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<br />The types of "off-field" habitats just described accounted for <br />more than one-fourth of the total mosquito production on a S;OOO-acn <br />study area in the Milk River Valley of northern Montana. In the <br />North Platte Valley of western Nebraska, wasteland areas, roadside <br />ditches, borrow pits and other undrained areas flooded by runoff <br />from irrigated fields and overflow from conveyance systems were <br />found to be important sources of both ~. tarsalis and Aedes <br />mosquitoes. Playas or land-locked depressions, flooded by runoff <br />from irrigated fields, were responsible for three-fourths of all <br />mosquito production on study areas in the Texas High Plains. These <br />'playas produced large numbers of ~. tarsalis and several other species <br />of irrigation mosquitoes. In certain western areas, large nonirrlg- <br />able areas are wildly flooded with excess irrigation water for use <br />by ducks and other waterfowl. Such areas are highly favorable for <br />the production of tremendous numbers of Aedes mosquitoes. Several <br />of the abatement districts in California and Utah spend large SUMS <br />each year to control mosquitoes produced on waterfowl areas of this <br />type. <br /> <br />Mosquito sources are often associated with the main irrigation <br />conveyance systems, portions of which are sometimes located on <br />nonirrigable lands. A large amount of irrigation water is often <br />lost by seepage and leakage through the beds and banks of unlined <br />canals and laterals which are located in pervious material. In <br />areaS having poor underdrainage, these seepage losses alone or in <br />combination with deep percolation from irrigated fields often result <br />in marshes and ponded water in roadside ditches, borrow pits, and <br />other areas. In many western irrigated areas, aquatic habitats <br />created by seepage produce large numbers of mosquitoes. Permanent <br />and semipermanent seepage areas have been found to be especially <br />favorable for the production of ~. tarsalis. a vector of encepha- <br />litis. Large numbers of Aedes mosquitoes are also produced in <br />seepage areas that have fluctuating water levels. <br /> <br />Mosquito production may also occur in various other "off-field" <br />aquatic habitats associated with the main irrigation conveyance <br />systems. These mosquito sources include flooded areas resulting <br />from overflow of poorly maintained canals and laterals,ponded areas <br />where conveyance channels block natural drainageways; residual <br />water in turnouts, stilling basins, and other hydraulic structures; <br />surface pools created by leakage from water control structures, <br />and residual water in conveyance channels during nonoperational <br />periods. Although these types of aquatic habitat contribute to <br />the total mosquito problem, they are generally of less importance <br />than those caused by seepaae and the nUlllerous other "off-field" <br />mosquito sources. <br /> <br />D-J <br />