Effects of temperature and diet on development and interspecies competition in Aedes aegypti and Aedes albopictus

We asked whether climate change might affect the geographic distributions of Aedes aegypti (L.) and Aedes albopictus (Skuse) (Diptera: Culicidae). We tested the effects of temperature, diet and the presence of congeneric species on the performance of immature stages of these two aedine species in the laboratory. Mosquitoes in three different species-density combinations were reared at four constant temperatures (20 °C, 25 °C, 30 °C, 35 °C) on low- or high-level diets. Of the four temperatures tested, mortality increased only at 35 °C in both species. Mortality was higher on the high-level diet than on the low-level diet at 35 °C, but not at other temperatures. The presence of congeneric species had a significant positive effect on mortality in Ae. albopictus, but not in Ae. aegypti. Both species developed more quickly at higher temperatures within the range of 20-30 °C; development was not enhanced at 35 °C. Population growth of Ae. albopictus was more stable, regardless of diet and temperature; that of Ae. aegypti varied more according to these two factors. These species-specific attributes may help to explain the latitudinal distribution of the mosquitoes and degree of species dominance where they are sympatric.     

Mark-release-recapture study to measure dispersal of the mosquito Aedes aegypti in Cairns, Queensland, Australia

In Queensland, Australia, in response to isolated cases of dengue infection, larval control of the vector Aedes aegypti (L.) (Diptera: Culicidae) is targeted at breeding sites within 200 m of a case and interior spraying with a pyrethroid adulticide is targeted at premises within 100 m. To ascertain whether these limits are appropriate, we conducted a mark-release-recapture study to measure the dispersal of female Ae. aegypti in the city of Cairns where transmission occurs. Female mosquitoes reared from wild collected eggs were differentially marked with fluorescent dust depending on whether they were to be released blood-fed or non-blood-fed, and a total of 1,948 females was released. A total of 132 sticky ovitraps was set at 64 premises within a 200 m radius and collections of trapped adults were made at 5-15 days post-release. Sixty-seven females (3.4%) were recaptured, with the furthest being caught 200 m from the release point, and the mean distance travelled was 78 m. Overall, 23.1% of the recaptures outside the release site were taken beyond 100 m by day 15. Dispersal was comparable for both blood-fed and non-blood-fed releases. There was a significant tendency for dispersal to be in a north-westerly direction, probably because of the presence of numerous containers and heavy shading by trees in this direction and a busy road to the south of the release point that appeared to inhibit dispersal. The results suggest that adulticiding may have to be extended beyond 100 m if more than 8 days have elapsed since female Ae. aegypti could have fed upon a viraemic dengue case. The study also shows that dispersal is not random, and that it may be possible to maximize vector control by taking into account environmental factors that affect the direction of female mosquito flight.     

Effects of intraspecific larval competition on adult longevity in the mosquitoes Aedes aegypti and Aedes albopictus

Abstract Larval competition is common in container‐breeding mosquitoes. The impact of competition on larval growth has been thoroughly examined and findings that larval competition can lead to density‐dependent effects on adult body size have been documented. The effects of larval competition on adult longevity have been less well explored. The effects of intraspecific larval densities on the longevity of adults maintained under relatively harsh environmental conditions were tested in the laboratory by measuring the longevity of adult Aedes aegypti (L.) and Aedes albopictus (Skuse) (Diptera: Culicidae) that had been reared under a range of larval densities and subsequently maintained in high‐ or low‐humidity regimes (85% or 35% relative humidity [RH], respectively) as adults. We found significant negative effects of competition on adult longevity in Ae. aegypti, but not in Ae. albopictus. Multivariate analysis of variance suggested that the negative effect of the larval environment on the longevity of Ae. aegypti adults was most strongly associated with increased development time and decreased wing length as adults. Understanding how larval competition affects adult longevity under a range of environmental conditions is important in establishing the relationship between models of mosquito population regulation and epidemiological models of vector‐borne disease transmission.     

