the southern edge of the Mar Chiquita Lake in Córdoba Province. The degree of development of the primary ovarian follicles also was determined in host-seeking females. Abundance of the larvae and the females were noticeably different during the 2 yr of the study and matched changes in the pattern of precipitation. Although most females had ovarian follicles in Christophers stages Ib-IIa, 14% and 4% in 1996 and 1997, respectively, were in stages III-V (i.e., exhibited gonotrophic discordance). Females appeared to continue taking blood meals and laying eggs even during the autumn-winter period.

Age structure of adult mosquito (Diptera: Culicidae) populations from Buenos Aires Province, Argentina

In order to detect seasonal trends in the age structure of adult mosquitoes from Buenos Aires province, Argentina, female populations were sampled with CDC traps during 1989-1991 in Punta Lara and La Plata. The mosquitoes were dissected and age-grouped according to ovarian tracheation and ovariolar stages. All Runchomyia paranensis females were parous, suggesting that this species could be autogenous. Aedes albifasciatus showed parous peaks following population peaks, with shorter delays in spring-summer and longer in fall-winter. Ae. crinifer and Culex dolosus showed wide fluctuations in age structure due to adult emergences during all months. Psorophora ferox showed high population replacement rates. Mansonia indubitans and Ma. titillans have few generations per year during their activity period. This is the first report on age-grading of adults of field mosquito populations from Argentina.