Mosquito Almanac
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Mosquitoes in Arizona

By Region Updated Jul 2026 6 min read
Swarm-of-mosquitoes-in-the-sky

Short answer

Arizona’s deserts are dry, but they are not mosquito-free. Summer monsoon rains and standing urban water (neglected pools, irrigation, retention basins) give mosquitoes the breeding sites they need, and West Nile virus is the state’s leading mosquito-borne disease. In 2021, Maricopa County recorded 1,487 human West Nile cases, the largest documented focal outbreak in a single county on record (Sources 1, 2). The invasive yellow fever mosquito, Aedes aegypti, is now established in Phoenix, Tucson, and other urban areas, and Maricopa County reported its first locally acquired dengue infection in late 2022 (Sources 3, 6). If you live in or visit the low deserts, mosquito bites are a year-round consideration, not just a summer one.

Mosquito season

In Arizona’s low deserts, the mosquito season is long. Warm temperatures let some species stay active well beyond the typical spring-to-fall window, and in the warmest zones activity can persist into late fall or early winter.

The clearest seasonal driver is the North American monsoon, which usually arrives in mid-July and runs into September. Monsoon storms flood retention basins, roadside channels, and low-lying ground, creating fresh breeding water across the metro areas. Maricopa County reports that mosquito-borne illness is most likely to spread from roughly May through October, which lines up with monsoon breeding conditions (Source 4). West Nile virus detections in mosquitoes typically climb through August and into fall, so the highest-risk period sits in the late summer and early autumn months.

The mosquitoes you meet

Two groups matter most in Arizona.

Culex mosquitoes are the West Nile carriers. Culex quinquefasciatus (the southern house mosquito) drives most West Nile transmission across the urban metros, while Culex tarsalis fills in across the irrigated low-desert farmland of counties such as Yuma, Pinal, and Maricopa (Source 5). These are the species that state and county surveillance programs trap and test for virus. (See our Culex species page for identification and behavior.)

Aedes aegypti, the yellow fever mosquito, is the invasive daytime biter of concern in cities. It was first detected in Tucson in 1994 and has since spread through residential and business districts, and it is now established in urban areas statewide with population centers in Phoenix (Maricopa), Tucson (Pima), and Yuma (Sources 6, 7). This species is a competent vector of dengue, chikungunya, and Zika, which is why its spread changes the disease picture in Arizona’s cities. (See our Aedes aegypti species page.)

Disease risk

West Nile virus is the dominant mosquito-borne disease in Arizona. Most people who are infected have no symptoms, but a small share develop serious neuroinvasive illness. The 2021 Maricopa County outbreak shows the scale that is possible: of the 1,487 human cases identified that year, 956 (64.3%) had neuroinvasive disease and 101 (6.8%) died, making it the largest documented focal West Nile outbreak in a single county on record (Sources 1, 2). That season the mosquito vector index reached its highest recorded level, and an unusually heavy monsoon is widely cited as a contributing factor. (See our West Nile virus disease page.)

Dengue is the growing concern tied to Aedes aegypti. On November 7, 2022, Arizona identified dengue virus (serotype DENV-3) in a Maricopa County resident who had not traveled, the first evidence of locally acquired (autochthonous) dengue in the county. Mosquito surveillance detected dengue virus in a trap in the same area, and an environmental assessment found Aedes aegypti and breeding sites at the residence. The local outbreak consisted of two autochthonous infections and was declared over on January 4, 2023, after more than 45 days without additional local cases (Source 3). As Aedes aegypti populations expand in the cities, the risk of local dengue transmission is likely to remain a live concern. (See our dengue disease page.)

Local control

In the Phoenix metro, mosquito surveillance and control are run by the Maricopa County Environmental Services Department Vector Control Division. Staff operate weekly trapping at more than 850 locations, sorting captured mosquitoes into species pools of 1 to 50 and testing female Culex for West Nile virus and Saint Louis encephalitis virus (Sources 6, 8). When virus activity or nuisance levels rise, the division applies larvicide to breeding sites and, in the worst weeks, fogs with adulticide; during the 2021 outbreak season it treated large acreages and thousands of larval sites (Source 8).