Productivity and population density estimates of the dengue vector mosquito Aedes aegypti (Stegomyia aegypti) in Australia

New mosquito control strategies centred on the modifying of populations require knowledge of existing population densities at release sites and an understanding of breeding site ecology. Using a quantitative pupal survey method, we investigated production of the dengue vector Aedes aegypti (L.) (Stegomyia aegypti) (Diptera: Culicidae) in Cairns, Queensland, Australia, and found that garden accoutrements represented the most common container type. Deliberately placed ‘sentinel’ containers were set at seven houses and sampled for pupae over 10 weeks during the wet season. Pupal production was approximately constant; tyres and buckets represented the most productive container types. Sentinel tyres produced the largest female mosquitoes, but were relatively rare in the field survey. We then used field‐collected data to make estimates of per premises population density using three different approaches. Estimates of female Ae. aegypti abundance per premises made using the container‐inhabiting mosquito simulation (CIMSiM) model [95% confidence interval (CI) 18.5–29.1 females] concorded reasonably well with estimates obtained using a standing crop calculation based on pupal collections (95% CI 8.8–22.5) and using BG‐Sentinel traps and a sampling rate correction factor (95% CI 6.2–35.2). By first describing local Ae. aegypti productivity, we were able to compare three separate population density estimates which provided similar results. We anticipate that this will provide researchers and health officials with several tools with which to make estimates of population densities.     

Polymorphic microsatellite markers for studies of Aedes aegypti (Diptera: Culicidae), the vector of dengue and yellow fever

A significant challenge to population genetic studies of the dengue vector, Aedes aegypti, has been the lack of polymorphic microsatellite loci. In an effort to develop useful markers, we evaluated the genetic variation at 17 microsatellite loci identified in the A. aegypti genome. Nine loci with at least five alleles were identified in field‐collected specimens from Thailand. An additional two loci carried five alleles if samples from an A. aegypti laboratory colony were included. Our results greatly increase the number of highly variable markers available for the study of the genetics and the population structure of this medically important species.     

Genetic differentiation of the dengue vector, Aedes aegypti (Ho Chi Minh City, Vietnam) using microsatellite markers

Dengue haemorrhagic fever emerged in the 1950s and has become a major public health concern in most Asian countries. In Vietnam, little is known about the intraspecific variation of the vector and its consequences on vectorial capacity. Here we report the use of microsatellite markers to differentiate Aedes aegypti populations in Ho Chi Minh City, a typical, overcrowded Asian city. Six microsatellite loci, with 5-14 alleles per locus, were scored in 20 mosquito samples collected in 1998 in Ho Chi Minh City. We found substantial differentiation among Ae. aegypti populations from the outskirts, whereas populations from the centre of the city showed less differentiation. These results are consistent with the hypothesis that populations of Ae. aegypti in central Ho Chi Minh City are panmictic because there are abundant larval breeding sites and an abundance of humans for adults to feed upon. In contrast, populations on the outskirts become differentiated largely through the processes of genetic drift because larval breeding sites are not as abundant. These findings implicate human activities associated with urbanization, as factors shaping the genetic structure of Ae. aegypti populations.     

A replicated comparison of breeding‐container suitability for the dengue vector Aedes aegypti in tropical and temperate Australia

In Australia, the dengue vector Aedes aegypti is presently restricted to Queensland but was historically more widespread. Future spread may be facilitated by changes in the availability of suitable ovipositing sites (artificial containers) in response to climate change. We undertook a replicated comparison of thermal and hydric conditions in a selection of water containers commonly used by Ae. aegypti under sun and shade conditions in a tropical (Cairns) and temperate (Melbourne) location. We assessed the implications of thermal and hydric regimes for development rates and thermal stress. Container type had no effect on potential development rate in Cairns but mosquitoes in tyres were predicted to have consistently slower development than those in other containers in Melbourne. Our dataset included the hottest day on record for Melbourne (46.4°C) yet few containers exhibited lethal water temperatures in this location. Similarly high water temperatures were reached in Cairns at more benign air temperatures due to high solar radiation loads. The tyres had unique thermal profiles that exhibited a plateau at shaded air temperature even when in full sun. Overall, our results suggest that chronic cold stress would prevent development in Melbourne during spring, drying of containers would be limiting in Melbourne in summer, and heat stress in unshaded small containers would be limiting in Cairns. Tyres could be an important and unappreciated buffered habitat in open areas in the tropics. These results are of value for directing water storage and waste policy to prevent the further spread of Ae. aegypti and dengue fever as well as other mosquitoes. The methodology can be applied to identify priority containers for surveillance in other parts of the world.     

