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<br />showed that large numbers of both ~. tarsal is and Aedes mosquitoes <br />are produced in ponded areas on irrigated pastures and hay meadows <br />along the North Platte River. The use of irrigated mountain meadows <br />for the production of hay and supplemental grazing of livestock is <br />widespread in the Rocky Mountain States. In many of the IIlOUntain <br />valleys, the bottomland meadows are flooded almost continuously <br />throughout the spring and early summer months. Ponding also occurs <br />in numerous low areas following irrigation later in the season. The <br />extensive aquatic habitats that occur on many irrigated mountain <br />meadows produce tremendous numbers of encephalitis and other irriga-' <br />tion mosquitoes. <br /> <br />In addition to the depressions on irrigated fields, water some- <br />times remains ponded in the field laterala and draiUll long enough to <br />produce encephalitis and other irrigation mosquitoes. This type of <br />mosquito source occurs most often in areas having fine textured soils <br />and poor underdrainage. Field laterals and drains that contain <br />excessive amounts of vegetation are more favorable for mosquito <br />production. Mosquitoes may also be produced in the residual water <br />in various hydraulic structures, such as drops in ditches, particu- <br />larly when they contain tumbleweeds and other debris. Mosquito <br />production in hydraulic structures is usually of minor importance in <br />compariSon with that whicb occurs in other types of "on-field" aquatic <br />habitats. <br /> <br />Rice fields constitute a special type of "on-field" mosquito <br />source, since the flooding used for growing rice is very favorable <br />for mosquito production. In rice fields of the Mississippi Delta <br />and Gulf Coast areas, large numbers of Psorophora confinnis and <br />Psorophora discolor are produced by intermittent irrigation; and <br />Anopheles quadrimaculatus. the malaria mosquito, is produced in the <br />constantly flooded rice fields. Rice fields in California are <br />usually flooded throughout the growing season and they provide <br />favorable aquatic habitats for production of Culex tarsalis. tbe <br />encephalitis mosquito, and Anopheles freeborni. the western malaria <br />mosquito. Large numbers of ~. ni~romaculis and ~ . dorsalis mosquitoes <br />are frequently produced by the initial flooding of rice fields in <br />California. ' <br /> <br />Off-Field Mosquito Sources <br /> <br />In many western irrigated areas, widespread and prolific mosquito <br />production occurs in a variety of aquatic habitats located outside the <br />boundaries of irrigated fields. These ~squito sources include roadside <br />ditches, borrow pits, depressions on nonirrigable land, and numerous <br />other undrained areas ~ are flooded by surface runoff from irrigated <br />fields and other excess irrigation water. Such aquatic habitats often <br />contain dense vegetation which makes them more favorable for mosquito <br />production. The semipermanent and permanent water areas are highly <br />favorable for ~. tarsal is. the encephalitis mosquito. Those with <br />fluctuating water levels produce large numbers of this mosquito as <br />well as several vicious-biting Aedes species. <br /> <br />D-2 <br />