Neglected swimming pools are a specific Arizona problem. A single untended “green pool” can produce very large numbers of mosquitoes, so counties treat pool reports as a control priority. Residents can report a stagnant or green pool, or heavy mosquito activity, to Maricopa County Vector Control at (602) 506-6616 (Source 8). Other urban water sources (irrigation runoff, retention basins, clogged drainage, and containers around the home) matter for the same reason.

Protect yourself

The basic steps are the same ones public health agencies recommend nationally, and they work against both Culex and Aedes species:

  • Use an EPA-registered repellent (such as DEET, picaridin, IR3535, or oil of lemon eucalyptus) and follow the label.
  • Wear long sleeves and long pants when mosquitoes are active. Culex mosquitoes bite mostly at dawn and dusk, while Aedes aegypti bites during the day, so cover up accordingly.
  • Repair window and door screens and keep them closed.
  • Dump and drain standing water around your home at least weekly: buckets, plant saucers, tires, toys, and clogged gutters. Keep pools maintained or report abandoned ones.
  • Report green pools and heavy mosquito activity to your county vector control program.

Sources

  1. CDC, Morbidity and Mortality Weekly Report, “Unprecedented Outbreak of West Nile Virus, Maricopa County, Arizona, 2021,” MMWR 2023;72(17). Primary source for the 2021 case total (1,487), neuroinvasive share, and deaths. https://www.cdc.gov/mmwr/volumes/72/wr/mm7217a1.htm (page returned HTTP 403 on 2026-07-20; figures confirmed via the peer-reviewed abstract in Source 2). Accessed 2026-07-20.
  2. Kretschmer M, et al., PubMed record for the MMWR outbreak report (PMID 37104168). Abstract states 1,487 cases, 956 (64.3%) neuroinvasive, 101 (6.8%) deaths, “largest documented focal WNV outbreak in a single county,” and record vector index of 53.61. https://pubmed.ncbi.nlm.nih.gov/37104168/ Accessed 2026-07-20.
  3. CDC, Morbidity and Mortality Weekly Report, “Notes From the Field: First Evidence of Locally Acquired Dengue Virus Infection, Maricopa County, Arizona, November 2022,” MMWR 2023;72(11). Source for DENV-3, the November 7, 2022 detection, two autochthonous cases, and the January 4, 2023 outbreak close. https://www.cdc.gov/mmwr/volumes/72/wr/mm7211a5.htm (page returned HTTP 403 on 2026-07-20; details confirmed via CDC search listing and PMC copy PMC10027408). Accessed 2026-07-20.
  4. Maricopa County, “Mosquito-Borne Disease Surveillance / Vector Control” public health guidance. Source for the May-through-October transmission window and monsoon breeding conditions. https://www.maricopa.gov/2476/Mosquito-Borne-Disease-Statistics Accessed 2026-07-20.
  5. Peer-reviewed and extension sources on Arizona Culex vectors, including work on Culex quinquefasciatus (urban West Nile) and Culex tarsalis (irrigated farmland). PMC review of Sonoran Desert Culex. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3835805/ Accessed 2026-07-20.
  6. Maricopa County, “Vector Control” division page. Source for weekly trapping at more than 850 sites, pool testing methodology, and the (602) 506-6616 report line. https://www.maricopa.gov/632/Vector-Control Accessed 2026-07-20.
  7. Engelthaler DM, et al., “The reemergence of Aedes aegypti in Arizona” and related Tucson surveillance work (Emerging Infectious Diseases / PMC). Source for the 1994 Tucson detection and urban establishment statewide. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2627602/ Accessed 2026-07-20.
  8. Maricopa County Environmental Services Department, Vector Control Division program materials on trapping, larviciding, adulticiding, and green-pool response during the 2021 season. https://www.maricopa.gov/632/Vector-Control Accessed 2026-07-20.

Related

  • Culex mosquitoes (species) [placeholder]
  • Aedes aegypti (species) [placeholder]
  • West Nile virus (disease) [placeholder]
  • Dengue (disease) [placeholder]
  • Mosquito repellents and bite prevention [placeholder]
  • Regions index: mosquitoes by state [placeholder]

Educational note: This guide is for general information and public health awareness only. It is not medical advice. If you have symptoms of a mosquito-borne illness, contact a healthcare provider or your local health department. For local mosquito complaints and green-pool reports, contact your county vector control program.

Last reviewed 20 July 2026.