7 mosquitoes to watch in the U.S. as warming and urbanization expand disease risk

Mosquito-borne disease risk in the United States is rising as several species that transmit dangerous pathogens expand their ranges, public health experts say. Seven mosquito types are responsible for the bulk of infections and merit close surveillance because warming temperatures, changing rainfall patterns and human land use are making more of the country hospitable to them.

Mosquitoes are not uniformly dangerous: only a subset of the more than 200 species in the United States are efficient vectors of human disease. Still, the combination of longer warm seasons, more extreme precipitation events and urban conditions that create breeding sites has increased the odds that infected travelers or animals will seed local outbreaks. West Nile virus remains the largest mosquito-borne threat in the country; first detected in 1999, it annually causes about 1,200 severe illnesses and more than 120 deaths, according to the Centers for Disease Control and Prevention.

Transmission of mosquito-borne pathogens is a multistep process and requires that the female mosquito feed on an infected host, allow the pathogen to survive and replicate inside her, and then bite another susceptible person or animal. Only female mosquitoes bite to obtain blood for egg production. Because so many factors must align, outbreaks are difficult to predict, but warmer conditions increase mosquito survival, accelerate reproductive cycles and can speed pathogen replication inside insect hosts, all of which raise transmission potential.

Culex mosquitoes, which include several species common across the country, are the principal vectors of West Nile virus as well as other encephalitis-causing viruses. Culex pipiens, the common house mosquito, and Culex quinquefasciatus, the southern house mosquito, are widespread and often live near humans in urban and suburban areas. Culex tarsalis is more prevalent in the Western United States and is associated with wetlands and irrigated agricultural lands. Culex species tend to bite in the evening and at night and breed in standing water ranging from storm drains to poorly maintained urban ponds.

Anopheles mosquitoes were once responsible for most malaria transmission in the United States before coordinated public health efforts largely eliminated local transmission by the mid-20th century; malaria was declared eliminated in the U.S. in 1951. The insects remain present and are rebounding in some regions. Anopheles freeborni, common in parts of the West, breeds in vegetated ditches and culverts and is brownish-black with spotted wings. Anopheles quadrimaculatus is more typical of the Eastern United States and derives its name from the dark spots often seen on its wings. In 2023, public health authorities documented locally acquired malaria cases in Florida and Texas, underscoring that presence of competent Anopheles species can permit local transmission when infected people reintroduce parasites.

Aedes mosquitoes present another set of concerns because they are effective transmitters of dengue, Zika, chikungunya and yellow fever viruses and are highly adapted to human environments. Aedes aegypti, the yellow fever mosquito, is notable for white lyre-shaped markings on its thorax and a tendency to bite indoors. Aedes albopictus, the Asian tiger mosquito, has distinctive black-and-white banding, is a daytime biter and breeds in small containers of water such as flower pots, discarded tires and gutters. Both Aedes species are container breeders that prosper on the margins of urban development and are difficult to control because they need only tiny pools of water for larvae to mature.

Mosquito life history traits also affect control strategies. Most adult mosquitoes live only a few weeks and usually remain within a few miles (often much less) of their birthplace, though flight distance varies by species. They are sensitive to cold: many species stop biting below about 50 degrees Fahrenheit, and freezing temperatures can kill adults even as eggs survive to hatch when warming returns. Humidity and rainfall patterns shape breeding opportunities; heavier, more frequent rainfall can produce the standing water mosquitoes require. In many parts of the U.S., rising average temperatures are lengthening the breeding season and allowing species to survive at higher elevations and farther north than in previous decades.

Human actions have amplified these ecological shifts. Urbanization increases impervious surfaces that create standing-water microhabitats, while deforestation and habitat loss reduce predators such as bats and birds that historically helped limit mosquito numbers. International travel increases the chance that infected persons will return to the United States and be bitten by local mosquitoes, which can pick up and then spread pathogens domestically. Recent introductions of invasive mosquito species as stowaways have also contributed to range shifts.

Public health officials say containment requires layered, coordinated responses at local, state and federal levels. Early surveillance to detect both the insects present and the pathogens they carry can guide interventions such as targeted pesticide applications, community education to eliminate container breeding sites, and focused spraying when necessary to curb outbreaks. Preserving habitat for mosquito predators, improving water management to prevent pooling, and investing in diagnostic and reporting systems are additional measures experts recommend. In the longer term, limiting greenhouse gas emissions to slow climate change would reduce the expansion of mosquito-suitable habitat.

For individuals, practical steps remain effective: remove standing water around homes, use EPA-registered repellents, maintain window and door screens, and be aware of peak biting times for different species. Public health authorities continue to monitor mosquito populations and the diseases they can carry; awareness of which species are present locally can help residents and officials prioritize prevention and response.

Mosquito-borne diseases in the United States have declined markedly from their early-20th-century peaks, but that progress is not guaranteed. With ranges of key vector species expanding and climate and land-use changes providing more opportunities for transmission, experts emphasize vigilance, targeted mosquito control, and sustained public health investment to prevent a resurgence of mosquito-borne illness.