The predatory bug Orius majusculus (Reuter) was reared on 2 different diets during the nymphal stages. The 1st group was exclusively offered eggs of Ephestia kuehniella Zeller (Lepidoptera: Pyralidae), a standard diet for O. majusculus production. The 2nd group was exclusively offered 4th instars of the pea aphid, Acyrtosiphon pisum (Harris). Subsequently, adult predatory behavior in experimental arenas containing A. pisium was recorded using 2 video cameras. One camera permitted observation of the predator’s contact with the prey, where the 2nd camera viewed the arena from above to record the path taken by O. majusculus adults before and after contact with prey. When O. majusculus were reared on aphids, adult bugs successfully located and consumed 55% of experimental prey and continued prey search behavior after each aphid meal. O. majusculus adults that had no experience of aphid predation as nymphs, did not prey on aphids in the experimental arena. The mean walking speed of this group of predators increased from 5.9 ± 1.2 mm/s to 9.8 ± 0.7 mm/s after contact or detection of prey, indicating that predators rapidly moved away from unfamiliar prey. Moreover, for egg-reared O. majusculus, all contacts between aphid and predator were lateral, along the side of the prey and were effectively repelled by an aphid kicking response. In contrast, 83% of attacks by aphid-reared O. majusculus were directed at the head or posterior abdomen for which the prey could not defend themselves adequately. When egg-reared O. majusculus were exposed to novel aphid prey for 1–8 d, the frequency of aphid attack increased significantly. We conclude that the standard diet used for rearing O. majusculus may adversely affect the efficiency of this predator as an agent of biological control.