Geographic variation of host use in the leaf beetle Agelasa nigriceps suggests host range expansion

Host range expansion is an important event in the evolution of host use in phytophagous insects. Herein, we report geographic variation of host use in the chrysomelid leaf beetle, Agelasa nigriceps Motschulsky (Coleoptera: Chrysomelidae), and suggest that this beetle is expanding its host range. This beetle has been recently recorded on Pterostyrax hispidus Sieb. et Zucc. (Styracaceae) in addition to its common host plant Actinidia arguta (Sieb. et Zucc.) Planch. ex Miq. (Actinidiaceae). The A. arguta‐associated populations were widely found in Japan, whereas the P. hispidus‐associated populations were found only in central and southwestern Japan. In the present study, we examined adult feeding behavior and larval performance of 12 A. nigriceps populations collected from eight localities, four localities where beetles occurred only on A. arguta (allopatric localities) and four localities where A. arguta‐ and P. hispidus‐associated populations occurred sympatrically (sympatric localities). Beetles of all populations, irrespective of their host plants and localities, showed high acceptance of and high larval performance on A. arguta leaves. In contrast, we found considerable variation in the beetle response to P. hispidus leaves. The A. arguta‐associated populations of allopatric localities scarcely accepted P. hispidus leaves, whereas those of sympatric localities, particularly those of P. hispidus‐associated populations, accepted and grew on P. hispidus leaves, although the degree of acceptance and larval performance varied among localities. These results strongly suggest that A. arguta is the ancestral host for A. nigriceps, and host range expansion to the P. hispidus has occurred in this beetle.