A comparison of live versus kill pitfall traps to assess the diet of carabids through a metabarcoding approach

Metabarcoding approaches are powerful tools to unravel trophic relationships between predators and prey. To apply metabarcoding analyses on invertebrate gut contents, specimens must be well preserved from DNA degradation, thus the trapping method should be selected accordingly. Dry pitfall traps are commonly assumed to provide a better DNA preservation than traps that use a killing agent. However, this assumption has never been specifically tested for gut content analyses. In our study, we compared how two types of pitfall trapping, dry vs. with brine, affect the conservation of prey DNA contained in the digestive tract of predators and subsequent metabarcoding analyses. We placed dry and 'classic' pitfall traps in oilseed rape fields within an intensive agricultural area in the French Nouvelle-Aquitaine region. Traps were set up in autumn and compared for carabid trapping efficiency as well as our capacity to retrieve dietary information from the digestive tract of the main carabid species, Nebria salina (Fairmaire & Laboulbene) and Calathus fuscipes (Goeze) (both Coleoptera: Carabidae). The PCR success rate was higher in dry pitfall traps compared to classic ones for N. salina. We hypothesise that this was due to the presence of PCR inhibitors in the gut of this species. The ability to sequence prey DNA did not differ between specimens caught in both trap types. The list of preyed species was similar between both trap types. However, sequencing yielded more prey operational taxonomic units (OTUs) from specimens caught in dry pitfall traps, leading to differences in prey community composition and a greater ability to reconstruct prey community. Our analyses also shed light on the prey spectrum of C. fuscipes and N. salina in oilseed rape in autumn.