To investigate the indirect effects of browsing by sika deer on a gall midge (Procystiphora uedai) on dwarf bamboo, we tested the hypothesis that by changing the quality of plant modules, deer browsing indirectly affects multiple life history traits of P. uedai both before and after colonization. A previous study in the same system reported that following gall induction, larval survival was lower in galls on previously browsed shoots, due to higher levels of parasitism. We elucidated the indirect effects of deer browsing by examining the oviposition preference, neonate survival, and potential fecundity (i.e., weight of survivors) of P. uedai on browsed versus non-browsed dwarf bamboos using exclosures. Browsing caused opposite indirect effects on different parameters. Ovipositing females preferred large, non-browsed more than small, browsed dwarf bamboo shoots. Browsing increased neonate survival, presumably by producing relatively soft, small dwarf bamboo shoots. However, deer browsing decreased the weight of survivors, due presumably to the relatively smaller galls that produced smaller shoots. Thus, the present study suggests that deer browsing has lower impact on the total survival rate of larvae than the previous study as a result of increased neonate survival, which will offset the lower survival rate of successfully colonized larvae on browsed dwarf bamboo. Therefore, our study illustrates that the indirect effects of mammal browsing on insect herbivores are not as simple as previously reported. Our study emphasizes the need for evaluating both the colonization process and post-gall-induction survival and fecundity if we are to fully understand the indirect effects of mammal browsing on insect herbivores.
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Vol. 17 • No. 4