EFFECTS OF WATER-QUALITY ON THE VECTOR COMPETENCE OF CULEX TARSALIS (DIPTERA, CULICIDAE) FOR WESTERN EQUINE ENCEPHALOMYELITIS (TOGAVIRIDAE)AND ST.-LOUIS-ENCEPHALITIS (FLAVIVIRIDAE) VIRUSES

The effects of water quality during immature development on the vector competence of adult female Culex tarsalis Coquillett for western equi ne encephalomyelitis (WEE) and St. Louis encephalitis (SLE) viruses wa s evaluated during 6 field and 4 laboratory experiments. Immatures of the Bakersfield Field Station laboratory strain and the F-1 progeny of field-collected females were reared in the field or laboratory and th en infected by feeding on pledgets, after which remnants (head, thorax , abdomen), legs, and salivary secretions were tested for WEE or SLE v irus to estimate infection, dissemination, and transmission rates, res pectively. Although the salt content of the 6 larval habitats varied m arkedly (range, alkalinity 160-1,310 ppm CaCO3, conductivity 460-7,600 mu mhos/cm, chlorides 22-1,560 ppm) and significantly altered immatur e survival, development time, and female body size (wing length), cons istent changes in infection, dissemination, or transmission rates were not observed. Susceptibility (ID50) to WEE virus in field strains dec reased as a linear function of developmental timer with populations fr om a dry alkali lake bed (Goose Lake) least susceptible. Three subsequ ent laboratory experiments that tested the effects of rearing immature s in dilution series of water from Goose Lake failed to produce consis tent within or among experiment patterns in vector competence. A 4th l aboratory experiment tested changes in NaCl concentration with negativ e results. Changes in female size was not related to vector competence . These and previous temperature studies indicated that temporal chang es in vector competence observed within and among field populations pr obably were related to intrinsic genetic factors and were not related directly to extrinsic factors in the immature aquatic environment.