The effects of four commonly used pesticides, diazinon (EC 60%), phosalone (EC 35%), fipronil (Granular formulation 0.2%), and pyriproxifen (EC 10%), on functional response of Habrobracon hebetor (Say) (Hymenoptera: Braconidae) were investigated using two lepidopteran hosts, Ephestia kuehniella Zeller (Lepidoptera: Pyralidae) and Heliothis viriplaca (Hufnagel) (Lepidoptera: Noctuidae). Laboratory bioassays determined diazinon and fipronil as highly toxic insecticides for all developmental stages of the parasitoid, while the acute toxicity of phosalone and pyriproxyfen was moderate to very low depending on the life-stage studied. Larval, pupal, and adult stages of the parasitoid were exposed to sublethal concentrations (LC30) of insecticides, and the newly mated females were used to study functional response of the parasitoid to different host densities. With a single exception, a type II functional response was found for control and all insecticide treatments on all life stages and both host species. All insecticides tended to decrease the asymptote of the functional response curve and the maximum parasitism rate, probably by negatively affecting the searching efficiency (a') of the parasitoid. These results indicate that the control efficiency of parasitoids may be negatively affected by sublethal doses of pesticides, even though the type of functional response remains unchanged. As a potential solution, appropriate timing of pesticide application is required to avoid the antagonistic interactions with natural enemies under integrated management programs.