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Mosquito Control and West Nile Virus



West Nile Virus (WNV) was first recognized in the Western Hemisphere in 1999. It is a vector-borne disease, meaning that it requires an intermediary creature -- in this case, a mosquito -- to be transmitted from one person or animal to another.

Map of the United States showing human cases of West Nile Virus in the U.S. in  2003. (Diagram courtesy USCDC.)Although humans can become infected with WNV, it's actually most prevalent among birds. As with humans, mosquitoes circulate the disease among wild bird populations.

In addition to birds and humans, other vertebrate animals, such as horses, also vulnerable to WNV. Horses are particularly susceptible, in fact, and one-third of horses that become ill with WNV either die or must be put down.

In humans, West Nile Virus is most dangerous to the very old, the very young, and those who already have weakened immune systems. Fewer than one percent of humans who are bitten by an infected mosquito will become seriously ill.

As the diagram above shows, all but five U.S. states reported human cases of West Nile Virus in 2003, despite aggressive attempts in many parts of the country to control the spread of the disease through wide-area spraying.

Mosquito Control

Because mosquitoes are the only known vector of West Nile Virus in the United States, the only effective way to control the spread of WNV is to control mosquito populations. Mosquito control may consist of any or all of the following:

Source Reduction

Mosquitoes breed in water, so reducing sources of standing water is a very important part of any successful mosquito-control plan. Many mosquito species can breed in very small bodies of water, such as a birdbath, an old tire, or a child's toy pail.

Homeowners should walk carefully through their property and eliminate man-made objects that may serve as breeding pools whenever possible. In addition, artificial fish ponds should be kept aerated, permanant swimming pools kept properly chlorinated, kiddy pools emptied and turned upside-down after use, and birdbaths emptied, cleaned, and refilled on a daily basis.


There are several pesticides labeled for use in bodies of water to kill larval mosquitoes before they pupate into adults.

One of the oldest mosquito control products is Bacillus thuringiensis (commonly called BTi), which is a bacterium that is deadly to larval mosquitoes but essentially non-toxic to other creatures. BTi briquettes, or "mosquito dunks," are simply tossed onto the surface of the water in the correct quantity for the surface area being treated, during the several days before or after mosquito eggs start hatching.

BTi is one of several over-the-counter mosquito larvicidesthat can be purchased without a license in most jurisdictions in the U.S., but we recommend that you consult with your state's agriculture or Cooperative Extension department prior to purchasing or using BTi or other mosquito larvicides.

There also are a number of chemical insecticides that are labeled for use in water, but they generally must be applied by specially trained individuals who hold advanced certifications in Aquatic and Public Health Pest Control. We offer a variety of chemical and non-chemical mosquito control products at our online pest control store.


Several years ago, fish of the species Gambusia affinis ssp. began to be used for mosquito control. This practice has fallen out of favor due to the finding that so-called "mosquito fish" consumed many times more fry (young fish) and beneficial insects than they did mosquitoes; so nowadays, their use is pretty much limited to artificial ponds, drainage pools, ditches, and so forth.

Dragonflies are perhaps a more promising predator. Although many people are afraid of them, dragonflies are completely harmless (and in my opinion, very beautiful) insects who can consume several hundred adult mosquitoes each on an average night. Reintroducing dragonflies into areas where their populations have been decimated (ironically, in many cases, by the very insecticides intended to control mosquitoes) is a possibility worth considering.

Trapping and Electrocution

There are a wide variety of mechanical devices designed to capture or kill mosquitoes. Consider installing a pesticide-free, wide-area mosquito-control device on your property, such as the Mosquito Magnet or a Flying Insect Electrocutor. Non chemical mosquito control reduces the need for wide-area insecticide applications. We offer quite a few mosquito traps at our online pest control product store.


When large numbers of adult mosquitoes are found to be carrying West Nile Virus, it may be necessary to use pesticide sprays or fogs to quickly reduce their numbers. Of course, such applications should be made only by properly trained and licensed individuals working closely with their local regulatory and public health agencies.

Mosquito Repellent Barriers

There are many over-the counter liquid mosquito repellents that may be sprayed on the exterior of buildings and in other outdoor areas to temporarily reduce mosquito populations. These are very useful when planning outdoor events such as barbecues and outdoor weddings and parties.