Contra Costa Mosquito District Targets Invasive Species Capable Of Spreading Serious Diseases

Mosquitoes are considered the deadliest animals in the world due to their ability to spread diseases that kill hundreds of thousands of people each year. While Contra Costa County is home to more than 20 mosquito species, local officials are particularly focused on controlling an invasive mosquito known as Aedes aegypti, or the yellow fever mosquito. Unlike the larger mosquitoes commonly seen at dusk, Aedes aegypti is a small, aggressive daytime biter that often targets people around the ankles and lower legs. Health officials say the species is especially concerning because it can transmit viruses that cause dengue fever, Zika, yellow fever and chikungunya.

The mosquito is not native to California. Since first appearing in Southern California about 15 years ago, it has steadily expanded its range throughout the state. The species spreads easily because its eggs can survive in small containers holding water, including flower pots, pet bowls, toys and other items commonly found in residential yards. The Contra Costa Mosquito and Vector Control District first detected Aedes aegypti in Martinez in 2022. After an extensive control effort, no additional mosquitoes of that species have been found in the area. However, the mosquito was detected in Antioch in late 2024 and has continued to be found there each year, including in 2026.

To combat the invasive mosquito, district employees have been conducting door-to-door inspections in affected neighborhoods, searching for standing water and signs of mosquito activity. Crews have also used larvicide treatments to target mosquitoes before they reach adulthood. The district is also evaluating newer mosquito-control methods already being used in parts of Southern California. One approach involves releasing sterile male mosquitoes that mate with females, preventing viable offspring from being produced. Another method uses a naturally occurring bacterium called Wolbachia, which causes mosquito eggs to fail to hatch after mating. Because only female mosquitoes bite, officials say releasing treated males poses no risk of disease transmission.

Officials continue to stress that residents play a critical role in preventing mosquito breeding. Mosquitoes need standing water to complete their life cycle, and even small amounts can support their development. Residents are encouraged to empty containers holding water at least once a week and to contact the Contra Costa Mosquito and Vector Control District if they experience unusual daytime mosquito activity, particularly bites around the legs and ankles. District services are provided to county residents through existing property taxes and assessments, and there is no additional charge for residential mosquito-control assistance.