The preference of Microplitis mediator for larval Leucania separata in different instars and the suitability of host instar for the parasitoid were investigated under laboratory conditions at 25 [plus or minus] 1[degree]C, 65% [plus or minus] 5% RH and 14:10 (L: D) h. When the parasitoids were exposed concurrently to host larvae from the 1st to 5th instars, they could parasitize the 1st to 4th instar larvae, but refused the 5th instar larvae. Parasitism rates were higher for the 2nd and 3rd instar larvae than for the 4th and 1st instars . The preference index of M. mediator for the 1st , 2nd, 3rd and 4th instar larvae were 0.21, 0.31, 0.27 and 0.21, respectively. Host mortality after exposure to the parasitoid decreased with the increase of host ages. The parasitoid developed in the 4th instar larvae had longer development period from egg to cocoon stage and had heavier cocoon weight than those in the 3rd and 2nd instar larvae. Young M. mediator adults had higher parasitism capacity than the old ones. However, age of the adult wasp had no significant effect either on the development period from the egg to cocoon or on the cocoon weight in their offsprings. Leucania separata (Noctuidae) : Host Microplitis mediator (Braconidae) : Parasite Biometrics; Life cycle and development; Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts; Ecology; Animal constructions Leucania separata; Age; Hymenopteran parasites; Host parasite interactions influence of parasite & host age; Microplitis mediator Microplitis mediator; Age; Weight; Cocoon weight; Development; Development period; Lepidopteran hosts; Host parasite interactions influence of parasite & host age; Leucania separata; Cocoons none