Aedes aegypti, commonly known as the yellow fever mosquito, is an invasive mosquito. They come from tropical and subtropical areas of the world. Since they are not native to California, they do not live well in the state's natural areas, but they do live and thrive in cities and backyards near people.
Not all mosquitoes spread the same diseases, and not all mosquitoes are infected. That’s why certain species, like Aedes aegypti, are a major concern—this invasive mosquito can spread diseases like Zika, dengue, yellow fever, and chikungunya. These diseases are not currently in the county because they cannot be spread by the mosquitoes that already live here but can be spread by these new invasive mosquitoes.
This invasive mosquito prefers to bite humans, which causes a higher risk of spreading these diseases to people.
Sometimes, people become infected with diseases spread by mosquitoes - like Zika, dengue, yellow fever, or chikungunya - while traveling to areas where these illnesses are common. These are known as “travel cases.” If someone returns home while still infected with one of these diseases and is bitten by the right kind of mosquito species that can spread those diseases - like Aedes aegypti - there’s a risk that the disease could begin spreading locally. This has already happened in parts of Southern California and the Central Valley, where Aedes aegypti populations and the spread of local infections have increased.
We often investigate travel cases related to these diseases to determine whether the types of mosquitoes that can spread them are present in our area. If these mosquitoes are found in our county, it increases the risk of these diseases spreading within our community
Yes. Two adult Aedes aegypti mosquitoes were detected in 2022, and we successfully controlled them. In 2023, we continued monitoring for their presence but did not detect any.
However, in 2024, a total of 388 Aedes aegypti mosquitoes were found at multiple locations throughout the county. Despite our efforts to control them in 2024, they were detected again in April 2025.
For the most up-to-date detection numbers and locations, visit our Aedes aegypti detection timeline.
Aedes aegypti is currently present in more than 18 counties in California.
We are uncertain of how this invasive mosquito got here; however, they can easily spread as unhatched eggs. This mosquito looks for damp soil or container-like items with standing water, for example potted plants, old tires, toys, plant cuttings stored in water, unused water fountains, and bird baths. They lay eggs on the walls of these items or on damp soil. When these containers are moved to a new area, they may already have mosquito eggs. If the containers are filled with water under the right environmental conditions, the eggs can hatch into larvae, which then develop into flying and biting adult mosquitoes.
These mosquitoes do well in urban and suburban parts of cities, often in people’s backyards, patios, and balconies – private properties that District staff cannot easily access.
Aedes aegypti mosquitoes are known as “container breeders” because they like to lay their eggs inside containers-like items filled with water, on plant stems growing in water or on damp soil. Their eggs can survive for several months. Once the eggs get covered with water and the environmental conditions are just right, the eggs will hatch. They can lay eggs in a container as small as a bottle cap.
The flying-biting adult stage is small (about ¼ inch). It has black and white stripes on their back and legs as pictured below.
Their eggs, shown below under a microscope, are about half a millimeter in size. They look like tiny bits of dirt to the naked eye, so it is very hard to tell them apart from soil.
(Aedes aegypti eggs seen under a microscope)
(Aedes aegypti eggs seen from naked eye)
Aedes aegypti is an aggressive mosquito that will bite any time of day (even indoors) and will bite multiple times. Aedes aegypti can bite at dawn and dusk, just like the mosquitoes that already live in the county. But unlike the mosquitoes that are already in the county, Aedes aegypti also bite during the day.
This mosquito will permanently affect our quality of life if their population keeps growing.
They are aggressive biters and can be so annoying that people go inside just to get away from them. Aedes aegypti can spread Zika and other viruses. Once their population gets too big, they are hard to control because they do well in cities and their eggs can survive dry conditions.
Aedes aegypti, commonly known as the yellow fever mosquito, is an invasive mosquito. They come from tropical and subtropical areas of the world. Since they are not native to California, they do not live well in the state's natural areas, but they do live and thrive in cities and backyards near people.
Not all mosquitoes spread the same diseases, and not all mosquitoes are infected. That’s why certain species, like Aedes aegypti, are a major concern—this invasive mosquito can spread diseases like Zika, dengue, yellow fever, and chikungunya. These diseases are not currently in the county because they cannot be spread by the mosquitoes that already live here but can be spread by these new invasive mosquitoes.
