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Pest Control—Mosquitoes


When a mosquito slips in to your home through a broken window screen, a high-pitched buzzing in the ear and a couple of itching welts are probably the only immediate consequences. But the mosquito is a deceptively minor irritation, because contained within that tiny insect--light as a feather, almost transparent--is a tangle of issues, where infectious disease, chemical toxins, ecological damage, and local politics all intersect.


Mosquitoes transmit some of the most devastating diseases on earth: malaria, dengue fever, yellow fever, filariasis (which causes elephantiasis of the limbs), and encephalitis viruses, including West Nile fever, which first appeared in the Western hemisphere in New York City in 1999. The female insect, which needs blood so she can lay her eggs, picks up parasites and viruses from the mammals on which she feeds. (Males never bite animals; they feed on flower nectar.) When she goes for another meal, these pathogens travel from her mouth into the blood of her victim.

The diseases transmitted by mosquitoes kill millions--malaria alone is responsible for close to a million deaths every year--but the U.S. is relatively safe from mosquito-borne illness. Here, mosquitoes carry encephalitis viruses and, very rarely, dengue fever. Immigrants and travelers sometimes bring yellow fever and malaria with them to these shores, but these diseases are almost never transmitted within the U.S.

But while the U.S. offers little risk of malaria-borne disease to its inhabitants, the emergence of West Nile virus raised public concern to new heights about the importance of reducing mosquito populations and limiting mosquito bites, not only because they are a nuisance but also because they are a health threat. At the same time, concerns about the health and ecological effects of reducing mosquito populations have also emerged.


There are about 2,500 species of mosquito, only some of which transmit diseases. Local mosquito populations are monitored and controlled by Mosquito Control Districts or Departments of Health on a state-by-state basis. These mosquito brigades use a range of strategies, from setting up hotlines for nuisance complaints to spraying chemicals over entire communities. In those places where sprays are used, different kinds of sprays may have different health and ecological impacts.

Organophosphates are a class of chemicals that act against mosquitoes by interfering with the nervous systems of insects--and sometimes other species, including humans. Malathion is one of the least toxic of the organophosphates. Nevertheless, it has been linked to eye disorders, immune system disruptions, and genetic damage. It also can cause headaches, dizziness, nausea, and respiratory distress. Malathion is highly toxic to fish and to some beneficial insects such as honeybees, and it may cause ecosystem damage if used improperly.

Another organophosphate (OP) used for mosquito control is naled, which also causes dizziness and nausea. At very high doses, naled, like all OPs, can cause convulsions and death. Naled is highly toxic to all insects, including honeybees, whose pollinating skills are vital to agriculture.

Fenthion, or Baytex, is an OP used for mosquito control in some parts of Florida. Exposure to even tiny amounts of fenthion can kill birds, and it is also harmful to shrimp, crabs, and other aquatic species if sprayed near water. Due to Florida's importance as a stopover point for migrating birds, use of fenthion there is particularly troubling.

Another OP found in sprays, pest strips and pet collars is Dichlorvos or DDVP, a known carcinogen and developmental toxicant which can cause headaches, nausea and vomiting. Despite an EPA health assessment indicating risks, the EPA has not banned DDVP for residential use. Because pest strips can result in chronic expsoure, it is best to avoid the chemical in products such as Alco No-Pest Strip, Amvac Insect Strip, and Swat Pest Strip and check labels of other products.

Another class of anti-mosquito chemicals is the pyrethroids. Like organophosphates, pyrethroids kill insects by interfering with their nervous system. Insecticides that fall into this category include permethrin, resmethrin and d-phenothrin. Experts have relatively few concerns over the acute toxicity of pyrethroids, but this class of chemicals may over the long term interfere with the immune system and disrupt hormone functions. In one study, pyrethroid insecticides caused estrogen-sensitive breast cancer cells to multiply in a test tube, for example. Pyrethroids may also damage the liver and thyroid.

Spraying is a tactic used to kill adult mosquitoes. Some Mosquito Control Districts focus instead on the larval stage of mosquito development, adding chemicals to water bodies in which mosquitoes breed. One such chemical, the insect growth regulator methoprene, is applied to water to prevent larvae from developing into adults. The Northwest Coalition for Alternatives to Pesticides reports that methoprene, while posing little harm to humans and other mammals, nevertheless delays the development of other invertebrates and is toxic to some fish. Methoprene also interferes with the maturation of several non-target insects, including beneficial ones.

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Last Updated: March 30, 2002

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