none A proteinaceous factor that represses plasma juvenile hormone esterase activity in parasitized insect larvae has been isolated and partially characterized from last instar larvae of the armyworm Pseudaletia separata parasitized with the wasp Apantales kariyai . Purification procedures consisted of extraction with 25% ethanol, gel filtration and reversed phase high performance liquid chromatography. Plasma juvenile hormone esterase activity in Day 3 last instar larvae was repressed by 50% when larvae were injected on Days 1 and 2 with 6.5 pmol of the purified peptide, which has a molecular weight of about 4,500 Da. The application of the factor also causes more than a 2-day delay in the onset of pupation. Biochemistry; Proteins; Life cycle and development; Development; Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts; Host parasite relationships Pseudaletia separata (Noctuidae); Enzymes; JH esterase activity, effects of hymenopteran parasite; Metamorphosis; Hymenopteran parasite effects; Hymenopteran parasites; Apanteles kariyai; Parasite effects on JH esterase activity & metamorphosis; Reaction of host to parasite; Apanteles kariyai (Hymenoptera) effects on JH esterase, mechanisms Apanteles kariyai (Braconidae); Lepidopteran hosts; Pseudaletia separata; Effects on host JH esterase & metamorphosis; Reaction of host to parasite; Pseudaletia separata (Lepidoptera) host; Effects on host JH esterase activity & metamorphosis, mechanisms