To test the possibility that the free glycine has a role related to the resistance of the pink bollworm larvae to phenolic compounds (PCs), newly hatched larvae were fed on artificial diets containing different concentrations of the selected two PCs (pyrocatechol and resorcinol). Data indicated that the PCs have a great efficacy against pink bollworm larvae. Mortality percentages had a positive relationship with both the concentration and the period after treatment (r= 0.72 ± 0.13). Based on the LC₅₀ values, pyrochatechol was the most potent extract after two hours of treatment (toxicity index (Ti) = 100.0%). Two days after treatment, resorcinol became the most effective extract (Ti= 100.0%), as it had the lowest LC₅₀ value (0.73 g/100 g diet). Seven days following treatment, the efficacy of the tested PCs follows the same trend as previously observed after 2 hours of treatment.LC₅₀ values reached 0.70 and 1.06 g/100 g diet with Ti of 100.0 and 65.91% for pyrochatechol and resorcinol, respectively. Also, we determined and compared the glycine content as well as free amino acids in the total body homogenate of 4^th instar larvae of PBW treated as newly hatched larvae with LC₅₀ of pyrochatechol and resorcinol and the untreated ones. Results showed that the PCs caused a reduction in the amino acid contents; the most obvious was in resorcinol (the most toxic compound). With audited look for glycine and lysine contents, it could be noted that the pyrochatechol caused an increased while resorcinol caused a lost in the glycine content by 18.56, 6.53%, respectively. The loss in the concentration of lysine by resorcinol (9.09%) was less than that induced by pyrochatechol (13.40%). So, the amount of free glycine in PBW appears to have a clear correlation with the detoxification activity of the PCs, especially for the resorcinol. Further study on this phenomenon will give us a chemical basis for under­standing how plant-herbivore relationships have evolved.