Abstract: Effects on toxicity of insecticides by sublethal doses of anticholinesterase agents (phoxim, malathion and methomyl) and their induction of AChE specific activity, the Michaelis constants(K_m) were investigated in the larvae of the cotton bollworm, Helicoverpa armigera(Hübner). The results indicated that toxicity of five insecticides had significant differences at 24 hours after treatments. The AChE specific activity of cotton bollworm was moderately depressed at 24 hours after treatment with LD₁₀ of phoxim. The timecourse study revealed that the maximum induction of AChE specific activity by malathion and methomyl was observed at 3 h and 12 h after treatment, resulting in a 1.45fold and 1.92fold increase of AChE activity, respectively. The sublethal dose of phoxim had no effect on the affinity of AChE toward acetylthiocholine in 48 hours after treatment, but AChE affinities were significantly reduced after treatment with LD10 of malathion and methomyl. The density gradient centrifugation showed that the distribution of AChE activity in the 5% gradient layer was higher than the control at 24 hours after treatment with sublethal doses of anticholinesterase agents, but lower in the 20% gradient layer. It was inferred that treatment of sublethal doses may cause the changes in molecular forms and the distribution of different molecular forms of AChE.

Key words: Helicoverpa armigera, acetylcholinesterase, anticholinesterase, toxicit y, sublethal doses