1. Species diversities of some insect lineages have been attributed to differentiation of feeding habits among species. Our objective was to determine variation in diet composition among harpaline ground beetle species occurring in a riverside grassland.
2. We examined the diet compositions of 14 species from six genera in the spring and 10 species from two genera in the autumn. We performed measurements of nitrogen and carbon stable isotope ratios in consumers and in their potential food items, and estimated relative contributions of different food items with two mixing models, IsoSource and MixSIR.
3. IsoSource and MixSIR software gave similar results, but IsoSource tended to calculate higher contributions of principal food items and smaller percentile ranges than MixSIR. Among harparine beetle species, there were diverse food utilisation patterns among four food categories (detritivorous invertebrates, herbivorous invertebrates, C3 plants, and C4 plants). Detritivores comprised the main diets of abundant harpaline species in the spring, whereas abundant harpaline species in the autumn were primarily herbivores feeding on C4 plants, or omnivores feeding on herbivorous invertebrates and C3 plants. Seasonal changes in food use were related to seasonal changes in the abundance of each food resource.
4. Mixing model analysis of stable isotope ratios is a convenient and effective method for roughly estimating diets of many species with diverse food habits (such as ground beetles). This method can contribute to determining the trophic relationships of related insects in one ecosystem.