Flavonoids in the hemolymph of European pine sawfly (Neodiprion sertifer) larvae that were feeding on Pinus sylvestris needles were identified. HPLC-ESI-MS analysis revealed that the main components in the hemolymph were flavonol di- and triglucosides and a catechin monoglucoside. These compounds were isolated from the larval hemolymph and their structures were established by HPLC-MS, GC-MS, and NMR spectroscopy. The isolated flavonoids were identified as (+)-catechin 7-O-β-glucoside, isorhamnetin 3,7,4'-tri-O-β-glucoside, kaempferol 3,7,4'-tri-O-β-glucoside, and quercetin 3,7,4'-tri-O-β-glucoside. The combined concentration of these four compounds in the hemolymph was 3.7 mg/ml. None of these compounds was present in the needles of P. sylvestris. Therefore, we propose that the flavonoid glucosides were produced by the larvae from flavonoid monoglucosides and (+)-catechin obtained from the pine needles.