Orange County Mosquito and Vector Control District
This website includes CSS elements that your browser does not support. Please upgrade your browser to a current version, then come back and try again.


The Orange County Mosquito and Vector Control District's (District) primary objective is to protect the people of Orange County from the dangers of vector-borne disease. A major component of our program is to educate the public about the shared responsibility of vector control. The District works hard to eliminate existing mosquito breeding sources and prevent new ones. The common goal is public health and enjoyment of our backyards and the many recreational facilities within the County. We also want to promote mosquito-free agricultural and industrial working conditions.

Mosquito Life Cycle

Mosquitoes have four different stages of their life cycle- egg, larva, pupa, and adult. During each stage of their life cycle the mosquito looks distinctly different than any other life stage.

Mosquito Life Cycle


The most common mosquitoes lay egg rafts that float on the water. Each raft contains from 100 to 400 eggs. Within a few days the eggs hatch into larva.


The larva or "wiggler" comes to the surface to breathe through a tube called a siphon. It then sheds its skin or molts four times during the next several days. It grows rapidly between each molt. After the fourth molt, it changes into a pupa.


The pupa or "tumbler" cannot eat. It breathes through two tubes, called trumpets, on its back. The adult mosquito grows inside the pupa and in two days, when it is fully developed, it splits the pupal skin and emerges to the adult stage.


The newly emerged adult rests on the surface of the water until it is strong enough to fly away and feed. For a diagram of the mosquito's life cycle, click here.
There are some insects that look very similar to mosquitoes. To see a list of these insects, click here.

Vector-borne Diseases
Mosquitoes Are Not Just a Pest

As well as being a pest, mosquitoes can carry numerous diseases that can make people and animals sick.

West Nile Virus

The disease that is of most concern right now in Southern California is West Nile virus (WNV). West Nile virus has been in the United States since 1999 and in California since 2003. West Nile virus is spread to humans by mosquito bite. WNV is a blood-borne disease; there have not been any cases of a person contracting the virus through everyday contact with infected individuals. Although, there are records of people contracting the disease through organ transplants and blood transfusions. There are three different gradations of WNV: West Nile virus, West Nile fever and West Nile neuroinvasive disease. Most of the public who contract WNV (about 80%) will never show any symptoms; this gradation of the virus is referred to as West Nile virus. About 20% of the population that contracts the virus will contract West Nile fever. Symptoms of this include fever, headache, nausea, possible rash. These symptoms are very similar to flu- like symptoms and many people who contract West Nile fever think they have the flu. The more severe cases of West Nile are the neuroinvasive diseases. These can include encephalitis, meningitis and mengioencephalitis. This gradation of the virus can lead to stroke-like symptoms, coma, paralysis and even death. About 1% of the population that contracts WN will get these more severe symptoms. Click here for more information on West Nile virus.


There are three forms of viral encephalitis capable of being transmitted by mosquitoes in Orange County: West Nile, St. Louis and Western Equine. All are carried by wild birds, most of which show no symptoms. Infected birds are then bitten by local mosquitoes that can pass the virus on to humans through future bites. Symptoms of encephalitis range from mild flu-like illness to severe brain inflammation that can cause death. West Nile virus and Western Equine encephalitis can affect horses and other animals as well as humans.


Malaria is much less likely to occur in Orange County due to the necessity for human reservoirs of the disease. Anopheles mosquitoes, the vectors of malaria, are found in some areas of California, and there have been isolated instances where human reservoirs from other countries temporarily provided a source of malaria infection to local residents.


