We evaluated preferences of the parasitoid Glyptapanteles militaris (Walsh) among various host-diet complexes in an effort to understand the abundance of the parasitoid in Azorean pastures. We also examined effects of host diet on the quality of the parasitoid as a biological control agent. The larvae of Pseudaletia unipuncta (Haworth) were reared on three different diets (Zea mays L., Lolium perenne L., and artificial diet) and were exposed to G. militaris. The percentage of parasitized larvae (yielding parasitoid cocoons), of host mortality unrelated to successful parasitism (host death without cocoon formation) and of pupating hosts did not differ significantly among diet treatments. Also, the total number of parasitoid larvae per host and the mean number of cocoons yielded per host did not differ significantly when hosts were fed on different diets. Time from egg to cocoon formation (16.1 and 18.6 d, respectively), pupal period (8.0 and 8.5 d, respectively), and total developmental time (24.1 and 26.9 d, respectively) for G. militaris were significantly shorter (at 22°C) when hosts were reared on L. perenne compared with artificial diet. Rate of adult parasitoid emergence from cocoons was significantly lower when hosts were fed artificial diet (82.5%) than when fed L. perenne (91.2%) or Z. mays (91.1%). The longevity of the adults of G. militaris ranged from 5.1 to 7.2 d among treatments differed significantly for the three types of diets and between sexes. When adults of G. militaris could choose among hosts reared on Z. mays, L. perenne, or artificial diet, they preferred the hosts fed L. perenne leaves. These results suggest that the parasitoid may be well adapted to the Azorean agricultural ecosystems, characterized by the prevalence of L. perenne throughout the year. Results show that hosts fed fresh leaves of L. perenne are the most suitable for the mass rearing of G. militaris.