IRRIGATION-WATER MOSQUITOES

Background shows irrigated area in which low spots in fields and pastures and seepage from ditches combine to create a mosquito problem (x). A. Life stages of Aedes dorsalis. a typical mosquito that breeds in irrigation water; a. egg, which is laid on dry land in areas subject to periodic flooding; b. adult female; c, young larva; d, full grown larva e, pupa. (Life stages all about five times natural size.

IRRIGATION-WATER MOSQUITOES

Several species of mosquitoes breed in irrigation ditches, rice fields, and pastures. On thousands of acres in the West that have become valuable because of irrigation, mosquitoes reproduce so abundantly as to be a menace to man and beast. Beef cattle held on summer pasture lose weight, milk flow drops, a serious labor turn-over occurs during haying and fruit harvesting operations, and frequently otherwise desirable property loses value.

The irrigation-water mosquitoes over-winter in the egg stage. Eggs are laid singly on the damp soil in late summer and hatch the following summer when covered with the irrigation water. The number of eggs per acre may be as high as 20 million. The larvae, or wrigglers, reach the pupal stage and emerge as adults in from 5 days to 2 weeks, according to species and weather. One to eight broods may occur during the season or after each flooding. Little is known about their flight habits, but they have been observed several miles from their breeding grounds.

Control of larvae: Because the wrigglers cannot live without water, the simplest way to keep them from developing into adults is to have the water soak into the fields or drain off before the wrigglers have had enough time to develop. The problem of furnishing an adequate supply of water to the fields and drying them up within a week's time is therefore the important consideration in the reduction of the pest about irrigated fields. The border method solves the problem and is economical of water.

If larvicides can be used, DDT formulations applied at the rate of 0.02 to 0.4 pound of DDT per acre and toxaphene or TDE applied at the same rate destroy wrigglers.

Control of adults: Adult mosquitoes that have migrated over large areas may be killed economically by spraying DDT from ground equipment or aircraft. A spray made of 1 pound of 50 percent DDT wettable powder in 2 gallons of water can be applied with hand-operated garden-type sprayers to shrubbery around homes. The use of a 5 percent DDT oil solution in a fine spray will give relief to fruit pickers and lumber crews and in recreational areas for several hours.

Repellents: The best materials for use on the skin and on clothing for repelling mosquitoes and other biting flies are dimethyl phthalate, dimethyl carbate, Indalone, and Rutgers 612. The materials may be used alone or in combination. They are safe to use and are effective for several hours.

Do not apply over-dosages of DDT, TDE, or toxaphene in places where fish and animals are present. Do not pasture milk cows on treated fields for about a month after treatment. Use repellents with caution near synthetic cloth and plastic watch crystals, since they are solvents of plastics.