Optimality models of host-parasitoid relationships have traditionally assumed that host quality increases as a function of host age (size) at parasitism. The current study contributes to the growing base of literature suggesting that this generalisation is not necessarily valid for many koinobiont. parasitoid species, which allow the host to continue development after parasitism. Enormonia formosana Scopoli (Lepidoptera: Tortricidae) larval size at the time it is parasitised by the ichneumonid Compoplex dubitator Horstmann was shown to have highly variable effects on selected fitness correlates, ranging from linear to non-linear to non-existent. For mate progeny, increasing initial host size led to a reduction in development time but had little or no association with body size or longevity. Host size effects were more apparent for female offspring, with the youngest hosts producing later emerging but larger and more fecund females and the oldest hosts producing less fecun! d, smaller females that emerged first. Host mortality before parasitoid egression was more common for hosts parasitised at early instars, indicating a significant fitness cost associated with development in these host stages. Overall, our findings that C. dubitator maximises body size and egg load at the expense of increased development time agree with recently developed models of host utilisation patterns of parasitoids attacking concealed hosts. However, we found a greater trade-off between development time and body size than predicted by these models. The fitness consequences of delayed development could be traded off against increased body size (i.e. egg load), thereby enabling C. dubitator to attack a wider range of host stages and the parasitoid offspring to undergo optimal development for the environment (host stage) in which it finds itself. [copyright] 2005 Gesellschaft fur Okologie. Published by Elsevier GmbH. All rights reserved. BR Biometrics. Life cycle and development. Evolution. Parasites diseases and disorders. Parasites. Insect parasites. Hosts. Insect hosts. Enarmonia formosana (Tortricidae): Host; Campoplex dubitator (Ichneumonidae): Parasite. Campoplex dubitator: Development, Natural selection, Fitness, Lepidopteran hosts, Enarmonia formosana, Host size effect on parasite development and fitness; Enarmonia formosana: Size, Body size, Hymenopteran parasites, Campoplex dubitator, Host size effect on parasite development and fitness. ST Insecta, Lepidoptera, Glossata, Heteroneura, Tortricoidea, Tortricidae; Insecta, Hymenoptera, Apocrita, Terebrantia, Ichneumonoidea, Ichneumonidae. Arthropods; Hymenopterans; Insects; Invertebrates; Lepidopterans. none