Spatial and temporal dynamics of Aedes aegypti larval sites in Bello, Colombia

Counts of immature stages of the mosquito Aedes aegypti have been used to calculate several entomological indices of dengue vector abundance. Some studies have concluded that these indices can be used as indicators of dengue epidemic risk, while other studies have failed to find a predictive relationship. Ecological niche models have been able to predict distributional patterns in space and time, not only of vectors, but also of the diseases that they transmit. In this study, we used Landsat 7 ETM+ images and two niche-modeling algorithms to estimate the local-landscape ecological niche and the dynamics of Ae. aegypti larval habitats in Bello, Colombia, and to evaluate their potential spatial and temporal distribution. Our models showed low omission error with high confidence levels: about 13.4% of the area presents conditions consistently suitable for breeding across the entire study period (2002-2008). The proportion of neighborhoods predicted to be suitable showed a positive association with dengue case rates, whereas the vector-focused Bretau index had no relationship to case rates. As a consequence, niche models appear to offer a superior option for predictive evaluation of dengue transmission risk and anticipating the potential for outbreaks.     

Evaluation of entomopathogenic fungi as potential biological control agents of the dengue mosquito,Aedes aegypti (Diptera: Culicidae)

Dengue is a global health concern. Growing insecticide resistance in the primary mosquito vector, Aedes aegypti, limits the effectiveness of vector control, so alternative tools are urgently needed. One approach is the use of biopesticides comprising entomopathogenic fungi, e.g., Beauveria bassiana and Metarhizium anisopliae. These fungi may decrease disease transmission by reducing mosquito vector longevity and also occur worldwide, although many isolates have not been tested for virulence against mosquitoes. Ninety-three isolates of entomopathogenic fungi representing six species (B. bassiana, M. anisopliae, Isaria fumosorosea, I. farinosa, I. flavovirescens, and Lecanicillium spp.) were screened as potential biological control agents of Aedes aegypti. A hierarchical, multi-criteria experimental design was undertaken to find suitable isolates. Initial screening was performed via in vitro assays measuring radial growth and spore persistence, eliminating isolates with poor growth or viability on nutrient-rich substrate. Subsequent measurements of spore persistence revealed that only nine of 30 strains tested had half-lives exceeding 3 weeks. Ten isolates were chosen for in vivo bioassays against adult Ae. aegypti. From these assays, two Australian isolates of B. bassiana, FI-277 and FI-278, appeared to be most promising. Both isolates were shown to be virulent against Ae. aegypti at 20, 26, and 32°C. Spreading spores manually onto substrate was found to be more efficacious than spraying. Ae. aegypti infected by manually-spread spores on cotton substrate were found to have an LT₅₀ of 3.7±0.3 days. These characteristics suggest that FI-277 has promise as a dengue mosquito biocontrol agent, either alone or combined with conventional chemical insecticides.     

