Interspecific Competition between Aedes albopictus and A. sierrensis: potential for Competitive Displacement in the Western United States

Towards the invasion of wild and rural forested areas in Gabon (Central Africa) by the Asian tiger mosquito Aedes albopictus: Potential risks from the one health perspective

BACKGROUND

Since its first record in urban areas of Central-Africa in the 2000s, the invasive mosquito, Aedes albopictus, has spread throughout the region, including in remote villages in forested areas, causing outbreaks of Aedes-borne diseases, such as dengue and chikungunya. Such invasion might enhance Ae. albopictus interactions with wild animals in forest ecosystems and favor the spillover of zoonotic arboviruses to humans. The aim of this study was to monitor Ae. albopictus spread in the wildlife reserve of La Lopé National Park (Gabon), and evaluate the magnitude of the rainforest ecosystem colonization.

METHODOLOGY

From 2014 to 2018, we used ovitraps, larval surveys, BG-Sentinel traps, and human landing catches along an anthropization gradient from La Lopé village to the natural forest in the Park.

CONCLUSIONS

We detected Ae. albopictus in gallery forest up to 15 km away from La Lopé village. However, Ae. albopictus was significantly more abundant at anthropogenic sites than in less anthropized areas. The number of eggs laid by Ae. albopictus decreased progressively with the distance from the forest fringe up to 200m inside the forest. Our results suggested that in forest ecosystems, high Ae. albopictus density is mainly observed at interfaces between anthropized and natural forested environments. Additionally, our data suggested that Ae. albopictus may act as a bridge vector of zoonotic pathogens between wild and anthropogenic compartments.

Laboratory study on larval competition between two related mosquito species: Aedes (Stegomyia) albopictus and Aedes (Stegomyia) cretinus

Aedes albopictus (Asian tiger mosquito) and Aedes cretinus are closely related mosquito species with similar morphological and bio-ecological characteristics. These species have been detected in specific areas of Athens, Greece, with Ae. albopictus developing significantly higher population densities than the native mosquito Ae. cretinus. In a laboratory factorial experimental design, we investigated the potential of interspecific and intraspecific competition between larvae of these species under various food and population density conditions. Duration of larval development, survival rate from the first larval instar until adulthood and the wing length of females were measured. When these two species developed on their own, larvae developed faster and the females were larger at high food provision, indicating intraspecific competition. When the two species developed in the same environment and food provision was low, Ae. albopictus outcompeted Ae. cretinus. In particular, the larval developmental time when these species competed with each other was 1.3 to 2.4 days shorter for Ae. albopictus and 0.9 to 1.4 days longer for Ae. cretinus, compared with single species development. Interspecific competition resulted to larger Ae. albopictus females at limited food availability and low density of individuals. Our findings indicate that Ae. albopictus is a superior competitor to Ae. cretinus, primarily at limited larval food resources, and this may account for the expansion of Ae. albopictus and the limited presence of Ae. cretinus in areas of Athens, Greece, where these related species co-exist.

Predicting the response of disease vectors to global change: The importance of allometric scaling

The distribution of disease vectors such as mosquitoes is changing. Climate change, invasions and vector control strategies all alter the distribution and abundance of mosquitoes. When disease vectors undergo a range shift, so do disease burdens. Predicting such shifts is a priority to adequately prepare for disease control. Accurate predictions of distributional changes depend on how factors such as temperature and competition affect mosquito life-history traits, particularly body size and reproduction. Direct estimates of both body size and reproduction in mosquitoes are logistically challenging and time-consuming, so the field has long relied upon linear (isometric) conversions between wing length (a convenient proxy of size) and reproductive output. These linear transformations underlie most models projecting species' distributions and competitive interactions between native and invasive disease vectors. Using a series of meta-analyses, we show that the relationship between wing length and fecundity are nonlinear (hyperallometric) for most mosquito species. We show that whilst most models ignore reproductive hyperallometry (with respect to wing length), doing so introduces systematic biases into estimates of population growth. In particular, failing to account for reproductive hyperallometry overestimates the effects of temperature and underestimates the effects of competition. Assuming isometry also increases the potential to misestimate the efficacy of vector control strategies by underestimating the contribution of larger females in population replenishment. Finally, failing to account for reproductive hyperallometry and variation in body size can lead to qualitative errors via the counter-intuitive effects of Jensen's inequality. For example, if mean sizes decrease, but variance increases, then reproductive outputs may actually increase. We suggest that future disease vector models incorporate hyperallometric relationships to more accurately predict changes in mosquito distribution in response to global change.

