Inter- and intra-specific density-dependent effects on life history and development strategies of larval mosquitoes

Context-specific variation in life history traits and behavior of Aedes aegypti mosquitoes

Aedes aegypti, the vector for dengue, chikungunya, yellow fever, and Zika, poses a growing global epidemiological risk. Despite extensive research on Ae. aegypti's life history traits and behavior, critical knowledge gaps persist, particularly in integrating these findings across varied experimental contexts. The plasticity of Ae. aegypti's traits throughout its life cycle allows dynamic responses to environmental changes, yet understanding these variations within heterogeneous study designs remains challenging. A critical aspect often overlooked is the impact of using lab-adapted lines of Ae. aegypti, which may have evolved under laboratory conditions, potentially altering their life history traits and behavioral responses compared to wild populations. Therefore, incorporating field-derived populations in experimental designs is essential to capture the natural variability and adaptability of Ae. aegypti. The relationship between larval growing conditions and adult traits and behavior is significantly influenced by the specific context in which mosquitoes are studied. Laboratory conditions may not replicate the ecological complexities faced by wild populations, leading to discrepancies in observed traits and behavior. These discrepancies highlight the need for ecologically relevant experimental conditions, allowing mosquito traits and behavior to reflect field distributions. One effective approach is semi-field studies involving field-collected mosquitoes housed for fewer generations in the lab under ecologically relevant conditions. This growing trend provides researchers with the desired control over experimental conditions while maintaining the genetic diversity of field populations. By focusing on variations in life history traits and behavioral plasticity within these varied contexts, this review highlights the intricate relationship between larval growing conditions and adult traits and behavior. It underscores the significance of transstadial effects and the necessity of adopting study designs and reporting practices that acknowledge plasticity in adult traits and behavior, considering variations due to larval rearing conditions. Embracing such approaches paves the way for a comprehensive understanding of contextual variations in mosquito life history traits and behavior. This integrated perspective enables the synthesis of research findings across laboratory, semi-field, and field-based investigations, which is crucial for devising targeted intervention strategies tailored to specific ecological contexts to combat the health threat posed by this formidable disease vector effectively.

Mosquito Egg Raft Distribution Is Affected by Semiochemicals: Indication of Interspecific Competition

Numerous species of animals alter their behavior in response to increasing competition. To do so, they must possess the ability to detect the presence and density of interspecific competitors. We studied the role of semiochemicals released by increasing densities of larval Culiseta longiareolata Macquart on female oviposition habitat selection in two field experiments. Similarly to C. longiareolata larvae, subordinate Culex laticinctus Edwards are periphyton grazers who dwell in rain-filled pools in the Mediterranean region. We show that C. laticinctus females oviposited significantly less in mesocosm pools that were treated with crowding signals originating from C. longiareolata larvae. In the second experiment, we placed a similar number of larvae directly inside the 50 L mesocosms. These low-density mesocosms did not affect C. laticinctus oviposition but were attractive to conspecific oviposition. These results increase our understanding of the female ability to detect species-specific signals, indicating increased larval competition.

Fish-Released Kairomones Affect Mosquito Oviposition and Larval Life History

Several species of mosquitoes respond to the presence of kairomones released by larvivorous predators during oviposition habitat selection and larval development. These responses may differ among mosquito species and do not always correlate with larval survival. This study examined the responses of the mosquito Culiseta longiareolata Macquart (Diptera: Culicidae) to kairomones released by three species of fish, Gambusia affinis (Cyprinodontiformes: Poeciliidae), Aphanius mento (Cyprinodontiformes: Cyprinodontidae) and Garra rufa (Cypriniformes: Cyprinidae) during oviposition. In addition, the study examined the effects of kairomones released by Gm. affinis on larval development and survival. Results show that ovipositing female avoided cues from the two larvivorous fish species but not the algivorous Gr. rufa. In addition, developing larvae metamorphosed slower and showed increased mortality when exposed to fish-released kairomones. Culiseta longiareolata larvae are known as dominant competitors, and the straightforward responses of both larvae and adult female to fish-released kairomones may be explained by the lack of additional sources of larval stress other than the presence of predators.

