Identification And Control Of Mosquitoes

Mosquitoes are important vectors of numerous diseases such as malaria, yellow fever, filariasis, dengue and encephalitis. Besides their disease-carrying abilities, biting mosquitoes can be terribly annoying to people working, camping or relaxing outdoors. It is at these times that they become nuisance pests.

CLASSIFICATION. Approximately 3,000 species of mosquitoes exist throughout the world, including more than 160 species in North American north of Mexico. Mosquitoes are classified with flies in the order Diptera. Dipterans, as the name implies, have only two wings (the front pair). The hind wings are shrunken and modified into small knobbed structures called halteres. It is believed that halteres function as organs of equilibrium.

Mosquitoes belong to the family Culicidae; they are characterized by having scales or "hairs" on their wings as well as on their legs and on parts of their body. Their mouthparts consist of a pair of palpi (sensory organs) and a long proboscis with which the female is able to pierce the skin of animals and suck blood.

Most mosquito species require a blood meal before they can produce a batch of eggs. However, some species can produce a brood shortly after they emerge from the pupal case without having to take a blood meal. They obviously have enough protein accumulated from their larval feeding. They must, however, have blood to produce future broods.

Gillett (1972) and Harrison (1978) reported the following on the amount of blood mosquitoes ingest: "Although a mosquito takes only some 2 to 8 milligrams of blood per meal, depending largely on the size of the mosquito, the total amount taken when thousands upon thousands are involved can be considerable. There are cases on record of dogs and even cattle dying following mass attack by mosquitoes."

Mosquitoes feeding on their victims introduce foreign proteins into their host and this can produce an allergic response which on occasion can be very severe. Usually, however, only the familiar itching red wheal is produced (Gillett, 1972).

Male mosquitoes do not feed on blood (their probosci are too thin and flexible to pierce through animal skin). They feed on nectar and plant juices. Males differ from females, having very bushy antennae and palpi as long as their proboscis. Female Anopheles also have long palpi, but they are not swollen and hairy at their tips.

Mosquitoes are classified in three subfamilies:

• Toxorhynchitinae. This subfamily contains the genus Toxorhynchites, mosquitoes which, as adult females, do not feed on blood. Their larvae are beneficial since they feed on other mosquito larvae.

• Anophelinae. Anophelinae contains the important genus Anopheles, many species of which transmit malaria to man.

• Culicinae. Culicinae contains the bulk of the mosquito genera and species. Important genera in this subfamily are Aedes, Culex, Culiseta, Psorophora and Mansonia. Many of the species in these genera are known vectors of such diseases as encephalitis, yellow fever, filariasis and heartworm.

Among the culicine mosquitoes we find some species that Bishopp (1939) has designated as "domestic mosquitoes." He states, "This grouping is made because these mosquitoes breed in and about houses, are always associated with man and have a restricted flight range… It should be borne in mind that other kinds of mosquitoes may breed near houses and may enter buildings and attack man, as in the case of malaria mosquitoes." Three of the most common species are the northern house mosquito, Culex pipiens L., the southern house mosquito, Culex quinquefasciatus Say and the yellow-fever mosquito, Aedes Aegypti (L.) The eastern salt-marsh mosquito, Aedes sollicitans (Walk.), and the black salt-marsh mosquito, Aedes taeniorhynchus (Wied.), may migrate many miles from their breeding places.

MOSQUITO DEVELOPMENT. Mosquitoes undergo complete metamorphosis. They develop through four states: egg, larvae, pupa and adult. Complete development from egg to adult usually takes from 10 to 14 days (Matheson, 1950), but varies according to the species and the temperature. The egg stage requires water in which to hatch. The larval and pupal stages also require an aquatic environment.

BREEDING HABITATS. Mosquito larvae and pupae must have water in which to live. An object capable of retaining water for five or more days is a potential breeding area for some species of mosquito

Laird (1988) reclassifies the habitats in which mosquito larvae can develop. He discusses 11 such habitat categories:

1. Flowing streams (including drainage ditches)
2. Ponded streams
3. Lake edges
4. Swamps
5. Shallow permanent ponds
6. Shallow temporary pools (snow melt pools)
7. Intermittent ephemeral puddles
8. Natural containers (abandoned snail shells, leaves of the pitcher plant, tree holes)
9. Artificial containers (flower pots, tires, construction depressions)
10. Natural subterranean waters (holes dug by crabs, hoofprints)
11. Artificial subterranean waters (septic tanks, storm and sewage drains)

PCOs should keep in mind that an understanding of the various places in which mosquitoes can breed is vital to any control program.

