Forest Preserve District of Cook County, Illinois
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Mosquito Control in the Forest Preserves
Nature Bulletin No. 454-A   April 22, 1972
 Forest Preserve District of Cook County 
George W. Dunne, President
Roland F. Eisenbeis, Supt. of Conservation

Mosquitoes, although not a menace here, are a very real nuisance in Cook County. The bloodthirsty little pirates pester picnickers, hikers, fishermen, and nature lovers in our preserves. In many suburban areas, after sundown, people gardening or sitting on their lawns are driven indoors.

There are four mosquito abatement districts, created by vote of the people, which include large sections of the preserves along the DesPlaines River and Salt Creek valleys, the northwestern and southern parts of Cook County, and the North Branch of the Chicago River. The problem is to determine what successful methods of mosquito control may be employed without causing appreciable damage to the wildlife, the vegetation, and the scenic beauties of the Forest Preserve District .

There are more than 20 species of mosquitoes very common in this locality and they may be classified in groups according to their breeding habits. Most numerous are the kinds called "domestic" mosquitoes because the adults hibernate during winter in basements and other protected hide-outs and sally forth in spring to lay their eggs in every conceivable spot where water collects. Their principal breeding places are in polluted or stagnant waters; catchbasins, basement floor drains, cisterns, clogged roof gutters, old tin cans, lily ponds and, of course, municipal dumps. They are controlled by spraying such spots with powerful insecticides.

The "woodland" mosquitoes, all large and showy, include one species that lays its eggs in water collected in hollow trees or stumps, and some others that hatch out, in spring, from eggs laid the previous autumn in the little depressions so numerous on our forest floors. Control of these mosquitoes by drainage or by spraying is obviously unfeasible. Fortunately, they have a short night range and are not a nuisance in most residential areas.

There are several kinds of marsh or "floodwater" mosquitoes which lay their eggs in bottomlands and temporary marshes. These have a long flight range and, aided by wind, may travel as much as 15 miles. A fourth group includes several species which breed in polluted streams, lakes, ponds and sloughs. They, too, have a long flight range and are a serious nuisance.

The permanent lakes, ponds and sloughs in our forest preserves normally support enough small fish and predatory insects to destroy most mosquito larvae. But the shallow semi-permanent marshes, which dry up after rainy seasons, can be prolific breeding places of mosquitoes. In cooperation with the abatement districts, a number of these have been converted into what have been widely publicized as "wildlife oases" -- permanent sloughs, each with a deep central pool, stocked with fish and the little Gambusia or mosquito-fish. These no longer produce swarms of mosquitoes; they add to the scenic beauty of the preserves; and they attract many kinds of wildlife: ducks, wading birds, songbirds, muskrats, mink, raccoon and other animals.

The South Cook County Mosquito Abatement District, in close cooperation with the Forest Preserve District, experimented with other mosquito control methods -- such as the dusting of test areas, in early spring, -- in order to determine their effect upon beneficial insects, aquatic life, and the many interrelationships of animals and plants which are so essential to the health and naturalness of the preserves.

Nature's own controls are being sought and will be used wherever possible.

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