Mosquito Control Operations

The District is dedicated to protecting the public from both the discomfort of mosquito bites and mosquito-borne diseases. Monitoring and surveillance for abundance of immature (larvae/pupae) and adult mosquitoes and mosquito-borne diseases are key components of our mission. The District’s approach to controlling mosquitoes is called Integrated Pest Management (IPM), which includes using a wide variety of methods or alternatives to achieve its goal of minimizing mosquito numbers. The brief description provided here is a summary of operations that will be shown in much greater detail in the District’s upcoming Environmental Evaluation.

The term “integrated” is the first and most important concept in IPM. An IPM program is made up of a number of components or alternatives which need to be evaluated and considered. More than one alternative may be used simultaneously or in sequence. IPM alternatives do not always require that a choice has to be made between one and the other. Chemicals are but one component or alternative in a sound IPM program.

It is common in the public discussion for people to introduce the idea of “natural” or biological control measures as an alternative to fogging. There are currently no viable biological control agents for adult mosquitoes. Research has shown that predators, such as, bats, swallows or dragonflies, do not feed exclusively on adult mosquitoes, but take advantage of a wide variety of available prey. The only truly biological controls are directed against the larval stage. The judicious use of mosquitofish, and some of the bacterial products, are the only widely adopted biological control methods.

The table below (not comprehensive) shows how the Vector Control District integrates IPM alternatives during the life cycle of a mosquito.

Mosquito Life Cycle

IPM Alternatives





Public Education, Outreach and Advertising

The public is informed about how to recognize eggs, and about the need to eliminate water.

The public is informed about how to recognize larvae, and about the need to eliminate water.

The public is informed about how to recognize pupae, and about the need to eliminate water.

The greatest impact of public education on adults is achieved by effectively educating about the aquatic larvae, protective clothing and insect repellents.

Physical Control, Vegetation Removal

Modifying or eliminating the water habitat can remove the opportunity to lay eggs

Larvae are subject to the same IPM alternatives as the eggs. Larvae already alive can be reduced or eliminated

Pupae are subject to this alternative in the same way as eggs and larvae.

In some cases, removal or thinning of vegetation can eliminate resting places. The greatest impact of these methods occurs in the water stages.

Biological Control

Mosquitofish will consume egg rafts. Will not tolerate chloramines in water. About 60% of neglected pools are planted with mosquitofish for season-long control

Mosquitofish can consume large numbers of larvae, especially where the larvae are the most abundant food source. They are used most often in isolated water such as swimming pools and backyard ponds. They are not used in natural habitats due to endangered species concerns

Mosquitofish consume pupae just as they do the other stages. Certain nematodes, fungi and bacteria have been used for mosquito control, but their use has been restricted to certain narrowly defined habitat characteristics which do not frequently occur in our County.

Currently there are no effective biological control methods for adults. Research has shown that most predators, such as bats, swallows or dragonflies do not feed exclusively on mosquitoes, nor consume enough mosquitoes to have much of an impact. Genetically modified or sterilized adult releases are currently under development but have not proven effective over large areas.

Pesticidal Control

Thin films break the water surface tension and sink both eggs and adults trying to lay eggs

A) Thin films break the water surface tension and sink both eggs and the adults trying to lay eggs. B) Bacterial pesticides contain special proteins which are toxic to mosquitoes. In recent years, extended release formulations have made this a much more viable alternative for swimming pools C) Thin films interfere with the larval breathing function

A) Subject to thin surface films which break the water surface tension. B) Methoprene interferes with development of adult structures and the pupae die before they can transform into adults.

A) ULV applications of pyrethrins or pyrethroids are currently used to reduce adult numbers of mosquitoes only when WNV positive mosquitoes are detected. B) Infrequently, vegetation or surfaces may be treated with residual pyrethroids to kill adults resting there. C) The District will be testing a new organic (garlic-based) "bait" to be applied to foliage and surfaces.

Tools for larval control include specialized pesticides, surveillance and elimination of breeding sources, and public education efforts. The pesticides used include bacterial products that are very specific for mosquitoes, insect growth regulators (like those found in pet shampoos for parasite control), and surface films that block the breathing process in the larvae. Most of the larvicidal applications are done in areas like creeks, curbs, catch basins and swimming pools. In a typical year, a summary of these activities would include:

  • Aerial Survey Program: Approximately 250 square miles surveyed annually, resulting in 875 potential new pool inspection opportunities, averaging 2,225 swimming pool inspections (newly identified plus already known pools) requiring 502 treatments.
  • Inventoried Sources: Over 3,200 inspections made on 517 known mosquito breeding sources, resulting in 1,627 treatments.
  • Storm Drains/ Curbs/Gutters: There are over 11,000 inspections of curbs and storm water catch basins, with about 15% of samples positive for mosquitoes. These inspections result in 1,671 treatments.
  • Mosquitofish Biological Control Applications: Over 12,600 mosquitofish are delivered to residents in response to service requests or breeding source inspections.
  • Mosquito Service Requests: An average of about 817 mosquito service requests are received each year. About 85% of those result in free home and yard inspections and delivery of informational material.

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