Zika virus is transmitted by Yellow Fever and Asian Tiger mosquitoes. Both species are now found in Orange County. No Zika virus infected invasive mosquitoes have been found in the United States. Invasive mosquitoes must acquire the virus from an infected human. Example: Infected traveler returning from an area with active transmission of Zika. A strong correlation has been made to microcephaly (small head size) and other congenital birth defects, but no definitive medical causation has been determined. The California Department of Public Health acknowledges that the risk of transmission of Zika virus in California is low. Factors contributing to our lowered risk are the use of window screens, air conditioning, and comprehensive mosquito control and education programs. There is no cure for Zika. Mosquito breeding source elimination and bite prevention is the only way to prevent the spread of this virus. Even in the presence of colder winter temperatures, the District continues to find new invasive mosquito breeding. The formula is simple: no invasive mosquitoes = no Zika virus transmission. Mosquito control is a shared responsibility. Every resident must redouble their efforts to eliminate ALL standing water from their property, including standing water found indoors. Click here to visit our Invasive Species page.

What We Do

Surveillance & Control

The District controls mosquitoes in the County's wetlands and other breeding sources created by standing water in street catch basins, drains, roadside ditches, flood channels, ravines and similar places on public land. The procedures are routine larviciding operations throughout the year and treating for adults only when necessary. We work with city, county, state, and federal agencies toward permanent correction of these sources whenever it is advisable.


The District has about 100 mosquito traps placed all through the County to collect mosquitoes to see how many are in that area as well as to test them for possible diseases they could be carrying (i.e. West Nile virus, St. Louis and Western Equine Encephalitis). Orange County Mosquito and Vector Control District uses two different types of traps: a carbon dioxide baited trap and a gravid mosquito trap.

CDC Trap The carbon dioxide trap is used as an attractant for recently mated females. After females mate, they need to find a blood-source to be able to produce eggs. Carbon dioxide is what all animals exhale when they are breathing so the trap mimics a potential blood-meal to the mosquito. Every time you exhale when you are outside you are attracting mosquitoes!
Gravid Trap The gravid trap is a foul-smelling trap that egg-laying female mosquitoes are attracted to as a potential place to lay their eggs.

Dipping to Locate Larvae

Treating Gutters

Potential Mosquito Breeding Ditch

Besides surveillance conducted by our lab every week, our Inspectors are also doing surveillance every day by looking for mosquito breeding sources in their areas. In between service requests, our Inspectors are checking drains, gutters, and ditches. Anything that can hold water for longer than one week has the potential to be a breeding source.


The District uses many different means of controlling mosquitoes:

Source Reduction

Manipulating or eliminating potential mosquito breeding sources can provide a dramatic reduction in mosquito populations. District staff educate property owners as to ways in which they can remove these sources like by emptying containers holding water. Staff also works with large land managers to restore and maintain water flow or circulation in systems that become clogged with sediment and debris or over run with vegetation.

Wetlands Marsh

Biological Control

The District's primary source of biological control is a little fish, the mosquito fish (Gambusia affinis). These fish are indispensable to our mosquito control program. They eat mosquito larvae as fast as the larvae hatch from their eggs. Mosquito fish are provided free of charge at our District for stocking ornamental ponds, unused or "out-of-order" swimming pools, and animal watering troughs. They feed themselves and care is limited to protecting them from garden sprays and from chlorine or other chemicals used to clean the pond. The District also stocks thousands of these fish each year in artificial lakes, reservoirs, waste water disposal lagoons, and drainage channels to eliminate the need for frequent spraying with a mosquito insecticide. The District is careful not to place mosquito fish where they could enter a natural system.

Mosquito Fish

Mosquito Fish
Gambusia affinis

The District uses mosquito fish (Gambusia affinis) as biological control agents in water sources that can sustain immature mosquitoes. These fish are not native to California, so they are only used in water sources that do not connect or drain to natural water bodies. Sources that would fall under this category would be unused swimming pools and or spas, ornamental ponds, water troughs, etc.

Mosquito fish are a critical part of Orange County Mosquito and Vector Control District's integrated approach to mosquito control. Mosquito fish are opportunistic feeders, have a tremendous appetite for mosquito larvae, and are very effective at preventing the production of mosquitoes in isolated water containers or systems that are too large to dump out or easily drain. For example, these fish are ideal for controlling mosquitoes in non-chlorinated, out-of-service swimming pools and ornamental ponds. They should not be used in situations where they might escape into natural waterways and become pests, as these fish are not native to California and all distribution of these fish is regulated by the California Department of Fish and Wildlife.

