ANR-1116, New Aug 1998. Renee R. Anderson, Graduate Student, and Faith M. Oi, Extension Entomologist, Assistant Professor, both in Entomology at Auburn University

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Mosquitoes in and Around the Home

Mosquitoes are familiar and annoying pests of humans and animals. Their rapid wing movement produces a distinctive high-pitched hum, and their bites cause red, itchy welts. Mosquitoes are more than just a nuisance, however, because they also serve as carriers (vectors) of several disease-causing agents.

Mosquitoes are small, slender flies that are members of the family Culicidae. When viewed under a hand lens, adult mosquitoes are easily recognizable--scales cover the wing veins and the hind wing margin. The scale and setal (bristle) patterns of the adults are distinctive characteristics for species identification. There are at least 50 known species of mosquitoes in Alabama.

Mosquito Biology

Like butterflies, mosquitoes undergo complete metamorphosis and have egg, larval, pupal, and adult stages. Only the adult females ingest blood. However, not all species are "blood-suckers." The cannibal mosquitoes in the genus Toxorhynchites are large and conspicuous, but they do not bite humans. The larvae of these mosquitoes eat the larvae of other mosquito species. As adults, the Toxorhynchites feed exclusively on nectar and other plant juices.

Of the blood-sucking mosquito species, a blood meal is usually (but not always) necessary for egg maturation. The eggs may be laid singly or in rafts but always in association with water and where it is quiet and protected. Females may lay up to 100 or 200 eggs per batch and deposit an egg batch every 7 or 10 days. Eggs can take from days to months to hatch. In general, during warm periods and under favorable conditions, eggs will hatch in a few days. However, the eggs of "flood-water" mosquitoes can remain dormant and are capable of surviving for months out of water until they are flooded. Eggs may be laid at the water surface, at the water-soil interface along banks, or on soil or vegetation that is subject to periodic flooding, depending on the species involved. Other sites where eggs may be deposited include tree holes, temporary puddles, freshwater and salt marshes, and containers in which water stands, such as discarded tires, cans, pet water dishes, and bird baths.

The larvae have no legs and have an enlarged thorax that is much wider than the head and abdomen. Larvae prefer quiet, still water, and are commonly referred to as "wrigglers," which describes their movement in water (Figure 1). They feed on decaying organic matter that includes microorganisms. After hatching, the larva goes through four molts, with the final molt resulting in a pupa. The larval stage typically is completed in about 7 to 10 days.

The pupa is a mobile but nonfeeding stage. The pupae are frequently called "tumblers" because when they are disturbed at the water surface, they will quickly tumble downward and then rise slowly to the water surface. After 2 or 3 days, the pupa molts into the winged adult.

The longevity of the adult (Figure 2) varies greatly. During the heat of summer, most adult females do not live more than 2 weeks. The males have a shorter life span. During both the adult and immature stages, mosquitoes serve as valuable prey to numerous forms of wildlife such as bats, birds, and aquatic animals including sport fish.

Although many species of mosquitoes have a wide geographic distribution in the United States, their local abundance varies greatly. Warmth, precipitation, and favorable microhabitats influence potential population levels of the immature stages. Eggs laid in flood-prone sites can accumulate for years until they hatch under favorable environmental conditions, thus resulting in explosive population levels within a relatively short period of time.

Feeding Habits of Adult Mosquitoes

Initially, mosquitoes use odor and chemical cues to locate their food source. As the mosquito comes near a potential host, moist air from the host and visual cues become important. The chemical cues help the mosquito decide where to land on a host. Carbon dioxide exhaled by animals, including humans, is an example of a chemical cue that mosquitoes use to find their hosts. Repellents (discussed in the following section) work by "confusing" the mosquito, keeping it from landing on or biting the intended host.

Mosquitoes may be either "specialists," feeding only on birds, mammals, reptiles, or amphibians; or "generalists" that will readily attack whatever comes along. Others feed primarily on birds in the spring and then switch to mammals later in the season. Some mosquitoes will actively bite only at night, while others will readily feed during the day or at sunrise and sunset.

Diseases Spread by Mosquitoes

Eastern Equine Encephalomyelitis (EEE)

EEE virus is the most severe of the disease agents transmitted by mosquitoes in Alabama. EEE may cause disease and death in humans, horses, and introduced birds such as pheasants and emus. Disease is most severe in infants but also in the elderly and those who have underlying medical conditions. The death rate may approach 60 percent even with medical treatment, and mental and physical problems usually persist in those individuals who do survive. Fortunately, human cases in Alabama are rare.

EEE is caused by a virus that is maintained in the wild bird population by the avian-feeding black-tailed mosquito (Culiseta melanura). The preferred habitat of the black-tailed mosquito is freshwater swamps shaded by heavy vegetation. It is not definitely known how the virus is introduced into the human population because the black-tailed mosquito rarely feeds on mammals. It is suspected that other mosquito species are involved in the disease-transmission cycle and are responsible for introducing the virus from the wild bird population to mammals. Currently, there is no vaccine available for humans.

Heartworm

Pet owners should be concerned with mosquito control because mosquitoes transmit heartworm to dogs. Your veterinarian can prescribe a drug treatment that prevents the worms from reaching the adult stage in your dog.

Myth: Mosquitoes can spread AIDS.

