The Winter Biology of Culex tarsalis (Diptera: Culicidae) in Kern County, California1

Abstract

Adult Culex tarsalis Coquillett were collected each week for more than 5 years in the vicinity of Bakersfield, California. In females, blood-engorgement and ovarian development were greatly diminished in the autumn and early winter of each year only to be resumed in January. The cessation of blood-feeding each autumn is interpreted as a diapause probably initiated by the short autumn photoperiod and terminated by a cumulative heat budget. Emergence of adults from November to late March was minimal. The great majority of winter females were fertile, so mating preceded overwintering. Birds were apparently almost the only hosts for blood-feeding in late winter and early spring, since nucleated erythrocytes were found in nearly all blood-engorged specimens at this season. Much importance attaches to the question as to whether or not the large population of overwintering female C. tarsalis includes appreciable numbers of individuals that have taken a blood-meal prior to overwintering, since such specimens might serve as an overwinter vehicle of encephalitis viruses if they occur with frequency in the natural population. Experimental data show that specimens may take a blood-meal, mature and deposit ova, and then survive through the winter, but observations of field populations do not indicate that this is a common event.