Host-plant resistance and biological control are considered to be cornerstones of integrated pest management programs, but there are few studies that elucidate the intricate relationships among host plant, phytophagous insect and natural enemies. Our knowledge of interactions in systems that include three or more trophic levels is limited, yet the need for such knowledge is great. First, for understanding how selection forces shape these relationships, and secondly, for knowing how we can manipulate the systems to our benefit. Two aspects related to studies of tritrophic interactions deserve special attention for those working on biological control: (i) the incorrect assumption that host-plant resistance always positively contributes to the suppressive effect of natural enemies, and (ii) the correct assumption that plants can be selected that promote the effectiveness of natural enemies. Examples are presented to illustrate the first point. A study of the relationships among cucumber, greenhouse whitefly [Trialeurodes vaporariorum (Westwood)] and the parasitoid Encarsia formosa Gahan illustrates the second point—selection of less-hairy plants leads to the finding and killing of more hosts per unit of time by the parasitoid, and thus to improved biological control.