Even in the absence of disease transmission, mosquito bites can result in allergic reactions producing significant discomfort and itching. In some cases excessive scratching can lead to bleeding, scabbing, and possibly even secondary infection. Children are very susceptible to this because they find it difficult to stop scratching. Frequently, they are outside playing and do not realise the extent of their exposure until it is too late.
Female mosquitoes can produce a painful bite during feeding, and, in excessive numbers, can inhibit outdoor activities and lower property values. Mosquitoes can be a significant burden on animals, lowering productivity and efficiency of farm animals.
Adult mosquitoes are small, fragile insects with slender bodies; one pair of narrow wings (tiny scales are attached to wing veins); and three pairs of long, slender legs. They vary in length from 4-mm to 1.27-cm. Mosquitoes have an elongate “beak” or piercing proboscis. Eggs are elongate, usually about 0.6-mm long, and dark brown to black near hatching. Larvae or “wigglers” are filter feeders that move with an S-shaped motion. Larvae undergo four growth stages called instars before they moult into the pupa or “tumbler” stage. Pupae are comma-shaped and non feeding and appear to tumble through the water when disturbed.
LIFE CYCLE & HABITS
Mosquitoes may overwinter as eggs, fertilised adult females or larvae. Eggs, larvae, and pupae must have water to develop. Some female mosquitoes lay their eggs directly on the water surface. Others lay their eggs on substrates above the water line (flood pool mosquitoes); the eggs hatch upon flooding. In some cases, the eggs will remain viable for several years until further flooding occurs. Mosquitoes belonging to the genus Culex lay their eggs in bunches or “rafts.” Each raft may contain up to 400 individual eggs. Larvae feed on bits of organic matter dispersed in the water, becoming full grown in about one week. The pupal stage lasts two to three days. Female mosquitoes are ready to bite one to two days after adult emergence. Male mosquitoes do not bite but feed on flower nectar or plant juices. Some mosquitoes have only one generation per year, whereas others may have four or more. Adults may fly 8 to 16-km, but usually rest in grass, shrubbery or other foliage close to the water breeding area.
Mosquitoes may transmit diseases such as dengue, yellow fever, and malaria to humans. Mosquito-borne encephalitis is a viral inflammation of the brain. Encephalitis can infect humans, horses, and a variety of other mammals and birds. Eastern equine encephalomyelitis (EEE), although very rare is frequently fatal. A small rural outbreak in late 1991 resulted in more than 20 farm animal fatalities, most of which were horses. Transmission of the disease occurs when an infected mosquito takes a blood meal. Birds serve as natural hosts for EEE and St. Louis encephalitis (SLE). St. Louis encephalitis, like EEE is an epidemic disease, meaning that it is usually rare. It can be absent from an area for several years and then reoccur suddenly without warning.
Mosquitoes can also transmit filariasis (heartworm) to animals. Dog heartworm is the most significant of these, however in some areas, veterinarians are beginning to see more heartworm in cats.
WHAT DOESN'T WORK
There have been a number of natural and man-made mosquito repellents, attractants, and predators touted as effective against mosquitoes. In truth, they don’t do much good and cannot be used to effectively control mosquitoes.
A company has been marketing a “mosquito repellent plant” that produces citronella and consequently repels mosquitoes. Citronella oil is produced by a number of different plants. At relatively high concentration, Citronella oil is repellent to mosquitoes. Thus far, there does not appear to be adequate scientific literature to substantiate the claim that enough Citronella is released by a stationary plant to repel mosquitoes. Most likely the plant would have to be physically damaged in order to release enough citronella to repel mosquitoes and the effect would be very short lived.
Dietary studies indicate that mosquitoes are insignificant in the purple martin diet. Studies of bat stomach contents show beetles as the dominant food. Ultraviolet or black lights and sonic devices indicate ineffective control.
Since most of the mosquitoes that transmit encephalitis will not travel very far, the risk of contracting encephalitis can be minimised by controlling the mosquito breeding sites which are in close proximity to your home. Water management, to prevent mosquito breeding, is essential for control. Eggs do not hatch unless they are in water. Remove old tires, buckets, tin cans, glass jars, broken toys and other water-catching devices. Change water in bird baths and wading pools once or twice a week; clean out roof gutters holding stagnant water; and place tight covers over cisterns, cesspools, septic tanks, barrels, and tubs where water is stored. Never over-apply lawn and garden irrigation; fill, drain or treat tree holes; and drain or fill stagnant water pools, puddles, ditches, or swampy areas. Inspect water in plant containers, water-holding stumps, keep grass mowed around bodies of water, stock ponds and reservoirs with fish.
Use adequate screens with 16 x 16 or 14 x 18 mesh on windows and doors. Screen doors should open outward and close automatically.
Repellents applied to the skin and clothing will prevent mosquito bites for one to five hours depending on the person, type, and number of mosquitoes and the type and percent of active ingredient in the repellent. Repellents are available as aerosol sprays, pump sprays, creamsticks, lotions, or foams.
N, N-Diethyl-m-toluamide (Deet) is very effective and widely used as a repellent but it should not be used indiscriminately as severe allergies can develop. Formulations containing high concentrations of Deet, 50% or more, should not be used on children. Formulations containing 5 to 10% Deet will work just as well as those containing 90% or more, however, they will not last as long.
Avon Skin-So-Soft has been widely used as a mosquito “repellent” for a number of years without being labelled. Avon Products, Inc. has recently obtained EPA approval and is now marketing some of its Skin-So-Soft products for use as a mosquito repellent.
Space sprays or aerosol “bombs,” containing synergised pyrethrins 0.1%, are effective against adult mosquitoes. Frequent treatments may be needed during problem periods.
Space sprays or aerosol foggers, containing pyrethrins, will give rapid knockdown of adult mosquitoes. However, it is a temporary treatment with little residual effect. Residual sprays applied to tall grasses, weeds, trees, shrubs, and outbuildings, one to two days before use of the area, is effective. Use water solution or emulsions instead of oil-based formulations to prevent plant injury. Some insecticides registered for residual mosquito control include: carbaryl (Sevin), chlorpyrifos (Dursban) and malathion. There are a number of different formulations available. Follow specific label directions when applying.
Note: Malathion and carbaryl (Sevin) are extremely toxic to honey bees. Do not spray plants when in bloom. Mow weedy areas before treatment. Bee losses are minimized by spraying late in the afternoon when bees are gone or when temperatures are below 45 deg F. Malathion and methoxychlor are highly toxic to fish.
Homeowners may apply Mosquito Dunks (made with Bacillus thuringiensis Berliner var. israelensis or B.t.i.) to kill mosquito larvae in the water. This natural ingredient is harmless to other living things and is biodegradable. (Summit Chemical Co. 800-227-8664).
Methoprene (Altosid XR) is another safe material for control of mosquito larvae. It is an insect hormone which retards the development of larvae ( disrupts molting) and prevents mosquitoes from developing into adults (Clarke Mosquito Control Products, Inc. 800-323 -5727).
Altosid XR Briquets can be placed even on ice for season-long control. Treat swamps, ponds, and marsh areas in early spring before thawing. These extended-release briquets will provide up to 150 days of uninterrupted mosquito control once they hit the water. It can be applied by hand and the product is labeled for use in known fish habitats.
William F. Lyon
Keith L. Smith, Associate Vice President for Ag. Adm. and Director, OSU Extension.
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