Mosquito fish grow to a maximum size of about 2 inches in length and typically live 2-3 years, but may exceed this lifespan if conditions are favorable. They are live-bearing and can produce up to 3-5 broods per year, with each brood containing from 30-100 young fish. Young fish will be eaten by larger fish unless they are provided with aquatic vegetation or other refuge that is dense enough to offer protection. Mosquito fish can tolerate a wide range of temperatures and water quality conditions, and are quite tolerant of pollution. They prefer sunlit areas of water and do not thrive in heavily shaded waters. During the summer these fish are most active, whereas in winter they become inactive, move to the bottom of waters, and reduce their feeding.

Mosquito fish eat a broad range of food sources (e.g., plants and animals) and will consume most types of available prey; one fish is capable of eating over 100 mosquito larvae per day. Feeding these fish is not necessary unless stocking pond used to hold such fish is new and bare of vegetation. In this case, tropical fish flakes or other commercial fish food are suitable to use as feed.

To see a video of mosquito fish eating mosquito larvae, click here.

To pick up your own mosquito fish, click here.

Chemical Control

The District routinely applies "biorational" pesticides countywide to control mosquitoes. The term "biorational" relates to the application of naturally occurring mosquito pathogens and predators in a manner that provides effective mosquito control with the least amount of impact on the environment. Currently, the District uses several biorationals including two microorganisms, Bascillus thuringiensis israelensis (Bti) and Bascillus sphaericus plus an insect growth regulator, methoprene.

The District uses chemical control products when source reduction and biological control are not possible or efficacious.  Chemical control of mosquitoes is often grouped into larviciding and adulticiding.  Larviciding is a general term for the application of nonliving natural materials or synthetic chemical products to aquatic habitats to kill mosquito larvae or pupae or to otherwise prevent emergence of adult mosquitoes.  Larvicides can be applied in a wide variety of formulations using a broad range of application technologies.   Adulticides target adult mosquitoes.  Chemical control efforts in Orange County are largely confined to larvicides, as they are the most effective means of control in the majority of breeding sources found in the County. Breeding sources treated for mosquitoes can include, but are not limited to, gutters, underground storm drains, marshes, unused swimming pools, etc. In certain habitats, such as coastal marshes, larviciding and adulticiding are used to target larvae and adult mosquitoes.

To learn more about adult mosquito applications, see the treatment schedule listed here. To learn more about mosquito IVM practices please refer to the District's IVM Manual.

IVM Products

Integrated vector management (IVM) is a decision-making process for the optimal use of resources in the management of vector populations. These decisions are made in order to reduce or interrupt transmission of vector-borne diseases, and prevent nuisance vector populations from impacting the quality of life.

The approach seeks to improve the efficacy, cost-effectiveness, ecological soundness and sustainability of vector control activities.

The key objectives of Integrated Vector Management include:

  • The selection of proven vector control methods based on knowledge of vector biology and ecology, and disease transmission.
  • Utilizing of a range of interventions, separately or in combination and often synergistically, integrating all available and effective measures, whether chemical, biological, or environmental.
  • Collaborating within the health sector and with other public and private sectors that impact vector control.
  • Engaging local communities and other stakeholders.
  • Knowledge and compliance with public health regulatory and legislative frameworks.
  • The rational use of insecticides.

The District’s management policy for mosquito control is to use the objectives of IVM in the following four fields: education, source reduction, biological control, and chemical control. These four fields are necessary for the success of a balanced and comprehensive Integrated Vector Management program.

To learn more about mosquito IVM practices please refer to the District's IVM Plan.

