About Mosquitoes

There are about 20 species of mosquitoes in Santa Clara County, including the common house mosquito (Culex pipiens), cool weather mosquito (Culiseta incidens), Western encephalitis mosquito (Culex tarsalis), the summer salt marsh mosquito (Aedes dorsalis), and the winter salt marsh mosquito (Aedes squamiger). Mosquitoes are the most dangerous insect in the world. Some mosquitoes are capable of transmitting diseases such as West Nile virus, dengue, f​ever, and malaria causing close to one million deaths per year. Mosquitoes need water to lay eggs and can do so in as little as a bottle cap of water. Under ideal conditions, they can reach adulthood in less than a week.



Small Insect with Piercing Mouth Part

Mosquitoes are often confused with other flying insects such as crane flies. Both need water to reproduce, but only mosquitoes are capable of biting and transmitting diseases. Mosquitoes are much smaller than crane flies and have a tube-shaped mouth part (called proboscis) used to pierce flesh and suck blood. Crane flies may look intimidating because of their similarity to mosquitoes, but lack a proboscis making them incapable of taking blood.

Mosquito and cranefly


Need Blood Meals and Standing Water

Male and female mosquitoes feed on nectar but only female mosquitoes require blood meals in order to lay eggs. Female mosquitoes lay their eggs in standing water and damp soil, which then hatch into mosquito larva. The larva then develops into a pupa, a non-feeding and resting stage. Adult mosquitoes emerge from the pupa and the cycle begins again.

Mosquito lifecyles: eggs to larva to pupa to adult mosquito


Mostly Active During Warm Months

Common mosquitoes are mostly active during dusk and dawn, with the highest activity during warm months, but some invasive mosquitoes bite aggressively during the day. Review our most frequently asked questions regarding the day-biting invasive Aedes mosquito and call the District office at (408) 918-4770 if you encounter day-biting mosquitoes. If you are active outdoors during that time, prevent mosquito bites by wearing long sleeves, pants, and insect repellent approved by the Environmental Protection Agency (EPA). For more information about repellents, visit the EPA or the California Department of Public Health.

A person sprays mosquito repellent on her forearm.

Public Health Concern

Disease Transmission

Mosquitoes pose a serious health risk to humans as they can transmit deadly diseases such as West Nile virus. Different mosquitoes species can carry different diseases, but not all mosquitoes are infected. Most people infected with West Nile virus do not develop any symptoms. However, some people may develop flu like symptoms including headaches, body aches, joint pain, vomiting, diarrhea, or a rash. Elderly individuals and people with compromised immune systems are at a higher risk of getting sick and they are more likely to develop complications.

Virus transmission: bird (virus reservoir) to mosquitor (vector) to incidental hosts (people and animals)


Inspect, Drain, Remove, and Turn Over

Mosquitoes need standing water to reproduce. The best way to prevent mosquitoes in your area is to drain, remove, or turn over anything that can hold water. This includes flowerpots, planter bases, wheelbarrows, bird baths, toys, cans, rain gutters, pet dishes, buckets, old tires, and tarps. Drill holes in tire swings and other containers so they can drain, and clear roof gutters of debris. Change the water in bird baths at least one time per week, place tight fitting screens on doors and windows, and keep lids tight on rainwater collection systems.

Do not allow water levels in your swimming pool to fall below the pump circulation area. Thousands of mosquitoes can emerge from one neglected/abandoned swimming pool. You can request free mosquitofish (fish that eat mosquito larvae) for placement in pools, ornamental ponds, fountains, and rain barrels (see our Mosquitofish page for more information).

Report mosquito activity and abandoned or neglected pools to the District by submitting a service request, or by calling our District office at (408) 918-4770. Help conserve water and prevent mosquito breeding by reporting water waste to Valley Water. Valley Water offers many rebates and services including helping convert pools or lawns to low-water use landscapes, or upgrade irrigation equipment to reduce runoff through their Landscape Rebate Program. Learn more at

Dump and Drain Water at least once a week; people pour water from watering cans


Mosquito Abatement Program

The District has a robust mosquito prevention program in place, focusing on the use of Integrated​ Pest Management (IPM) to eliminate dangerous mosquitoes before they reach adulthood, and begin infecting people and animals. Staff inspect potential breeding sources such as catch basins, curbs, marshes, creeks, ponds, or anything that holds standing water. The District uses biological control such as mosquitofish and products approved by the Environmental Protection Agency that specifically target mosquito larvae and pupae.

Typical abatement procedures are used to access properties to eliminate the source of a mosquito problem (for example a neglected swimming pool in the backyard). The District makes every effort to work with landowners to eliminate mosquito sources and other nuisances. However, if a landowner is unwilling to cooperate or fails to respond, the District has the authority to access private properties to inspect and abate the source of the problem. The District also has the authority to charge a landowner a fee, and place liens upon property (California Health and Safety Code, Division 3, Chapter 1 §2000-2067).

A person sprays insecticide into trees


Monitoring for Diseases

Detecting disease activity early can prevent problems from occurring, making surveillance one of the most essential District programs. District staff routinely monitor for mosquito-borne diseases such as West Nile virus, Western equine encephalitis, and Saint Louis encephalitis. The District’s Surveillance Program is composed of mosquito trapping, sentinel chicken monitoring, and dead bird testing. A variety of carbon dioxide and light traps are placed throughout the county, to capture mosquitoes and help the District understand mosquito abundance and the presence of mosquito-borne diseases

A person inspects an insect trap

The District manages chicken flocks located in Palo Alto, Milpitas, Campbell, San Jose, San Martin, and Gilroy, with eight chickens at each location. Sentinel chickens are tested every two weeks through the summer months for mosquito-borne diseases. Blood samples are processed and tested by the California Department of Public Health. Test results are used to determine if control measures and inspections need to be increased in areas where viral activity is found. Due to COVID-19, our sentinel chicken program is currently on hold.

A worker inspects a chicken

The District encourages the public to report dead birds by submitting a service request or by calling the District office at (408) 918-4770.



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