Variations in the susceptiblility of field and laboratory populations of Culex tarsalis to experimental infection with western equine encephalomyelitis virus.

Four field populations and four laboratory colonies of Culex tarsalis from California were evaluated for their susceptibility to infection with western equine encephalomyelitis (WEE) virus by intrathoracic inoculation and by feeding on pledgets soaked with a virus-blood-sucrose mixture. All mosquito strains were uniformly susceptible to infection by intrathoracic inoculation, but 50% infective doses for field and colonized strains varied as much as 1000- to 1500-fold, respectively, by pledget feeding. Moreover, dose-response curves obtained by pledget feeding revealed that some field and laboratory strains of C. tarsalis were quite heterogeneous for susceptibility since some individual females could be infected after ingestion of small concentrations of virus and others could not 0e infected by increasing the concentration of virus by 1000- to 10,000-fold. Variability in viral susceptibility of different mosquito strains could not be correlated with differences in autogeny rates or organophosphorus insecticede resistance.