Water chestnuts (Trapa spp.) dominate many shallow eutrophic lakes, and impact aquatic fauna. Use of Trapa beds by animals may vary with the growth stage of Trapa, but little is known regarding when and how animals use Trapa and nearby, non-Trapa habitat. To clarify seasonal habitat use by the Trapa-feeding chrysomelid beetle (Galerucella nipponensis), we examined seasonal changes in the density of G. nipponensis from Trapa beds and from adjacent emergent vegetation. Furthermore, to determine whether habitat use by beetles is associated with their physiological state, we measured their oviposition and foraging activities. The density of G. nipponensis in each life-stage changed seasonally, with a high density of adults inhabiting emergent vegetation in July, after beetle densities in Trapa beds had declined. Adults collected from Trapa beds showed active egg-laying and foraging activities, while beetles collected from emergent vegetation were in reproductive diapause. This is the first report of G. nipponensis leaving its host plant to initiate summer diapauses in another habitat type.