Abstract
The mode of inheritance of malathion resistance among larvae of Culex pipiens fatigans was studied. A homozygous susceptible colony (SS) was separated from a mixed field population by the same method as adopted in the separation of dieldrin- or DDT-susceptible colonies. This method is as follows : Larvae from an egg-raft are reared in a vessel. Twenty or thirty of 3 rd-instar larvae from each egg-raft are exposed to a dipping test. If higher mortality is evident, the remaining larvae were pooled to produce the next generation. After repeating the procedure for 8 generations, a homozygous susceptible colony was established. The homozygous resistant colony (RR) was obtained by selection with the pressure of malathion. The ld-p line (log dosage-mortality regression line) of the hybrids (F1) after crossing the susceptible colony with the resistant one was straight and was located intermediate to the RR and SS lines as shown in Fig. 3. The progeny of the back cross of F1 with SS (P) showed a curved ld-p line with one plateau at about 50% mortality, and of F1×RR (P), an inflection point at about 50% mortality as shown in Fig. 5. F2 obtained from mass-mating of F1 showed a curved ld-p line with one plateau at about 25% mortality and with one inflection point at about 75% mortality as shown in Fig. 4. In view of the results mentioned above, the authors were led to the conclusion that the mode of inheritance of malathion-resistance in C. p. fatigans was autosomal and monofactorial, and that the resistant factor was partially dominant to the susceptible one. By drawing an ld-p line of malathion on logprobability paper using the larvae of any population and by determining the plateau in the ld-p curved line, the configuration ratio of SS : (RS+RR) in the population might be estimated easily.
