West Nile virus (family Flaviridae, genus Flavivirus, WNV) was first reported in Chicago in 2001, where it has since been the most common vector-borne pathogen in the region, with consistently high mosquito infection rates and persistent cases of human and veterinary illness. As in most vector-borne systems, one focus of current WNV research is studying the biology of the mosquito vector. Areas with high vector densities are likely to have high rates ofpathogen transmission from avian reservoir hosts to mammalian hosts, so understanding the ecological drivers of vector abundance and distribution patterns is critical for obtaining accurate estimates of risk for human infection.The most important enzootic vectors for WNV in the midwestern United States are Culex pipiens Linnaeus (Diptera: Culicidae) and Cx. restuans Theobald. Both species have complex life cycles with aquatic juvenile stages and terrestrial adult stages. Because exposure to the bite of a mosquito is clearly linked to risk for WNV infection, the majority of previous field research has focused on the habitat and behavior of adult females. The relative paucity of attention to the aquatic stages is surprising because laboratory studies consistently have demonstrated that many physiological traits that later determine the vector competence of adults develop as products ofenvironmental exposures in the larval stage. Furthermore, most mosquito abatement focuses onthe larval stage, so it is in the interest of mosquito control programs to understand the ecological conditions that favor Culex production.In this study, I examine multiple potential environmental determinants of the abundanceand spatial distribution of larval Cx. pipiens and Cx. restuans in suburban Cook County, Illinois.Over the course of two seasons from May to September 2009 and 2010, mosquito larvae were sampled from a total of 80 storm water catch basins, a common oviposition site in urban locales, in three Chicago municipalities (Oak Lawn, Alsip, and Evergreen Park). Larval sampling was supplemented by daily temperature and precipitation measurements taken at Midway Airport and Cook County Precipitation Network weather stations, as well as quantification of the trees and shrubs within 40 m of each catch basin and, in 2009, measurements of aquatic pH, ammonia, nitrate, and phosphate content.I found that weather conditions and neighborhood structural characteristics contribute toboth the seasonality and geographic patterns of mosquito abundance in the urban storm water catch basin system. Field-collected data from 2010 demonstrated that large precipitation events (>3.48 cm rainfall over a period over2 consecutive hours) dramatically reduced the number of Culex larvae while high ambient temperature (>23.9°C) was associated with high larval abundance. Data from 2009 indicated that local density of certain tree genera was an important predictor of Culex production, with arborvitae (Thuja spp.) and magnolia (Magnolia spp.)positively associated with larval abundance and spruce (Picea spp.) negatively associated withlarval abundance, while shrub are was positively correlated to presence of Culex. Finally, aquaticchemistry was shown to alter larval abundance, with ammonia decreasing and nitrate increasing the number of juvenile mosquitoes per catch basin. Collectively, these results may be used by local mosquito abatement districts to target limited resources for treating common urban breeding sites when they are particularly likely to produce WNV vectors. These findings also provide a foundation for further laboratory and field research investigating mosquito abundance in relation to vegetation characteristics.
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Influence of neighborhood structure and weather conditions on density of vectors and risk for West Nile Virus in Cook County Illinois
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