This invasive mosquito prefers to bite humans, which causes a higher risk of spreading these diseases to people.
Sometimes, people become infected with diseases spread by mosquitoes - like Zika, dengue, yellow fever, or chikungunya - while traveling to areas where these illnesses are common. These are known as “travel cases.” If someone returns home while still infected with one of these diseases and is bitten by the right kind of mosquito species that can spread those diseases - like Aedes aegypti - there’s a risk that the disease could begin spreading locally. This has already happened in parts of Southern California and the Central Valley, where Aedes aegypti populations and the spread of local infections have increased.
We often investigate travel cases related to these diseases to determine whether the types of mosquitoes that can spread them are present in our area. If these mosquitoes are found in our county, it increases the risk of these diseases spreading within our community
We have a specific plan to respond to this aggressive mosquito. Our first steps include increasing surveillance, going door-to-door to educate community members, and inspecting the outside of homes for sources of standing water. If we find any mosquito larvae, and are unable to dump out the water, we use larvicides (pesticides that target mosquito larvae) after getting approval from the individuals of that home. All pesticides we use are registered with the Environmental Protection Agency and California Department of Pesticide Regulation.
Surveillance: We place traps in the affected area and nearby streets to determine the level and extent of infestation. With permission from residents, traps are also placed in front and back yards overnight to monitor the mosquito’s presence.
Education: We work directly with community members, property owners, and businesses to remove sources of standing water, provide education on personal protection from bites, and remind the public to report any mosquito activity to us. Educational materials are sent to community members in the immediate area and can include flyers, mailed postcards, and email alerts.
Mosquito Control Methods: We use various methods to control mosquitoes like removing standing water, placing free mosquitofish in ponds and neglected pools, and using products registered with the Environmental Protection Agency and the California Department of Pesticides that target mosquito larvae (this is known as larviciding). We apply larvicides in the properties we have been granted access to. Larvicide treatments prevent mosquito larvae from reaching the flying-biting adult stage. In large-scale control, the same larvicides are sprayed using truck-mounted equipment to cover many homes in a short amount of time. The method is called wide area larvicide spraying. If needed, we will also use adulticides (pesticides that target adult biting mosquitoes), to kill mosquitoes that have reached the biting stage. Adulticides are used only when needed to quickly prevent the spread of diseases or reduce the spread of invasive mosquitoes. We also use bait stations to reduce the population of these aggressive mosquitoes.
We’ve stepped up our efforts to find different types of mosquitoes year-round by purposefully setting traps for flying-biting adult mosquitoes in randomized populated areas of the county. We place a higher number of traps in and around the places where Aedes aegypti have been found to track the spread. Visit our mosquito trap website to learn more about the different mosquito traps we use.
We are committed to keeping our affected community members informed. We are taking several approaches to get the word out.
Door-to-Door
Our team goes door-to-door in the affected neighborhoods to talk to residents and inspect for mosquitoes and control them if they are found. We are working closely with the affected community members to get rid of standing water where this mosquito can lay eggs.
- Notices: If community members in the affected area received a notice like a flyer or door hanger from us, but did not get a chance to speak to one of our technicians, please contact us as soon as possible to request a free mosquito inspection. This will help us check for invasive mosquitoes.
Detection and Treatment Notifications
We are committed to providing affected community members notice of any detection or treatment for this mosquito taking place in their community.
- Postcards: Community members in new detection areas will get a postcard in the mail with information about the invasive mosquito issue and links to more resources.
- AlertSCC Notifications: Residents who are signed up for the County’s emergency alert system, AlertSCC, will receive a notification (phone call, text, and/or email) about the mosquito detection in their neighborhood and any scheduled treatment plans. If you are not yet signed up for AlertSCC, visit the County's Office of Emergence website to sign up.
- Email Notifications: To protect public health, we get email addresses of residents in affected neighborhoods from the County of Santa Clara Registrar of Voters. These community members receive an email about the mosquito detection and information on any scheduled treatments in their area.
- General Mosquito Alerts: To stay informed about invasive mosquito detections and mosquito control treatments happening in the county, sign up for our mosquito alert system.
If community members in the affected area are worried about their privacy, please know our staff is only allowed to set traps, check for mosquitoes, and control mosquitoes that are found. They are not allowed to do anything else on the property. Our staff will have clear ways to identify themselves as district employees.
Our priority is to focus on mosquito larval control first. We would consider using adulticides, one of the methods within Integrated Pest Management (IPM), if needed.
Treating Aedes aegypti adult mosquitoes with adulticides would take place during the day because these flying-biting adult mosquitoes are mostly active during the day. These treatments would be different from our West Nile virus adult-mosquito-control treatments because those mosquitoes are mostly active during dusk and dawn.
To control mosquitoes and other vectors, we don’t use only one tool. We use the integrated pest management method (also known as IPM). IPM is the best method to use to provide long term mosquito prevention by using a combination of multiple methods at the same time.
Physical Control: This involves changing the environment to reduce the number of areas with standing water where mosquitoes can lay their eggs. This action reduces their population. It includes actions like emptying containers and removing leaf litter or other obstructions to allow water to flow, which eliminates the need to use pesticides.
Cultural Control: Education is an important part of mosquito control. This involves providing knowledge to community members with new habits that help reduce the mosquito population, leading to fewer problems and more knowledge.
Biological Control: This involves the use of natural predators, like mosquitofish, to eat and control vectors like mosquitoes. Mosquitofish are a great mosquito control option to stop mosquitoes from growing in artificial bodies of water like neglected pools and spas, water troughs, ponds, and rain barrels. Mosquitofish are available for free to all the Santa Clara County community. Visit our mosquitofish website for more information on our mosquitofish program or to request them.
Chemical Control: Chemical control includes using pesticides like larvicides and adulticides. Larvicides are used to target mosquito larvae and adulticides are used to target flying-biting adult mosquitoes. Larvicides are often based on naturally occurring materials, like bacteria found in soils (including Bacillus thuringiensis subspecies israelensis and spinosad). We also use insect growth hormones (methoprene) to stop mosquito larvae from becoming flying-biting adults. All larvicides and adulticides we use are registered with federal, state, and local authorities.
Surveillance: To focus our efforts in specific areas and what actions to take, we monitor and collect information on mosquitoes to find problem areas around the county. This includes monitoring mosquito populations year-round and actively checking for the presence of invasive mosquitoes across the county.
We use different methods to apply larvicides depending on the size of the area and type of area that needs to be treated. Wide area larvicide spraying is a specific method that we use to apply larvicides (pesticides that target mosquito larvae) to prevent them from reaching the flying-biting adult stage. It uses specialized truck-mounted equipment to spread larvicide (pesticide that targets mosquito larvae) over larger areas. This method lets us control mosquito larvae in pockets of standing water we can’t reach.
Wide area larvicide treatments use a different pesticide, takes place at a different time of day, and targets mosquito larvae in the water instead of flying-biting adult mosquitoes.
We will continue to monitor the affected areas where we found Aedes aegypti and will conduct wide area larvicide spraying treatments as needed if more Aedes aegypti mosquitoes are found.
The flying-biting Aedes aegypti adults will negatively change our community’s quality of life because they are aggressive biters. One of the best ways to control mosquito populations is to target mosquito larvae because they are concentrated in one area and are not flying, allowing us to reduce their population in larger numbers. Larvae can be controlled by either removing standing water or applying larvicides. Controlling Aedes aegypti larvae helps lower the number of flying-biting adult mosquitoes and means fewer bites for our community. This also gives us a better chance at preventing this mosquito from spreading and a better chance of getting rid of this mosquito.
Wide area larvicide spraying allows us to treat large areas efficiently, reaching many potential mosquito breeding sites in a short amount of time. It's especially useful for targeting locations that are difficult to access or where mosquito larvae are widely dispersed. This approach helps reduce mosquito populations before they mature into flying, biting adults and complements our other control methods for a more effective strategy overall.
We use a naturally occurring bacterium, Bacillus thuringiensis subspecies israelensis , that target mosquito larvae. It is nontoxic to beneficial insects, wildlife, and humans.
Our licensed technicians follow the instructions on the pesticide label, which is registered with the Environmental Protection Agency. We use about seven ounces per acre. This amount does not pose any health risks to humans or animals.
