Population dynamics of Culex tritaeniorhynchus in relation to the epidemics of Japanese encephalitis in Kochi Prefecture, Japan

Abstract

Mosquitoes were collected by light traps at a pigstay near paddy fields in Kochi, Japan, during 9 years from 1966 to 1974 and 8 years from 1982 to 1989. Major species were Culex tritaeniorhynchus (77.6%), Anopheles sinensis (7.0 %) and Cx. pipiens pallens (15.4%) in 1966. However, the population size was gradually reduced especially in latter two species in recent 1980s. Cx. tritaeniorhynchus was caught through the end of March to mid-October, and it showed a peak of occurrence in June and July in 1966 and subsequent several years. Six larval generations were estimated in a year. A high degree of correlation was observed between the number of Cx. tritaeniorhynchus collected from May to September during 1966 and 1973 and the total area of rice fields or the proportion of two-crop fields in Kacho Plain in this period. There was, also, a high correlation coefficient (r=0.949) between the number of vector mosquitoes collected and the number of Japanese encephalitis patients in Kochi Prefecture during the same period. The number of vector mosquitoes caught by light traps decreased abruptly from the end of August even when the temperature is around 25.degree.C. This phenomenon was partly explained from the results of laboratory experiments in which phototaxis and blood feeding behavior of adult females were suppressed when the larvae were bred under day length of 10 hr or less at 20 or 25.degree.C. The vector mosquito was not found in caves and other places in Kochi Prefecture throughout the winter surveys. Japanese encephalitis virus was not isolated from three other species than Cx. tritaeniorhynchus collected in winter.