The resistance to methamidosphos and methomyl and the effect of synergists on the insecticides in susceptible (S) and resistant (R) strains of Diaeretiella rapae (Hymenoptera: Aphidiidae) were detected using the residual film method. The kinetic parameters of acetylcholinesterase (ACHE), the activity of detoxication enzymes in D. rapae and its host Lipaphis erysimi (Homoptera: Aphididae) and the inhibition of synergists on enzyme activities in D. rapae in vivo were also investigated. The results show that compared to the susceptible strain, resistance to methamidosphos and methomyl was 5.6-fold and 9.1-fold respectively in the field D. rapae collected from Fuzhou, Fujian Province, China. Obvious synergisms of piperonyl butoxide (PB), triphenyl phosphate (TPP) and diethyl maleate (DEM) to these insecticides were found in D. rapae, the synergism of PB being the highest. The synergisms of PB to the two insecticides in the R strain of D. rapae were significantly higher than those in the S strain. PB, TPP and DEM could not inhibit AChE activity, but strong inhibition of CarE activity was caused by PB and TPP, and GST activity by DEM. The apparent Michaelis-Menten constant (Km) and maximal velocity (Vmax) of AChE and the activities of carboxylesterase (CarE) and glutathione S transferase (GSTs) in the S strain were similar to those in the R strain, but the values of the bimolecular rate constant (Ki) in the S strain of D. rapae were far higher than those in the R strain of D. rapae. In addition, a comparison of kinetic parameter of AChE and the activities of CarE and GSTs between D. rapae and L. erysimi were conducted. The results indicate that the resistance mechanism to methamidophos and methymol in D. rapae is related to AChE insensitivity and detoxication enzymes. Biochemistry; Proteins; Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts; Abiotic factors; Chemical factors; Land and freshwater zones; Palaearctic region; China Diaeretiella rapae (Aphidiidae); Enzymes; AChE, carboxylesterase & glutathione S transferase; Hemipteran hosts; Lipaphis erysimi; Enzyme activities, role in insecticide resistance; Fertilizers and pesticides; Methamidosphos & methomyl; Fujian; Fuzhou; Enzyme activities role in insecticide resistance Lipaphis erysimi (Aphidoidea); Hymenopteran parasites; Diaeretiella rapae; Parasite insecticide resistance mechansims none