Abstract
In 1970 we selected 9 spots, i.e. 2 spots in the northern part, 1 in the middle part, and 6 in the southern part of Okayama Prefecture. The day-light type light-traps were settled from 30 minutes before the sunset till the following morning, twice a week, from the beginning of April (except for 4 spots in the southern and 1 in the northern parts where the day-liht type light-traps were settled from June) till the end of September. Comparing to the whole number of collected mosquitoes last year, the mosquitoes are considerably decreasing in number this year with a few exceptions. In Tsuyama, the northern part, there is an increase of 56%, 2562 (1969) to 4002 (1970). In the southern part there is a little increase at the two spots, 284 to 361 at Hama in Kurashiki and 1967 to 2197 at Okayama Prefectural Health Center.
The thmperature has exceeded 25°C from July 18 to September 13. Only during this term Culex tritaeniorhynchus (C. t.) appeared and the C. t. concentration period in every part was during the time between the end of July and the beginning of August. The number of C. t. has been decreasing these few years, and Japanese B encephalitis patients only counted 2 this year. There is a correspondence of the decreasing number of C. t. to that of patients.
