Apolygus lucorum (Heteroptera: Miridae), an important herbivore feeding on a broad range of cultivated and wild plants, always shows a strong preference for flowering host plants. In a prior study, we found A. lucorum fecundity to be higher on flowering plants than on plants still at the vegetative stage. This led us to hypothesize that ovarian development and vitellogenin (Vg) expression in A. lucorum, which are highly correlated with its fecundity, might respond positively to the presence of host plant flowers as food. In this study, two types of plant foods, i.e., cotton terminals and flowers, and cotton terminals only without flowers, were examined. Adult females on terminals with flowers had higher ovarian development levels than those of similar stage held on terminals without flowers. Moreover, adults on terminals with flowers had more follicles per ovary throughout adult life than those held on terminals without flowers, and the length of the most developed ovariole of the former treatment was significantly longer than that of the latter diet. Use of qRT-PCR showed that the expression level of Vg was significantly higher in adults raised on a diet with flowers compared with those raised on the diet without flowers. These results suggest that host plant flowers significantly facilitate ovarian development and Vg gene expression of A. lucorum adults.