How to Manage Pests
Control of mosquitoes by applying biological or chemical treatments is a supplement to the preventive measures of good water and weed management. It does not replace them. Prevention is key because the need to use biological or chemical treatments may be overlooked until it is too late, or the treatments may not be effective for various reasons. Reducing mosquito habitat reduces the production of mosquitoes.
MVCD personnel may use biological or chemical controls on mosquitoes. Pesticide applicators need special permits to apply insecticides for mosquito control in California, so you will usually need to work with MVCDs to control mosquitoes using insecticides. This section discusses substances commonly used against larval or adult mosquitoes. Visit the California Department of Pesticide Regulation Product/Label Database Web site for further information on specific pesticide formulations. Insecticide formulations change frequently as methods are improved.
Control of Larvae and Pupae
Mosquito larvae can be controlled by substances added directly to the water. These substances may be organisms that consume them, biological compounds that poison them or cause fatal infections that are specific to them, chemicals that disrupt their development or physiology, or oils and films that suffocate them. Biological compounds tend to be more expensive than chemical controls but they affect fewer nontarget organisms. Conserving nontarget organisms helps maintain natural control of mosquitoes by predators that eat mosquito larvae, such as water bugs, dragonflies, and backswimmers.
The mosquitofish (Gambusia affinis) is a small top-feeding minnow that is often used in mosquito control. The mosquitofish is not native to California but is now found in many waterways due to introductions for mosquito control or escapes. It is not legal for private citizens to introduce fish into natural waters of California, but mosquitofish may be used in tanks, containers, or artificial ponds. Do not stock areas that could connect to natural waters during floods. The mosquitofish is most effective in waters that are not too weedy, because in weedy areas they may harm other predators more than mosquitoes. Mosquitofish are not appropriate in vernal pools because many vernal pool invertebrates and other species do not have defenses against fish. Contact your local MVCD if you think you have a situation that is appropriate for mosquitofish.
One commonly used biological control is the bacterium Bacillus thuringiensis israelensis (Bti). This bacterium contains a toxin that is activated by the mosquito gut. Bti is toxic to mosquitoes and a limited number of other flies, including blackflies and some midges. It is manufactured as a noninfectious bacterial concentrate. Formulations include a sprayable liquid, granules that can sift through vegetation, and as floating briquettes for mosquito control in small bodies of water. Bti kills mosquitoes over the course of a few days (liquid or granules) or up to 3 weeks (floats). It is often available at garden stores. One limitation of Bti is that it is not effective against mosquito pupae or larvae near pupation, because pupating mosquitoes do not feed and may not ingest a lethal dose.
Another bacterium, Bacillus sphaericus (Bs), is also toxic to mosquitoes. Bs is sold as a living form that can cause infections in mosquitoes. This bacterium is most effective against mosquitoes in the genus Culex, and it also affects some Anopheles, Ochlerotatus, and Psorophora spp. mosquitoes. It is not effective against several species of mosquitoes, so you must know the species of mosquito present for effective use. Bs is safe for nontarget organisms, but it is not effective against mosquito pupae. It can kill mosquitoes for up to several weeks, depending on the amount used.
Other biological controls
Some fungi, nematodes, and copepods (freshwater crustaceans) are also effective against mosquitoes, but these are not widely produced commercially.
Chemical controls are typically very effective against mosquitoes. However, if the same chemicals are used against many generations of mosquitoes over a large area, the mosquitoes may develop resistance. This happens because some mosquitoes have genes that make them less sensitive to the toxin. These mosquitoes survive the treatment and gradually create a resistant population that cannot be controlled with the same chemical. Rotating the classes of chemicals used to control mosquitoes prevents resistant populations from building up, because mosquitoes with resistant genes are less likely to survive different treatments in different years. It is very important to prevent the emergence of resistant mosquitoes, because chemical control is a strong defense against serious mosquito-borne diseases such as West Nile fever and malaria.
Insect growth regulators
Some chemical agents mimic natural substances that regulate insect development, preventing mosquitoes from maturing. Methoprene is a commonly used insect growth regulator. It can be applied as a liquid, which kills a single batch of mosquitoes, or as pellets or briquettes that can give control for 30 days or several months, respectively. Methoprene does not kill mosquito larvae immediately; it kills them as they begin to transform into adults. It may affect a few other insects, such as midge larvae, and possibly some crustaceans, but it is safe for other forms of life at rates used in mosquito control. Insect growth regulators have the advantage of not killing insects immediately so that they are available as prey in the food chain.
As with biological controls, methoprene does not work against pupating mosquitoes. One concern with the pellet and briquette forms of methoprene is that they may cause resistance in mosquitoes because they persist for so long. They should be used in limited areas or rotated with other control methods.
Oils and monomolecular films
Mosquito larvae need to breathe air to live. Oils suffocate virtually all species of mosquito larvae and pupae by forming a barrier at the water's surface. In the past, diesel oil was used, but this is unsightly and toxic to many aquatic animals and some plants. The oil that is currently used, Golden Bear Oil, is less toxic and persists in the environment for just a few days. Golden Bear Oil is the only material available in California that is very effective against mosquito pupae. Oil drowns any aquatic insects that need to breathe air, including mosquitoes, diving beetles, and many bugs. Air-breathing insects are important natural predators of mosquitoes, so this product can temporarily disrupt biological control. To preserve predators, it is important to control mosquitoes by other methods before they reach the pupal stage. However, oil is a necessary tool in cases when mosquitoes have already pupated by the time they are discovered.
