Road salt pollution alters sex ratios in emerging mosquito populations.

De-icing road salt is a persistent emerging pollutant in temperate freshwater systems, where salting is necessary for road and pedestrian safety. Experts argue that road salts may increase salt-tolerant mosquito populations and, thus, disease transmission in urban areas. Furthermore, there is a natural male bias in sex ratios within some mosquito species. However, it is unclear whether road salt differentially affects the number of male and female mosquitoes - only adult females consume bloodmeals. We hypothesized that road salts would masculinize emergence sex ratios and decrease female success because females may face higher exposure to stressors during their lengthy juvenile development than males. We measured mosquito emergence sex ratios of control (0 g/L added salt) and salt (4.5 g/L added salt) mesocosms in Mississauga, Canada across the West Nile Virus season (May to October). We found female-biased sex ratios (i.e., <50% male frequency) in both 0 and 4.5 g/L. While mosquito abundance was significantly higher in 4.5 g/L compared to 0 g/L, road salt significantly increased the proportion of emerging males from 32.8% to 40.8% (Negative Binomial Model; Estimate ± SE = 0.283 ± 0.108; P = 0.009); mosquitoes shift their sex ratios from female-biased towards parity (50:50) in response to salt. Our study illustrates the need to evaluate sex-specific abundance in pollution-related population studies. By showing a shift toward more male mosquitoes emerging in high salinity compared to control treatments, our results suggest that road salts may have the potential to decrease female mosquito success and indirectly reduce disease transmission in cities.