none Selectivity of 9 insecticides in diamondback moth, P. xylostella (L.), and its natural enemy, A. plutellae Kud. was investigated. Among the insecticides tested, acephate, dichlorvos, profenofos and fenvalerate, with smaller selective ratios, were recomended to use with integrated biocontrol. Synergistic studies with piperonyl butoxide, the microsomal mixed-function oxidase inhibitor, and EPN (O-ethyl-O-p-nitrophenylphenylphosphonothioate), the hydrolase inhibitor, showed that the additions of either synergist to most insecticides in this experiment created an adverse effect on natural enemy conservation. The data confirmed a hypothesis proposed previously by other authors, i.e, differential tolerance to insecticide occurs between the herbivorous insect pest and its natural enemies, basically a result of the coevolution among the host plant, the pest insect and the natural enemies; that a toxic secondary substance in the plant species, working as a selecting agent, enlarges the difference of oxidative activity between the insect pest and its natural enemies but not the nonoxidative enzymes during the evolutionary process. This theoretical basis is useful for proposing the proper candidate insecticides in integrated pest management system.