Culture-dependent variation in esterase isozymes and malathion susceptibility of diamondback moth, Plutella xylostella L.

Five cultures of the Sheh-Tzu diamondback moth, Plutella xylostella L., sho wing different levels of malathion resistance, were successfully raised by the single male-female mating method. The LD50 Of malathion to ST10 was 29- and 22-fold that to ST15 and ST12, respectively. ST26 and ST34 possessed 1 4- and 13-fold the resistance to malathion as did ST12. The difference in t he adult emergence rate of treated larvae between ST10 and ST12 was 920 fol d. Toxicity of malathion to ST26 larvae was mostly synergistically enhanced by S, S, S- tributyl phosphortrithioate and diethyl maleate. A zymogram st udy using l-naphthyl acetate as a substrate on PAGE gel showed that ST10, 2 6, and 34 were possessed of significantly higher frequencies of esterases 8 b/9b which were rarely found in ST12 larvae. Esterases 9 and 8null were fou nd in most susceptible larvae. ST26 shared toxicological characteristics ag ainst malathion and in biochemical properties of the slow-moving esterase w ith ST34. Subcultures of ST26, which showed differential susceptibility to malathion, were significantly and linearly correlated with the differential frequency of esterases 8b/9b of the moth. Frequencies of esterases 3 and 4 b of subcultures were either positively or negatively correlated with a low titer of resistance. The significance of the correlation between isozyme f requency and malathion resistance of DBM is discussed. The possibility of u sing the recessive alleles of esterases 4b and 9b as indicator proteins for monitoring malathion resistance of the DBM is also discussed.