Effects of interspecific competition on development and reproduction in two giant water bugs, Diplonychus japonicus vuillefroy and Diplonychus major esaki (Hemiptera: Belostomatidae)

Summary We studied the interpecific competition between 2 species of predatory aquatic bugs, Diplonychus japonicus and D. major by conducting a field experiment. We set up 3 types of experimental plots in the paddy fields where D. major predominated. The two plots contained single species of either D. japonicus or D. major , respectively, and one plot had both species in equal number. We compared the development and the reproductive performance between plots in each species. In D. japonicus , the number of eggs and early instar nymphs were significantly smaller in the plots containing both species than in the monospecific plots. However, the numbers of late instar nymphs and newly emerged adults were not significantly different between plots. The proportions of starved nymphs in both plots were larger than those in the D. japonicus's natural habitats. The final densities of adults in both plots were lower than those in the natural habitats. These results suggest that lower density of D. japonicus in these paddy fields is due to the lack of available food for nymphs rather than the effects of interspecific competition with D. major . In D. major , significant differences were not found in the number of eggs, each instar nymphs and adults. These results suggest that the effects of interspecific competition did not affect the reproductive performance of D. major .