Larvae & Adult Mosquito Control

Bacillus thuringiensis israeliensis and Bacillus sphaericus:

Both of these microorganisms produce natural metabolic by-products that are only toxic to mosquitoes and show no toxic effects on organisms other than midges and buffalo gnats. The latter two groups of aquatic flies are considered pests, particularly buffalo gnats that are well known for their painful bites and human blood feeding habits.

The protein toxins produced by these two bacteria control mosquitoes by destroying (rupturing) the gut of the larva (wiggler). Once ingested, death usually follows quickly within 24 hours and sooner under ideal conditions. Unlike B. sphaericus, which remains in the water and regenerates from the corpses of dead mosquito larvae, Bti is short lived and only effective for one generation of control. Under suitable conditions, B. sphaericus can remain effective for several generations and even longer.

Recent studies have indicated that when Bti and B. sphaericus are combined, overall control is enhanced. Together, the Bti reduces natural resistance to B. sphaericus and at the same time extends the effective period of one application from 10 days to over three weeks. Unlike Bti, which is least effective in polluted water, B. sphaericus is unaffected by organics and therefore, preferred for controlling mosquitoes like the Southern House Mosquito (Culex quinquefasciatus), which breeds throughout Orange County in foul water situations (storm drains, catch basins, etc).


Methoprene, a juvenile hormone mimic, and insect growth regulator (IGR) is an active ingredient in vector products. This products acts as a "natural hormone" of insects and effectively retards the completion of the life cycle of the mosquito. The IGR works by preventing the larva from transforming to the pupa (stage between the larva and adult) and/or the adult from emerging from the pupa. Methoprene is formulated in ways to provide both short and long-term control by one-time application mixtures and slow release presentations using granules and briquettes.

Adult Mosquito Control Application FAQ

Q. What will trigger adult mosquito control application?
A. The District's aggressive campaign against mosquito larvae is intended to minimize the need to use public health pesticides targeting adult mosquitoes. If West Nile virus is detected in the community, the District's initial response will be to intensify its efforts to reduce mosquito breeding sites and increase its levels of larviciding in those areas in which West Nile virus has been found. Reducing the population of adult mosquitoes with public health pesticides (adulticides) that are registered by the US Environmental Protection Agency (US EPA) will be done if necessary to prevent human illness or to suppress a heavy nuisance infestation of mosquitoes. The decision to use adult mosquito control application either by truck-mounted, handheld, or aircraft application equipment will be based on surveillance information or the documentation of West Nile virus activity at a level that indicates a threat to human health. Adulticing will be concentrated in areas most at risk for disease occurrence and will be conducted by certified and licensed applicators.

More information on triggers for larvacide and adulticide application can be found in the District’s Integrated Vector Management and Response Plan.

Q. Where will the public health pesticides application against adult mosquitoes take place?
A. Application of the public health pesticides targeting adult mosquitoes will take place in areas of concern, as determined by our mosquito and disease surveillance indicators. Our trained and certified technicians use a variety of surveillance techniques and treatment criteria to ensure effective mosquito control with the least amount of risk to our residents and our environment. Click here for current treatment location schedule.

Q. What is the District’s primary form of mosquito control?
A. We practice Integrated Vector Management relying on biorational larvacides, with public health pesticides reserved for situations where other methods would be ineffective to protect public health.

Q. How can I learn about adult mosquito control application activities?
A: Announcements about adult mosquito control applications are posted on our District website; click here for our weekly schedule. Please visit our social media sites regularly for additional District events. You may also subscribe to our e-mail list to receive automatic Alerts regarding District activities by clicking here.


Another integral facet of vector control is education and outreach. The District participates in hundreds of outreach events each year to educate the public on vector control issues. The District has a website and uses social media as another means for educating the public on vector issues and what the homeowner can do to manage his/her vector problem. The District also gives presentations throughout the County to numerous groups and community organizations on vector topics. The District has pamphlets at the office that can be picked up or we can send them to a residence free of charge. The District has also created public service announcements for both radio and television to help educate the public on health issues. For educational mosquitoes material Click Here.

