Gonocephalum japanum, Gonocephalum sexuale are the pioneer beetles populating the volcanic desert of Mt. Fuji. The typical pioneer plant in this area is abundant Polygonum cuspidatum. This plant forms vegetation patches of various sizes. The plant species composition in the patches of P. cuspidatum is different: the central part is dominated by P. cuspidatum, Aster microcephalusand Miscanthus oligostachyus, while edges have mainly P. cuspidatum. These differences may provide different habitats to herbivorous beetles.

We set pitfall traps at center, edge and outside of these patches from July to September 2010. We conducted observation experiments on behavior and manipulative experiments on feeding habits in the laboratory.

Two beetles showed clearly different association with different parts of the patches. Whilst G. japanum appeared both at centers and edges in the patches, G. sexuale were seen only at the edges. Six-week feeding experiments showed different rates of survival between two beetles. Survival was highest in G. japanum and G. sexuale fed with P. cuspidatum. Survival was significantly less in beetles fed with A. microcephalus, particularly in G. sexuale. This indicates that G. sexuale is a specialized consumer of only P. cuspidatum litter. These results suggested that feeding preference is more important factor in determining their habitat.