Influence of landscape composition and configuration on the richness and abundance of potential sylvatic yellow fever vectors in a remnant of Atlantic Forest in the city of São Paulo, Brazil

The landscape's structure can play a relevant role in epidemic patterns of arboviruses, influencing factors such as abundance, movement, and dispersal ability in arthropod vectors and vertebrate hosts, besides promoting alterations in the rate of potential infectious contacts between these organisms. In the Americas, yellow fever (YF) exhibits only the sylvatic cycle, in which the virus circulates in sylvatic areas among non-human primates, being transmitted by mosquitoes of the Haemagogus and Sabethes genera. In this study, we investigate some aspects of the landscape in relation to diversity and abundance of culicid species associated with YF transmission. Studies were performed in the Cantareira State Park, a remnant of the Atlantic Forest located in Greater Metropolitan São Paulo, Brazil, where the YF virus circulated recently with dozens of deaths in howler monkeys (Alouatta guariba), in addition to reported human cases. Mosquito collections were carried out monthly from February 2015 to April 2017. Mosquitoes were collected from three sites using battery-powered aspirator (12-volt battery), CDC, and Shannon traps for adults, and suction samplers and entomological spoons in breeding sites to collect immature forms. 703 mosquitoes belonging to 12 species of the Aedini and Sabethini tribes were collected. Aedes scapularis and Psorophora ferox exhibited higher abundance, while Haemagogus leucocelaenus, the main vector of YF in São Paulo state, showed lower abundance in all sampled areas. The site with longer edge between forest area and anthropic area presented more richness and abundance of YF vector species, while the site with larger forest cover area and shorter edges between forest and anthropic areas exhibited an inverse pattern. Statistically significant differences were observed between the composition of potential YF vector species among the investigated sites. Although Hg. leucocelaenus occurred in all sampled sites, the different patterns of distribution and abundance of other mosquitoes such as Aedes scapularis and Psorophora ferox suggest that these species may be involved in the transmission of sylvatic YF in the study area.

Quantifying the Influence of Larval Density on Disease Transmission Indices in Culex quinquefasciatus, the Major African Vector of Filariasis

Larval crowding is one of the abiotic factors affecting biological fitness in mosquitoes. This study aims at elucidating, quantitatively, the influence of more larval crowding on aspects of fitness in Culex quinquefasciatus mosquito. To this end, day-old larvae of the species were reared in 4 density regimens equivalent to 1 larva in 1.25, 2.5, 5, and 10 mL of distilled water. Developmental indices, adult fitness indices, and accumulation and utilisation of teneral reserves for metamorphosis were determined at these density regimens. The results revealed varying significant negative effects of larval density on all fitness indices measured for the species. The study also revealed high utilisation of teneral reserves for metamorphosis at high larval densities. The information generated will be useful in making informed-decisions in allocating scare resources for vector control, although field trials are advocated to establish these laboratory findings.

Competition among Aedes aegypti larvae

Adult Aedes aegypti mosquitoes are important vectors of human disease. The size of the adult female affects her success, fitness, and ability to transmit diseases. The size of the adults is determined during the aquatic larval stage. Competition among larvae for food influences the size of the pupa and thus the adult. In these experiments, the food level (mg/larva) and the density (larvae/vial) both affect intraspecific competition, which shows up as the interaction of the two factors. Furthermore, the total food per vial affects the nature of competition among the larvae, also apparent in the interaction of food and density. Male larvae are affected by the percent of males in the vial, but females are not. Seven biologically significant dependent variables were examined, and the data analyzed by multivariate analysis of variance to gain insight into the relationships among the variables and the effects of these factors on the larvae as they grew in small containers. Male and female larvae compete differently from one another for the particulate yeast cells in this experiment; female larvae outcompete males through larger size and by retaining cells within their gut at low total food levels. Under conditions of more intense competition, the pupal masses of both males and females are smaller, so the effect of competition is a reduced apparent food level. The age at pupation is also affected by food and density. Across the twenty treatment combinations of food/larva and larvae/vial, female larvae grew as though there were six different ecological environments while male larvae grew as though there were only four different environments. No interference competition was observed. Eradication efforts aimed at adult populations of this mosquito may inadvertently increase the size and robustness of the next generation of larvae, resulting in a subsequent adult population increase in the second generation.

