Reduced susceptibility of the diamondback moth, Plutella xylostella LINNÉ, to pyrethroid insecticides

Abstract

The diamondback moth, Plutella xylostella L., is known to be resistant to various kinds of insecticides. In Japan some strains of the moth are resistant to organophosphorus and carbamate insecticides, but no obvious resistance was previously observed for pyrethroid insecticides. This paper is intended to report a new evidence of the declined susceptibility of the diamondback moth to pyrethroid insecticides. A first failure in controlling the moth population with a mixture of fenvalerate and malathion was noted at a cabbage field in Mizobe, Kagoshima Prefecture in summer of 1984. In that field, a mixture of 0.01% fenvalerate and 0.03% malathion has been sprayed since summer of 1983 at a rate of 200 to 300l per 10 a. LC₅₀ values were determined in the laboratory by the leaf or body dipping method. The laboratory experiments indicated that the diamondback moth extremely reduced its susceptibility to the pyrethroids tested. The moth was also reconfirmed to be less susceptible to organophosphate and carbamate insecticides, but it was still susceptible to Bacillus thuringiensis preparations.