Host age selection by the host-feeding pupal parasitoid Diadromus subtilicornis (Gravenhorst) (Hymenoptera: Ichneumonidae)

Laboratory experiments were conducted to determine the effects of host age on oviposition and host-feeding by Diadromus subtilicornis, a pupal endoparasitoid of the diamondback moth Plutella xylostella. When given 6 hosts of single ages, females oviposited in hosts of prepupae, 1-d-old green and 2-d-old pupae 66-75% of the time. However, females oviposited in only 43% of 1-d-old beige pupae, 31% of 3-d-old pupae and none of 4-d-old pupae. They often rejected those hosts after antennal examination or drilling. Total time taken for oviposition increased with increasing host age. When given 36 hosts of 6 different ages, females preferred hosts of prepupae, 1-d-old green and 2-d-old pupae for oviposition and hosts of 1-d-old beige, 2-d-old and 3-d-old pupae for host-feeding, suggesting host-age selective oviposition and host-feeding. Although 93% of parasitized prepupal hosts yielded emerged adult parasitoids, successful emergence rate decreased with increasing age. No parasitized 3-d-old pupal hosts yielded adult parasitoids. Sex ratio (% females) of parasitoids emerged from hosts ranged from 28.6% for 1-d-old beige to 44.4% for prepupae. Host ages did not affect developmental time and size of emerged adults. We suspected that D. subtilicornis has mixed host-feeding strategies of non-concurrent and concurrent types according to host availability.