Tea saponin (TS) is extracted from the seeds of the tea plant and is generally regarded as a safe compound that has insecticidal properties and can act synergistically with other compounds. In this study, the activities of antioxidant enzymes superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and the levels of malondialdehyde (MDA) were compared in midgut tissues of third instar larvae of the diamondback moth (DBM), Plutella xylostella L. (Lepidoptera: Plutellidae). The larvae were fed on three different host plants, cabbage (Brassica oleracea L. var. capitata [Capparales: Brassicaceae]), radish (Raphanus sativus L. var. radiculus Persi [Capparales: Brassicaceae]), or rape (Brassica campestris L. [Capparales: Brassicaceae]), that had been treated with TS. Higher SOD, POD, and CAT activities were found in DBM larvae fed on cabbage after LC20 (concentration that induced 20% larval mortality) or LC50 (concentration that induced 50% larval mortality) treatment than on the control. On rape, TS treatments led to lower SOD and CAT activities than in the control and to higher POD activities after 24 h. MDA content increased in larvae fed on rape but decreased in larvae fed on radish after 12 h. Our results indicated that DBM larvae are more susceptible to TS on rape than on cabbage and radish, suggesting that this treatment may be an economic and effective means of controlling DBM on rape.