Dasineura mali Kieffer (Diptera: Cecidomyiidae) is an important pest of young apple trees in New Zealand and a serious quarantine pest in Asia, Australia, and western United States. Platygaster demades Walker (Hymenoptera: Platygastridae) is a parasitoid of D. mali. The developmental period of the parasitoid is longer than that of D. mail, suggesting a possible asynchrony between the two species in the field. Prior to the present study, this parasitoid was considered laying eggs only in the host eggs. Here we carried out experiments in the laboratory and field to determine whether P. demades also parasitized other stages of the host and explain the recently observed high parasitism rate and parasitoid—host synchronization in the field.We demonstrated for the first time that P. demades explored and accepted both eggs and first-instar larvae of D. mali as hosts. This feature widens the phenological window for parasitization, allowing the parasitoid to continue laying eggs after D. mali eggs hatch, and contributes to the high within- and between-generation parasitism rate and synchronization of the two species. Therefore, the difference in developmental period between the pest and its parasitoid cannot simply be considered evidence of asynchrony between the two. The consistent superparasitism rate and progeny sex ratio regardless of host stage at parasitization detected in the present study may be attributed to the fact that P. demades eggs do not hatch until the host larvae reach the third instar (mature) and thus all parasitoid larvae feed on the third-instar larvae with similar quantity and quality of nutrition. The higher parasitism rate in eggs than in larvae may be due to higher larval defense ability against attack by and lower attractiveness to the parasitoid.