The effect of mixing Brussels sprouts with potato plants on the foraging behavior of two parasitoid species was examined within the tritrophic system of Brassica oleraceae, the herbivore Pieris rapae L. (Lepidoptera: Pieridae), and two parasitoids, Cotesia glomerata L. (Hymenoptera: Braconiadae) and Cotesia rubecula Marshall (Hymenoptera: Braconidae). The two parasitoids differ in the spectrum of host used: C. glomerata has a wider host range, and C. rubecula has narrower host range, with P. rapae being its preferred host. The experimental design consisted of a completely crossed multifactorial design with the following factors: (1) plant diversity (monoculture and diculture), (2) species of parasitoid (C. glomerata and C. rubecula), and (3) level of experience (naive and experienced individuals). Results indicated that the effect of plant diversity was different for the two parasitoids. Naive C. glomerata, the parasitoid with the wider host range, were less efficient in the diculture than in the monoculture, but this difference disappeared after experience. In contrast, naive C. rubecula were more efficient in the diculture than in the monoculture with experience having no effect. Response level increased for both species after oviposition experience, with C. glomerata exhibiting a high degree of behavioral plasticity. Data indicates that the negative effect of polyculture on the foraging efficiency of C. glomerata is a result of an attraction to the nonhost plant (potatoes).