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Frequently Asked Questions

Q. Why is mosquito control spraying taking place?

A. The Orange County Mosquito and Vector Control District (OCMVCD) is conducting mosquito control spray applications from backpacks, trucks, helicopters, or airplanes based on elevated mosquito counts or elevated West Nile virus activity in a specific area. Mosquito control spray applications effectively reduce the mosquito population and the number of infected adult mosquitoes, thereby decreasing the risk of disease transmission to the public. To view the current spray schedule in Orange County, click here:

Q. How can I find out if mosquito control spray applications are scheduled to take place in my area?

A. Residents are encouraged to sign up for spray notifications to receive information on spraying occurring in their zip code:

Q. When do email notifications regarding mosquito treatment spray events go out?

A. Email notifications are sent prior to scheduled mosquito treatment spray events. For spraying conducted on the weekend, notifications will be sent on Friday, outlining the work that will be performed throughout the weekend. Emails are sent to residents subscribed to the eAlerts in the affected area ( Notifications are also posted online and on OCMVCD social media pages.  Spraying events can be canceled or rescheduled due to weather conditions or other unforeseen circumstances.

Q. What’s the life cycle of a mosquito and why should I care?

Mosquito Life Cycle

A. Mosquitoes have four stages in their life cycle: egg, larva, pupa, and adult. The first three stages are aquatic. The female mosquito lays eggs on or near a water surface; the eggs hatch a few hours or days later and the larvae emerge. The larvae feed on aquatic algae and bacteria. As they mature, they outgrow their skin, grow a new skin layer, and shed the old one, a process called molting. After the fourth growth-molt cycle, the mosquito enters the pupal stage. A few days later, the adult mosquito emerges from the pupal case and flies away.

Because three (3) of the four (4) life stages of a mosquito are aquatic, the best way to control them is to target their aquatic stages.  By denying them the water they need to develop, mosquitoes cannot complete their life cycle and they die. For this reason, the Orange County Mosquito and Vector Control District asks the public to eliminate any standing water around the home or workplace and to report standing water that cannot be eliminated. 

Q. What are larvicides and adulticides?

A. Control of mosquitoes while in the larval stage is the backbone of most mosquito control programs in California. Larvicides are products used to reduce immature mosquito populations when they are still in the water. Larvicides, which can be biological or chemical-based, are applied directly to water sources that hold immature mosquitoes, including eggs, larvae, and pupae. Larvicides reduce the overall mosquito population by limiting the number of biting adult mosquitoes produced from a water source.

Adulticides are products that rapidly reduce adult mosquito populations. This can become necessary when larval control measures no longer reduce the presence of biting-adult mosquitoes, or there is significant threat of disease transmission in an area. The most common method of adulticiding is ultra-low volume (ULV) spraying. ULV spraying is the process of putting very small amounts of liquid into the air as a fine mist of droplets. These droplets float on the air currents and quickly eliminate mosquitoes that come into contact with the droplets. Sprays can be applied from backpacks, trucks, helicopters, or airplanes. Adulticides immediately reduce the number of adult mosquitoes in an area, with the goal of reducing the number of mosquitoes that can bite people and possibly transmit disease.

Q. What type of insecticides are being used by the Orange County Mosquito and Vector Control District?

 A. For larval mosquito control, OCMVCD typically utilizes Bacillus thuringiensis israelensis (Bti), Bacillus sphaericus and Saccharopolyspora bacterial products), or insect growth regulators, (IGRs) which prevent the immature mosquito from becoming a biting adult mosquito.

Adult mosquito control products used by OCMVCD can be placed into two groups (Pyrethrins and Pyrethroids), both of which can be formulated with a synergist chemical to increase their efficacy:

  • Pyrethrins are the active ingredients in pyrethrum, an extract of the flower Chrysanthemum cinerariifolium. Pyrethrins are natural insecticides that act by blocking chemical signals at nerve junctions.
  • Pyrethroids are the synthetic version of pyrethrins that also act by blocking chemical signals at nerve junctions.
  • Piperonyl butoxide (PBO) is a synergist that is usually incorporated with pyrethrins and pyrethroids. PBO enhances the effect of these insecticides by inhibiting cytochrome P450, a class of enzymes that break down the insecticides. This allows the insecticides to be effective with less active ingredient than would otherwise be required.

The U.S. Environmental Protection Agency (US EPA) approves the use of insecticides nationally and the California Department of Pesticide Regulation (CDPR) approves their use in California. Before pesticides are registered by the US EPA or CDPR, they must undergo extensive and time-consuming laboratory testing for environmental impacts as well as test for acute and chronic health effects.

Q. When does the Orange County Mosquito and Vector Control District use chemical mosquito control (pesticide)?

A.  Chemical control is used for immature mosquitoes in water when biological control (mosquitofish) and source reduction is not plausible or effective.

Chemical control for adult mosquitoes is necessary when biological and physical control methods are unable to maintain mosquito numbers below a level that is considered tolerable, or when emergency control measures are needed to rapidly disrupt and reduce the transmission of disease to humans.

All products are registered with the California Environmental Protection Agency (EPA) and are applied by trained and state-certified Orange County Mosquito Vector Control District technicians.

More information on triggers for chemical mosquito control spraying (larvicide and adulticide) can be found in the District’s Integrated Vector Management and Response Plans.

