Each summer, hordes of these insects descend on backyards, parks and campgrounds. While most mosquitoes are merely nuisances, some can transmit serious diseases such as encephalitis and malaria in humans and heartworm disease in dogs.

What kinds of mosquitoes are common in Illinois?

Two different kinds of mosquitoes plague Illinoisans. Floodwater (temporary pool) mosquitoes deposit their eggs singly in low-lying areas that will be flooded later. Under normal summer temperatures, large numbers of biting mosquitoes will emerge about two weeks after heavy rains and can be a major nuisance problem for several weeks. The most common of these in Illinois is the inland floodwater mosquito. A vicious biter, this mosquito will commonly fly 10 or more miles from where they hatch, particularly along prevailing winds. Floodwater mosquitoes have not been significant disease carriers in Illinois.

Vector mosquitoes carry diseases and lay their eggs in stagnant ditches and sewage treatment ponds or water in treeholes, old tires, clogged gutters, old tin cans and anything else that will hold water. Eggs are laid on or just above the water surface, where they usually hatch within two to three days. Two of the more common vector mosquitoes in Illinois are the Culex, or house mosquito, and the tree-hole mosquito. Neither migrates long distances.

Another disease-carrying mosquito is the Asian tiger mosquito, which arrived in the United States in 1985 in old tires. An aggressive day-biting mosquito, it breeds in large numbers in water-filled artificial containers.

Do all mosquitoes carry disease?

Floodwater mosquitoes are not major vectors of human disease in Illinois, but they do transmit heartworm disease in dogs, as do Culex mosquitoes. The worms live and reproduce in the heart and pulmonary vessels and can severely weaken or kill the dog. Although difficult to treat, the disease is easily prevented by medication prescribed by a veterinarian.

In Illinois, the most common human illness carried by mosquitoes is encephalitis. This inflammation of the brain is caused by viruses and the disease can range from mild to severe. Severe symptoms include rapid onset of severe headaches, high fever and mental disturbances, such as confusion, irritability, tremors, stupor and coma. Severe cases sometimes end in death or with survivors suffering permanent physical and mental disabilities.

Mosquitoes do not carry all types of encephalitis virus, but they do carry at least three that can cause illness. The Culex mosquito, which bites from dusk to dawn, is a vector of St. Louis encephalitis and West Nile virus; it becomes infected by feeding on birds that carry these viruses. St. Louis encephalitis and West Nile virus affect mainly older adults. The tree-hole mosquito, which bites during the day, is the main vector of California (LaCrosse) encephalitis in Illinois. The virus infects chipmunks, squirrels and other small woodland animals; in humans, it affects mainly children. The Asian tiger mosquito transmits dengue fever in other parts of the world and could become involved in the California encephalitis cycle in Illinois.

The last major nationwide epidemic of mosquito-borne encephalitis occurred in 1975. In Illinois, 578 cases of St. Louis encephalitis, which resulted in 47 deaths, and 23 cases of California encephalitis were identified during that year.

When was West Nile virus found in Illinois?

West Nile virus was first identified in September 2001 when laboratory tests confirmed its presence in two dead crows found in the Chicago area. The following year, the state's first human cases and deaths from West Nile disease were recorded and all but two of the state's 102 counties eventually reported a West Nile positive, human, bird, mosquito or horse. By the end of 2002, Illinois had counted more human cases (884) and deaths (66) than any other state in the nation. In 2003, the number of human cases fell to 54 and only one death, and West Nile activity was reported in 77 counties.

What is the best way to reduce populations of mosquitoes?

The first and best defense against these pests and the illnesses they may carry is to eliminate the places where they breed. Here are a few suggestions:

• Remove or empty water in old tires, tin cans, buckets, drums, bottles or other places where mosquitoes might breed. Be sure to check clogged gutters and flat roofs that may have poor drainage. Make sure cisterns, cesspools, septic tanks, fire barrels, rain barrels and trash containers are covered tightly with a lid or with 16-mesh screen.
• Empty plastic wading pools at least once a week and store indoors when not in use. Unused swimming pools should be covered or drained during the mosquito season. (Note: If you choose to drain your pool, be sure the hydrostatic relief valve is open in order to keep it from floating out of the ground if the water table rises.)
• Change the water in bird baths and plant pots or drip trays at least once each week.
• Store boats covered or upside down, or remove rainwater weekly.
• Empty your pet's water bowl daily.
• Level the ground around your home so water can run off and not collect in low spots. Fill in holes or depressions near your home that accumulate water.
• Fill in tree rot holes and hollow stumps that hold water.
• If you have an ornamental water garden, stock it with mosquito-eating fish (e.g., minnows, "mosquito fish," or goldfish). They eat mosquito larvae.
• Keep weeds and tall grass cut short; adult mosquitoes look for these shady places to rest during the hot daylight hours.
• Use a flyswatter or household spray to kill mosquitoes, flies or other insects that get into buildings. Spray shrubbery and high weeds to kill adult insects. (Check the insecticide label to make the sure the spray will not damage flowers or ornamental plants.)
• Small impoundments of water can be treated for mosquito larvae with "Bti," a bacterial insecticide. Many hardware stores carry doughnut-shaped Bti briquets (Mosquito Dunks R) for this purpose. Be sure to follow the insecticide label directions exactly.
• Some mosquito control methods are not very effective. Bug zappers are not effective in controlling biting mosquitoes. Various birds and bats will eat mosquitoes, but there is little scientific evidence that this reduces mosquitoes around homes.
• Community-wide mosquito abatement efforts can be quite effective if they are conducted as part of an integrated pest management program. This includes monitoring and draining or treating areas where mosquitoes breed — such as street catch basins, occasionally flooded marshes, river backwater areas, swamps and other low-lying areas.

How can people protect themselves from mosquito bites?

• Avoid places and times when mosquitoes bite. Generally, the peak biting periods occur just before and after sunset and again just before dawn. Each species, however, has its own peak period of biting. Tree-hole and Asian tiger mosquitoes, for example, feed during daylight hours in or near shaded or wooded areas.
• Be sure door and window screens are tight-fitting and in good repair.
• Wear appropriate clothing. Long-sleeved tops and long pants made of tightly woven materials keep mosquitoes away from the skin. Be sure, too, that your clothing is light colored. Keep trouser legs tucked into boots or socks.
• Use mosquito netting when sleeping outdoors or in an unscreened structure and to protect small babies any time.
• When it is necessary to be outdoors, apply insect repellent as indicated on the repellent label. The more DEET a product contains, the longer the repellant can protect against mosquito bites. However, concentrations higher than 50 percent do not increase the length of protection. For most situations, 10 percent to 25 percent DEET is adequate. Apply repellents to clothes whenever possible; apply sparingly to exposed skin if label permits. Consult a physician before using repellents on young children.