MOSQUITO IDENTIFICATION. Mosquito identification is of primary importance. Only by knowing the correct identification of the mosquitoes you collect can you determine the following:

• The important mosquito species (those which bite man or are potential vectors of disease) versus those which are not. Obviously the control of disease vectors would take priority.

• The source of annoying mosquitoes. This is determined by the type of water they breed in and the distance they normally travel from their breeding area (i.e. flight range).

• The correct pesticide and adequate concentration for control.

• The timing for control efforts.

To identify mosquitoes to species, refer to publications pertaining to or near the state where they occur

REPRESENTATIVE COMMON MOSUITOES. Some of the more common mosquitoes likely to be encountered by PCOs or of public health importance include:

• The northern house mosquito, culex pipiens L. This mosquito commonly is found in the northern states and Canada. It is a cosmopolitan mosquito that has been introduced into the United States and is pale brown with whitish bands across the abdomen. This mosquito also is known as "the common house mosquito" since it breeds in tin cans, rain barrels, tubs, hollow stumps, flat roofs, roadside and sewer ditches, sluggish streams and cesspools. In fact, they can breed practically anywhere around the house where water is found. At times, the fertilized females may hibernate in tremendous number in hollow trees, street drain traps, cellars, etc. They are usually inactive in these locations, but may breed during the winter when it is sufficiently warm.

The hibernating females emerge early in the spring and lay their eggs in raft-like masses, each egg standing on end in this mass. There are 50 to 400 eggs in such a raft and the female may lay several such masses.

The larvae or "wrigglers" emerge from the eggs during the summer and molt four times before becoming mature. Under optimum conditions they pupate in one week. The pupae are active but do not feed. The pupal stage may be completed in one day. When conditions are optimum the life cycle may require seven to 10 days, but often it is extended far beyond this.

Culex pipiens L. breeds continuously during the summer and is known to fly up to a distance of 14 miles (Clarke, 1943) from the breeding place. When present in persistent numbers the common house mosquito is probably breeding in the immediate vicinity of the house. This mosquito is known to carry parasitic worms that infect man. It usually bites at dusk and after dark, hiding in some dark place during the day. Moreover, its persistent singing hum is very annoying to many individuals.

• The southern house mosquito, Culex quinquefasciatus Say. This mosquito is very similar in appearance, life history and habits to the northern house mosquito. King et. al. (1944) note it is "generally the most abundant night-biting house mosquito in the cities and towns of the southern states. In the North it is replaced by its very near relative, Culex pipiens, and the range of the two overlap in Virginia, northeastern Tennessee, North Carolina and other intermediate states."

• The yellow-fever mosquito, Aedes aegypti (L.). The yellow-fever mosquito easily is recognized by the white or silver-colored lyre on its thorax an the silver stripes on its abdomen and legs. At one time this was a common house mosquito in the South. While Aedes aegypti is not common in the United States presently, it is included in this section because it is a most important disease vector and has been reintroduced into the United States many times in the past. It continues to be a potential health threat. The United States Department of Health lists 10 southern states in which Aedes aegypti can still be found including Oklahoma, Texas, Louisiana, Arkansas, Mississippi, Tennessee, Alabama, Georgia, North Carolina and Florida.

Bishopp (1939) states, "… the yellow-fever mosquito prefers relatively clear water and seldom, if ever, breeds in natural ponds or pools. Its wrigglers are found in tanks or cisterns, and in cans, bottles, pots, jars and other vessels that will hold even very small quantities of water. They often occur in unexpected places, such as discarded automobile casings, flower vases, holy water and baptismal fonts, urns in cemeteries, water troughs, water pans for chickens, grid-stone cans, dishes of water placed under legs of refrigerators and unused toilet bowls or tanks." This mosquito usually breeds close to the house.

• The eastern salt-marsh mosquito, Aedes solicitans (Walk.). This is the most important of the salt-marsh mosquitoes and breeds along the Atlantic and Gulf coasts. This species can migrate for long distances, often in enormous swarms. The black salt-marsh mosquito, Sedes taeniorhynchus (Wied.), is another important salt-marsh mosquito pest which is found widely distributed throughout the United States, particularly in the South.

MOSQUITO CONTROL. Mosquito control is a problem that is best handled by the community and public health agencies rather than by the individual because the breeding places of mosquitoes are often situated at considerable distances from the place of annoyance. However, there are some methods that the homeowner and experienced pest control operator may utilize in reducing a mosquito pest problem.

• Screening. There is seldom any serious mosquito problem in houses protected with 16 to 18 mesh wire screening.

