Insecticidal resistance of Culex tritaeniorhynchus in Chinen, Okinawa Prefecture, with special reference to the mechanism of pyrethroid-resistance

Abstract

Culex tritaeniorhynchus mosquitoes collected at Chinen Village, Okinawa, in 1987 and colonized in the laboratory were tested for resistance to insecticides. High resistance levels of the Chinen strain to organophosphorus (OP) and carbamate insecticides were similar to those of 16 samples in the 1984 country-wide survey in which no material from Okinawa was included. Extremely high levels of resistance to malathion and temefos were observed in the Chinen strain; the resistance ratio in larvae was about 10,000 for malathion and >125,000 for temefos. However, the Chinen strain was less resistant to fenitrothion, fenthion, and carbamates than the mosquitoes tested in 1984. The electrophoretic analysis indicated that the Chinen strain showed higher activity of carboxylesterases which hydrolyze β-naphthyl acetate than in the susceptible (Taiwan) strain. Therefore, the enhanced carboxylesterase activity was estimated to be a mechanism of OP resistance in the Chinen strain, although the role of acetyl-cholinesterases has not been analyzed. The Chinen strain was also resistant to pyrethroids with the resistance ratios of 50-1,000 for larvae. This finding was contrary to the results in the 1984 survey. When piperonyl butoxide was applied, LC_<50>'s of permethrin and cypermethrin for Chinen larvae were decreased by 1/3 and 1/23,respectively, but a considerable difference between LC_<50>'s for Chinen and Taiwan larvae remained in each of these pyrethroids. This indicates mfo's are not major factors regulating pyrethroid resistance in the Chinen strain. Application of an inhibitor of DDT-dehydrochlorinase (DMC) and an inhibitor of mfo's (sesamex) did not affect the LC_<50> of DDT for Chinen larvae. Experimental crosses of the Chinen strain with the susceptible (Taiwan and e ma) strains showed that pyrethroid resistance of the Chinen strain is regulated by genetically recessive factor(s). This indicates that kdr-like factor(s) might participate in resistance of this strain to pyrethroids and DDT.