Oviposition dynamics of Aedes aegypti in Central Argentina.

Aedes (Stegomyia) aegypti (L.) (Diptera: Culicidae) is the vector of multiple arboviruses. To evaluate the association between environmental factors and the oviposition activity of Ae. aegypti in Argentina, data on the presence and abundance of eggs were collected using ovitraps, between September of 2018 and May of 2019, in the cities of Villa María, Río Cuarto and Salsipuedes (Córdoba province, Argentina). We analysed the relationships between oviposition and five environmental factors: Temperature, precipitation, vegetation cover, human population density and distance to sites with a potential high density of larval habitats, like cemeteries and trash dumps. Environmental factors' data were collected using satellite image products. The oviposition activity was randomly distributed in three cities. Using generalized linear mixed models, we show that the house where each ovitrap was placed was a source of variability in oviposition, suggesting the relevance of microsite factors and the importance of domestic control actions. Ae. aegypti oviposition was positively correlated with night-time temperature of the previous 3 weeks, and in a context-dependent manner, it was positively correlated with human population density, vegetation cover and precipitation. The consistency and magnitude of these relationships varied between cities, indicating that oviposition is related to a complex system of environmental variables.

Triatominae in furnariid nests of the Argentine Gran Chaco.

Triatomines (Hemiptera, Reduviidae) are bloodsucking insects involved in the transmission of Trypanosoma cruzi, the causative agent of Chagas disease, an important public health problem in Latin America. The triatomine species found in sylvatic habitats generally play a limited epidemiological role compared to domestic species, but they may act as a reinfestation source of dwellings after insecticide spraying and have to be carefully considered in control strategies of Chagas disease transmission. The objectives of this work were to carry out a survey of the sylvatic triatomine species colonizing Furnariidae nests in a typical area of the Chaco region of Argentina during the winter and to study the parasites and natural enemies associated with the collected triatomines. Sixty-three triatomine specimens were collected from Furnariidae nests (Coryphistera alaudina and Phacellodomus sibilatrix) randomly selected within the study area. Fifty-four were identified as Psammolestes coreodes, seven as Triatoma platensis, and two as Triatoma infestans. Specimens of T. infestans and T. platensis were found in one nest. The first finding of instar nymphs of T. infestans x T. platensis in a sylvatic habitat is reported. For the first time, sylvatic collected specimens of T. platensis were found infected by T. cruzi. Triatoma virus was found in one Ps. coreodes specimen.

Control of experimental Triatoma infestans populations: effect of pour-on cypermethrin applied to chickens under natural conditions in the Argentinean Chaco region.

Among peridomestic structures, chicken coops are sites of major importance for the domestic ecology of Triatoma infestans (Hemiptera: Reduviidae). The aim of this study was to evaluate in an experimental context the effects of a cypermethrin pour-on formulation applied to chickens on blood intake, moulting and mortality in T. infestans, under the natural climatic conditions of a region endemic for Chagas' disease. Experimental chicken huts were made of bricks and covered with plastic mosquito nets. Ninety fourth-instar nymphs were maintained in each hut. The study used a completely random design in which chickens in the experimental group were treated with a cypermethrin pour-on formulation. Five replicates (= huts) of the experimental and control groups were conducted. The number of live T. infestans, blood intake and moults to fifth-instar stage were recorded at 1, 5, 20, 35 and 45 days after the application of cypermethrin. Cumulative mortality was higher in nymphs exposed to treated chickens (> 71%) than in control nymphs (< 50%) (P < 0.01). Blood intake and moulting rate were lower in nymphs fed on treated chickens than in control nymphs (P < 0.05). Pour-on cypermethrin was able to cause significant mortality, although it did not eliminate the experimental population of T. infestans.

Development of a geographical distribution model of Rhodnius pallescens Barber, 1932 using environmental data recorded by remote sensing.

