Population Ecology of Preimaginal Culex tarsalis (Diptera: Culicidae) in Kern County, California

Abstract

The effects of abiotic and biotic mortality factors on preimaginal survivorship and the production of adults were investigated for populations of Culex tarsalis Coquillett at a stable foothill breeding site during 1985 and at seven ephemeral breeding sites during 1986. Female abundance was correlated temporally with larval abundance and spatially with first instar to adult survivorship. Catastrophic events, such as fluctuations in water level and pollution, drastically reduced larval and pupal abundance. Under relatively stable conditions, the effect of natural mortality factors was estimated by comparing preimaginal survivorship rates, estimated horizontally for fed and unfed cohorts in predator exclusion cages, with survivorship, estimated vertically from the age structure of the natural population using a life table approach. Mortality for fed cohorts ranged from 8.5 to 90.0% and was attributed to abiotic factors, including temperature and water quality. Mortality caused by lack of food was comparatively low (< 15%); however, the low nutrient level at three of six sites lengthened the duration of the fourth stadium, delayed pupation, and resulted in the emergence of small-sized adults with a reduced expression of autogeny. Predation mortality ranged from 3.7 to 84.5% and was the most important cause of larval death at five of six study sites. Based on relative abundance and correlation over time and space, zygopteran naiads were the most important predator at a stable foothill breeding site during 1985, and coleopteran larvae were the most important predator at ephemeral breeding sites sampled during 1986.