• Larviciding (Control of mosquito larvae). Larvae may breed wherever suitable water is available. If it is not possible to eliminate this water, insecticides should be used. The following larvicides are widely used in the United States: chlorpyrifos (Dursban), fenthion (Baytex), malathion, methoxychlor, prethrins and temephos (Abate). Check the labels of these products for specific uses and rates. Rates vary according to specific environmental situations (catch basin; clear water; highly polluted water).

Oils also are used to kill larvae. Fuel oil no. 2, diesel oil no. 2 or kerosene may be used. Only temephos should be used if fish are present.

Aerosurf MSF is effective in controlling larvae and pupae. Aerosurf is a monomolecular surface film which Hester et al. (1989) found not to be phytotoxic to rice and four other aquatic plants.

The biological larvicide Bacillus thuringiensis var. israelensis is safe for non-target organisms and effective in controlling many mosquito species. Commercial Bti is available as Bactimos, Skeetal, Teknar and Vectobac.

An insect growth regulator (IGR) such as methoprene (Altosid) is used to control mosquito larvae when fish are present. Methoprene does not kill mosquito larvae. It does, however, prevent them from becoming normal adults. Methoprene is also available as a slow release briquet formulation. This product is claimed to release effective levels of methoprene over a 30-day period (longer if the briquets dry out periodically).

Adulticiding (Control of mosquito adults). Adulticides can be applied in three ways:

1. Thermal aerosols (fogging). Insecticides such as fenthion, malathion and naled are used. The fog is produced by heating fuel oil in which the insecticide is diluted. Disadvantages of thermal fogging are the hazard of reduced visibility for passing motorists from the smoke, the cost of using fuel oil as a carrier and the pollution of the environment with fuel oil. This method, once widely used to kill adult mosquitoes, has been almost entirely replaced by ULV application.
2. Ultra-low volume application (ULV). Special machines are used with nozzles which break up an undiluted insecticide into microscopic droplets. With ULV application very small amounts of chemical are used (2 to 4 ounces per acre). Fuel oil is not burned and smoke is not seen coming out of the nozzle. Insecticides applied by ULV include chlorpyrifos, fenthion, malathion, naled, pyrethrins and resmehin. Bendiocarb shows promise in this type of application. Both thermal fogging and ULV application are very temporary in nature. They kill only those adults which pass through the fog which disperses quickly.
3. Mist blower and spraying. This method places the insecticides on the foliage of trees shrubs, tall grass, tree trunks and the sides of structures where resting mosquitoes may contact it. Diluted amounts of pesticides are used including carbaryl, chlorpyrifos, malathion, mehoxychlor, naled, permethrin, pyrethrins, propoxur and resmethrin.

CONCLUSION. Although mosquito control is generally addressed by municipalities and public health agencies, PCOs need to have an understanding of the biology and control of this pest in order to answer customer inquiries when they arise.

REFERENCES

Bishopp, F.C. - 1939. Domestic mosquitoes. U.S.D.A. Leaflet No. 186. 1951. Domestic mosquitoes,

U.S.D.A. Leaflet No. 186.

Clarke,J.L. - 1943. Studies of the flight range of mosquitoes. J. Ecn.Entomol. 36: (1):121-122.

Gilett, J.D. - 1972. Mosquitoes. Weidenfeld - Nicholson, London. 274 pp.

Gjullin, C.M. - 1947. Effect of clothing color on the rate of attack of Aedes mosquitoes. J. Econ. Entomol.

40 (3): 326-327.

Harrison, G. - 1978. Mosquitoes, Malaria and Man. E.P. Dutton, N.Y. 314 pp.

Hester, P.G., J.C. Dukes, R. Levy, J.P. Ruff, C.F. Hallmon, M.A. Olson, and K.R. Shaffer - 1989. Jour.

Amer. Mosq. Control Assoc. 5(2):272-274.

James, M.T. and R.F. Harwood - 1969. Herm’s Medical Entomology. McMillan Co., London. 484 pp.

King, W.V., G.H. Bradley and T.E. McNeel - 1944. The mosquitoes of the southeastern states. U.S.D.A.

Mis. Publ. 336.

Laird, Marshall - 1988. The Natural History of Larval Mosquito Habitats. Acad. Press, London. 555 pp.

Matheson, R. - 1950. Medical Entomology. Comstock Publ. Co., Ithaca, N.Y. 612 pp.

The preceding article was excerpted from Chapter 18 (Mosquitoes) of The Handbook of Pest Control. The chapter was revised by Dr. Stanley Green of Pennsylvania State University.