Induced resistance in plants affects insect growth and development as a result of the up-regulation of defence-related secondary metabolites or enzyme-binding proteins. In the present study, the effects of jasmonic acid (JA) and salicylic acid (SA) induced resistance in groundnut on Helicoverpa armigera (Hübner) are examined. Larval survival, larval weights and the activities of digestive enzymes (total serine protease and trypsin) and of detoxifying enzymes [glutathione S-transferase (GST) and esterase (EST)] are studied in insects fed on four groundnut genotypes with moderate levels of resistance to H. armigera (ICGV 86699, ICGV 86031, ICG 2271 and ICG 1697) and a susceptible genotype (JL 24). The plants are pre- and/or simultaneously treated with JA and SA, and then infested with H. armigera, which are allowed to feed for 6 days. Significantly lower serine protease and trypsin activities are observed in H. armigera fed on plants treated with JA. Greater GST activity is recorded in insects fed on JA and SA treated plants, whereas EST activity is low in H. armigera larvae fed on plants treated with JA and SA. Serine proteases, trypsin and GST activities and larval weights (r = 0.74–0.95) and larval survival (r = 0.77–0.93) are positively correlated, whereas EST activity and larval weight (r = −0.55) and larval survival (r = −0.65) are negatively correlated. The results suggest that midgut digestive and detoxifying enzymes can be used as indicators of the adverse effects of constitutive and/or induced resistance in crop plants on the insect pests and the role of JA and SA in insect pest management.