The District has a multifaceted mosquito education program that includes:

  • Presentations and participation in local outreach events to educate the public about mosquito control practices.
  • The creation and distribution of numerous Public Service Announcements (PSAs) that explain how to reduce mosquito populations.
  • Extensive literature on mosquito control practices, insects resembling mosquitoes, diseases that are transmitted by mosquitoes and many other publications are available in print and electronically.
  • Inspectors educate homeowners and neighborhood groups about mosquito-breeding sources on their property and how to prevent or eliminate them.
  • Staff attends local and national educational conferences to learn about advances in mosquito control techniques, products, and other measures to better educate and protect the public from mosquitoes and mosquito-borne diseases.

Outreach Program

General Information

The District offers some of the most comprehensive public vector education programs in the United States. Programs, including a large library of information pamphlets and "how to" instructions, are anchored by a dedicated Public Affairs Coordinator. District education programs are noted for their informative style, content, and professional delivery. Citizen groups, city counsels, and other public entities are encouraged to contact the District and request a program on vectors, on services provided by the District, or on what private citizens and commercial interests can do to prevent vectors. Programs emphasize rats, flies, mosquitoes, and Red Imported Fire Ants.

Educational Themes and Program Content

The intent of the District's education programs is to inform our citizens about vectors and how they impact human health, property, and life style. In addition, common sense pointers are explained to guide audiences to a reasonable understanding that vectors are "manageable" if proper exclusion and preventative measures are used.

If your school or civic organization is interested in having a vector program presented by the District, please contact Mary-Joy Coburn at (714) 971-2421 or at

Learn more about the Outreach Program

What You Can Do

The owner of any property on which a breeding source is located is responsible for the abatement of the nuisance and for the prevention of its recurrence. The District will inform the property owner of the mosquito breeding source and assist them in working out a satisfactory correction. In extreme cases, where the owner does not accept their responsibility to the public, the nuisance may be abated and a lien filed against the property as enforced by the California Health and Safety Code.

Where to Look and What to Do

Ornamental Ponds

Stock with mosquito fish, which are available free of charge to the public at our office. Add goldfish for looks if desired, but they do little for mosquito control. Avoid spraying with garden insect sprays. Remove leaves and thin out pond lilies. Keep water level up. Screen inlet of recirculation pump. Chlorine kills fish- transfer fish to glass bowl when cleaning pond. If pond is no longer desired, break holes in bottom and fill with dirt or sand.

Concrete or Plastic Swimming Pools

Operate filter and skimmer everyday to remove egg rafts and larvae. Provide drainage for filer and pump sumps. Chlorine will NOT kill mosquito larvae. If a pool cover is used, keep it tightly sealed. Remove rainwater from the top of the pool cover. Stock unused or "out-of-order" pools with mosquito fish. To see a video of mosquito larvae in an unused pool, click here.

Boats and Boat Covers

Prevent accumulation of bilge water. Store small boats upside down or cover to keep out rain and water from sprinklers.

Animal Water Troughs

Stock large troughs with mosquito fish. Clean small troughs every week.

Other Kinds of Containers

Remove and dispose of all unused containers that will collect rain or water from sprinklers:

  • Cans
  • Old Tires
  • Lawn and patio drains
  • Jars
  • Buckets
  • Irrigation valve boxes
  • Barrels
  • Tubs
  • Tarps covering outdoor equipment

Home gardeners rooting plant cuttings in vases, buckets, etc. should change water every week.

Rain gutters should be clear of leaves and debris. Click here to see a video of mosquito larvae in a rain gutter.

Usable containers should be stored upside down.

Vector Reduction Manual For the District's specific guidelines on how to avoid or reduce mosquitoes, rats, red imported fire ants, and flies please refer to recommendations in the Vector Reduction Manual: Procedures and Guidelines.
 Orange County Mosquito and Vector Control District