Drastic changes in host preference in a gall‐parasitic flea weevil,Orchestes hustachei,during the last decade

1. Factors affecting host preference in herbivorous insects are actively discussed. Larval performance, competition and predation on each host, and host abundance are reportedly factors affecting the host preference of ovipositing females. In the present study, chronological changes in female host preference were examined by employing a flea weevil species that uses the original host and a newly introduced host simultaneously. 2. Orchestes hustachei Klima (Coleoptera: Curclionidae) is an aphidophagous flea weevil that oviposits on aphid galls, and the larvae feed on aphids inside. The weevil's native hosts are galls of the Tetraneura species on Ulmus davidiana, while the novel hosts are galls of an introduced species, Paracolopha morrisoni on Zelkova serrata, an introduced hardwood. Choice experiments were conducted using Tetraneura galls and Paracolopha galls and the results were compared with experiments conducted 10 years ago. 3. More than 90% of ovipositing females selected Paracolopha galls. This result is in marked contrast with the result of the 2002 experiment, in which 66.3% of females selected Paracolopha galls. To explore driving forces of the preference change, mortality factors, pupal mass on the two hosts, and temporal changes in the abundance of the host galls were examined. 4. Abundance of Tetraneura galls decreased gradually throughout the last 26 years. By contrast, weevil survival and performance did not vary significantly between the two hosts. Therefore, it is concluded that temporal changes in the relative abundance of two hosts are main factors of the change in host preference.