Mosquito population dynamics is shaped by the interaction among larval density, season, and land use.

Understanding how variation in key abiotic and biotic factors interact at spatial scales relevant for mosquito fitness and population dynamics is crucial for predicting current and future mosquito distributions and abundances, and the transmission potential for human pathogens. However, studies investigating the effects of environmental variation on mosquito traits have investigated environmental factors in isolation or in laboratory experiments that examine constant environmental conditions that often do not occur in the field. To address these limitations, we conducted a semi-field experiment in Athens, Georgia using the invasive Asian tiger mosquito (Aedes albopictus). We selected nine sites that spanned natural variation in impervious surface and vegetation cover to explore effects of the microclimate (temperature and humidity) on mosquitoes. On these sites, we manipulated conspecific larval density at each site. We repeated the experiment in the summer and fall. We then evaluated the effects of land cover, larval density, and time of season, as well as interactive effects, on the mean proportion of females emerging, juvenile development time, size upon emergence, and predicted per capita population growth (i.e., fitness). We found significant effects of larval density, land cover, and season on all response variables. Of most note, we saw strong interactive effects of season and intra-specific density on each response variable, including a non-intuitive decrease in development time with increasing intra-specific competition in the fall. Our study demonstrates that ignoring the interaction between variation in biotic and abiotic variables could reduce the accuracy and precision of models used to predict mosquito population and pathogen transmission dynamics, especially those inferring dynamics at finer-spatial scales across which transmission and control occur.

Para poder predecir la distribución y abundancia de las poblaciones de mosquitos y la transmisión potencial de patógenos a humanos, es crucial comprender cómo factores abióticos y bióticos clave para el éxito reproductivo y la dinámica poblacional de los mosquitos interactúan a escalas relevantes. Sin embargo, los estudios que han investigado los efectos de variables ambientales en las características demográficas de los mosquitos han considerado su efecto de forma aislada o en experimentos de laboratorio bajo condiciones ambientales constantes que, a menudo, no reflejan lo que ocurre en el campo. Para abordar estas limitaciones, llevamos a cabo un experimento de semi-campo en Athens, Georgia, utilizando el mosquito invasor tigre asiático (Aedes albopictus). Seleccionamos nueve sitios que abarcaban variaciones naturales en la superficie impermeable y cobertura vegetal para explorar los efectos del microclima (temperatura y humedad) en los mosquitos. También manipulamos la densidad de larvas de tigre asiático en dos experimentos que fueron realizados en el verano y otoño. Evaluamos los efectos de la cobertura vegetal, la densidad de larvas, la temporada climática, y la interacción entre estas variables en la proporción de hembras que emergieron, el tiempo de desarrollo de las larvas, el tamaño al momento de la emergencia, y el crecimiento demográfico per cápita previsto (éxito reproductivo). Encontramos efectos significativos de la densidad de larvas, la variación de la cobertura vegetal y la estación del año en todas las variables de respuesta. Más notablemente, observamos un fuerte efecto de la interacción entre la temporada climática y la densidad de larvas en todas las variables de respuesta, incluyendo una disminución no intuitiva en el tiempo de desarrollo con el aumento de la competencia intraespecífica en el otoño. Nuestro estudio evidencia que ignorar la interacción entre variables abióticas y bióticas podría reducir la exactitud y precisión de los modelos utilizados para predecir las dinámicas de las poblaciones de mosquitos, y por tanto, de la transmisión de patógenos. Esto, especialmente en modelos que infieren estas dinámicas a escalas espaciales más finas, en las cuales ocurre la transmisión y el control.

Microclimate and Larval Habitat Density Predict Adult Aedes albopictus Abundance in Urban Areas.

The Asian tiger mosquito, Aedes albopictus, transmits several arboviruses of public health importance, including chikungunya and dengue. Since its introduction to the United States in 1985, the species has invaded more than 40 states, including temperate areas not previously at risk of Aedes-transmitted arboviruses. Mathematical models incorporate climatic variables in predictions of site-specific Ae. albopictus abundances to identify human populations at risk of disease. However, these models rely on coarse resolutions of environmental data that may not accurately represent the climatic profile experienced by mosquitoes in the field, particularly in climatically heterogeneous urban areas. In this study, we pair field surveys of larval and adult Ae. albopictus mosquitoes with site-specific microclimate data across a range of land use types to investigate the relationships between microclimate, density of larval habitat, and adult mosquito abundance and determine whether these relationships change across an urban gradient. We find no evidence for a difference in larval habitat density or adult abundance between rural, suburban, and urban land classes. Adult abundance increases with increasing larval habitat density, which itself is dependent on microclimate. Adult abundance is strongly explained by microclimate variables, demonstrating that theoretically derived, laboratory-parameterized relationships in ectotherm physiology apply to the field. Our results support the continued use of temperature-dependent models to predict Ae. albopictus abundance in urban areas.

Carry-over effects of urban larval environments on the transmission potential of dengue-2 virus.

BACKGROUND: Mosquitoes are strongly influenced by environmental temperatures, both directly and indirectly via carry-over effects, a phenomenon by which adult phenotypes are shaped indirectly by the environmental conditions experienced in previous life stages. In landscapes with spatially varying microclimates, such as a city, the effects of environmental temperature can therefore lead to spatial patterns in disease dynamics. To explore the contribution of carry-over effects on the transmission of dengue-2 virus (DENV-2), we conducted a semi-field experiment comparing the demographic and transmission rates of Aedes albopictus reared on different urban land classes in the summer and autumn season. We parameterized a model of vectorial capacity using field- and literature-derived measurements to estimate the bias introduced into predictions of vectorial capacity not accounting for carry-over effects. RESULTS: The larval environment of different land classes and seasons significantly impacted mosquito life history traits. Larval development and survival rates were higher in the summer than the autumn, with no difference across land class. The effect of land class on adult body size differed across season, with suburban mosquitoes having the smallest wing length in the summer and the largest wing length in the autumn, when compared to other land classes. Infection and dissemination rates were higher in the autumn and on suburban and rural land classes compared to urban. Infectiousness did not differ across land class or season. We estimate that not accounting for carry-over effects can underestimate disease transmission potential in suburban and urban sites in the summer by up to 25%. CONCLUSIONS: Our findings demonstrate the potential of the larval environment to differentially impact stages of DENV-2 infection in Ae. albopictus mosquitoes via carry-over effects. Failure to account for carry-over effects of the larval environment in mechanistic models can lead to biased estimates of disease transmission potential at fine-scales in urban environments.