The plant growth regulator gibberellic acid (GA₃) is known to negatively impact growth and development of insects. In this study, larvae of Galleria mellonella L. (Lepidoptera: Pyralidae) were fed a diet with varying dosages of GA₃ to investigate how antioxidant enzymes are influenced. Activity levels in last instars reared in laboratory at 25±2°C, 60±5% relative humidity, and a photoperiod of 12:12 (L:D) h were measured for superoxide dismutase (SOD), glutathione S-transferase (GST), and catalase (CAT). Treatment with GA₃ in diet resulted in a remarkable increase in the activities of both SOD and GST at lower GA₃ doses (50–1,000 ppm) with respect to control and higher doses. The activity of CAT in the hemolymph of last instars significantly increased at all doses when compared with that in the hemolymph of untreated larvae. This trend in the increase of CAT was not dose-wise, except for the significant increases at 2,000 and 5,000 ppm when compared with that of untreated and all treated groups. Consequently, our results showed that GA₃ is effective at activating the antioxidant defense system of insects as a source of free radical and can be toxic for larvae in a dose-dependent manner. Therefore, we suggest that the increase in the activity of GST, SOD, and CAT in larvae may indicate a physiological adaptability to compensate for GA₃-induced stress.