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Host range expansion of herbivorous insects is a key event in ecological speciation and insect pest management. However, the mechanistic processes are relatively unknown because it is difficult to observe the ongoing host range expansion in natural population. In this study, we focused on the ongoing host range expansion in introduced populations of the ragweed leaf beetle, Ophraella communa, to estimate the evolutionary process of host plant range expansion of a herbivorous insect. In the native range of North America, O. communa does not utilize Ambrosia trifida, as a host plant, but this plant is extensively utilized in the beetle's introduced range. Larval performance and adult preference experiments demonstrated that native O. communa beetles show better survival on host plant individuals from introduced plant populations than those from native plant populations and they also oviposit on the introduced plant, but not on the native plant. Introduced O. communa beetles showed significantly higher performance on and preference for both introduced and native A. trifida plants, when compared with native O. communa. These results indicate the contemporary evolution of host plant range expansion of introduced O. communa and suggest that the evolutionary change of both the host plant and the herbivorous insect involved in the host range expansion.

The need to understand the mechanisms enhancing species coexistence and diversity is one of the central issues of community ecology. Theoretical work suggests that resource partitioning based on body size contributes to the coexistence of competing species, resulting in increased species diversity. Nevertheless, few empirical studies have investigated this. The species composition of wood‐dwelling arthropods was surveyed to examine resource partitioning based on body size in the dead wood of the ubame oak (Quercus phillyraeoides A. Grey) trees. Greater resource sizes (larger dead wood diameter) retained a lower overall density of wood‐dwelling beetles per resource volume, greater total beetle biomass per resource volume and larger bodied individuals of the dominant species (Cerambycidae; Anaglyptus nipponensis Bates). A gradient in the species composition of arboreal ant assemblages along resource size and a positive correlation between resource size and ant body size was observed. These results suggest that the incidence of wood‐boring beetles was limited by the size of the available resource, resulting in resource partitioning by the ants according to the sizes of galleries made by the beetles. Our findings indicate that resource size leads both directly and indirectly to body size‐dependent resource partitioning in beetle and ant assemblages, and contributes to the maintenance of species coexistence in the assemblages present in dead wood.

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Abstract. Ophraella communa (Coleoptera: Chrysomelidae) is an oligophagous herbivorous beetle that feeds on Ambrosia artemisiifolia. It is native to North America, but was accidentally introduced into Japan in 1995 and Europe in 2013. We analyzed partial DNA sequences of the mitochondrial cytochrome oxidase II gene for O. communa collected from 29 locations in the United States, Japan and Italy. Overall, the results of our analyses indicate that the introduced Japanese populations have lower genetic variation than the native populations. The sequences for the Italian specimens did not share haplotypes with Japanese specimens. These results indicate that the introduced Japanese populations originated from a single introduction, and that the Italian and Japanese populations have different origins.