Mosquitoes are one of the most important insect pests that affect the health and well being of humans and domestic animals worldwide. If environmental conditions are favorable vast populations can occur, anywhere in the U.S., and even the dry Southwest.
Female mosquitoes require a blood meal for egg production, and they produce a painful bite as they feed. While feeding, they can transmit a number of disease-causing organisms to humans and animals. The diseases these organisms cause include: encephalitis, dengue fever, filariasis, yellow fever and malaria. Both encephalitis and dengue (caused by different mosquito born viruses) are potential threats in the United States.
Mosquitoes are flies with slender bodies, delicate legs, and scaled wings. The mosquito life cycle is an example of complete metamorphosis. There are four distinct stages in the life of a mosquito; egg, larva, pupa and adult. A few days after acquiring a blood meal the female mosquito lays eggs. Depending on the species the eggs can be laid singly or in rafts, on the surface of the water, on the sides of containers, or on damp soil.
After the eggs hatch, the larvae or “wrigglers”, swim in the water and feed on microorganisms or decaying matter. There are four growth phases in the larva’s life, called instars. The larvae must come to the surface of the water to breath (with the exception of a few specialized mosquitoes). Mosquito larvae have a siphon at the tail end of the body.
The siphon permits larvae to breathe by penetrating the surface of the water to access air directly. After the fourth larval instar, the pupa forms. Pupae, often called “tumblers”, do not feed. Mosquito pupae breathe by using their respiratory “trumpets” to draw air directly from the atmosphere. Larvae and pupae can be killed by cutting off their access to air with oils or monomolecular films.
After a few days, the adult mosquito emerges from the pupal skin and flies away. Only female mosquitoes take blood meals, and the blood is used to produce eggs. Male and female mosquitoes also feed on flower nectar. Male mosquitoes look similar to females but can be identified by having feathery antennae. The long hairs on their antennae help them to locate flying females.
Encephalitis is an inflammation of the brain and the causal virus is primarily transmitted by the mosquito, Culex tarsalis. The mosquito transmits the virus to birds, horses, mules and occasionally people. Birds serve as the most important host reservoir for the virus in the disease cycle. During the mosquito season public health officials routinely trap and test mosquitoes for viruses, and undertake sentinel chicken flock testing. The chickens are bled once or twice a month and tested for antigens indicating the presence of viruses. Health officials may also survey local and migrating bird populations to determine the incidence of virus and the potential for transmission. However, this is only done under special circumstances such as during severe floods.
Mosquitoes carrying Western equine encephalitis (WEE) are most common during the summer and early fall months. Symptoms of WEE in horses include neurological signs such as depression and lack of coordination. A sick horse may also go down and not be able stand back up. The illness is fatal in 20-50% of horses that are stricken with the disease.
Human symptoms of WEE include high fever, convulsions, delirium and other characteristic central nervous system dysfunctions. Medical assistance should be obtained quickly if symptoms occur. Saint Louis encephalitis (SLE) is a second viral disease transmitted by Culex tarsalis. Birds are again the most important hosts, but humans can also be infected.
Unlike WEE, horses are not involved with the SLE disease cycle. Fatal human cases are uncommon and occur sporadically. SLE is a much more serious threat to humans than WEE. Children and the elderly are most susceptible to fatal infections.
Dengue is a disease caused by a complex of viruses transmitted between humans by Aedes aegypti. No other animals are involved in the disease cycle. This disease is currently endemic (naturally transmitted) in Mexico and sporadically occurs in southern Texas. The disease has three forms. Dengue fever is characterized by a high fever, severe joint pain, vomiting and a rash. Recovery normally occurs in a few weeks. Dengue hemorrhagic fever and/or dengue shock syndrome are much more serious forms of the disease that occur in people that have had dengue previously. Both hemorrhagic dengue and dengue shock syndrome can be fatal and children and the elderly are most susceptible.
Mosquitoes also transmit heartworm in dogs. Heartworm can cause severe circulatory problems and produce symptoms such as coughing, labored breathing and general loss of vitality in advanced stages. Because of the impracticality of protecting dogs from mosquito feeding, the most effective means of controlling heartworm is to prevent worms from reaching the adult stage inside the dog. Veterinarians can prescribe excellent drug treatment to protect pets from heartworm. Mosquitoes infect healthy dogs while taking a blood meal.
