Variation in weed seed DNA detectability among arable carabids with different trophic specialization

While most ground beetles (Coleoptera: Carabidae) include seeds in their diet, preferences for seed feeding vary among carabid species and range from facultative diet supplementation to obligate seed feeding. DNA-based diet analyses have been used to study their regulatory effect on soil seedbanks. It is unknown whether specialized granivores digest seed species they are adapted to (‘essential seeds’) faster, and whether this affects food web construction based on molecular data. We hypothesized that specialized granivores digest their essential seed faster than other seeds, and at faster rates than generalist granivores or carnivores. Further, we assumed that generalist granivores digest different seeds equally fast, while carnivorous carabids digest seeds slower than granivores. In feeding experiments, three carabid species—Amara similata (specialized granivore), Harpalus affinis (generalist granivore), and Poecilus cupreus (generalist carnivore)—were fed either a broadly accepted seed or the specialist's essential seed. Gut content samples were collected 0, 6, 12, 24, 48, and 72 h after feeding and screened with plant-primers to trace seed DNA. Time until 50% detection probability in the specialists was shorter for its essential than the broadly accepted seed and shorter than in the generalist granivore, which digested both seed species equally fast. The carnivore was reluctant to feed on the seed species offered, and detection probabilities did not significantly decrease with digestion time. Our findings suggest that the strength of specialized granivores’ feeding interactions and their role in weed seed regulation might be underestimated when assessed with DNA-based diet analysis, due to their more efficient seed digestion.