Mosquitoes are present across California, thriving in diverse habitats from coastal regions to inland valleys and mountainous areas. Understanding the types of mosquitoes found here, their activity patterns, and the potential health concerns they pose can help residents navigate their environment more effectively.
Key Mosquito Species and Activity Patterns
California is home to over 50 mosquito species. The Culex species, including Culex tarsalis, Culex pipiens, and Culex quinquefasciatus, are widely distributed. These mosquitoes prefer to breed in stagnant water sources like vegetated ponds, ditches, and neglected swimming pools. They are active during dusk and nighttime hours, seeking blood meals from birds, other animals, and humans.
Invasive Aedes species, such as Aedes aegypti and Aedes albopictus, are established in many urban areas, particularly in Southern California and the Central Valley. These “container breeders” lay eggs in small water collections found in flowerpot saucers, pet dishes, old tires, and clogged gutters. Aedes mosquitoes are aggressive daytime biters, sometimes called “ankle biters” due to their preference for biting around the lower legs.
Mosquito activity increases in spring as temperatures rise, peaking during July and August. As fall approaches, cooler temperatures and reduced daylight lead to a decline, with activity diminishing by late October to November. However, in some warmer microclimates and urban environments, certain species, particularly invasive Aedes, can remain active year-round.
Understanding Mosquito-Borne Illnesses
Mosquitoes in California can transmit several human diseases, with West Nile Virus (WNV) being the most identified concern. WNV is endemic in California, recurring annually, with transmission primarily in summer and early fall. About one in five infected people develop mild symptoms such as fever, headache, body aches, nausea, or a skin rash.
A small percentage of individuals can develop severe WNV symptoms affecting the nervous system, including high fever, neck stiffness, disorientation, paralysis, or coma. Mosquitoes, mainly Culex species, become infected with WNV when they feed on infected birds, then transmit the virus to humans or other animals.
Other mosquito-borne illnesses, such as St. Louis Encephalitis (SLEV) and Western Equine Encephalitis (WEE), are also present but less common. SLEV, transmitted by Culex mosquitoes, is similar to WNV in its symptoms and transmission cycle. WEE, also transmitted by Culex tarsalis, has historically caused outbreaks in California.
While Zika and Dengue are associated with mosquitoes, local transmission in California was historically rare, with most cases linked to international travel. However, the presence of invasive Aedes mosquitoes increases concern. In 2023 and 2024, California confirmed locally acquired Dengue cases in Los Angeles County, indicating a growing potential for these diseases to spread within the state.
Strategies for Mosquito Protection and Control
Protecting oneself from mosquito bites involves personal and environmental strategies. For personal protection, use EPA-registered insect repellents containing active ingredients such as DEET, Picaridin, IR3535, or Oil of Lemon Eucalyptus. Apply these to exposed skin and clothing, following product label instructions. Wearing long-sleeved shirts and long pants, especially during peak mosquito activity times like dawn and dusk, also reduces skin exposure.
Ensure windows and doors have intact screens to prevent mosquitoes from entering indoor spaces. For household and community control, eliminate mosquito breeding sites. The “Tip ‘n Toss” method involves regularly emptying and scrubbing containers that hold standing water, such as pet bowls, bird baths, and flowerpot saucers. Unused containers, old tires, and clogged rain gutters should be cleaned out to prevent water accumulation.
Local mosquito and vector control districts conduct surveillance, monitor mosquito populations, and implement control programs. These agencies use methods like applying larvicides to water sources to prevent larvae development, and adulticides to control adult populations. They also provide public education and respond to resident requests.