The Impact of Aerial Applications of Ultra-Low Volume Adulticides on Culex Tarsalis Populations (Diptera: Culicidae) in Kern County, California, USA, 1982

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Abstract
Three ultra-low volume (ULV) adulticide sprays applied at 3-day intervals failed to alter the relative abundance or age structure of Culex tarsalis populations sufficiently to interrupt virus transmission in Kern County, California, USA. Adulticides were applied by single-engine, fixed-wing aircraft during late afternoon. Mortality among caged sentinels indicated acceptable coverage of the 260–365-ha study areas. Laboratory susceptibility tests, field-cage exposures, and mark-recapture studies indicated that a foothill Cx. tarsalis population was susceptible to chlorpyrifos, while a valley population from an agricultural area was partially resistant to chlorpyrifos and resmethrin. Relative abundance of Cx. tarsalis, Aedes melanimon, and Cx. quinquefasciatus at CDC light traps baited with dry ice and located in the center of the spray zone decreased during all adulticide applications when compared to unsprayed comparison traps. However, suppression was transient and Cx. tarsalis abundance never declined below the minimum threshold presumed to be necessary for the maintenance of arbovirus transmission. The minimum infection rates for Hart Park and Turlock viruses in female Cx. tarsalis did not change during chlorpyrifos applications in foothill and valley study areas. However, the minimum infection rate of western equine encephalomyelitis virus in female Cx. tarsalis declined concurrently with resmethrin adulticiding at a valley study area.