none Comparative toxicity tests showed adult, late larvae, and early larvae of the host tortricid, A. citrana (Fernald), less susceptible to propoxur than similar life stages of the braconid ectoparasite O. americanus. Propoxur was not less synergized by piperonyl butoxide (PBO) for all of these same life stages of the host vs. the parasite. Levels of aldrin epoxidase were low in life stages of both species when measured in whole-body assays; however, gut assays of host larvae demonstrated high levels of this enzyme. Higher levels of MFO (mixed function oxidase) (cytochrome P-450 monooxygenase) activity were observed when a tyrosinase inhibitor was added to host aldrin epoxidation reactions, but not in the parasite test. Considering data from whole-body, gut and inhibitor tests, aldrin epoxidase levels were probably higher in the lepidopteran life stages than in the braconid life stages. Similar levels of trans-epoxide hydrolase, glutathione transferase and general esterases, but lower levels of cis-epoxide hydrolase were found in the lepidopteran host as opposed to the braconid life stages. The consistently higher ratios of trans/cis epoxide hydrolase in the herbivore versus entomophage are similar to data found earlier in detoxification comparisons between several species of prey and predators. The epoxide hydrolase enzyme system may be an appropriate target site for exploiting selectivity differences between pests and natural enemies.