Two house mosquitoes (Culex spp.) are present in the United States: the northern house mosquito and the southern house mosquito. The northern house mosquito was introduced from North Africa and is present across the northern US; the southern house mosquito was introduced from Southeast Asia and is present across the southern US. There is a hybrid zone where these mosquitoes meet and are able to produce viable offspring, indicating they are probably subspecies instead of two separate species.
House mosquitoes can vector West Nile virus (and other diseases that may cause encephalitis) from birds to humans and horses. They are more likely to vector these diseases in urban areas because of their preference for living and breeding near humans. Since house mosquitoes overwinter as adults, they are often found in structures during cold temperatures. Females may bite when indoor heating or the weather becomes warm enough. Although they will breed anywhere water is allowed to accumulate, they prefer to lay eggs in stagnant water rich in organic matter, as in sewage or drainage systems, and in pastures.
Removing standing water on properties reduces house mosquito populations. Otherwise, apply an insect growth regulator (IGR) labeled for Culex mosquitoes to natural or manmade sites that hold water, to prevent larvae from developing into adults. Apply a liquid residual insecticide to adult mosquito resting sites under shady and secluded areas such as foliage, eaves and decks. A longer-term approach is to add mosquito traps designed to kill the larvae inside, and contaminate adult females with a larvicide they leave behind in their next egg-laying sites. They are also contaminated with a fungal pathogen that will kill them in several days.