APPENDIX I-V:  Pest Control—Mosquitoes


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Pest Control—Mosquitoes

The Problems

Environmental Health

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.

The Solutions

Preventive Strategies

Prevent Breeding

In most parts of this country, where the risk of catching mosquito-borne disease is slim, it is possible to control mosquito populations without resorting to potentially toxic chemicals. Mosquitoes do not fly far during their short life spans: the mosquito bothering you in your backyard was likely born and raised there. This means you have some control over the mosquito population near your home.

Mosquitoes need water to complete their life cycle. Adult females lay their eggs on the surface of slow-moving or stagnant water. When the eggs hatch into larvae, the larvae feed underwater while breathing air from the surface. Eliminate water suitable for the breeding of eggs and larvae, then, and you can eliminate the mosquitoes. This was the thinking behind the draining of coastal wetlands over the past two centuries. (That practice, while ecologically destructive, did contribute to the virtual disappearance of malaria from North America, where cities such as Philadelphia, Washington, D.C., and even New York had been hotbeds of the disease.)

But removing water doesn't have to have such devastating environmental consequences, especially when it targets artificial breeding habitats. Begin around your home. Go outdoors and look for any container that can hold water--cans, buckets, saucers under plants, plastic wading pools, birdbaths, old tires--and remove them. If you can't remove them, turn them over so they don't collect water. Check street gutters and other drainage ditches to make sure they are clear of obstructions such as leaves.

Water bodies that can't be drained, such as wells or ponds, can be stocked with natural predators, such as the larvae-eating fish Gambusia. They can also be sprayed with the anti-larvae bacteria Bacillus thuringiensis var. israelensis. This bacteria kills larvae when they ingest it, and is recommended by the National Coalition Against the Misuse of Pesticides as an effective, least-toxic biological control. It is fatal to mosquitoes, blackflies, and other aquatic flies, and may be purchased as inexpensive "mosquito control rings" or "mosquito dunks" from the Gardener's Supply Company,, 888/833-1412; from Real Goods,, 800/762-7325; or from March Biological,, 800/328-9140.

Of course, any alteration to a water body in order to reduce mosquitoes may affect other organisms, including populations of beneficial insects. These potentially adverse impacts should be considered before taking action.

Prevent Biting

Make sure every window, door, and porch is fully screened, and check your screens often for tears. Wear light-colored long sleeves and pants made from tightly woven material. (There is some evidence that mosquitoes are attracted to dark colors.) Try to stay inside when the mosquitoes in your area are most active, which in many regions is at dusk. If you are outside when mosquitoes are biting, burning citronella candles may work to repel them. If mosquitoes bother you in the night, consider sleeping under mosquito netting, available at camping stores.

Insect repellent often contains the ingredient N,N-diethyl-m-toluamide, or DEET. Unless you are traveling outside the U.S., where you may need to use precautions against malaria and dengue fever, avoid the use of bug sprays containing DEET, which is a skin and eye irritant and has occasionally caused severe neurological problems, including three deaths. Instead, use DEET-free repellents such as Bite Blocker or Green Ban. These and others should be available at your local natural foods store (See our Product Report on Insect Repellents, for more recommendations).

If you decide that you wish to use DEET-containing repellents, select a product formulated with 10% DEET or less, as recommended by the American Academy of Pediatrics. Such levels have been shown to be effective in repelling mosquitoes yet minimize exposure levels. Some products containing low levels of DEET include Off! Skintastic Family Formula and Cutter All Family. Avoid products that combine sunscreen with insect repellent. Sunscreen should be applied copiously and often, whereas reducing exposure to DEET requires applying as little as possible. Do not apply to children's hands or face, to reduce potential contact with eyes or ingestion. Do not apply on cuts, wounds, or sunburned skin. Apply as little as possible, to exposed skin only. Be conservative about reapplication, following label directions regarding length of effectiveness. Never use under clothing. Finally, wash product off with soap and water when coming indoors.

Get involved with your Mosquito Control District

Backyard mosquito control can significantly reduce the population of mosquitoes disturbing your sleep at night, but it will never have the same impact as a coordinated prevention and control strategy on a larger scale. This is where the Mosquito Control District becomes important, because it (or your Department of Health) sets the course of mosquito control for the whole community.

In evaluating the activities of your Mosquito Control District, keep in mind that chemical insecticides should be used against adult mosquitoes only as a last resort. If the adult population is out of hand, that means that control efforts targeting breeding sites and larvae have failed. Get involved to make sure that your area improves its strategies for controlling mosquitoes before they get to be adults. That way, potentially toxic spraying can be avoided in the future.

If spraying is necessary, it should be done based on careful monitoring, targeting only those sites that need treatment, not entire neighborhoods. Least-toxic chemicals should be used, and applicators should be trained in strict compliance with regulations, which can help reduce harm to non-target organisms. Different species of mosquito have different habits and preferences; make sure your Mosquito Control District is taking the local species mix into account when formulating its plans.

Finally, many mosquito control programs are inadequately staffed and funded. Advocating for generous funding for your local Mosquito Control District is one way to ensure that mosquito control does not come at the expense of human health and the environment.

Resources and References


The American Academy of Pediatrics:

The American Bird Conservancy's Ban Fenthion site,

American Mosquito Control Association (to find out more about mosquito control in your area):

EXTOXNET Pesticide Information Profiles:

Fradin, Mark S. "Mosquitoes and Mosquito Repellents: A Clinician's Guide." Annals of Internal Medicine, No. 128 (1998), pp. 931-40.

Go, V. et al. Estrogenic Potential of Certain Pyrethroid Compounds in the MCF-7 Human Breast Carcinoma Cell Line. Environmental Health Perspectives, Vol. 107, No. 3 (March 1999).

"Least-Toxic Control of Mosquitoes." Beyond Pesticides/National Coalition Against the Misuse of Pesticides.

"Naled for Mosquito Control." U.S. Environmental Protection Agency.

"Roll Back Malaria":

"Seizures Temporally Associated with Use of DEET Insect Repellent--New York and Connecticut." Morbidity and Mortality Weekly Report, October 6, 1989, pp. 678-680.

Spielman, Andrew and Michael D'Antonio. Mosquito: a natural history of our most persistent and deadly foe. Hyperion, 2001.

Swadener, Carrie. "Community mosquito control." Journal of Pesticide Reform. National Coalition for Alternatives to Pesticides, Vol.14, No. 2 (1994), pp. 37-39.

"Synthetic Pyrethroids for Mosquito Control." U.S. Environmental Protection Agency.

Toward Safer Mosquito Control in New York. New York Coalition for Alternatives to Pesticides (NYCAP), Environmental Advocates, New York Public Interest Research Group. January 2000.

U.S. Environmental Protection Agency, Office of Pesticide Programs,

- By Molly Rauch, M.P.H.
March 30, 2002

© 2007 The Green Guide Institute