Graduate Thesis Or Dissertation
 

Using stable isotopes and visual gut examination to determine the diet composition of Pterostichus melanarius (Coleoptera: Carabidae) in western Oregon vegetable row crops

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  • The omnivorous ground beetle, Pterostichus melanarius Illager (Coleoptera: Carabidae), is a ubiquitous inhabitant of western Oregon crop land. Because it is often the most abundant ground beetle, and attains densities that could lead to serious impacts on its prey sources, I sought to understand the overall diet composition, and specifically the importance of weed seeds in its diet at three sites in western Oregon. I compared the use of stable isotope analysis and manual gut dissection as tools for determining diet composition, and examined the influence of site characteristics. Visual gut inspection identified broad groups of food items that included arthropods, worms and plant material, while the analysis of δ¹³C and δ¹⁵N signatures revealed more defined prey groups including small ground beetles, spiders, soil larvae and C3 and C4 seeds. Visual examination of gut contents, and comparing the δ¹³C and δ¹⁵N signatures of potential prey sources with gut content each lead to diet compositions that varied between sites and seasons. However, both techniques indicated that the primary contributors to the diet of P. melanarius are ground dwelling arthropods and earthworms. Specifically, the δ¹³C and δ¹⁵N signatures of small ground beetles, earthworms, spiders, centipedes and soil dwelling larvae made the largest contributions to the δ¹³C and δ¹⁵N signature of the diet at each of the sites. Weed seeds did not contribute significantly to the beetle's diet, however there is preliminary evidence that suggests further examination of the larval diet may reveal that seeds are important at that stage of life. I did find overall differences in δ¹³C and δ¹⁵N diet signatures between sites—the isotopic signatures at the two most geographically similar sites were different than those at the third site. Because general categories of food that were included in the diet did not differ between sites, we attribute isotopic differences to as yet undetermined site characteristics.
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