The effects of temperature and shading on mortality and development rates of Aedes aegypti (Diptera: Culicidae)

Urbanization has caused an increase in favorable habitats for Aedes aegypti (Diptera: Culicidae), given their ability to reproduce in small and often non‐degradable artificial water‐containers. While much work has been done on Ae. aegypti biology and ecology in urban landscapes, the role of shading on immature stages as an independent factor from temperature, and any possible interactions between these factors, remains unexamined. We assessed how temperature and shading affected egg hatch‐rate, larval/pupal mortality, and larval development to adult stage under different factorial temperature (28; 31; 34; 37; 40° C) and shade (0%, 3,100 lux; 40%, 1,860 lux; 75%, 775 lux; 100%, 0 lux) regimes. Hatch‐rate was significantly lower at 37° C (57 %), and no eggs hatched at 40° C. There was no significant effect caused by shading on hatchability. Larval and pupal mortality at 37° C was significantly higher (35%) compared to lower temperature groups, while the effects of shading were emergent at low temperatures. Developmental times from hatching to adult emergence were significantly reduced with increasing temperatures and with greater light exposures. The eco‐physiological response of Ae. aegypti larvae to temperature and light regimes suggest a photosensitivity previously unstudied in this species.     

Horizontal and vertical dispersal of dengue vector mosquitoes, Aedes aegypti and Aedes albopictus, in Singapore

To study the dispersal of dengue vector mosquitoes in Singapore, females of Aedes aegypti (L.) and Aedes albopictus (Skuse) (Diptera: Culicidae) were fed blood containing rubidium (Rb), which was detectable in their eggs by means of Graphite Furnace Atomic Absorption Spectrophotometry (GFAAS). Laboratory calibration of the Rb reading, for a range of egg numbers from Rb-fed females, indicated a reasonably linear relationship and an unequivocal distinction between results with zero and one marked egg. Rb-marked female Aedes mosquitoes aged 3-5 days were released in semi-rural and urbanized parts of Singapore, with an array of ovitraps extending to a radius of 320 m from the release point. Subsequently, Rb-marked Aedes eggs were detected throughout the array, with similar distributions on each of the 4 days after release. More Rb was detected nearer the release point. However, when correction was made for the greater areas of zones further from the release point (and therefore presumably existence of more alternative oviposition sites), there were no significant differences in the numbers of marked eggs per ovitrap in the zones nearer or further from the release points. It is concluded that females of both these Aedes (Stegomyia) species could disperse easily and quickly throughout areas of radius 320 m in search of oviposition sites. This contrasts with the general belief that Ae. aegypti seldom flies more than 50 m and that control operations can safely be based on such an assumption. Releases on level 12 of a 21-storey apartment block, with ovitraps on each storey, showed similar easy and rapid dispersal to the top and bottom of the block.     

Elimination of Aedes aegypti in northern Australia, 2004–2006

The Northern Territory (NT) of Australia is currently free of the dengue mosquito Aedes (Stegomyia) aegypti (L). However, on 17 February 2004, two Ae. aegypti adults were captured in two routine CO₂‐baited encephalitis virus surveillance traps in Tennant Creek, located 990 km south of Darwin in the NT. The detection triggered an immediate survey and control response undertaken by the NT Department of Health and Community Services, followed by a Commonwealth of Australia‐funded Ae. aegypti elimination program. This report details the methods and results of the detection and subsequent elimination activities that were carried out between 2004 and 2006, returning the NT to its dengue vector‐free status. There have been very few successful Ae. aegypti elimination programs in the world. This purposeful mosquito elimination for Australia was officially declared on 5 April 2006.     

Geographic and ecological distribution of the dengue and chikungunya virus vectors Aedes aegypti and Aedes albopictus in three major Cameroonian towns

Aedes albopictus (Diptera: Culicidae) was first reported in Central Africa in 2000, together with the indigenous mosquito species Aedes aegypti (Diptera: Culicidae). Because Ae. albopictus can also transmit arboviruses, its introduction is a public health concern. We undertook a comparative study in three Cameroonian towns (Sahelian domain: Garoua; equatorial domain: Douala and Yaoundé) in order to document infestation by the two species and their ecological preferences. High and variable levels of pre‐imaginal Ae. aegypti and Ae. albopictus infestation were detected. Only Ae. aegypti was encountered in Garoua, whereas both species were found in Douala and Yaoundé, albeit with significant differences in their relative prevalence. Peridomestic water containers were the most strongly colonized and productive larval habitats for both species. No major differences in types of larval habitat were found, but Ae. albopictus preferentially bred in containers containing plant debris or surrounded by vegetation, whereas Ae. aegypti tended to breed in containers located in environments with a high density of buildings. These findings may have important implications for vector control strategies.     

