Ovarian Diapause in Culex Peus (Diptera: Culicidae)1

The effect of photoperiod and temperature on ovarian follicle development and other factors related to hibernation was studied in a colonized population of Culex peus mosquitoes from Philomath, Benton County, Oregon, USA. When females were subjected to a combination of short daily photophases and low temperatures from the time of pupation to 8 days after adult emergence, we observed a retardation of follicular development (ovarian diapause), a reduction in the blood-feeding rate, and the occurrence of hypertrophic fat. Cool temperatures caused a retardation of follicle growth in non-blood-fed females regardless of photoperiod, but only under the influence of both low temperature and short photophase did ovaries remain in a diapause condition for as long as 21 days after adult emergence. Based on experimental exposure to 12 different photoperiods at 18°C, we estimated that 50% of adults would have diapause-state ovaries at a photoperiod of ca. L:D 13:11. Routine sampling of Cx. peus larvae from a log pond from April to November in 1980 and 1981 showed that numbers decreased sharply after mid-September. This coincides well with the occurrence of 13 h of light per day (15 September) at this site. The significance of these results in relation to the geographical range of this and other Culex species is discussed.