Rhodnius pallescens, main vector of Trypanosoma cruzi in Panama and secondary vector in Colombia, Costa Rica and Nicaragua, represents an important epidemiological risk in those countries. It occupies sylvatic ecotopes, and because of this its distribution and abundance could be conditioned by environmental factors. In this work, we integrated environmental variables recorded by remote sensing and data of R. pallescens presence in the countries mentioned above in order to know the environmental variables with better capacity to describe the insects' distribution, which will help to entomological surveillance and control. Standard discriminant analysis (DA) was used to determine if there is a significant difference in the environmental variation between the presence and the absence sites of R. pallescens. Forward stepwise discriminant analysis (fDA) was used to determine the environmental variables which better discriminated between presence and absence sites, and to construct a predictive map of geographical distribution. Univariate analyses were used to determine the discriminatory power of individual variables. The model derived from DA showed 89% of sensitivity and 92.8% of specificity. Multivariate and univariate analyses showed the vapor pressure deficit minimum as the principal variable among the nine most important to describe the distribution of the species, which is in agreement with the R. pallescens stenohydric status. Map shows insects' distribution predicted by environmental variables, and moreover includes the distribution of most species belonging to Rhodnius genus, except R. domesticus, R. nasutus and R. neglectus. We interpreted these results as a reflection of the common evolution of the most Rhodnius species, except for the last ones that probably evolved isolated due to particular environmental conditions. In conclusion, this study showed that a reduced number of environmental variables can predict the distribution of R. pallescens and related species. This methodology can be very useful to make critical decisions for vectorial surveillance and control of Chagas disease vectors.

The association between the geographic distribution of Triatoma pseudomaculata and Triatoma wygodzinskyi (Hemiptera: Reduviidae) with environmental variables recorded by remote sensors.

In this study, predictive models of geographic distribution patterns of Triatoma pseudomaculata (Tps) and T. wygodzinskyi (Twy) were carried out. They were based on biophysical variables estimated from information provided by the satellite remote sensors AVHRR (Advanced Very High Resolution Radiometer) and MODIS (MODerate-resolution Imaging Spectroradiometer). Our goal was to analyze the potential geographic distribution of Tps and Twy and to assess the performance of three predictive models (one for each species and one for both species together) based on temperature, vapour pressure deficit, vegetation and altitude. The geographic distribution analysis shows that all models performed well (>85.7% of overall correct classification of presence and absence point data). The MODIS-based models showed lower correct classifications than the AVHRR-based models. The results strongly suggest that environmental information provided by remote sensors can be successfully used in studies on the geographic distribution of poorly understood Chagas disease vector species.

Phenotypic diversity of Triatoma infestans at the microgeographic level in the Gran Chaco of Argentina and the Andean valleys of Bolivia.

Triatoma infestans (Klug) is the main vector of Trypanosoma cruzi, etiologic agent of Chagas disease. The phenotype of the species varies at different geographic scales. The objective of this study was to compare the antennal phenotype of spatially close T. infestans populations and to evaluate its usefulness as a marker of exchange of individuals between populations. The antennal phenotype of 190 T. infestans from Argentina and Bolivia was analyzed using uni- and multivariate techniques. This study shows heterogeneity of the antennal phenotypes of closely related T. infestans populations living in different habitats. Specimens collected in a goat corral and a rabbit cage in La Rioja (Argentina) were dissimilar to the specimens collected in the nearby intradomestic environment. Similarly, specimens from peridomestic corrals in Cochabamba (Bolivia) were different from the intradomestic and sylvatic specimens, indicating some degree of isolation between these populations. In contrast, T. infestans collected in chicken coops and the intradomestic environment were similar, in all studied regions, suggesting a frequent exchange of individuals between the two habitats and/or because of the similarity of the habitat due to the presence of chickens in the intradomestic environment. We propose that each habitat affects in a particular mode the insect morphology and these changes could be used to identify recolonizing T. infestans. These results support the hypothesis of different degrees of isolation between intradomestic and peridomestic habitats. Within this context, special attention should be given to chicken coops and other close peridomestic structures in relation to the recolonization process of domestic habitats by T. infestans.

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.

Infestation of rural houses by Triatoma infestans in the region of Los Llanos (La Rioja, Argentina).

Vectorial transmission of Chagas disease has been strongly reduced in most parts of the Southern Cone countries of South America, except in the Gran Chaco region of Argentina, Bolivia, and Paraguay. Given periodical interruptions of the vector control programmes in the endemic region of the Gran Chaco of Argentina, the vectorial transmission of the disease has been increasing during the last years. From the beginning of 2004, the provincial Ministry of Health of La Rioja, Argentina, started a vector control programme to cover the rural houses of the Los Llanos area in the southwestern area of the Gran Chaco region. This article reports the result of a standardized entomological survey and insecticide application against Chagas disease vectors in the intra and peridomestic structures of the rural houses of Los Llanos. A total of 4062 houses were inspected, of which 46.8% were found to be infested by Triatoma infestans. Infestation by vector species other than T. infestans was less than 0.5%(T. eratyrusiformis and T. platensis). Intradomestic infestation was found in 27.2%, whereas peridomestic infestation was found in 39.3% of the houses. The lowest figure of intradomestic infestation was 6.6% (Department F Varela), and the highest value of intradomestic infestation was 45.1% (Department Independencia). In spite of the demonstrated success of vector control elsewhere, this study shows that the vector populations are susceptible to pyrethroid insecticides in the southern area of the Gran Chaco of Argentina, that there still are regions where rural houses show heavy infestation by T. infestans associated with big peridomestic structures and that the vectorial transmission of the Chagas disease will continue, unless a sustained and well organized vector control effort is installed in the region.

