Changes in the activity of some adaptive enzymes of the bird cherry-oat aphid R. padi after transfer from primary (bird cherry) to secondary (triticales) host plants were assessed. The following groups of enzymes were studied: (1) transferases-glutathione S-transferase (GST) and UDP-glucosyltransferase (UDPGT); (2) antioxidant enzymes-superoxide dismutase (SOD) and catalase (CAT); (3) oxidoreductases-polyphenol oxidase (PPO) and peroxidase (PX); and (4) glucoside hydrolases--and -glucosidase. The activity of the transferases and the antioxidant enzymes increased after transfer to the secondary host, but the level of activity was closely associated with feeding duration on the secondary host. The strongest induction was noted for SOD, the activity of which was more than three times greater on the secondary than on the primary host. In contrast, transfer of the bird cherry-oat aphid was accompanied by a decline in activity of PPO, PX, and -glucosidase; PPO and PX activity was 50% less in aphids fed on the secondary host rather than on the primary host. Activity of -glucosidase increased after prolonged feeding on the secondary host. The results indicated that the adaptive enzymes of the bird cherry-oat aphid enable it to feed on distantly related host plants.