none Microplitis kewleyi Muesebeck is a gregarious internal parasite of larvae of the black cutworm, Agrotis ipsilon (Hufnagel). Studies of the biology of the parasite revealed that there was an inverse relationship between host instar and parasite preference. Duration of development from egg to pupa ranged from 18 days at 27 degree C to 68 multiplied by 7 days at 16 degree C. Development from egg to pupa took 13 multiplied by 5-21 multiplied by 6 days when fourth and first instar host larvae, respectively, were parasitized. A larger number of parasites emerged from hosts parasitized in the fourth instar (22 multiplied by 4) than the first instar (11 multiplied by 5). Parasite pupation occurred when the host was in the fifth/sixth instar, depending on the instar parasitized. Thirty-nine per cent of host larvae exposed as first instars to parasites died before parasite emergence. This decreased to 0% for host larvae exposed as fourth instars. The sex ratio was 1:1 multiplied by 2 (M:F). Biometrics; Reproduction; Life cycle and development; Development; Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts; Ecology; Population dynamics Agrotis ipsilon (Noctuidae); Age; Larval instar; Larval development; Instar, hymenopteran parasite preference & development relationships; Hymenopteran parasites; Microplitis kewleyi; Parasite biology & host parasite relationships; Mortality; Parasitism by Hymenoptera effect, influence of host age Microplitis kewleyi (Braconidae); Reproductive productivity; Daily progeny production, lepidopteran host; Development; Duration, influence of temperature & lepidopteran host age; Lepidopteran hosts; Agrotis ipsilon; Parasite biology & host parasite relationships