noneEnter Host-instar selection behaviors of Meteorus pulchricornis in small and large arenas under lab conditions were observed using all larval instars of Spodoptera exigua as hosts. The host distribution on potted plants in the large arena was manipulated to simulate field spatial aggregation in L1 - L2 and dispersed distributions in L3 - L5. In the large arena, the parasitoid performed more encountering and rejecting acts toward young host larvae (L1 - L2) than toward old host larvae (L3 - L5), but in the small cage the above pattern was reversed. Host-handling time was significantly longer in the large cage than in the small cage for each instar host, and increased linearly with increase in host instars both in the large and small cages. The parasitoids in the small cage resumed attacks on young larvae after 653.17 s and on old larvae after 171.32 s, whereas the reverse was observed in the large cage. The young larvae (L1 - L2) were parasitized at higher rates in the large cage (23.24% - 38.54%) than in the small cage (7.88% - 21.43%), but old larvae (L4) were parasitized more than in the small cage (34.65%) compared to the counterparts in the large cage (16.86%). Our studies suggest that arena space can have major impacts on their host selection behaviors. Finally, mechanisms of differences in host selection behaviors in M. pulchricornis in different-sized spaces were discussed. BR Parasites diseases and disorders. Parasites. Insect parasites. Hosts. Insect hosts. Abiotic factors. Physical factors. Meteorus pulchricornis (Braconidae): Parasite; Spodoptera exigua (Noctuidae): Host. Meteorus pulchricornis: Lepidopteran hosts, Spodoptera exigua, Host instar selection behaviour, Spatial environment, Available space; Spodoptera exigua: Hymenopteran parasites, Meteorus pulchricornis, Host instar selection behaviour and available space. ST Animalia, Arthropoda, Insecta, Hymenoptera, Apocrita, Terebrantia, Ichneumonoidea, Braconidae; Animalia, Arthropoda, Insecta, Lepidoptera, Glossata, Heteroneura, Noctuoidea, Noctuidae. Arthropods; Hymenopterans; Insects; Invertebrates; Lepidopterans.