Biting midges of the Nearctic Culicoides variipennis complex, C. sonorensis Wirth and Jones, C. variipennis (Coquillette), and C. occidentalis Wirth and Jones, are widespread and locally abundant throughout the United States. An understanding of environmental factors associated with the distributions of respective species is of value because risk for bluetongue disease in livestock is defined by the distribution of C. sonorensis, the primary vector of bluetongue viruses in the United States. This study tested the relationship between dissolved salts and immature populations of the C. variipennis complex by examining widespread and diverse aquatic habitats with larval populations and using discriminant analysis to classify larval habitats by species on the basis of soil chemistry. Aquatic habitats with immature C. sonorensis in British Columbia, Illinois, New Mexico, and Washington were moderate-high in levels of dissolved salts, as is typical for habitats with this species. In contrast, a nearby site with the sister species C. variipennis in British Columbia, as well as aquatic habitats in Illinois, were low in dissolved salts, similar to habitats with this species in the eastern United States. A saline stream in central Washington with C. occidentalis was extremely high in dissolved salts, like the playa lakes where this species occurs in the far western United States, whereas a stock pond near the saline stream was lower in dissolved salts and occupied by C. sonorensis. These data confirm that conspecific populations of the C. variipennis complex occur in habitats that have similar levels of dissolved salts, which differ between species, regardless of geographic location. Reverse stepwise discriminant analysis using new and archived soil chemistry data showed that electrical conductivity and chloride discriminated best between habitats with C. variipennis or C. sonorensis and that the combination of electrical conductivity and calcium had the most power of discrimination between habitats with C. sonorensis or C. occidentalis. Collectively, these findings represent convincing evidence that levels of dissolved salts influence the suitability of aquatic habitats for immature populations the C. variipennis complex and hence in part determine respective species’ distributions.
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