Abstract: The dynamics of topically applied ³H-abamectin to the fourth instar larvae was compared between the ABM-S and ABM-R strains. The results showed that during the period of 5 to 360 minutes, the number of ³H-abamectin penetrated the cuticular of ABM-R strain was 1.5-fold reduction to that of ABM-S strain. Treatment after 24h, the extraneous³H-abamectin still remained 45.9% in the ABM-R strain. While in the ABM-S strain, there was 98.4% of the ³H-abamectin penetrated the cuticular. Radioligand binding assay indicated that the change of GABAA receptor binding was also involved in abamectin resistance. Scatchard assay showed that there was no significant difference or receptor affinity between the ABM-S strain (K_d=10.9368±0.4374 nmol/L), and the ABM-R strain (K_d=9.8328±0.3933nmol/L). However, the maximum number of GABA_A receptor was decreased 63.6% in the ABM-R strain (B_max= 71.2842±4.9910 fmol/mg protein) compared with the ABM-S strain (B_max=112.0255±7.8418 fmol/mg protein). Therefore, the change of GABA_A receptor binding was caused by the reduction of receptor number, not the change of its molecular structure.

Key words: Plutella xylostella, abamectin, resistance, cuticular penetration, GABAA receptor