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., showing different levels of malathion resistance, were successfully raised by the single male-female mating method. The LD sub(50) of malathion to ST10 was 29- and 22-fold that to ST15 and ST12, respectively. ST26 and ST34 possessed 14- and 13-fold the resistance to malathion as did ST12. The difference in the adult emergence rate of treated larvae between ST10 and ST12 was 920 fold. Toxicity of malathion to ST26 larvae was mostly synergistically enhanced by S, S, S- tributyl phosphortrithioate and diethyl maleate. A zymogram study using 1-naphthyl acetate as a substrate on PAGE gel showed that ST10, 26, and 34 were possessed of significantly higher frequencies of esterases 8b/9b which were rarely found in ST12 larvae. Esterases 9 and 8null were found in most susceptible larvae. ST26 shared toxicological characteristics against malathion and in biochemical properties of the slow-moving esterase with 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 4b of subcultures were either positively or negatively correlated with a low titer of resistance. The significance of the correlation between isozyme frequency and malathion resistance of DBM is discussed. The possibility of using the recessive alleles of esterases 4b and 9b as indicator proteins for monitoring malathion resistance of the DBM is also discussed.