The beet armyworm(Spodoptera litura) is a global insect pest that feeds on various agricultural crops including vegetables,cotton,and ornamentals.To investigate the physiological mechanism that Microplitis pallidipes parasitizes larval S. litura,we collected the hemolymph of host larvae (parasitizated and unparasitized),and measured the content of free amino acid using an amino acid analyzer.The results indicated that the content of total free amino acid in the hemolymphs of larval S. litura were consistently higher from 1 to 4 d,but lower on 5 d after parasitization in the parasitized group than in the control.The content of 17 free amino acids in the hemolymph of the S. litura larvae changed after parasitization with significant differences in some free amino acids.The content of Glu,Pro and Tyr in the larval hemolymph were consistently higher after parasitization than that in the control.The content of Cys and Ala significantly increased on 1 and 3 d after parasitization and the content of Leu,Ile,Lys and Arg showed similar trend on 4 d after parasitization.Our results reveal that the content of free amino acid varies in host larvae at different developmental stages of the parasitoids and that this variation benefits the growth of parasitoids.