Q. What training is required for workers who apply pesticides?

A. Each state has mandated training and experience requirements that must be met before an individual can commercially apply insecticides. In California, the California State Health and Safety Code requires that every employee of a mosquito abatement or vector control district who handles, applies, or supervises the use of any pesticide for public health purposes be certified by the Vector-Borne Disease Section of the California Department of Public Health (CDPH) as a Certified Vector Control Technician. Upon certification, technicians must also meet established continuing education hours. In addition, they must follow the instructions and precautions that are printed on the pesticide label. All EPA-approved pesticide product labels provide information, including instructions on how to apply the pesticide and precautions to be taken to prevent health and environmental effects.

Q. What is the Centers for Disease Control’s (CDC) position regarding the use of chemical mosquito control?

A. The CDC has guidance for all aspects of larval and adult mosquito control. For their updated guidelines and planning resources that provides detailed guidance about the use of mosquito control measures, including suggestions for an elevated response and the actions that are possible at different levels of virus activity, see the CDC resource links here:  

Q. How will these pesticides affect me and my family?

A. At the rates these products are applied (2 tablespoons or less per acre), they do not pose a risk to you or your family. In fact, some lice control products that are applied directly to a person’s head, contain an active ingredient that is often used in adult mosquito control products but the lice products are applied at a much higher rate. For more information on insecticides and public health, consult the U.S. Environmental Protection Agency, which oversees the registration of these chemicals. The National Pesticide Information Center (NPIC) and the Centers for Disease Control and Prevention also post their mosquito control resources online.

For treatment schedules, including locations see our Ground-Based Mosquito Control Efforts page at

Q. What should I do if I think that I am having health problems because of pesticides used in my area?

A. If you are experiencing health problems for any reason, it is important to see your health care provider promptly.

Q. Do I have to go indoors during spraying?

A. The District sprays public health pesticides approved by the US EPA for use on and over outdoor residential and recreational areas. It is not necessary to close doors or windows. The spray will dissipate from the treated area and degrade quickly in sunlight. In some instances, mosquito control backpack sprays may leave wet surfaces and should not be contacted until dry.

Q. Can pets go outside during spraying?

A. The materials used for controlling mosquitoes, when used in accordance to the label, are not harmful to pets. Many times, it is the same materials used to treat cats and dogs for fleas and ticks. However, if you choose to reduce your pet’s exposure, keep them inside during spray applications.

Q. Should I close my windows when a spraying is scheduled in my area?

A. It is not necessary to close doors or windows. The spray will dissipate from the treated area and degrade quickly in sunlight. However, residents may take additional measures to achieve personal comfort during the application.

Q. Why are you treating my neighbor’s yard?

A. When the presence of a mosquito-borne virus is identified in a neighborhood, the District deploys personnel to investigate the residential and commercial properties in the area. Products to control mosquitoes are applied on residential properties with homeowner/property owner consent.

Q. Will the adult mosquito spraying affect my swimming pool water, lawn furniture, play equipment, toys, etc.?

A. Your swimming pool water and items found in your yard will not be affected.

Q. What if I have a vegetable or fruit garden?

A. Just as you normally would, wash your vegetables and fruit before you eat them.

Q. Will the adult mosquito spraying affect bees?

A. When adult mosquito control spraying is conducted using ULV applications at the label rates at night, there should be no impacts to bees. In some instances, the OCMVCD may apply adult mosquito control products during the day to either control resting mosquitoes infected with West Nile virus; or to control invasive Aedes mosquitoes.  If a daytime application is necessary, applicators are trained to avoid blooming plants, bee hives, or other areas where bees may congregate.

Q. How do I protect the bees in hives?

A. If residents are still concerned about their hives, and bees are currently bearding on the side of the hive, the hive can be draped in wet burlap.  The burlap is wet with water and then draped like a curtain over the hive with an opening in the front for bees to enter and exit.

Q. Are there special notices for bee keepers?

A. Beekeepers can register hive locations with the Orange County Agricultural Commissioner’s office.  Hives registered with OC Agricultural Commissioner’s office will be notified 48 hours before an aerial spray event by OCMVCD.  You may contact the Orange County Agricultural Office at 714-955-0100.

Beekeepers are encouraged to sign up for spray notifications to receive information on spraying occurring in their zip code:

Q. I have an air conditioner. Should I turn it off if spraying is scheduled in my area?

A. It is not required to turn off air conditioning units during or after mosquito control spray events. However, if you have a window or wall air-conditioning unit that is running on the fan setting, you may turn it off so that air is not brought in from the outside. Window and wall units running on cooling settings do not draw air from outside of the home, so there is no need to turn your air conditioning unit off. Central air-conditioning units cool recirculated air in your house. Since a central air-conditioning unit does not pull in outside air, there is no need to turn it off.

Q. Where can I get additional information regarding specific insecticides?

A. Questions concerning specific insecticides can be directed to the U.S. Environmental Protection Agency, as this agency has responsibility for the registration of insecticides. Many issues are addressed on the EPA’s Mosquito Control Web site.

The National Pesticide Information Center (NPIC) provides insecticide information and questions about the impact of insecticide use on human health. NPIC is cooperatively sponsored by Oregon State University and the U.S. Environmental Protection Agency.

NPIC can be reached online or toll-free: 1-800-858-7378. The Center for Disease Control and Prevention have also posted guidance and planning resources online.