Using stable isotopes and gut dissection to determine the diet of a polyphagous ground beetle

Pterostichus melanarius Illiger (Coleoptera: Carabidae) is polyphagous ground beetle that is common in agricultural fields in western Oregon. It has been the subject of dietary studies focusing on agricultural pests and weed seeds, and many suggest that it and other carabids are important invertebrate pest and post-dispersal seed predators in agricultural ecosystems. Molecular analyses have focused on identifying and quantifying specific invertebrate prey within the guts of P. melanarius, and field and lab-based trials focus on seed predation and food preference. Consumption of food items such as worms, slugs, seeds and various insects has been established, but it is unclear what the relative frequency of food items are in their diets. Our objective was to test the efficacy of using stable isotopes and manual gut analyses to determine the relative occurrence of plant material to invertebrate material in the diet of P. melanarius, and whether it changes seasonally. This methodology may help predict and/or quantify P. melanarius’ impact on weed and invertebrate pest populations in agricultural lands. Visual examination of the gut contents of beetles from three farms in western Oregon revealed that the most frequent identifiable prey items are insect parts, followed by earthworm remains and plant material. Isotopic analysis of stomach content sampled in August and September 2009 are more depleted in δ¹³C than would be expected from a primarily plant-based diet, but enriched in δ¹³C compared to soil dwelling invertebrate prey. These differences suggest a mixed diet that is more influenced by invertebrate prey than plant material.