Wing morphology variations in a natural population of Phlebotomus tobbi Adler and Theodor 1930

Cutaneous leishmaniasis (CL) is highly endemic in the Cukurova region, located on the crossroads of main refugee routes from the Middle East to Europe on the eastern Mediterranean part of Turkey. Our purpose was to investigate the phenotypic variation of Phlebotomus tobbi, the known vector of CL in the region, during one active season. Sand flies and microclimatic data were collected monthly from May to October, 2011, from five locations in six villages in the study area. A geometric morphometric approach was used to investigate wing morphology. Shape analyses revealed that males collected in May and June comprised one group, while specimens collected in August, September, and October formed a second group. Specimens from July were found to be distributed within these two groups. A similar distribution pattern was observed for females, but specimens from October were represented as the third district group. Significant size variation was detected for both sexes between months. Wing size and temperature were negatively correlated for females, but there was no temperature effect for males. Wing size of both sexes was increased in correlation to increasing relative humidity. Males were found to have smaller wings with increasing population density.

Detrivory, competition, and apparent predation by Culiseta longiareolata in a temporary pool ecosystem

Larvae of the mosquito Culiseta longiareolata Macquart have been suggested as important species in desert and Mediterranean temporary pond ecosystems through their strong competitive abilities and as intra-guild predators. We examined their potential predatory effect on larvae of the abundant saltmarsh mosquito Ochlerotatus caspius. We did not find evidence for predatory effects of C. longiareolata on O. caspius larvae. We suggest that, at least in our system, C. longiareolata is an apparent predator. Namely, it does not actively prey on mobile victims, but rather feeds on the carcasses of its fallen competitors additional to its generally immobile food. Hence, we do not expect the occurrence of anti-predator behaviors in response to C. longiareolata presence, including larval development characteristics and oviposition habitat selection.

Diversity and abundance of mosquitoes (Diptera:Culicidae) in an urban park: larval habitats and temporal variation

Urban parks are areas designated for human recreation but also serve as shelter and refuge for populations of several species of native fauna, both migratory and introduced. In Brazil, the effect of annual climate variations on Aedes aegypti and dengue epidemics in large cities like São Paulo is well known, but little is known about how such variations can affect the diversity of mosquito vectors in urban parks and the risk of disease transmission by these vectors. This study investigates the influence of larval habitats and seasonal factors on the diversity and abundance of Culicidae fauna in Anhanguera Park, one of the largest remaining green areas in the city of São Paulo. Species composition and richness and larval habitats were identified. Seasonality (cold-dry and hot-rainy periods) and year were considered as explanatory variables and the models selection approach was developed to investigate the relationship of these variables with mosquito diversity and abundance. A total of 11,036 specimens from 57 taxa distributed in 13 genera were collected. Culex nigripalpus, Cx. quinquefasciatus and Aedes albopictus were the most abundant species. Bamboo internodes and artificial breeding sites showed higher abundance, while ponds and puddles showed greater richness. Significant relationships were observed between abundance and seasonality, with a notable increase in the mosquitos abundance in the warm-rainy periods. The Shannon and Berger-Parker indices were related with interaction between seasonality and year, however separately these predictors showed no relationship with ones. The increased abundance of mosquitoes in warm-rainy months and the fact that some of the species are epidemiologically important increase not only the risk of pathogen transmission to people who frequent urban parks but also the nuisance represented by insect bites. The findings of this study highlight the importance of knowledge of culicid ecology in green areas in urban environments.

Population Dynamics of Culex restuans and Culex pipiens (Diptera: Culicidae) Related to Climatic Factors in Northwest Ohio

Two species of Culex mosquitoes are common throughout much of North America. Culex restuans Theobold is a native species, whereas Culex pipiens L. is a European immigrant that has been in North America since the 1600s. Larvae of Cx. restuans are numerically dominant in spring and early summer but Cx. pipiens dominates by mid-summer. This transition is termed the "Culex crossover" and has been previously explored in larval populations, largely because Cx. pipiens is more likely to transfer West Nile virus to humans. Adult mosquitoes of both species were captured in 14 light traps in Lucas County, OH, between May and October 1980-2011. We examined this 31-yr, continuous record of adult populations for signs of a species crossover, relationships between abundances of both species and climate factors, and evidence of interspecific competition. The total cumulative degree-days (above 0°C), total cumulative precipitation, and total number of each species were calculated for each day of January-September (annual) and May-September (mosquito season) of each year. On average, adult Cx. pipiens became numerically dominant over Cx. restuans on day 175 ± 21 (June 24), consistent with the Culex crossover reported for their larvae. Pearson correlations showed that abundances of both species were related to temperature and precipitation, but Cx. pipiens tended to be positively related to climatic factors, whereas Cx. restuans showed negative correlations. Moreover, abundances of the two species were more positively than negatively related to one another, providing no evidence of interspecific competition.

