Overwintering Studies on Culex tarsalis (Diptera: Culicidae) in Kern County, California: Survival and the Experimental Induction and Termination of Diapause

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Abstract
More than 90% of female Culex tarsalis Coquillett collected from shelters during late autumn survived without blood or carbohydrate feeding for 35 days in a bioenvironmental chamber (BOD) set at 9°C and 10:14 (L:D) photoperiod. Mortality was reduced subsequently by offering females 10% sucrose. Ten females imbibed a blood meal and oviposited after transfer to an insectary (25 ± 2°C, 16:8 photoperiod), 106–132 days after collection. Three laboratory strains of C. tarsalis from California and Arizona failed to enter diapause uniformly when reared under simulated midwinter or autumnal conditions in a BOD. Vitellogenesis progressed to or beyond follicular stage I-II in 25-80% of the dissected females. Only 0-60% of females dissected after a 2-week postemergence period were inseminated. Although females imbibed a blood meal and oviposited when exposed to insectary conditions after a simulated winter period, fecundity and, especially, fertility were low, reflecting the low insemination rate. Thus, diapause would not be an efficient addition to Culex mosquito insectary culture methodology. Mosquitoes from a laboratory colony and the F1 progeny of field-collected females originating from the same site were reared and maintained as adults under natural, seminatural, and experimental diapause induction regimens. A higher proportion of the F1 progeny of field-collected females entered diapause than did laboratory-selected females. Some females in all regimens failed to respond to diapause induction cues and arrest follicular maturation at stage I. Thus, field populations from Kern County, Calif., responded heterogeneously to diapause induction cues and the genome that failed to enter diapause was selected by laboratory colonization.