Which molecular marker is better? Comparative analyses of COI and ITS2 in molecular identification of Trichogramma (Hymenoptera: Trichogrammatidae)

The accurate identification of pests and their natural enemies is critically important in pest management. Without requiring specialized taxonomic expertise, molecular identification has been widely used in the recognition of pests and their natural enemies, as well as in the assessment of their trophic interactions. However, systematic comparative studies on the selection of molecular markers are often lacking. Trichogramma are the most widely used parasitoid wasps for controlling various lepidopteran pests in both agriculture and forests. The most commonly used molecular markers for Trichogramma are cytochrome oxidase subunit 1 (COI) and internal transcribed spacer 2 (ITS2). It is still controversial which marker is superior in Trichogramma molecular identification. We surveyed Trichogramma COI and ITS2 sequences available on NCBI and BOLD systems, performed PCR and Sanger sequencing for seven laboratory-reared Trichogramma species, and compared these two markers in several aspects. We found that ITS2 covers more species than COI in the public dataset, but its sequence misidentification rate is also higher than that of COI; ITS2 generally shows a faster substitution rate than COI, but COI sequences can be more divergent than ITS2 in some Trichogramma species; ITS2 has a higher copy number and PCR success rate than COI, but the proportion of double peaks in chromatograms of Sanger sequencing is also higher than that of COI. In summary, the molecular marks ITS2 and COI have different characteristics and can complement each other in the molecular identification of Trichogramma.