Infestation of rural houses by Triatoma infestans in the region of Los Llanos (La Rioja, Argentina)

Vectorial transmission of Chagas disease has been strongly reduced in most parts of the Southern Cone countries of South America, except in the Gran Chaco region of Argentina, Bolivia, and Paraguay. Given periodical interruptions of the vector control programmes in the endemic region of the Gran Chaco of Argentina, the vectorial transmission of the disease has been increasing during the last years. From the beginning of 2004, the provincial Ministry of Health of La Rioja, Argentina, started a vector control programme to cover the rural houses of the Los Llanos area in the southwestern area of the Gran Chaco region. This article reports the result of a standardized entomological survey and insecticide application against Chagas disease vectors in the intra and peridomestic structures of the rural houses of Los Llanos. A total of 4062 houses were inspected, of which 46.8 percent were found to be infested by Triatoma infestans. Infestation by vector species other than T. infestans was less than 0.5 percent(T. eratyrusiformis and T. platensis). Intradomestic infestation was found in 27.2 percent, whereas peridomestic infestation was found in 39.3 percent of the houses. The lowest figure of intradomestic infestation was 6.6 percent (Department F Varela), and the highest value of intradomestic infestation was 45.1 percent (Department Independencia). In spite of the demonstrated success of vector control elsewhere, this study shows that the vector populations are susceptible to pyrethroid insecticides in the southern area of the Gran Chaco of Argentina, that there still are regions where rural houses show heavy infestation by T. infestans associated with big peridomestic structures and that the vectorial transmission of the Chagas disease will continue, unless a sustained and well organized vector control effort is installed in the region.

Infestation of rural houses by Triatoma infestans (Hemiptera: Reduviidae) in southern area of Gran Chaco in Argentina.

The impact of control activities against Triatoma infestans (Klug) (Hemiptera: Reduviidae) in South America has a marked contrast within and outside the Gran Chaco region. Development of a geographic information system, as part of an improvement in control program activities, allowed analysis of the spatial pattern of house infestations by T. infestans before and after house spraying with deltamethrin in the San Martin Department (an arid Chaco region of central Argentina). The overall peridomestic infestation index decreased from 48.2 to 28.2% after insecticide application. House infestation was spatially clustered in regions with low or high infestation levels that were located east and southwest of the department, respectively. This pattern was detected both before and after the insecticide application. Three environmental variables calculated from a temporal series of MODIS imagery (average of night temperature, maximum of day temperature, and temporal variation of vegetation index) were capable of correctly discriminating 96% of the places belonging to either high or low house infestation observed after the insecticide application.

Antennal phenotype of Triatoma dimidiata populations and its relationship with species of phyllosoma and protracta complexes.

Triatoma dimidiata (Latreille 1811) Reduviidae Triatominae is the main vector of Chagas disease in several countries of Latin America. As for other vector species, the characterization of T. dimidiata subpopulations within particular geographical regions or occupying different habitats could help in better planning of vector control actions. A first objective in this study was to evaluate the antennal phenotype as a phenetic marker to characterize populations of T. dimidiata collected in different geographic areas and domestic and sylvatic habitats. A second objective was to evaluate the phenetic relationships of T. dimidiata with other species of the phyllosoma complex: longipennis, pallidipennis, and phyllosoma. The antennal sensilla of T. dimidiata specimens collected in Mexico, Central America, and Colombia were analyzed and compared with the antennal sensilla of T. longipennis, T. pallidipennis, and T. phyllosoma. T. barberi was used as an outgroup in the analysis. For each specimen, the ventral side of the three distal segments of the antennae was drawn, identifying and counting four types of sensilla. In T. dimidiata, univariate and multivariate analysis showed differences between sexes, among populations collected in different habitats within the same region, and among populations collected in different geographic regions. Two types of antennal sensilla showed a latitudinal variation. Domestic specimens showed intermediate characteristics of the antennal phenotype, between sylvatic cave- and sylvatic forest-collected specimens. The antennal phenotypes show high similarities among T. pallidipennis, T. phyllosoma, and T. longipennis, with a better differentiation of T. pallidipennis. T. dimidiata is separated from the other members of the complex by a similar distance to T. barberi, of the protracta complex.

