Natural phenolics and reactive oxygen chemical species can have metabolic and fitness costs in insects and thus represent potential insecticidal agents. The response levels of total protein, phenoloxidase, proteases, chitinases, and total antioxidant activities to dopamine and hydrogen peroxide (H₂O₂) injected into third-instar flesh fly, Sarcophaga surcoufi Villeneuve (Diptera: Sarcophagidae), larvae were measured. In comparison to control groups, total protein initially increased following treatment with H₂O₂, but decreased at later sampling intervals. Treatment with dopamine did not affect total protein or protease activity for up to 12 h after treatment, but a slight decrease was observed in both at 24 and 36 h after treatment. Treatment with H₂O₂ decreased protease activity at all sampling intervals. H₂O₂ treatment increased phenoloxidase activity at each sampling interval, while treatment with dopamine decreased the activity at each interval. Total antioxidant activity increased substantially 12 h after treatment with H₂O₂ and increased only slightly 24 h after treatment before decreasing at 36 h, while dopamine increased antioxidant activity at all sampling intervals. Chitinase activity increased 12 h after H₂O₂ treatment, but then decreased at 24 and 36 h, but dopamine had no effect on chitinase activity. Antioxidants, phenoloxidase, and chitinase are important metabolic defense components in insects, and increases or decreases in each impacts the physiological integrity of the insect exposed to stressors that increase or decrease their quantity and activity.