Invasive Mosquitoes Have Spread Across 28 California Counties
Residents in these areas should be alert for daytime biting insects that thrive in small containers of water.
Updated on Sept. 25, 2026 in Diseases — General
Live Poll
Do you regularly eliminate standing water in your yard to help prevent invasive mosquito populations?
The invasive Aedes aegypti mosquito has established populations in 28 California counties, leading to an increase in reports of daytime bites. While these mosquitoes are known vectors for diseases like dengue and Zika, no viruses have been detected in local samples this year.
Why it matters
The expansion of this species poses a unique public health challenge because these mosquitoes thrive near human activity and breed in small containers that typical municipal control programs cannot reach. This shift means that managing mosquito risks now depends largely on the efforts of individual homeowners to eliminate breeding sites.
Official surveillance from the California Department of Public Health confirmed the presence of Aedes aegypti in 28 counties. Although these insects are capable of carrying viruses like chikungunya and yellow fever, mosquitoes collected in Shasta County have not tested positive this year.
The players
California Department of Public Health
The state agency responsible for monitoring infectious diseases and implementing public health surveillance programs.
The details
Aedes aegypti mosquitoes are distinct because they are daytime biters that often live in close proximity to human homes. They breed in tiny amounts of standing water, such as those found in bottle caps, plant saucers, and buckets, allowing them to proliferate in residential environments. The species has spread to new regions primarily by hitching rides in vehicles, outdoor equipment, and plant containers.
Timeline
The mosquito was first detected in Shasta County in 2020.
Established populations were confirmed across 28 counties in 2026.
Mosquito season is expected to continue well into the fall of 2026.
Health Landscape
The proliferation of Aedes aegypti represents a significant evolution in the state's vector-borne disease landscape compared to native mosquito varieties. It follows a pattern of range expansion for invasive species that thrive in human-modified environments.
To reduce the risk of bites, check your property weekly for any standing water in small containers that could serve as breeding grounds. If you notice an increase in daytime biting activity or are concerned about localized risk, discuss vector control strategies with your local public health office.
The takeaway
The arrival of Aedes aegypti means that the best defense is eliminating small sources of standing water on your own property. It is wise to track any unusual clusters of daytime mosquito bites and report them to your local vector control agency.
Further reading
For more information on monitoring disease vectors in your community, visit our Diseases — General section.
More information
For official updates on mosquito-borne disease monitoring, visit the California Department of Public Health portal.
Live Poll
Do you regularly eliminate standing water in your yard to help prevent invasive mosquito populations?