Mating,ovariole number and sperm production of the dengue vector mosquito Aedes aegypti (L.) in Australia: broad thermal optima provide the capacity for survival in a changing climate

Little is known about mating, ovariole number and sperm production of Aedes aegypti (L.) in Australia, especially in relation to climate. To determine the extent of interpopulation variation and thermal dependence of reproductive traits in A. aegypti, laboratory studies are conducted using colonies originating from up to four locations in Queensland, Australia. Observations of insemination reveal that these behaviours are temperature‐dependent, although humidity levels appear to have little effect, with only small increases in insemination at higher humidities. No noteworthy variations in thermal optima (temperature ranges within which maximal performance occurs) for such behaviours are observed between colonies, with all showing high levels of insemination between 25 and 35 ° C. Both male and female maximum fecundity for A. aegypti are also found to be temperature‐dependent. Sperm counts, not hitherto obtained for Australian A. aegypti, range from approximately 3000–5000 per male, with counts increasing with increased rearing and holding temperature, despite a decreasing body size. Conversely, ovary size decreases with temperature and body size, from approximately 100 ovarioles per female at 17 °C, to 85 at 35 °C. The lack of variation in reproductive capacity between colonies of different geographical origin indicates that any locally‐acting selective pressures are not driving divergence in key reproductive traits such as insemination ability and fecundity. This may be because the source populations used are not from sufficiently diverse climates. Nonetheless, the broad thermal optima for reproductive traits in A. aegypti are suggestive of limitations on these traits not being responsible for limiting distribution and population growth in the event of projected mean temperature rises.     

Effects of temperature on the life cycle,expansion, and dispersion of Aedes aegypti (Diptera: Culicidae) in three cities in Paraiba,Brazil

The mosquito Aedes aegypti is the primary vector of dengue and is common throughout tropical and subtropical regions. Its distribution is modulated by environmental factors, such as temperature. This study aimed to evaluate the influence of temperature on the life cycle and expansion of Ae. aegypti populations in the cities of Campina Grande, João Pessoa, and Patos. Samples of Ae. aegypti were collected in the three cities and raised in the laboratory. We assessed the life cycles of the three Ae. aegypti populations under six constant temperatures (16, 22, 28, 33, 36, and 39°C), selected on the basis of historical temperature tendencies of each city. We also used existing climate data to calculate projected temperature increases for all three areas. Our results suggest that Campina Grande, João Pessoa, and Patos will experience, respectively, maximum temperature increases of 0.030°C/year, 0.069°C/year, and 0.061°C/year, and minimum temperature increases of 0.019°C/year, ?0.047°C/year, and ?0.086°C/year. These projected increases will result in temperatures favorable to the Ae. aegypti life cycle, causing rapid population growth. Therefore, Ae. aegypti populations are likely to expand in the mesoregions represented by these cities.     

Seasonal population dynamics and the genetic structure of the mosquito vector Aedes aegypti in São Paulo,Brazil

Population genetic studies of insect vectors can generate knowledge to improve epidemiological studies focused on the decrease of pathogen transmission. In this study, we used nine SNPs across the Aedes aegypti genome to characterize seasonal population variations of this important dengue vector. Mosquito samples were obtained by ovitraps placed over Botucatu SP from 2005 to 2010. Our data show that, regardless of the large variation in mosquito abundance (deduced from the number of eggs obtained from ovitraps), the effective population size remained stable over the years. These results suggest that Ae. aegypti is able to maintain a sufficiently large active breeding population during the dry season to keep genetic frequencies stable. These results open new perspectives on mosquito survey and control methods.     

