Seasonal changes in population structure of Culex pipiens quinquefasciatus Say (Diptera: Culicidae): study of an isolated population.

In order to study population growth characteristics, an isolated population of Culex pipiens quinquefasciatus was established and maintained over a 2-1/2 year study period near Houston, Texas. A1J life stages of C. p. quinquefasciatus were present throughout the year. Despite severe cold weather with several snowfalls, egg production was maintained by the slow but constant emergence of adults from the immature population. A circannual (about yearly) rhythm in sex ratio was detected such that during colder months of the year females predominated; during warmer months males were most abundant. Circannual variation was also found for the population where build-up in the spring was aided by an increased number of eggs per raft. This might have resulted from increased feeding on readily available nestling birds. The population reached higher levels about the same time as the seasonal peak of high temperature. The relationship of all immature stages to the egg stage shows a positive correlation, as does the daily low temperature (the most significant of all weather phenomena tested). The mortality rate of the population was determined from cohorts initiated several times during the year. The survivorship to the adult stage reached a peak of 78% in July and dropped to a low of 15% in winter. The greatest mortality was experienced in the 4th instar and pupal stages. Crowding seemed to have a greater effect on these later stages, as revealed in the comparative studies between the tank population and cohorts. The duration of each larval stage was determined throughout each season. Ten days is the shortest time recorded from egg to adult (in August), as contrasted with a mean of 48 days for cohorts initiated in January. The combined effect of prolonged development and a preponderance of females in winter probably enhances the potential of C. p. quinquefasciatus to survive the winter of the subtropics.