ACETYLCHOLINESTERASE SENSITIVITY IN RESISTANT PLUTELLA XYLOSTELLA (L.)
TANG ZHEN-HUA ZHOU CHENG-LI(Shanghai Institute of Entomology, Academia Sinica, Shanghai 200025)
Larvae of the diamondback moth, Plutella xylostella (II.), from different geographical regions were tested for their resistance to malathion and carbaryl, using topical application recommended by FAO. The Shanghai and Guangzhou populations were found to be 27.4 and 113.8 fold more resistant for malathion and 3.1 fold more resistant for carbaryl than the Nanchang population. Acetylcholinesterase (AchE) activities of the larvae from Shanghai (R) and from Nanchang (S) were examined to know whether their insensitivity in AchE contributed to insecticide resistance, The relative activities of cholinesterase of R and S populations toward acetylthiocholine, propionylthiocholine and butyrylthiocholine were 0.3, 0.7 and 6.8 fold respectively. The kinetic parameters, Vmax and Km, were also determined with the three substrates. Vmax ratios with the S population were 0.4, 1.4 and 7.0, and Km ratios were 201.9, 87.0 and 37.8, tespectively. It is shown that there is a qualitative difference in the AchE between the R and S popultuions . In order ro further confirm the diffetence in inseusitiviry of AchE to inhibition, I50 and the bimolecular rate constant Ki were measured in the B and S popula tions. The I50 ratios of R to S populations was 333.0 for eserine, 66.7 for carbaryl, 17.2 for paraoxon, 32.5 for DDVP and 10.0 for propoxur, and the Ki ratios of insensitivity of AchE to paraoxon, DDVP, propoxur and carbaryl were 126.4, 86.5, 32.9 and 12.3, respectively. It is concluded that resistance to OPs and carbamates in larvae of R populations is associated with a mutant form of AchE of broad insensitivity. Resistance management strategy of the diamondback moth is discussed.