Competition between vectors of Chagas disease, Triatoma infestans and T. sordida: effects on fecundity and mortality.

Interspecific competition between two species of triatomine bugs (Hemiptera: Reduviidae), vectors of Chagas disease, was assessed for 16 months through comparative fecundity and mortality of experimental populations in chicken nests, maintained indoors with ambient conditions. Triatoma sordida (Stål), the secondary vector in north-eastern Argentina, was compared with Triatoma infestans (Klug) the more widespread domestic vector in the southern cone of South America. Both species populations originated from females collected in 1995 from the community of Empedrado, Corrientes, Argentina. Three population units were monitored: T. infestans alone, T. sordida alone and both species together in equal proportions. Each population started with six male and six female adults, 116 eggs, and nymphal instars I to V numbering 82, 48, 16, 11 and 19, respectively. Numbers and weight of individual bugs were recorded monthly (August 1995 to December 1996). The pure populations of T. infestans and T. sordida showed temporal changes in abundance, rising in summer and falling in winter, similar to the typical trends under normal field conditions. In the mixed population, however, T. sordida fell to extinction after 6 months, whereas T. infestans reached similar abundance to the pure (control) population. For each nymphal instar of T. sordida, the mean body weight was significantly less and mortality rate was higher in the mixed population compared to the pure population, but there were no significant differences of adult longevity or fecundity between the pure and mixed populations of T. sordida. The apparent competitive displacement of T. sordida by T. infestans was attributed to the latter species having better ability to obtain bloodmeals. This might explain the rarity of mixed populations where these two species occur in sympatry.

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.

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.

Biosystematics of Old World Triatominae.

Morphometric analysis of Old World species of Linshcosteus and Triatoma showed the two genera to be well separated. However, although the analysis showed clear distinction between the five species of Linshcosteus, there was considerable overlap between the eight Old World species of Triatoma, including specimens of T. rubrofasciata from Old and New World populations. The results are interpreted as supporting the idea of a New World origin for the Old World species of Triatoma, with an independent Old World origin for Linshcosteus.

Monitoring the abundance of Aedes (Ochlerotatus) albifasciatus (Macquart 1838) (Diptera: Culicidae) to the south of Mar Chiquita Lake, central Argentina, with the aid of remote sensing.

Surges in the size of adult populations of the flood-water mosquito Aedes albifasciatus can produce important economical losses because of the way this species irritates livestock. Although this species is also the main vector of west equine encephalitis in Argentina, little is known about the factors affecting its population dynamics, as it is difficult to obtain data on its abundance over a large area. However, the results of intensive study of the mosquito in a few sites might reasonably be extrapolated to a regional scale by the use of remotely sensed data. The adult, larval and pupal stages of Ae. albifasciatus were sampled at five field sites to the south of Mar Chiquita Lake, either once a month (during the dry, cold season) or once a fortnight (during the warm, rainy season), between August 1992 and April 1993. The measured abundance of adults or pre-adults and a meteorological coefficient useful for the estimation of larval abundance each showed significant correlation with various statistics derived from normalized-difference, vegetation indices (NDVI) calculated from satellite (NOAA-AVHRR) imagery. A linear discriminant analysis, using data on NDVI, rainfall and temperature, accurately identified periods with and without pre-adults. The satellite imagery was also useful in the estimation of larval abundance and consequently could be used to predict adult abundance 7 days in advance. Even though the satellite data employed have poor spatial resolution, their high temporal resolution makes them very useful in studies of the population dynamics of mosquitoes in general, at least once the relevant variables and their relationships with mosquito breeding and survival have been identified.

Daily pattern of flight activity of Aedes albifasciatus in central Argentina.

Aedes albifasciatus is an important common mosquito in Central Argentina. It is a confirmed vector of the Western Equine Encephalitis and is responsible for loss of milk production in dairy cattle during peak populations. This paper reports the flight activity pattern of Ae. albifasciatus for different seasons, in the southern coast of the Mar Chiquita Lake (central Argentina). Data were collected by sampling two sites between 1992 and 1993 with CDC traps and human-bait captures. Adult mosquito population density, estimated by CDC trapping and human-bait, were highly correlated. However, when compared to other species, the proportion of Ae. albifasciatus was higher in human-bait collections. Adult female populations were active only when temperature were higher than 6 degrees C. Two daily biting peaks were observed (dusk and dawn) during the spring, summer and autumn, and only one peak during winter (around 15:00 pm). Adult abundance was significantly correlated (R2 = 0.71; p < 0.01) with temperature and illumination.