A considerable number of carabid beetle specimens are regularly collected in agroecosystems for biodiversity monitoring and they are often stored in ethanol. Analysing the diet of these formerly caught carabid beetles would allow to gain insight into potential trends of their invertebrate prey community through time and potentially relate such patterns to changes in their environment. However, the quality of genetic material may be a limiting factor. We used a COI gut contents metabarcoding approach to characterize the diet of carabids specimens from two dominant species, Poecilus cupreus and Nebria salina, caught in oil seed rape fields in western France and stored in ethanol for 0–9 years. Contrary to expectations, specimen storage time did not reduce detection of prey DNA but enhanced it. Indeed, prey detection was higher in older samples than in recent ones, which may be due to an overrepresentation of good quality carabid DNA in the digestive tract of recently caught specimens. The diet of the two studied carabid species varied annually, showing significant switches between years in particular in the case of Nebria salina, which stresses the importance of temporal studies to fully describe the diet of carabid beetles. In addition, we detected a significant temporal trend in the diversity of prey caught by the two carabid species. We found that the two carabid beetles that dominate carabid community displayed a decreasing trend over the last decade in the number and the diversity of prey, which supports the general hypothesis of a decline in the diversity of invertebrate communities in intensive agricultural landscapes. In this study, we highlighted the power of metabarcoding for biomonitoring in historical samples for the longitudinal analysis of diets. By relying on existing collections of carabid beetles, this method could open a promising avenue to explore mid to long-term temporal variation in the animal diet and/or to detect consequences of long-term changes, such as global warming or land use changes, on predators and prey communities.
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