Oil causes bird feathers to mat, and matted feathers cannot keep young birds warm and dry. Therefore, oil should be avoided or used sparingly if young birds are present and the weather is cold.
Monomolecular films are nontoxic surfactants that act by either coating the mosquito's breathing tube (siphon) or causing the siphon to flood. Agnique and Arosurf are currently available brands. Like oils, these can be effective against both larvae and pupae, but they also drown some beneficial insects. Holes may form in the film on windy days or around vegetation, providing refuges where mosquitoes can survive. The effectiveness of this method should be closely evaluated especially in vegetated areas or on windy days.
Control of Adult Mosquitoes
Unfortunately, there are at present no effective biological controls for adult mosquitoes. Bats, swallows, dragonflies, and damselflies consume adult mosquitoes, but they do not reduce mosquito populations enough to break disease cycles or significantly reduce annoyance by large populations of mosquitoes. Nevertheless, you may wish to install bat boxes and leave swallow nests undisturbed, since every little bit helps.
Mosquito traps attract mosquitoes with light, carbon dioxide (CO2), and/or other substances like octenol. Carbon dioxide is a component of breath, which mosquitoes use to find humans and animals. Studies have shown that it is difficult to deplete mosquitoes in an area using traps. While traps remove some mosquitoes from the air, they also attract biting mosquitoes. Traps should not be placed close to human or animal habitation, and they should not be relied upon exclusively for control.
Broad-spectrum insecticides: Organophosphates, pyrethroids and others
A number of broad-spectrum insecticides are approved by the federal and state governments for use in mosquito control. In California, only specially licensed public health personnel and commercial applicators may apply these pesticides for mosquito control. Broad-spectrum insecticides are toxic at some level to most insects and some other forms of life, as well as to mosquitoes. You can buy some of the same pesticides over the counter for use on your property, but you are bound by law to use these products only as directed. Uses not allowed by the pesticide label are illegal and may be dangerous to humans, animals, and the environment. Because of the restrictions on pesticide use for mosquito control, the following information on chemicals used against adult mosquitoes is provided only to give you a greater understanding of methods used by MVCDs.
Chemicals in use for adult mosquito control are not thought to persist for more than several weeks in the environment, although this can vary according to local conditions. Two of the most common families of chemicals are organophosphates (such as malathion and naled) and pyrethroids (such as permethrin, pyrethrin, and resmethrin). These neurotoxins interfere with nerve cell operation. The concentrates must be handled with care because they are toxic to humans and other higher animals, as well as to insects. However, they are considered safe for vertebrate animals at levels used in mosquito control. Broad-spectrum insecticides are usually less expensive than the biological controls described above.
Pesticides can be used against adults by fogging them over the landscape at dusk or dawn to kill flying mosquitoes when they are most active. Trained personnel employ this method to control outbreaks of adult mosquitoes. However, fogging requires atmospheric conditions with a minimum of wind, usually less than 10 miles per hour (16 km/h), and fogs are not very effective at penetrating dense vegetation. Also, the results of fogging are temporary: mosquitoes may fly back into an area soon after fogging. Even under the best conditions, some mosquitoes in secluded places will escape, and a total elimination of mosquitoes cannot be expected. Fogging also kills nontarget flying insects, including mosquito predators.
Mosquito Abatement and Vector Control Districts
Publicly supported mosquito abatement districts and mosquito vector control districts have been formed in many areas to carry out local mosquito control. The area may be a county or other political subdivision. California residents can find out whether they live within a mosquito abatement district by referring to the Mosquito and Vector Control Association of California Web site, http://mvcac.org, or by consulting the "Government" pages in a telephone book. Advice and some services are free but others may incur fees.
Mosquito abatement districts target most of their services toward mosquito control and public education about mosquito problems and prevention. Mosquito and vector control districts also offer these services, but additionally address problems created by a wider variety of arthropods and sometimes other kinds of animals (e.g. ticks, wasps, rats, etc.). The distinction between the two types of district is, however, in practice, informal.
If you and your neighbors are interested in forming a district, or are interested in the organization, duties, and financing of a district, consult the Vector Borne Diseases Section of the State Department of Public Health in Sacramento. Laws and regulations applying to MVCDs are contained in the California Health and Safety Code §§ 2000-2093.
MVCDs serve the public and are more than happy to work with landowners to help them solve particular problems. Because mosquitoes can pose a public health problem for entire communities, MVCDs have the legal right to access private property for the purposes of mosquito abatement under California Health and Safety Code §§ 2040-2055.
As you may have guessed by the impressive list of mosquito habitats in this publication, public agencies can never control all the mosquitoes on every property. Community cooperation is essential.
One of the most important steps in cooperative mosquito control work is to establish a master drainage plan for the area. All new construction such as land leveling, roads, canals, housing projects, and industrial developments that use water or produce wastewater should be tied into the master plan. Although this may appear difficult in rapidly growing communities, a little coordination and planning at the beginning of projects can avoid major headaches later. If you become aware of a new project in your area, it is helpful to make sure that mosquito control and drainage have been considered and that the local MVCD is aware of the project.
Copyright © 2005 The Regents of the University of California, Division of Agriculture and Natural Resources. All rights reserved.