Since mosquitoes need water to complete their life cycle, a mosquito problem can develop just about anywhere that water collects. Municipal and farm animal waste lagoons may become breeding sites. Farm ponds and lakes are typically not major mosquito breeding areas if they contain fish and are free of weeds, algae or floating debris in which mosquito larvae hide. Permanent natural bodies of water, such as swamps, may contain a wide variety of predatory insects and fish that keep mosquitoes from reaching severe nuisance levels, although storms and floods may disrupt this system and allow mosquito populations to rise rapidly. Unfortunately it is important to recognize that some natural environments also generate large mosquito populations.
In residential areas, human activities often create mosquito-breeding sites or increase the production of mosquitoes in natural bodies of water. For example, road building and maintenance often impede the drainage of rain runoff, creating a mosquito-breeding site. Clogged drainage ditches along roads can also become breeding sites.
Logging and construction activities often leave tire ruts in the soil. These depressions are ideal breeding sites for “floodwater” mosquito species. Around the home, objects such as birdbaths, boats, canoes, discarded tires, soda and tin cans, plant pots and similar objects collect water and allow mosquitoes to breed literally right in our own backyard. The stagnant water in unused swimming pools becomes an ideal breeding site.
However, mosquitoes will not breed in a maintained swimming pool. You can help reduce mosquito populations by eliminating or properly maintaining these problem spots. Do not store open containers, tires, etc. on your property where they can collect water. Discard them as soon as possible.
Because some mosquito species can fly quite a long way from breeding sites a community-wide effort may be needed to reduce mosquitoes to tolerable levels. This requires the formation of a local mosquito control program to organize community-wide “clean up” efforts and to determine the need to eliminate breeding sites or to apply insecticidal sprays to control the insects. Local pest abatement districts can be set up using property tax dollars to employ a local pest control operator to coordinate management strategies.
Some personal protection from mosquitoes can be achieved through the use of insect repellents. Many of these products contain DEET (N,N-diethyl-mtoluamide). Select the desired formulation (e.g. lotion, aerosol spray or cream) and apply it to exposed skin or clothing. Repeated use of repellents over a short period of time (several days) is not recommended, especially for pregnant women and children. Some individuals may be sensitive to DEET, discontinue skin application if irritation occurs. Certain insect repellents contain insecticides and are only suitable for application to clothing and not to the skin. Please read and follow directions carefully. The consumption of certain natural substances including garlic and the vitamin B complex is reputed to reduce the number of bites. But as yet there is no scientific evidence to confirm their effectiveness.
Installing and maintaining tight fitting window and door screens will help keep mosquitoes out of the home. Candles containing oil of citronella are often used outdoors to repel mosquitoes from around decks and picnic tables. These products work best when there is relatively little air movement to disperse the chemical too quickly. Avoid splashing water on lit citronella candles, as they can flair. In some areas, bats and birds may consume mosquitoes as part of their diet. Putting up nesting boxes around your property will attract these natural predators to the area. However, the feeding activity of insect-eating bats and birds will not be sufficiently selective to cause significant reductions in mosquito populations. Please monitor bird and bat boxes regularly as bees can invade the boxes and develop hives.
Electrocution traps (“bug zappers”) placed out-of doors are not effective in reducing or eliminating mosquito populations. Recent studies have shown that less than 1% of the insects “zapped” in such devices were actually biting insects. The majority of the insects killed in electrocution traps are actually beneficial or harmless. Electronic mosquito repellents that emit high frequency sound to “repel” mosquitoes have not been shown to be effective. Claims that certain plants placed around a porch or the deck will repel mosquitoes are not supported by scientifically based studies as yet. Outdoor chemical foggers will keep mosquitoes away for several hours, but once the chemical dissipates, the mosquitoes return. Insecticides are available for controlling larvae, but their application in either large bodies of water or small artificial breeding sites can be difficult, expensive or illegal, particularly for an individual homeowner. Chemical control programs targeting mosquito larvae are best left to trained individuals in county or local government agencies.