Our ongoing mosquito surveillance trapping monitors the effectiveness of the treatment results. One method includes placing cups with water around the neighborhood before the wide area larvicide spraying treatment and then bringing the water back to our lab. Once in the lab, we place mosquito larvae in the water to monitor how many larvae die from the larvicide that landed in the water.
The larvicide products only stay in the air while they’re being sprayed and drifting. Once the larvicide falls on water it can work anywhere from 7 to 14 days depending on things like sunlight, dirt, and how much the water moves.
No. We will not need to access your property for wide area larvicide spraying treatments. Once the larvicide is released from the truck-mounted machines, the rain-sized droplets distribute over buildings and yards, letting us do the treatment without entering properties.
No. The wide area larvicide spraying treatment is very diluted and will not reduce visibility for driving or other activities.
We encourage you to stay inside to reduce exposure to the larvicides during the treatment. You can go outside about 10 minutes after our truck has passed your street. You will be able to hear it as it sounds like a loud leaf blower.
We encourage you to keep pets inside to reduce exposure to the larvicides during the treatment. They can be let outside about 10 minutes after our truck has passed your street. You will be able to hear it as it sounds like a loud leaf blower.
To receive alerts on detections and scheduled treatments, you can subscribe to our mosquito alerts. Direct notifications are sent to community members in the treatment areas through emails to registered voters in specific precincts, Nextdoor, and AlertSCC. To receive notifications regarding your specific neighborhood and other County emergency alerts, subscribe to AlertSCC.
*Please note that, per California Code of Regulations, CCR3 §6620, the District does not need consent from property owners to conduct mosquito control treatments and is not required to provide notice to the community beforehand. However, we do our best to send courtesy notifications to the public before conducting wide area larvicide spraying treatments in residential areas.
According to the larvicide label, which is registered with the Environmental Protection Agency, there should be no health risks to you or your family. Please consult with your doctor if you have concerns.
No. The larvicide only targets mosquito larvae and has no effect on those items.
Yes. Just normal washing with water before eating is recommended.
There is no health risk to pregnant women, but we recommend staying inside during the treatment. If you have concerns, please consult your doctor.
Remove or cover any sources of drinking water before the treatment starts. If drinking water was left out, please pour it out and rinse the container before use.
Yes. The products used are not harmful to you or your pool at the amount applied.
The Environmental Protection Agency has conducted studies and has determined it has no harmful effects on humans or animals. For more information on Bti, visit the Environmental Protection Agency website.
No. Bacillus thuringiensis subspecies israelensis (Bti), has no effect on bees or any pollinators. It only targets mosquito larvae.
There are several ways to protect yourself from mosquito bites.
- Insect repellents - Use insect repellents that contain ingredients registered with the Environmental Protection Agency, like DEET, picaridin, IR3535 or oil of lemon eucalyptus. Always follow the instructions on the label.
- Attire - When spending time outside, wear long-sleeve shirts, long pants, socks, and shoes.
- Inside your home: Make sure your window and door screens are in good condition with no holes.
You can help by:
- Granting us access to the outside of your home, so we can inspect for mosquitoes and places where they could have laid eggs. If you are concerned about your privacy, our staff is only allowed to check and treat for mosquitoes. Our staff will also have clear ways to identify themselves as district employees.
- Request a mosquito inspection. Inspections are provided at no additional costs.
- Report mosquitoes that bite during the day and at night (even inside) and mosquitoes with black and white stripes through any of these options
- Call: (408) 918-4770
- Email: [email protected]
- Online: submit an online service request.
- Prevent water from collecting in containers like plant pots, saucers, barrels, bins, buckets, and old tires.
- Keep pet dishes, bird baths, and kiddie pools scrubbed and clean. To see full instructions, please visit our Aedes Egg Removal website.
- Cover outdoor trash cans and recycling bins, and keep items that could hold water, such as toys, out of the rain and away from sprinklers.
- Properly seal rain barrels since mosquitoes can lay thousands of eggs inside them.
- Keep rain gutters and drains free of debris.
- If you have medical concerns, please consult your doctor.
- Please report any day-bites or mosquitoes with black and white stripes through any of these options:
- Call: (408) 918-4770
- Email: [email protected]
- Online: submit an online service request.