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 equine encephalomyelitis (WEE) and St. Louis encephalitis (SLE) viruses was evaluated during 6 field and 4 laboratory experiments. Immatures of the Bakersfield Field Station laboratory strain and the F1 progeny of field-collected females were reared in the field or laboratory and then infected by feeding on pledgets, after which remnants (head, thorax, abdomen), legs, and salivary secretions were tested for WEE or SLE virus to estimate infection, dissemination, and transmission rates, respectively. Although the salt content of the 6 larval habitats varied markedly (range, alkalinity 160-1,310 ppm CaCO3, conductivity 460-7,600 micromhos/cm, chlorides 22-1,560 ppm) and significantly altered immature survival, development time, and female body size (wing length), consistent changes in infection, dissemination, or transmission rates were not observed. Susceptibility (ID50) to WEE virus in field strains decreased as a linear function of developmental time, with populations from a dry alkali lake bed (Goose Lake) least susceptible. Three subsequent laboratory experiments that tested the effects of rearing immatures in dilution series of water from Goose Lake failed to produce consistent within or among experiment patterns in vector competence. A 4th laboratory experiment tested changes in NaCl concentration with negative results. Changes in female size was not related to vector competence. These and previous temperature studies indicated that temporal changes in vector competence observed within and among field populations probably were related to intrinsic genetic factors and were not related directly to extrinsic factors in the immature aquatic environment.