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This paper investigated the effects of grazing by Galerucella nipponensis (Coleoptera: Chrysomelidae) on growth and seed production of its host plant Trapa japonica (Trapaceae). A water tank experiment demonstrated that the new rosette leaves grazed upon by the beetles had a shorter life span than those of ungrazed rosettes. In ponds, severely grazed T. japonica rosettes produced more new leaves than slightly grazed rosettes. The period of seed production was delayed in years when leaves were severely grazed during the flowering seasons. Grazing peaked in later growing seasons when rosettes produce seeds. The ratio of seed production in the latter half of the peak to that in the early half was higher under severe grazing conditions than slight or moderate grazing conditions. A higher density of seeds in ponds in the current year tended to produce more abundant rosettes in the following spring. These results suggest that the herbivory of G. nipponensis induces compensatory reproduction of T. japonica in a complicated manner. [ABSTRACT FROM AUTHOR]

The spatial structure-mediated indirect effects of an aquatic host plant, Trapa japonica (Trapaceae), on survival of a leaf beetle, Galerucella nipponensis (Coleoptera: Chrysomelidae), with regard to predation by a water strider, Gerris nepalensis (Hemiptera: Gerridae), were investigated in an irrigation pond and under indoor conditions. Beetle density, particularly in the first-instar stage of the larvae, became extremely low in the second generation in the pond, even though food resources were abundant. The biological check method, which excludes natural enemies by caging, suggested that a causal factor of the low density was predation by Gerris nepalensis in midsummer and thereafter. Normally growing rosettes extend their leaves horizontally on the water surface. However, some rosettes of T. japonica grew leaves vertically in the later seasons, and such overgrown rosettes were distributed patchily in the ponds. Beetle density was higher on the latter rosettes than on the former ones. The indoor experiment showed that the water strider can feed on the eggs of beetles on normally grown rosettes but not on the overgrown rosettes. Furthermore, female adults of Galerucella nipponensis preferred the overgrown rosettes over the normally grown rosettes for oviposition when water striders were present. These facts suggest that the spatial structure-mediated indirect effects of the host plant modify the interaction between herbivore and predator. [ABSTRACT FROM AUTHOR]