In tritrophic interactions, host plants could influence not only the population of insect pests but also that of their natural enemies. This study examined the effect of three wheat cultivars on demographic parameters of Schizaphis graminum Rondani (Hemiptera: Aphididae) and its predator, Hippodamia variegata (Goeze) (Coleoptera: Coccinellidae). Furthermore, the secondary metabolites, photosynthetic pigments, and defence enzymes of wheat cultivars were evaluated at different times. According to the results, the highest and lowest developmental times of S. graminum were found on cultivar (cv.) Morvarid and cv. Ehsan, respectively. However, it was vice versa for the predator, being highest on cv. Ehsan and lowest on cv. Morvarid. Adults of S. graminum lived shorter on cv. Morvarid, while males and females of H. variegata lived longer on this cultivar. The intrinsic rate of natural increase (r) of S. graminum ranged from 0.347 to 0.456 day−1, and that of H. variegata from 0.118 to 0.176 day−1 on different cultivars. The value of this parameter was lowest for aphids but highest for their predator on cv. Morvarid. Based on the results, cv. Morvarid at 120 hours post-infestation by S. graminum contained the greatest concentrations of secondary metabolites and enzyme activities. The time-dependent loss of photosynthetic pigments occurred in each tested cultivar. The results revealed that cv. Morvarid had a suitable potential for reducing the population of S. graminum and enhancing the performance of H. variegata. However, further studies are needed to evaluate the complementary interactions of cv. Morvarid and H. variegata in controlling S. graminum population under natural conditions.