Comparison of behavior and foraging ability between two congeneric species of large-bodied diving beetle (Coleoptera: Dytiscidae) larvae, a non-expanding species and a distribution-expanding species

Global warming has altered the distribution of several insect species, including those inhabiting aquatic environments and this range expansion may modify existing interspecific interactions by facilitating new encounters between species that were not originally sympatric. Global warming is highly likely to expand the distribution of Cybister tripunctatus lateralis (Coleoptera: Dytiscidae) in Japan, leading to interspecific competition for food resources among congeneric species. In particular, Cybister larvae share similar feeding habits, suggesting that interspecific competition is likely to occur, although this has not yet been verified. Therefore, this study aimed to compare the microhabitat use and foraging abilities of the larvae of two species - C. tripunctatus lateralis with an expanded distribution area and Cybister brevis, a non-expanding species of similar body size. We compared (1) behavior quantified by visual scan census, (2) time taken to reach food, and the number of prey animals consumed by each species in the laboratory experiments. The results indicated that the behavioral patterns of the larvae tended to involve mainly perch and rest on the plants. There were no clear differences between the two species, although there were variations among the instars. There was no difference in the number of prey animals consumed by the two species; however, it was evident that C. tripunctatus lateralis reached the food earlier than C. brevis. Therefore, it is suggested that the two species utilize a common underwater microhabitat, and that C. tripunctatus lateralis has a competitive advantage over C. brevis in terms of food resources. However, further experimental verification and continued expansion of C. tripunctatus lateralis distribution trends should be carefully monitored, as reproductive habitats in the field differ and competitive advantages may depend on water temperature.