An endoparasitic wasp, Cotesia plutellae, inhibits pupal metamorphosis of diamondback moth, Plutella xylostella. The parasitized larvae extended their larval stage about 2 days at 25 [degree]C died after escape of wasp larvae for pupation. We set up a hypothesis that the wasp parasitization inhibits juvenile hormone (JH) degradation, which leads to extend larval stage and blocks metamorphosis. In this study, the activities of JH esterase (JHE), a major JH degrading enzyme in hemolymph, were analyzed by use of a surrogate substrate, HEPTAT (methyl 1-heptylthioacetothioate). Hemolymph JHE activities of P. xylostella showed a typical Michaelis-Menten enzyme kinetics in response to HEPTAT, where it exhibited Km at 19.7,?M and Vmax at 228.8 ?M/min/ ? I. The parasitized C. plutellae did not exhibit decrease of the JHE activity, but rather kept high JHE activity during the extended larval period. Plutella xylostella (Plutellidae) : Host Cotesia plutellae (Braconidae) : Parasite Endocrinology; Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts Cotesia plutellae; Lepidopteran hosts; Host larval JH esterase activity; Plutella xylostella Plutella xylostella; Hormones; JH esterase; Hymenopteran parasites; Host larval JH esterase activity; Cotesia plutellae none