Indirect effect of deer browsing on a gall midge of the dwarf bamboo and its parasitoids

Abstract

At Mt. Odaigahara, in west-central Japan, we compared the survivorship of an unidentified gall midge (Cecidomyiidae: tribe Oligotrophini) that forms galls on the culms of dwarf bamboo (Sasa nipponica), the major forage for sika deer (Cervus nippon), between in- and outside deer exclosures. We also compared the parasitism rates by two parasitoid wasps, Pediobius sasae (Eulophidae) and Torymus sp. (Torymidae), on the gall midge between in- and outside deer exclosures. Deer browsing reduced the gall width indirectly. Both the survivorship of the gall midge and the parasitism rate by Torymus sp. was higher inside deer exclosures than outside deer exclosures, but the parasitism rate by P. sasae was higher outside deer exclosures than inside deer exclosures. The parasitism of P. sasae concentrated on the galls with smaller width inside deer exclosures but it occurred irrespectively of the gall width outside deer exclosures. Torymus sp. has a longer ovipositor and finishes its emergence period earlier than P. sasae. These results suggested that both the gall midge and Torymus sp. was able to survive in larger galls that P. sasae could not penetrate with its shorter ovipositor, making the different survivorship and parasitism between in- and outside deer exclosures. Thus, deer browsing on the dwarf bamboo indirectly reduces the gall width, and this favors P. sasae by improving access to host and disturbs both the survival of the gall midge and the parasitism of Torymus sp.