Abstract
The experiments on the separation of a dieldrinsusceptible and -resistant colony from a field population of Culex pipiens larvae were conducted using seven populations collected in Japan. The separating methods adopted here were as follows : For the separation of the susceptible colony, the larvae from an egg-raft were reared to 3rd or 4th stage, then some of them were tested by the discriminating dosage, in most cases 0.02ppm. The batch of the larvae some of which showed higher mortality were pooled in a bat and the next generation was obtained. This was repeated for three to seven generations. For the separation of the resistant colony, similar method was adopted using the discriminating dosage of 0.3ppm and the discriminating mortality of over 80% to 100%. The usual method of the selection by the pressure of dieldrin was also adopted for this purpose. After the results, the separation of the susceptible colony succeeded in Koniya and Furuichiba populations, and that of resistant colony in Koniya, Furuichiba, Denken and Kawasaki populations. The ld-p lines of the separated colonies showed straight lines and these were in good accord with the estimated ld-p lines by "ld-p line tests" shown by Suzuki et al. (1964). The trial of re-separation of the separated susceptible colony with the discriminating dosage of 0.004ppm ended in failure which proved that the colony obtained here was the most susceptible colony, and more susceptible colony could not exist. The LC-50 of dieldrin to the susceptible and resistant colony obtained here were 0.004-0.006ppm and 0.5-0.8ppm, respectively.
