Activities of Glutathione S-Transferase and Glutathione Peroxidases Related to Diet Quality in an Aphid Predator,the Seven-spot Ladybird,Coccinella septempunctata L. (Coleoptera: Coccinellidae)

Larvae of Coccinella septempunctata were reared on three aphid diets, consisting of pure Rhopalosiphum padi, pure Metopolophium dirhodum, and an equal mix of these aphid species. In the pupal stage, the activities of three detoxifying enzyme systems — glutathione S-transferase (GST) with 1-chloro-2,4-dinitrobenzene as substrate, glutathione peroxidase with hydrogen peroxide as substrate (GSH-Px[H₂O₂]), and glutathione peroxidase with tert-butyl hydroperoxide as substrate (GSH-Px[TBH]) — were assayed. Growth rate, measured as the total protein content of the pupae, was significantly higher in the mixed-diet group than in the R. padi group, whereas that of the M. dirhodum group was intermediate. GST showed lower activity in larvae on a pure R. padi diet and a mixed aphid diet than on a pure M. dirhodum diet, whereas the variation in GSH-Px[TBH] was independent of diet. GSH-Px(H₂O₂) showed a significantly higher activity in the R. padi group than in the M. dirhodum group, whereas that of the mixed-diet group was intermediate. Thus, feeding on the low-quality aphid, R. padi, inhibited GST and activated GSH-Px(H₂O₂). The induction of GSH-Px[H₂O₂] indicated elevated oxidative stress. This may have been caused by toxic compounds in the R. padi.