none Cohorts of apterous Myzus persicae reared at 16.3 degree C and 22.2 degree C were exposed at various ages of their development to the oviposition by Diaeretiella rapae. Irrespective of the age at the beginning of initial parasitization and rearing temperature, the developmental rates of the aphids were unaffected; but obvious decrease of reproductive rates, cessation of reproduction and death of the hosts occurred when the parasites inside their bodies had completed 30%, 40% and 60% of their total development into adults respectively. As a result, the impact of the parasite on the performance of the hosts declined rapidly as their age at the initial parasitization increased. When the aphids were attacked in the first to the second instars, they generally failed to produce any young. When the aphids were attacked at the late second instar and onward, the realized intrinsic rate of increase of the aphids increased rapidly as the age at the initial parasitization increased, and approached to that of the unparasitized ones when parasitized immediately after emergence to adults. These results thus indicate that the pattern of relative probability of being parasitized among individuals in different instars is one of the crucial factors affecting the dynamics of an aphid-parasite system, and the parasite must attack a large proportion of nymphs of the first three instars to have a substantial impact on the population increase of the host aphid. Biometrics; Reproduction; Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts; Host parasite relationships; Ecology Diaeretiella rapae (Braconidae); Hemipteran hosts; Myzus persicae; Reaction of host to parasite; Myzus persicae (Hemiptera); Impact, influence of host age at parasitization Myzus persicae (Aphidoidea); Age; Hymenopteran parasitization, influence on parasite impact; Reproductive productivity; Hymenopteran parasites; Diaeretiella rapae; Reaction of host to parasite; Diaeretiella rapae (Hymenoptera); Parasite impact, influence of age at parasitization; Population dynamics; Increase rate, hymenopteran parasite impact, age at parasitization effect