Aphidius colemaniViereck is an important parasitoid of green peach aphid [Myzus persicae (Sulzer)], a serious pest of greenhouse crops. Augmentation of natural enemies is the most used approach to biological control of greenhouse pests. Yet, the optimal release time, rate, and age of natural enemies are often unclear. Here we tested the influence of population density of both A. colemani and M. persicae, as well as parasitoid age, on parasitism and sex allocation. We aim is to provide information for cost-effective aphid biological control programs. We show that 1) parasitoid females increased parasitism and produced more female-biased offspring with an increase of aphid density, 2) a moderate increase of parasitoid release rate elevated parasitism and daughter production when aphid density was high, 3) parasitism rate declined with an increase of aphid density but the moderate increase of parasitoid release rate eased the decline rate, and 4) 3-d-old parasitoids parasitized more aphids than younger ones but parasitoid age had no effect on daughter production. These findings suggest that A. colemani females can adjust their ability to regulate the aphid population in response to aphid population dynamics. In augmentative programs, a moderate increase of parasitoid release when aphid density is high could achieve effective control and a release of 3-d-old parasitoids could lead to a better control outcome.