Abstract: Inhibition and toxicokinetic action of several insecticide-combinations on the esterases from the heads of cotton bollworm (Helicoverpa armigera Hübner) were assayed. Residue activity of the esterases assayed in vitro showed that the I₅₀ values affording 50% inhibition of monocrotophos, trichlorfon and carbofuran were 2.4443×10^-6, 3.4562×10^-7 and 2.6302×10^-5（mol/L） respectively. Graphic analysis on the toxicokinetic mode of action on the enzyme of each insecticide indicated that monocrotophos and carbofuran inhibited the enzyme competitively, while trichlorfon did the same non-competitively. Joint treatment of two insecticides with non-competitive inhibition and competitive inhibition resulted in a potentiated inhibition on the enzyme, and I₅₀ values of monocrotophostrichlorfon and trichlorfon-carbofuran were 1.0846×10^-6 and 5.1786×10^{-6 (mol/L)} respectively. On the other hand, joint treatment of two insecticides with competitive inhibition led to a competitive inhibition, but no potentiated joint action. These results suggested that the toxicokinetic interactions between the insecticides and the easterases play an important role in the joint action of the insecticide-combinations on the enzyme.

Key words: cotton bollworm, joint action, insecticide-combinations, esterases, toxicokinetics