Environmental <scp>DNA</scp> metabarcoding for insect biodiversity surveys: Current status and future prospects for potential applications in highly diverse insect taxa

Abstract Environmental DNA (eDNA) is a groundbreaking molecular tool that despite its challenges, offers a powerful and comprehensive method for surveying biodiversity. Insects, as key bioindicators of water quality in river ecosystems, make insect eDNA an especially promising tool for biomonitoring and water management. However, significant issues remain. This review explores historical and ongoing eDNA studies, focusing on the genetic diversity of insects, which underpins eDNA analysis. We address two key issues regarding eDNA metabarcoding: selecting suitable DNA barcoding regions for accurate molecular identification and the importance of DNA databases. A review of 82 papers showed that while the mitochondrial DNA (mtDNA) COI region is widely used, it often amplifies nontarget groups. Regarding primer concordance rates among insects, the mtDNA COI region, mixed nucleotide bases are often used to increase the match rate for insects. In contrast, primers targeting the mtDNA 16S rRNA region actually show higher match rates for insects. In addition, we highlight the critical role of regional DNA databases, particularly for species that exhibit genetic differentiation across regions. Also, using a local database, we demonstrated improved detection sensitivity, underscoring the importance of local databases for accurate species identification. To guide future database construction, we reviewed phylogeographic studies and using the Japanese Archipelago, which is a region with high species diversity, proposed genetic ecoregions that demonstrate genetic regionality globally. Our findings emphasize that insect eDNA is at the forefront of species identification technology, offering immense potential for future applications. With innovative approaches, it could revolutionize biomonitoring tools for diverse ecosystems.