Insecticidal Resistance of Culex tritaeniorhynchus (Diptera: Culicidae) in Japan: A Country-wide Survey of Resistance to Insecticides

Abstract

Sixteen samples of Culex tritaeniorhynchus Giles collected in 14 prefectures in Japan in 1984 were assessed for resistance to insecticides. All of the samples were highly resistant to organophosphorus (OP) and carbamate insecticides, but none was resistant to a pyrethroid (permethrin). Resistance levels for each of the OP's and carbamates were similar among samples except for diazinon and temephos. Considerable variation in LC⁵⁰'s of temephos may have resulted from exposure to other OP's used for crop protection because temephos has not been used in ricefields in Japan. A uniform susceptibility to permethrin accorded with lack of a history of pyrethroids as agrochemicals. Insensitivity of acetylcholinesterases (AChE's) was the principal factor regulating resistance to OP's. Mosquitoes with such insensitive AChE's began to appear in ca. 1978 and rapidly replaced most of the susceptible mosquitoes within 2 or 3 yr. Since 1980 C. tritaeniorhynchus has become more abundant, and some resurgence in human cases of Japanese encephalitis has occurred at the same time. However, a recurrence of an epidemic of the magnitude of those experienced in the 1960s is implausible because of changes in agriculture and demography.