Evidence in stable isotope ratios for lichen-feeding by Lithosiini moths from a tropical rainforest but not from a temperate forest

Lithosiini (Lepidoptera: Erebidae: Arctiinae) is distinctive in having some species that feed on lichens, whereas the majority of moths feed on vascular plants. However, the larval diet of most Lithosiini species is poorly known. This study examines whether Lithosiini species, collected in a tropical rainforest of Borneo (nine species) and a temperate forest of Japan (eight species), feed on lichens as larvae, based on stable isotope analyses. As a result, the δ¹⁵N values for eight of nine Lithosiini species collected from Borneo were notably lower than those of nine co-occurring herbivorous non-Lithosiini species, and were similar to those of sympatric, lichen-feeding termites; however, δ¹³C and δ¹⁵N values of one Lithosiini species (Adites sp.) were significantly higher than those of the other moth species and similar to those of humus-feeding termites and predatory insects occurring at the same site. These results have suggested that the Lithosiini in the Southeast Asian tropical rainforests contain some species that feed on lichens as their larval main diet and at least one species whose larvae feed on humus or animal-derived materials. In contrast, the δ¹³C and δ¹⁵N values of all examined Lithosiini species (eight species) in the temperate forest have suggested that their larvae fed on plants and not on lichens. Our stable isotope ratio analysis presented quantitative evidence suggesting lichen-feeding by Lithosiini moths in a tropical rainforest without observation of feeding behavior during the larval stages.