The preference-performance hypothesis predicts that adults should select high quality hosts which can provide offspring with good resources for development. Other factors, however, can also influence the parasitoids in making decisions on the choice. Previus such studies often use adult parasitoids as model systems, but give scant attention to insects in which host search and selection are made by neonate larvae. Here we used as the model system the ectoparasitic beetle Dastarcus helophoroides, which depends its neonate larvae to search and select hosts. We first, using the long-horned beetle Saperda populnea larvae and the flour beetle Tenebrio molitor pupae as hosts, investigated whether D. helophoroides first instar larvae showed parasitization preferences between hosts varying in body size or in parasitism by conspecifics. We then, using the experimental regression design, examined developmental performances in relation to host body weight at parasitism. D. helophoroides first instar larva showed a significant preference for larger sized S. populnea larva, which were 4.55 times as likely to be parasitized as the smaller sized larva. D. helophoroides first instar larva preferred the parasitized T. molitor pupa over the healthy for parasitization, 12.57 times as likely for the parasitized as for the healthy host pupae. D. helophoroides larval development time (averaged to 11.49 d), pupal duration (26.67 d), and larva-to-adult survival rate (> 50%) were not related with host body weight at parasitism. But adult body weight at emergence significantly increased linearly with host body weight at parasitism, by an increase of almost 0.08% with each 0.01 g increment in host body weight. ANOVA and multiple comparison tests showed that D. helophoroides adult at emergence was significantly larger in body weight from attacking larger than smaller sized hosts. The results of our study resuggest that D. helophoroides first instar larva may take a trade-off between host body size and parasitism in making decisions on parasitization. In addition, our results suggest that the prefernece for larger hosts in body size by D. helophoroides first instar larva is aligned with developmental performances of offpring, lending suport for the preference-performance hypothesis.