Fact: While mosquitoes can transmit agents that cause diseases such as encephalomyelitis, they have never been shown to transmit agents that cause AIDS. The relationship between mosquitoes and disease- causing agents is very specific. The concept of mosquitoes as "flying hypodermic needles" is inaccurate and misleading.

How Do I Protect Myself from Bites?

The best means of protection is to avoid mosquitoes by scheduling outdoor activities before or after peak mosquito activity, which is usually in the late afternoon and at dusk. Repellents containing diethyl toluamide (DEET) are effective in discouraging mosquitoes from biting. Drenching oneself in repellent is not necessary. You need only enough repellent to "confuse" the mosquito so it cannot detect a suitable spot to feed on you. There are repellents that boast 95 percent DEET content; however, a repellent of this concentration is unnecessary to provide adequate protection, may result in skin irritation, and is costly. Concentrations of 10 to 15 percent DEET are recommended for adults; products containing less than 8 percent are recommended for children. These concentrations are less prone to cause skin irritation, and they are more affordable. Repellents that are DEET-free, containing citronella, are also effective.

How Can I Obtain Relief from Itching?

Ointments that contain hydrocortisone, benzyl benzoate, or calamine can provide relief from itching. If you have questions about these products, be sure to discuss them with your health-care professional or pharmacist.

The saliva that is injected into the bite wound causes the itch from a mosquito bite. The saliva contains proteins that serve as an anticoagulant and help the mosquito to feed. The human immune system responds to the "foreign proteins," and the reaction is an itch.

How Can I Control Mosquitoes?

Effective and lasting mosquito control is complex, often requiring municipalities or local governments to legislate control efforts. You may not have containers with standing water, but your neighbor may, or water may be collecting in a ditch somewhere in the neighborhood. There are things homeowners can do to help prevent mosquito infestations. Three methods of prevention are to sanitize to reduce breeding sites, to use physical methods of control, and to use control products.

Sanitation

The first step in sanitizing is to eliminate the breeding sites listed above. It is also important to manage vegetation because adult mosquitoes rest on dense vegetation during the day. Cut tall weeds, and keep shrubs and trees trimmed away from the house to increase air circulation. Other steps you can take include the following:

• Clean debris from rain gutters.
• Eliminate standing water on and around structures such as flat roofs, air conditioner units, and leaky pipes and faucets.
• Eliminate seepage from cisterns, cesspools, and septic tanks.
• Change the water in birdbaths and wading pools weekly.
• Change the water in pet bowls daily.
• Water plants and lawns so that water is not left standing for several days.
• Stock ornamental ponds with Gambusia fish that feed on mosquito larvae.
• Encourage other insects such as dragonfly (nymphs) and certain aquatic beetles that feed on mosquitoes.

Physical Control

Physical control methods focus on excluding mosquitoes from the indoors and include the following:

• Install screens that are 16 to 18 mesh.
• Screen the chimney and other vent flues during mosquito season. (Remove screens during winter.)
• Repair broken screens on windows, doors, and porches.
• Keep doors closed if not screened.
• Caulk cracks and crevices where insects can enter.
• Use a fly swatter for the occasional mosquito that is inside.

Control Products and Materials

There are a number of products and materials that can be used alone or in combination to control mosquitoes. These control methods can be directed toward either larvae or adults and therefore are categorized as larvicides or adulticides.

Larvicides

Products to control larvae ("wrigglers") include the following:

• Bacillus thuringiensis var. israelensis (B.t.i) that is commercially available as Bactomos, Teknar, and Vectobac. B.t.i. can also be purchased as "mosquito dunks" that can be used in water that cannot be drained, such as unused swimming pools and retention ponds. B.t.i. is considered a "biological" method and poses a minimal threat to nontarget organisms.
• Insect growth regulators such as methoprene can be used in water when fish are present. Methoprene acts by preventing the larvae from becoming adults. Timing is important when applying methoprene as a liquid because it degrades rapidly when exposed to sunlight. Commercial formulations include Altosid in liquid and briquets. Briquets are reported to last up to 30 days under field conditions.
• Malathion, temephos (Abate), and pyrethrins are also commonly used larvicides. Read the label and hazard statements concerning fish and other aquatic life before using these larvicides to determine whether these chemicals are appropriate to use in your situation.

Adulticides

Products to control adult mosquitoes include the following:

• Foggers. Yard foggers typically contain pyrethrins. They can be set off shortly before outside activity and will provide temporary relief from mosquitoes.
• Sprays. Treat shrubs and the lower branches of trees where mosquitoes rest. Use insecticides registered for flying insects but that will not harm plants--for example, certain formulations of malathion, permethrin, pyrethrins, and chlorpyrifos.
• Ultra-low volume (ULV) applications. ULV applications require special equipment that local governments usually control.

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Other Control Methods

Other methods that may be used to control mosquitoes but that are less reliable than those listed above are citronella candles, which work temporarily to repel adult mosquitoes, and bug "zappers." Data indicate that less that 5 percent of the insects caught in a bug zapper are female mosquitoes (the ones that bite). Some models tend to attract more insects into an area than would be there if there were no zapper around. Before purchasing this type of control, consider that bug zappers, like insecticides, also tend to kill nontarget, beneficial insects.

One other control method that has been tested for mosquito control is a device that mimics the sound of dragonflies; however, no data are available to support the claims that this device is effective.

For more information, contact your county Extension office. Look in your telephone directory under your county's name to find the number.