Review of the ecology and behaviour of Aedes aegypti and Aedes albopictus in Western Africa and implications for vector control

Western Africa is vulnerable to arboviral disease transmission, having recently experienced major outbreaks of chikungunya, dengue, yellow fever and Zika. However, there have been relatively few studies on the natural history of the two major human arbovirus vectors in this region, Aedes aegypti and Ae. albopictus, potentially limiting the implementation of effective vector control. We systematically searched for and reviewed relevant studies on the behaviour and ecology of Ae. aegypti and Ae. albopictus in Western Africa, published over the last 40 years. We identified 73 relevant studies, over half of which were conducted in Nigeria, Senegal, or Côte d'Ivoire. Most studies investigated the ecology of Ae. aegypti and Ae. albopictus, exploring the impact of seasonality and land cover on mosquito populations and identifying aquatic habitats. This review highlights the adaptation of Ae. albopictus to urban environments and its invasive potential, and the year-round maintenance of Ae. aegypti populations in water storage containers. However, important gaps were identified in the literature on the behaviour of both species, particularly Ae. albopictus. In Western Africa, Ae. aegypti and Ae. albopictus appear to be mainly anthropophilic and to bite predominantly during the day, but further research is needed to confirm this to inform planning of effective vector control strategies. We discuss the public health implications of these findings and comment on the suitability of existing and novel options for control in Western Africa.

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.

Laboratory colonization of the European invasive mosquito Aedes (Finlaya) koreicus

Background

Aedes (Finlaya) koreicus (Edwards) is a mosquito that has recently entered Europe from Asia. This species is considered a potential threat to newly colonized territories, but little is known about its capacity to transmit pathogens or ability to compete with native mosquito species. The establishment of a laboratory colony is a necessary first step for further laboratory studies on the biology, ecology and vector competence of Ae. koreicus.

Results

A self-mating colony was established at QIMR Berghofer Medical Research Institute (Brisbane, Australia) from eggs of the F1 progeny of individuals collected as free-living larvae in northeastern Italy (Belluno province). Mosquitoes are currently maintained on both defibrinated sheep blood provided via an artificial membrane system and human blood from volunteers. Larvae are maintained in rain water and fed with Tetramin^® fish food (©2015 Spectrum Brands - Pet, Home and Garden Division, Tetra-Fish). Morphometric measurements related to body size were taken and a fecundity index, based on wing length, was calculated. An in vivo technique for differentiating male and female pupae has been optimized. Our findings provide the basis for further studies on the ecology and physiology of Ae. koreicus.

Conclusion

We describe the establishment of an Ae. koreicus colony in the laboratory and identify critical requirements for the maintenance of this mosquito species under artificial conditions. The laboratory colony will facilitate studies investigating the vector potential of this species for human pathogens.

What Can Larval Ecology Tell Us About the Success of Aedes albopictus (Diptera: Culicidae) Within the United States?

Aedes albopictus (Skuse) was introduced in the United States approximately 30 years ago, and since has become an important pest and vector of disease. This species uses small water-holding containers as sites for oviposition and larval development. Larvae can consume a wide range of detritus-based energy sources, including microorganisms, and as such the type and quantity of detritus that enters these systems have been studied for the effects on adult populations. This review examines the documented responses of Ae. albopictus to different larval environments within the United States, and some of its unique ecology that may lead to a better understanding of its spread and success. Field surveys generally find larvae in shaded containers with high amounts of organic detritus. Larvae have higher survival and population growth under high amounts of detritus and microorganisms, but they also can outcompete other species when nutrients are limiting. Allocation of time to feeding by larvae is greater and more focused compared with resident species. These latter two points also may explain differences in carbon and nitrogen composition (nutrient stoichiometry), which point to a lower need for nitrogen. Combined, these facts suggest that the Ae. albopictus is a species with a relatively wide niche that had been able to exploit container habitats in the United States better than resident species. After 30 yr of research, only a narrow range of detritus types and environmental conditions have been examined. Data on factors affecting the production of adults and its spread and apparent success are still needed.

The Importance of Interspecific Interactions on the Present Range of the Invasive Mosquito Aedes albopictus (Diptera: Culicidae) and Persistence of Resident Container Species in the United States

Aedes albopictus (Skuse) established in the United States over 30 yr ago and quickly spread throughout the entire eastern half of the country. It has recently spread into western regions and projected climate change scenarios suggest continued expansion to the west and north. Aedes albopictus has had major impacts on, and been impacted by, a diverse array of resident mosquito species. Laying eggs at the edges of small, water-holding containers, hatched larvae develop within these containers feeding on detritus-based resources. Under limited resource conditions, Ae. albopictus has been shown to be a superior competitor to essentially all native and resident species in the United States. Adult males also mate interspecifically with at least one resident species with significant negative impacts on reproductive output for susceptible females. Despite these strong interference effects on sympatric species, competitor outcomes have been highly variable, ranging from outright local exclusion by Ae. albopictus, to apparent exclusion of Ae. albopictus in the presence of the same species. Context-dependent mechanisms that alter the relative strengths of inter- and intraspecific competition, as well as rapid evolution of satyrization-resistant females, may help explain these patterns of variable coexistence. Although there is a large body of research on interspecific interactions of Ae. albopictus in the United States, there remain substantial gaps in our understanding of the most important species interactions. Addressing these gaps is important in predicting the future distribution of this species and understanding consequences for resident species, including humans, that interact with this highly invasive mosquito.