Isolation and variability of polymorphic microsatellite loci in Aedes aegypti,the vector of dengue viruses

We isolated five microsatellite sequences from an enriched‐(CAA)_n library of 5000 recombinant clones in Aedes aegypti. Two polymorphic microsatellites from our library and four from other sequence databases were tested: we investigated their polymorphism and Mendelian inheritance in mosquito populations. Our results indicate that trinucleotide repeat markers could be used to differentiate Ae. aegypti populations, making them valuable tools for the study of population genetic structure.     

Effects of larval diet on development rates and reproductive maturation of male and female Mediterranean fruit flies

Abstract Mediterranean fruit fly Ceratitis capitata (Diptera: Tephritidae) larvae are capable of developing in one of many hosts that may vary greatly in quality. We hypothesized that they will respond to the larval environment in a manner beneficial to their subsequent reproductive performance. Accordingly, we investigated the effects of various larval diets (varying in the amount of protein and sugar they contain) on the size, development time, nutritional status and reproductive maturation (ovarian development and onset of sexual behaviour) of females and males. We found that flies which undergo larval development in artificial host fruit that contain sugar and protein ('protein-fed') were larger, developed faster and emerged with more nutritional reserves than flies that were protein-deprived as larvae. Protein-fed males, regardless of their size, became sexually active before males that developed in hosts with no protein. Protein-fed females produced more mature eggs than protein-deprived ones. Moreover, protein-fed females tended to copulate sooner than females that developed in hosts with no protein. In addition, regardless of female larval diet, females with more mature eggs tended to copulate sooner than females with less mature eggs. In light of these results, the importance of the larval environment for adult reproductive success is discussed.     

Ovitrap surveys of dengue vector mosquitoes in Chiang Mai, northern Thailand: seasonal shifts in relative abundance of Aedes albopictus and Ae. aegypti

Aedes aegypti (L.) and Aedes albopictus (Skuse) were surveyed using ovitraps in residential areas in Chiang Mai, northern Thailand. Egg populations (both species inclusive) remained low in the dry season, but increased/decreased exponentially during the first/latter half of the rainy season, respectively. This seasonal pattern was similar to the seasonal distribution of dengue haemorrhagic fever cases in the area. During the dry season (November-March) Ae.aegypti was dominant in urban and indoor ovitraps. With onset of the rainy season in April, relative abundance of Ae.albopictus increased in rural and outdoor ovitraps. Ae.albopictus displaced Ae.aegypti in the latter half of the rainy season in the rural area. Possible mechanisms to account for this seasonal decline of Ae.aegypti and reciprocal fluctuations in relative abundance of Ae.albopictus are discussed in relation to food availability for larvae in container habitats.     

The influence of larval density, food stress, and parasitism on the bionomics of the dengue vector Aedes aegypti (Diptera: Culicidae): implications for integrated vector management

New larval control strategies for integrated vector management of Aedes aegypti are in high demand, including the use of biological control agents. Exposure of Aedes aegypti to parasites, starvation, and overcrowded conditions during larval development reduces the probability of survival to eclosion, can directly affect fitness parameters such as adult size and fecundity, and can affect the size, provisioning, and viability of eggs produced by females. We compared these parameters after exposing larvae to 1) abundant food at low larval densities, 2) food deprivation and high larval density, and 2) infection with the endoparasite Plagiorchis elegans, an entomopathogenic digenean trematode. Female mosquitoes that eclosed from larval conditions of starvation and overcrowding were smaller and laid fewer and smaller eggs than controls. The proportion of females to complete an oviposition cycle was reduced in the P. elegans-infected treatment group. Parasite load was negatively correlated with wing length and egg size. Infection of Ae. aegypti with P. elegans has sublethal effects and may reduce population-level reproductive output, but one-time low-density P. elegans exposure does not have sufficient effect on Ae. aegypti fitness parameters to be considered a viable biocontrol option.