Occurrence of a mosquito vector in bird houses: Developmental consequences and potential epidemiological implications

Even with continuous vector control, dengue is still a growing threat to public health in Southeast Asia. Main causes comprise difficulties in identifying productive breeding sites and inappropriate targeted chemical interventions. In this region, rural families keep live birds in backyards and dengue mosquitoes have been reported in containers in the cages. To focus on this particular breeding site, we examined the capacity of bird fecal matter (BFM) from the spotted dove, to support Aedes albopictus larval growth. The impact of BFM larval uptake on some adult fitness traits influencing vectorial capacity was also investigated. In serial bioassays involving a high and low larval density (HD and LD), BFM and larval standard food (LSF) affected differently larval development. At HD, development was longer in the BFM environment. There were no appreciable mortality differences between the two treatments, which resulted in similar pupation and adult emergence successes. BFM treatment produced a better gender balance. There were comparable levels of blood uptake and egg production in BFM and LSF females at LD; that was not the case for the HD one, which resulted in bigger adults. BFM and LSF females displayed equivalent lifespans; in males, this parameter was shorter in those derived from the BFM/LD treatment. Taken together these results suggest that bird defecations successfully support the development of Ae. albopictus. Due to their cryptic aspects, containers used to supply water to encaged birds may not have been targeted by chemical interventions.

Larval intraspecific competition for food in the European grapevine moth Lobesia botrana

Effective pest management with lower amounts of pesticides relies on accurate prediction of insect pest growth rates. Knowledge of the factors governing this trait and the resulting fitness of individuals is thus necessary to refine predictions and make suitable decisions in crop protection. The European grapevine moth, Lobesia botrana, the major pest of grapes in Europe, is responsible for huge economic losses. Larvae very rarely leave the grape bunch on which they were oviposited and thus cannot avoid intraspecific competition. In this study, we determined the impact of intraspecific competition during the larval stage on development and adult fitness in this species. This was tested by rearing different numbers of larvae on an artificial diet and measuring developmental and reproductive life history traits. We found that intraspecific competition during larval development has a slight impact on the fitness of L. botrana. The principal finding of this work is that larval density has little effect on the life history traits of survivors. Thus, the timing of eclosion, duration of subsequent oviposition, fecundity appears to be more uniform in L. botrana than in other species. The main effect of larval crowding was a strong increase of larval mortality at high densities whereas the probability of emergence, sex ratio, pupal mass, fecundity and longevity of mated females were not affected by larval crowding. Owing to increased larval mortality at high larval densities, we hypothesized that mortality of larvae at high densities provided better access to food for the survivors with the result that more food was available per capita and there were no effect on fitness of survivors. From our results, larval crowding alters the reproductive capacity of this pest less than expected but this single factor should now be tested in interaction with limited resources in the wild.

Species-specific non-physical interference competition among mosquito larvae

Individuals of different sex, size or developmental stage can compete differently and hence contribute distinctively to population dynamics. In species with complex life cycles such as insects, competitive ability is often positively correlated with larval developmental stage. Yet, little is known on how the development and survival of early-instars is influenced by interference from late-instar larvae, especially at low densities when exploitative competition is expected to be negligible. Furthermore, the specificity and mechanisms by which interference competition operates are largely unknown. We performed two complementary experiments aiming to quantify the competitive effects of late instar Ochlerotatus caspius on early instar larvae at low densities and under high resource supply rate. The first experiment examined the net effect of interference by 4(th) on 1(st) instar O. caspius larvae, relative to the effect of 1(st) instars on themselves. The second experiment examined the effect of species-specific, non-physical interference competition (i.e., cage larvae) by 4(th) on 1(st) instar O. caspius larvae at low or high densities. Specifically, we compared the responses of O. caspius larvae raised in the presence of caged con- or hetero-specific, Culiseta longiareolata, with that of larvae in the empty-cage control group. As expected, interference from late instar larvae had a net negative effect on the development rate of first instars. In contrast, the presence of caged con-specifics (non-physical interference) accelerated the development rate of O. caspius, however, this pattern was only evident at the low density. Notably, no such pattern was detected in the presence of caged hetero-specifics. These results strongly suggest the existence of species-specific growth regulating semiochemicals.