Culex pipiens

Culex pipiens, the northern house mosquito, is a widespread urban mosquito and the principal vector of West Nile virus in Europe and North America. It belongs to the genus Culex and is the namesake of the Culex pipiens complex, a group of closely related, interbreeding mosquitoes that includes the southern house mosquito, Culex quinquefasciatus.

The species is also famous in biology for a different reason: the bacterium Wolbachia was first discovered in its tissues in the 1920s and named Wolbachia pipientis after it.

The complex is a long-standing puzzle for taxonomists because its members look nearly identical but differ in behavior, physiology, and disease role:

• Culex pipiens* form *pipiens lives above ground in temperate regions, prefers to feed on birds, needs a blood meal before laying eggs, mates in open-air swarms, and overwinters as an adult female in a dormant state (diapause).
• Culex pipiens* form *molestus thrives in underground spaces such as subway tunnels, basements, and flooded cellars. It readily bites mammals including humans, can lay its first batch of eggs without a blood meal (autogeny), mates in confined spaces, and does not enter diapause. Populations in the London Underground were shown in the late 1990s to be genetically distinct from surface mosquitoes above them.
• Culex quinquefasciatus replaces Cx. pipiens in tropical and subtropical regions worldwide, and hybridizes with it where their ranges meet, such as in parts of North America.

Hybrids between the pipiens and molestus forms are especially important for disease, because they combine a taste for birds with a willingness to bite people.

Females lay eggs in floating rafts of up to a few hundred eggs on the surface of still water. They favor water rich in organic matter, such as storm drains, catch basins, septic overflow, sewage lagoons, discarded tires, and neglected pools, which makes the species thrive in cities. Larvae hatch within a few days and filter-feed on microorganisms and organic particles, breathing through a siphon at the water surface. In warm weather, development from egg to adult can take a week or two.

Adults are active at dusk and at night. Females of the pipiens form spend winter in sheltered places such as caves, culverts, and sheds, emerging in spring to begin feeding and laying eggs. Because the virus can persist in some overwintering females, this behavior helps explain how mosquito-borne viruses reappear each summer in temperate climates.

Culex pipiens and its relatives transmit several pathogens:

• West Nile virus: The mosquito acts both as an enzootic vector, spreading the virus among birds, its natural reservoir, and as a bridge vector, carrying it from birds to humans and horses, which are dead-end hosts. Shifts in feeding from birds to mammals in late summer help explain the seasonal peak of West Nile fever cases.
• Other viruses: St. Louis encephalitis virus, Usutu virus (which causes die-offs of blackbirds in Europe), and in some regions Japanese encephalitis virus.
• Filarial worms: Culex quinquefasciatus is the main urban vector of Wuchereria bancrofti, a cause of lymphatic filariasis.
• Avian malaria: Culex mosquitoes spread Plasmodium relictum among birds. Introduced Cx. quinquefasciatus contributed to the collapse of native Hawaiian forest birds, which had no evolved resistance.

The temperature dependence of virus development inside the mosquito, its extrinsic incubation period, links outbreaks to hot summers. Warmer conditions shorten the time before an infected mosquito can transmit the virus.

Because the species breeds in human-made containers and drains, control focuses on source reduction (removing standing water), treating unavoidable breeding sites with larvicides such as Bacillus thuringiensis israelensis or Lysinibacillus sphaericus, and adult spraying during outbreaks. Culex pipiens has become a classic model of insecticide resistance: populations have evolved resistance through amplified detoxifying esterase genes and through mutations at the insecticide's target sites.

Wolbachia infections in Cx. pipiens cause cytoplasmic incompatibility, in which matings between incompatible strains produce no offspring. This phenomenon underlies "incompatible insect technique" approaches to population suppression. It is part of the broader toolkit of vector control used against mosquito-borne disease.