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Resting Culex nigripalpus Theobald were collected with a ground aspirator three times a week in Indian River County, Fla., from January 1985 through December 1987. The number of green (newly emerged), empty, blood-fed, and gravid females in each collection was counted; up to 100 empty females were dissected to estimate the abundance of empty, parous females in the population. In general, this species was uncommon during the dry season from January through June but increased during the wet season in July and remained abundant through early winter. During the wet season, green, parous, and gravid females were periodically abundant. Time series analysis demonstrated that green and parous mosquitoes were positively associated through time and that gravid females were associated negatively with daily rainfall. The strongest correlations were recorded in 1985, a year during which St. Louis encephalitis virus was detected with increased frequency in Indian River County. Laboratory experiments in a large outdoor wind tunnel confirmed that high relative humidity was important for the initiation and successful completion of oviposition by gravid, field-collected Cx. nigripalpus mosquitoes. Thus, intermittent rainfall during autumn may serve to delay oviposition, increase longevity, synchronize blood feeding, and enhance transmission of St. Louis encephalitis virus by infected mosquitoes in the field.

In the laboratory, 8.5% and 5.1% of colonized Culex annulirostris from Brisbane and Mildura, Australia respectively were autogenous when reared and maintained on nutrient rich diets. Females reared and/or maintained on poor diets mainly had ovaries at Christophers' stage I and exhibited from 0 to 0.7% autogeny. All autogenous females had previously mated. Insemination rates in the Brisbane and Mildura colonies respectively, were 72.8% and 78.8%. No autogeny was detected in 997 females reared from 7 localities throughout Queensland but this may have been due to their poorer nutritional status (as indicated by wing size) or more likely to a low insemination rate of 0 to 16%. Our laboratory results, particularly with well-fed females, may have little relevance to the field situation, where adults are generally smaller and less well-nourished.

Culex annulirostris Skuse larvae were reared on two different amounts of powdered dog chow and yeast, low intake (2.8 micrograms/larva) and high intake (8.4 micrograms/larva), to produce adults with mean wing lengths of 3.25 and 3.7 mm, respectively. The resulting adults were fed different dosages of Murray Valley encephalitis virus and after 10 d extrinsic incubation at 28 degrees C, the infection rate, transmission rate, salivary gland, and body titers were determined. There were no significant differences in any of these parameters.

Mice infected with Plasmodium berghei, P. chabaudi, or P. yoelii became lethargic and ceased to display normal antimosquito behavior. Periods of reduced defensiveness corresponded with maximum mosquito engorgement and with periods of maximum gametocyte infectivity to mosquitoes. Increased feeding success of mosquitoes during periods of peak gametocyte infectivity may be important to the natural maintenance of these malaria parasites.

Culex nigripalpus females were given double and, in 2 cases, triple (interrupted) bloodmeals separated by various intervals. Mosquitoes given single meals served as controls. Using Azan-stained serial paraffin sections, we could identify multiple meals separated by 1-72 hr in 44 of 53 cases (84.6%). Among the histological parameters of importance in the identification of double and triple bloodmeals are the peritrophic membrane secreted around each bloodmeal, the plug which forms between the anterior and posterior midgut, the layer of heme which forms as a bloodmeal is digested, and remnants of the pupal meconium and/or pupal-pharate adult peritrophic membranes. The parameters we have identified will help determine the incidence of double bloodfeeding in wild populations which, in turn, should enhance our understanding of the transmission of pathogens by mosquitoes.

Blood-engorged Culex nigripalpus (Theob.) collected throughout the year in two Florida localities were serologically tested to determine the host range of this mosquito in nature. A proportional increase in feeding on mammals occurred in early summer, reached a maximum between July and October, and was followed by a shift back to avian hosts which dominated the feeding pattern during winter and spring. This finding strengthens the hypothesis that a biphasic pattern of feeding is a basic characteristic of an enzootic vector that, in epidemic years, also serves as the primary vector of avian arbovirus to man.