Homeowners wishing to treat small areas, such as birdbaths, garden ponds, etc, can use a bacterial insecticide available at many hardware and garden centers. There are a variety of products containing a bacterium known as Bacillus thuringiensis israelensis or Bti. This bacterium kills mosquitoes, but will not harm fish, birds or other wildlife. Most products containing Bti will control mosquito larvae for about 7 days during mosquito season. The product is effectively used in breeding sites (usually 1 Bti briquette per 100 sq. ft. or less). Simply treating all areas of standing water with Bti without knowing if these areas are actually sources of the problem is a waste of time and money. Look to see if you have mosquitoes breeding in the water source before treating. It can be difficult to see the eggs but it is easy to see the larvae and pupae if you remove some water using a white cup or saucepan.
Effective mosquito control is often a complex, expensive task, frequently requiring the cooperative efforts of communities as well as such groups as industry, agriculture, state and local governments.
Many people are concerned about harmful effects of pesticides on the environment, their animals and plants, and themselves. Pesticide toxicity and pesticide hazard is not the same thing. “Toxicity” is the “killing power” under experimental conditions, whereas “hazard” is the risk of poisoning when a product is normally used. Hazard includes both the chemical toxicity and the chance of exposure to the product.
The dosage used and the type of chemical compound determines the hazard level of the pesticide. For example, if an individual were to consume an oral dose of 400 milligrams of table salt per kilogram of body weight the person would become violently ill. In fact every year about 60 people die from aspirin overdose. At lower doses both aspirin and salt are not hazardous. Thus, it is important that the dosage levels recommended on pesticide labels be followed very carefully.
Adulticides are pesticides used to kill adult mosquitoes. Mosquito control districts in the U.S. commonly use chemical adulticides. Applications of chemical spray to vegetation, tree trunks and walls of buildings and catch basins will control certain adult mosquito species but not others.
Larvicides are products that are used to control mosquitoes in their larval stage and include the following:
Bacillus thuringiensis israelensis, or Bti, is a naturally occurring soil bacterium. The bacterium produces proteins in a crystalline form. When the mosquito larvae eat these crystals, the proteins attack their gut wall, killing the larvae. Bti has a highly specific mode of action, and is of minimal environmental concern. Bti is quickly biodegraded and leaves no residue. Always store Bti products under cool conditions prior to use.
Bacillus sphaericus (Bs) is a common soil-inhabiting bacterium. The bacterium produces a protein toxin that may be used to control mosquito larvae. Bs is commonly used to control mosquito larvae in highly polluted water, such as sewage treatment plants. Bs is nontoxic to nontarget organisms. Some natural recycling of this organism is likely and products usually last for about 21 days.
Some synthetic pesticides (like Methoprene) mimic insect juvenile hormone. When these are present, the development of the mosquito larvae is disrupted, and they do not develop to the adult stage. Mosquito control districts use these chemicals in situations like cisterns and abandoned swimming pools.
The breeding site needs to be treated periodically depending upon the formulation of the insect juvenile hormone used. Thirty day and 150-day briquettes are available as well as granular formulations (lasting a few days). Insect juvenile hormone is usually not destroyed by heat. Methoprene for example is toxic to some other insects but is safe for use around humans. Temephos is one example of an organic phosphate compound that is used occasionally in temporary pools that contain mosquito larvae but do not support non-target organisms.
Oils and monomolecular surface films are used to control pupae and late-fourth instar larvae by interfering with their ability to breath. These products are usually used when an adult emergence will occur without treatment (at this point it is usually too late to utilize the other options). However, oils and monomolecular films will control all immature stages.
Gambusia are mosquito-eating fish. Some mosquito control districts raise the fish and use them to stock man-made water bodies. These fish will reproduce and continue to eat mosquito larvae. Gambusia should never be released into natural watercourses as they out compete native fish species. The Gila top minnow is now a protected fish. In many areas where Gambusia have been released the Gila top minnow no longer exists. Most small minnow fish (such as guppies, flathead minnow and shad) are good at reducing mosquito larvae populations and are suitable for release into garden ponds. Be sure you know about any local ordinances regarding releasing fish into water bodies before you release any species of fish.
Fishermen using minnows as bait should avoid releasing live fish into water systems. Often the species of the bait minnow is not suitable for release into the fishing environment and in time such action may result in the collapse of the natural ecosystem and destruction of game fish populations.
Because of the complexity of controlling mosquito populations, technical assistance may be required. Mosquito control personnel may be necessary on a permanent basis and communities may wish to investigate the desirability of an area-wide approach.
The adults of the three most common genera of mosquitoes can be distinguished as follows:
Mosquitoes are often confused with: