The ability of two benzoic add derivatives; tannic and salicylic acids to affect digestive enzymes and their substrates was evaluated. Phenolic adds were incorporated into artificial diet at concentrations of 20X10*³ M through 160X 10'³ M. In vivo studies demonstrated that treatment of Spoobptera &tom& (Boisd.) 4^th larval instar for 5 days with phenolic acids; significantly reduced growth, main metabolites and digestive enzymes. In yam experiments indicated that phenolic adds not only had the ability to affect protein (casein), but also affected carbohydrates (sucrose) and their specific enzymes; protease and invertase, Indicating their possible ability to get food less digestible. The observed reduction in weight gain could be attributed, at least in part, to the effect on digestion, but not excluding the presence of other additional mechanisms. We suggested that the oxidative stress of phenolic adds could affect digestive enzymes and their dietary substrates, which ultimately could reduce larval growth.