Which mosquito species is often associated with transmitting West Nile Virus and St. Louis encephalitis?

Culex species mosquitoes, particularly Culex pipiens and Culex tarsalis, are recognized as primary vectors for transmitting West Nile Virus and St. Louis encephalitis. These species thrive in habitats that include stagnant water or poorly drained areas, where they lay their eggs. Their habit of feeding at night also contributes to their effectiveness as vectors for these viruses.

Other mosquito species listed, while they may carry other diseases or have specific ecological roles, are not primarily associated with transmitting these particular viruses. Culex mosquitoes have been extensively studied and shown to be significant contributors to the epidemiology of these encephalitic diseases, making them the correct answer in this context.